A seedling with green cotyledons and emerging radical
Most of us have witnessed dicot seed germination at some point in our lives – watching the coytledons transform from seed halves to green, photosynthetic structures, while the radicle developed into the seedling root system. This seedling root – or taproot – is important to seedling survival as it buries itself in the soil to provide structural support and to give rise to fine roots for water and nutrient absorption. But that’s where much of our visual experience ends – because we don’t see what’s happening underground. Without additional visual information we imagine the taproot to continue growing deep into the soil. And while this perception is borne out when we pull up carrots, dandelions, and other plants without woody root systems, the fact is that woody plants do not have persistent taproots – they are strictly juvenile structures. Understanding the reality of woody root systems is critical in learning how to protect and encourage their growth and establishment.
Mature carrots have taproots. Mature trees do not. Photo courtesy of Pixnio.
Trees, shrubs, and other woody perennials all have juvenile taproots just like their herbaceous counterparts. But these long-lived plants develop different morphologies over time, which are primarily determined by their soil environment. Water, nutrients, and oxygen are all requirements for sustained root growth. Gardeners always remember the first two of these needs, but often forget the third. And it’s oxygen availability that often has the biggest effect on how deeply root systems can grow.
Roots grow where they can. Sometimes that zone can be very shallow, as this coastal forest photo shows.
Whole-plant physiologists have known for a long time that “roots grow where they can” (Plant Physiology, Salisbury and Ross, 1992). But this knowledge has become less shared over time, as whole-plant physiologists at universities have been largely replaced by those who focus on cellular, molecular, and genetic influences (and can bring in large grants to support their institution). Sadly, many of these researchers seem to have little understanding about how whole plants function. Simply looking at the current standard plant physiology textbook (Plant Physiology and Development, Taiz et al., 2014) reveals as much. (To be fair, there is now a stripped-down version of this text called Fundamentals of Plant Physiology, [Taiz et al., 2018] but even this text has little to do with whole plants in their natural environment.) If academics don’t understand how plants function in their environment, their students won’t learn either.
The Table of Contents for Plant Physiology and Development. You won’t find a discussion of woody root ecophysiology in here.
Well. Time to move on from my soapbox moment on the state of higher education.
Roots grow where oxygen is plentiful. It becomes a limiting factor as soil depth increases. Photo courtesy of Wikimedia.
Let’s look at what happens with a young tree as it develops. The taproot grows as deeply as it can, but eventually runs out of oxygen so vertical growth stops. At the same time, lateral root growth increases, because the levels of oxygen closer to the soil surface are higher. These lateral roots, and their associated fine roots, develop into the adult root system, continuing to grow outwards like spokes on a wheel. When pockets of oxygen are found, roots dive down to exploit resources. These are called sinker roots and they can help stabilize trees as well as contribute to water and nutrient uptake.
Gardeners and others who work with trees and other woody species would do well to remember that woody root systems, by and large, resemble pancakes rather than carrots. These pancakes can extend far beyond the diameter of the crown – so this means protecting the soils outside as well as inside the dripline.
Typical root structure of a mature tree in its natural environment. No taproot here!
Given the growing (haha) popularity of vegetable gardening over the last several years, which has gone into overdrive during the pandemic, more and more people are looking for innovative ways to grow in all kinds of spaces. Container vegetable gardening can be as simple as popping a tomato into a bucket, but there are lots of different ways to successfully grow crops in small, mobile containers. It is possible to grow full sized crops in containers, given a large enough container and space to grow. But more and more plant breeders have been developing small and dwarf cultivars of lots of different kinds of crop plants to meet the burgeoning interest in container and small space gardening. Let’s talk a bit about growing in containers, about some of those crops that do well in containers (including some dwarf/small cultivars, and even some design to make those vegetable containers attractive on your patio or porch.
Container Culture
Growing vegetables and fruits in containers follows the same general rules that ornamentals and houseplants follow. We’ve covered several container questions here on the GP blog, which you can find here. Probably one of the biggest questions (and myths) that we encounter is the placement of rocks or other items in the bottom of pots for drainage. It is a common question over on our social media. So to just get that out of the way, don’t do it – it actually makes drainage worse. The only exception might be if you are using a really large, deep pot and need to fill it with something so you don’t have to fill it all the way with soil – but you still need to ensure that the soil is sufficiently deep so that you don’t end up with waterlogged soil in the root zone.
Here are some other best practices to keep in mind:
Use only good quality potting mix, not garden soil, top soil, or “bargain” potting mix. Container culture means that soil needs to be “light and airy” to ensure proper balance of soil, air, and water.
Leafy greens can be grown in shallower pots than bigger crops like tomatoes and peppers.
Choose the right size and shape of container for the job. You have to look at container diameter for the plant size, but also ensure the proper depth and volume of soil to support root growth. Small crops like leafy greens can make do in a shallower container, but large rooted plants like tomatoes and peppers will require a larger volume. For example, you can grow one tomato plant in a five gallon container (if you’re a “thrifty” gardener, this means you can drill some holes in the bottom of a food-safe 5 gallon bucket). But you can also grow 12 carrots in the same size container, given that the soil is deep enough to accommodate the carrots. For a good size and spacing chart for “standard” sized crops, check out this table from UF Extension.
Drainage is a must. Make sure your containers have good drainage holes (and don’t add rocks!). If your containers are in an area exposed to rain, it would be best not to have saucers under them so that they don’t sit in water.
Make sure the containers are food safe. This isn’t an issue if your using just about any purchased container meant for container gardening, but if you’re repurposing containers you want to make sure they won’t breakdown or leach chemicals into the soil. Some plastics will break down in sunlight, but most should be food safe. The one big exception is plasticized (softened) PVC. Hard/rigid PVC is OK, but the softer plasticized versions can release dangerous phthalates when breaking down. You can look for the number 3 in the recycling symbol to know if you have PVC, and if it is soft and pliable don’t use it. Galvanized metal is another risk, as it can release zinc or cadmium into the soil both of which are harmful to humans. This is alarming as metal containers and raised-bed garden kits have been hitting the market and lots of people grow in galvanized livestock tanks. Be sure if you are using metal containers that they are either not galvanized or are sealed (or you create a barrier) if they are.
Make sure the light is right. Growing in containers doesn’t mean that tomatoes and cucumbers will become shade-loving plants. You’ll still need a minimum of 6 (preferably 8-10) hours of full sun for most fruit or root crops. You can grow shade tolerant crops, like most leafy greens, in shaded areas such as covered porches and under trees.
Nutrients are limited to what is in the potting soil, so keep an eye out for signs of nutrient deficiency and fertilize accordingly. Most potting soil comes with an initial dose of fertilizer, but you’ll probably need to add more through the season.
Keep on the lookout for insects and diseases – they still happen in container plants, too.
Little Plants for Big Flavor
It is possible to grow most standard vegetable plants in containers, save for maybe giant plants like pumpkins and some squashes. However, breeders have been developing numerous crops in small, container-ready sized plants over the last decade or so. These cultivars can let you grow more plants in smaller containers. For many, the fruit or harvestable portion is similar to that of the standard sized plant, but for others the edible parts are miniature themselves. These plants are not just cutesy wootsy (though they really are that), but they are also great alternatives to pump variety into any sized garden.
As a trial judge for the All-America Selections program, I’ve had the pleasure of trialing several plants over the last few years that are great for containers, including the 2021 AAS regional winner Pot-a-peno jalapeno pepper bred by Pan- American seeds. I’m excited that this year, a new trial has been added to the program to specifically trial plants for container growing, so be on the lookout for more container garden winners in the future.
Container-sized vegetables come in all shapes and sizes. Some of my favorites are ‘Patio Choice Yellow’ Tomato, which grows 18 inches tall and produces numerous yellow cherry tomatoes and the 2-ft tall ‘Patio Baby’ eggplant that produces 2-3″ eggplants (both plants are AAS winners). There’s the cucumber with only 3 foot long vines called ‘Patio Snacker’ and a 4 inch cabbage head named ‘Katarina’. You can find a fairly good list on this document I put together for my container vegetable gardening workshop:
Mini varieties of plants have even created some community-driven projects, like the Dwarf Tomato Project that uses a co-op type process where home gardeners are crossing plants in their own gardens to develop new dwarf cultivars of tomatoes.
Vegetable Garden, but make it Pretty
Gardens like this tomato, pepper, basil and flower combo are common on places like Pinterest
Many who grow container gardens like to make attractive gardens to decorate their porches, patios, and decks. Vegetable gardens can range from the utilitarian (like a tomato plant in a 5 gallon bucket) to the beautiful. There are lots of ways to mix plants to get a good container design if that’s what you’re after. Mixing color, shape, and form of plants can be done just as easily with vegetables as it can be with petunias and geraniums. You can add in flowers for extra pops of color as well. All one needs to do is search the internet (especially places like Pinterest) to find ideas for dressing up container gardens. I talk about container designs with vegetables in my recent talk (recording shared below) and the plant list I shared above.
Last month in my blog My Soil Is Crap Part I, I tried to dispel the myth that you can diagnose soil problems by just looking at your soil. While the color of a soil does impart some diagnostic qualities, most soils are not easily analyzed without a soils test. A complete soils test will give a textural analysis including useful information about water holding capacity and a variety of chemical analyses. Soil reaction or pH is an essential component of any soil test (and is often unreliable in home soil test kits). Soil reaction affects the availability of plant required mineral salts. Most soil tests give a measure of the salinity sometimes call TDS, or total dissolved salts (solids). Finally specific mineral content of soil is usually analyzed – in particular macronutrients are usually quantified. With these data a great deal can be predicted about the “grow-ability” of your soil. Soil tests can also help guide attempts to modify soils. The biology of soils is not easily or routinely analyzed through soils tests.
Soil Harm
Soil can be “harmed” in several ways–making it less able to grow plants. Or another way to look at this is that soil can be enhanced in several ways to grow plants better. First let’s examine the harm. Soil can be physically harmed by tilling with a rototiller. Tillage destroys structure and the natural clods and peds that form over time because of a soil’s innate qualities. Structured soils support plants and help prevent disease. Tilled soils will in time resume their native structure, but the amount of time required is quite variable depending on soil type. Soil structure can also be squished– this is compaction. Compacted soils hold less water, take water in slowly (so more runoff) and have less air space for gas exchange. In severely compacted soils roots have difficulty penetrating so plants don’t grow well or at all in compacted soil zones. Compacted soils are common in parks, school yards and public areas. Finally soils can be damaged chemically and biologically. Excessive salts from fertilizers applied in excess can compromise roots causing fertilizer burn. Soil residual herbicides from overapplication can have toxic effects on plants growing there or nearby. Herbicides and salts often accumulate along roadsides where they are used to melt snow and ice or control weeds.
Compacted, saturated or layered soils can build toxic gases that reduce metals in the soil, creating hazardous conditions for plant growth
Climate affects on soil
Climate can modify soils making them less than optimal for growing plants. In areas of high rainfall, soils may become deficient in certain ions such as metals, which tend to leach from soil, leading to increased acidity because these ions help maintain pH neutrality. In areas where precipitation is less than evaporation, salts tend to accumulate in soil and soil reaction rises above neutral. The ideal soil pH for most plants is 6.8. At this pH, most plant-required minerals are available for absorption by roots. As pH moves above 8 or below 5, soils are said to be alkaline or acid and various minerals are less available to plants. Soil reactions between pH 6.8 and 7.2 usually pose few problems for most plants. Some plants that are “acid loving” like blueberries are adapted to grow in low pH soils where nutrients are supplied by decaying organic matter. For these kinds of plants, some soil modification may be necessary (unless you live in a climate where such plants are natives). Testing your soil pH is very important to understand nutrient availability in general.
Amending vs Mulching
Arid soils are usually low in organic matter. In climates with more rainfall where forests or grasslands naturally occur, soils have higher organic matter content. Typically organic matter ranges between 1 and 5% of total soil solids. Organic matter supplies carbon for soil microbes and is necessary to promote soil structure. Organic matter can hold and release positively charged (cations) soil mineral nutrients used by plants. Organic soils have the highest cation exchange capacity (CEC), a measure of soil fertility. Soil organic matter tends to bring soil pH back toward neutral. Very acid or alkaline soils can be modified by adding organic matter. Finally, organic matter may contain nutrients that help plants grow. Sometimes amending with a nutrient-rich compost will give annual plants quite a boost (see Calendula images below) While arborist chip mulches yield nutrients to soils slowly over years, composts provide nutrients immediately, and they can be easily over-applied depending on what is required for a given soil to grow the intended plants. If you are going to amend a soil, be sure that the amendment has enough nitrogen in it. Well-formed composts, high in plant required mineral nutrients but not overly salty, make excellent amendments.
Adding amendment to planting holes of perennials is not recommended because it has little long term effect
Perennials, including all woody plants, generally do not benefit from amending because they rapidly grow out of the amended zone in the planting hole. Unless you amend an entire site, not much will happen. Also, once perennials are set in the ground you can’t amend again. Mulches of arborist chips, fresh or aged, are best for perennial plantings. Mulches can be replenished as needed without disturbing root systems. Raised beds are often amended heavily, and rightly so, since these planting situations amount to large containers that need a more porous “soil”. Since raised bed plantings are usually annuals, amendment can be added again as needed between crops. Composts make suitable amendments. Compost qualities, especially salinity, should be carefully measured or monitored before using, or through a bio-assay as detailed in my last blog.
Compost applied to clay soil.
No compost on a clay soil
Calendulas left and right are the same age
Adding minerals and fertilizers
Gardeners generally buy and add fertilizers without concern to harming their plants. This is a big NO. Excess levels of phosphate can interfere with uptake of other needed minerals. Applying fertilizer to landscapes above what is needed can pollute creeks and other bodies of water. It is important to let your soil test guide fertilizer applications. Usually there are enough fertilizer elements in most soils that landscapes can remain unfertilized, especially if leaf litter and mulches are utilized. If plants show deficiency symptoms be sure to check your soil reaction to make sure that the pH is in a growing range for the plants you are cultivating. If the pH is right but you still have symptoms, then consideration of fertilizers based on soils tests is appropriate.
There is some confusion about use of minerals as amendments. Lime is used to raise pH and often brings soils back into production in high rainfall areas where soils are too acid. Gypsum does not alter pH of soils but is often called things like “clay buster” or “compaction reliever” This is because salt affected clay soils have too much sodium which is replaced by calcium when gypsum is applied to a sodic soil relieving some of the particle dispersion. Most gardeners do not have sodic soils (which are greasy and poorly productive) but just plain old clay or clay loams. Gypsum supplies sulfate as an anion and calcium as a cation and if sulfur or calcium are deficient gypsum can be helpful. Gypsum is not needed in most gardens. Gypsum does have a fungicidal effect against root rot organisms (Phytophthora) and can be added to reduce root rot hazard. Epsom salts (magnesium sulfate) are often recommended for rose culture, but there is no research showing any benefit from their application to roses. In our trials in California, application of Epsom salts had no effect on rose bloom quality or quantity. Some soils low in magnesium could benefit from magnesium sulfate but these are fairly rare.
Some Soil changes are not long lasting
The textural nature of soil (i.e., relative amounts of sand, silt and clay) does not change over time. While we can add organic matter, it breaks down and disappears rapidly. Water quality, evaporation, and rainfall drive soil change. These factors tend to bring soil back to its “native” conditions. Irrigated soils may be affected by the quality of the irrigation water. So if you are trying to grow blueberries in Las Vegas, this will be a challenge that likely can’t be met by soil modifications. Growing plants adapted to the type of soil and climate you have is best. Growing exotics that require a different soil formation process will always be an uphill battle better suited to container culture.
References:
Blakey, D. 2021. Adjusting soil pH in California Gardens. UCANR publication 8710. https://doi.org/10.3733/ucanr.8710
Downer. A.J. and B.A. Faber. 2021. Organic Amendments for Landscape Soils. UCANR publication #8711.
Downer, A.J., and B.A. Faber. 2019. Mulches for Landscapes UCANR publication #8672.
Faber, B.A., A.J. Downer, D. Holstege, and M.J. Mochizuki. 2007. Accuracy varies for commercially-available soil test kits analyzing nitrate nitrogen, phosphorus, potassium and pH. HortTechnology: 17:358-362.
Messenger, B.J., Menge, J.A., and E. Pond,. 2007. Effects of gypsum on zoospores and sporangia of Phytophthora cinnamomi in field soil. Plant Disease 84(6): 617-621
If you’ve been around as long as I have, you will no doubt remember the Creedence Clearwater Revival song “Have You Ever Seen the Rain”. This week I want to talk about sensing the rain using radar and how you can use it to provide you with local rainfall information if you don’t have a rain gauge of your own.
Source: Brocken Inaglory, Creative Commons
How does radar work?
Radar is what scientists call an active sensor, because it sends out a beam of electromagnetic radiation that is reflected back to the radar if it hits something reflective like raindrops or hail (it also works on birds, insects, and cars traveling along interstates, but that’s another story). By detecting how much of the original beam is returned and how long it takes to get back, the radar can determine how much precipitation there is and how far away it is falling. The radar emitter usually rotates around a circle to provide a 2-dimensional picture of the precipitation in the area around the radar instrument. They can make it 3-dimensional by tilting the radar up at different angles to see different levels in the atmosphere. Now, the newest doppler radars used by the National Weather Service can also sense the size of the falling particles and how fast they are moving towards or away from the sensor. The radar displays that are usually used on television or online show a color-coded map with the brightest colors corresponding to the highest radar returns and thus the heaviest rain rates.
Source: Environment Canada
Radars can be used to estimate rainfall, but some assumptions must be made about the rain to get a good estimate. The major estimate that is needed is what size or sizes are the raindrops and how many of them are present. That will allow the radar software to calculate the volume of water that is falling and relate it to the strength of the return “echo” of the radar beam.
But how do they know the distribution of raindrop sizes in a rainstorm?
Source: Jason Zhang, Creative Commons
I learned this week in a video on raindrop shapes that the first person to measure rainfall size distributions was William Bentley, a citizen scientist in Vermont who is best known for his spectacular photographs of snowflakes. Bentley used a tray filled with a shallow layer of flour and exposed it to falling rain. The drops landed on the flour and dried into balls that provided a measure of how the size of the drops varied in the storm. Of course, now there are more sophisticated ways of determining this using optical sensors and other devices, but this was surprisingly good for its time.
William Bentley, photographer (public domain)
Today, by measuring the amount of radar emissions returned to the sensor and calibrating it to rain gauge measurements at the surface, atmospheric scientists have been able to provide good estimates of the rain falling across the region that the radar is able to sense. That is usually within about 120 miles before the radar beam overshoots most of the rain clouds due to the earth’s curvature. Fortunately, with a network of radars across the country, we can get a pretty good estimate of rainfall that is spatially much more detailed than we can get with a network of surface observers from the National Weather Service, state networks like the agricultural weather network I manage at the University of Georgia, or the volunteer corps of observers in CoCoRaHS (for more on this network, see https://gardenprofessors.com/the-weather-where-you-are/). That allows us to have a pretty good sense of how the rain is varying across fairly short distances and provides a reasonable estimate of the rain at your house if you don’t have a rain gauge available.
Radar-estimated rain where you are
To find the rainfall estimates for your location, the easiest way to do it is to use the National Weather Service’s Advanced Hydrologic Prediction Service. This website provides a daily rainfall amount based on radar estimates for the period currently from 8 AM EDT on the previous day to 8 AM on the day of the map. They are usually available an hour or two after that time period ends so they can receive the data and perform quality control before releasing the maps. You can zoom in on the maps to your location and add county outlines or other backgrounds to help pin it to your exact location. The site also allows you to look at 7-day, 14-day, and longer accumulation periods and to compare those to normal or expected precipitation. The map below is one I created for a heavy rain event in Georgia this past week on April 25, 2021, where a few locations in southern Georgia got up to 10 inches in just a few hours, causing problems for farmers there due to standing water, erosion due to runoff, and scattered loss of seed and fertilizer.
Radar-estimated rainfall for 24 hours ending at 8 am EDT on April 25, 2021.
The radar maps are not perfect. You can only zoom down so far, and the smallest unit is still at least a few kilometers or miles on a side, so you will never be able to distinguish the exact edge of a summer thunderstorm that drops rain on one side of the road and leaves the other side dry. The estimates also tend to be too low in high-intensity rainfall because the relationships that the radar software uses to estimate the volume of water don’t work very well when it is raining harder than normal. But by calibrating the rainfall to observers’ reports, they are usually pretty reasonable. If you are not in the United States, you will need to check with your own nation’s weather service to see what radar information is available.
Coming in May…
Speaking of “normal”, in May NOAA is expected to update the normals for temperature and precipitation for the US from the 1981-2010 values to the 1991-2020 values. The new temperature values will be higher than the previous ones due to the upward trend in temperature in the US and the globe over time. Rainfall will also change but it will go up in some places and down in others due to wet and dry spells in different parts of the country over time. I will talk about the new normals and how they are created in my blog post in late May.
Masses of spring bulbs transform this lawn. Photo by Charlotte Scott.
Nothing seems to take homeowners more time, or generate more frustration, than maintaining their lawns. In addition to mowing, fertilizing, and applying pesticides for weeds, insects, and diseases, gardeners fret about removing thatch and aerating the soil. Commercial interests have taken note and pedal various “aerifying” products like soap (cunningly described in non-soap terminology), spiked sandals, and thatching rakes. Previous posts (here and here) have addressed ways to decrease fertilizer and pesticide use. This post will look at the science behind aeration of home lawns.
Iron maiden torture devices in sandal form
More lawn torture via thatching rakes
That first ingredient in this aerifying product? It’s soap.
The home lawn care industry is lucrative and full of questionable things.
First, let’s acknowledge that most research has focused on maintaining turf on golf courses and playing fields. Neither of these are good models for home lawn management because home lawns have different functions. The turf that one might find on a putting green, for instance, is devoid of most life except for closely mown monocultural (or oligocultural) grasses. The management of these grasses is chemically and physically intensive to preserve a completely unnatural system. Yet these management techniques, including core aeration and vertical mowing (aka verticutting), have seeped into the lucrative home lawn maintenance market, especially to address the dreaded thatch layer common in many home lawns.
Live and artifical putting green turfs look alike. Neither is a good substitute for a home lawn.
What is thatch?
Briefly, thatch is caused by organic material accumulating at the base of grass plants. (It is NOT caused by lawn clippings, which are small and nitrogen rich – they are broken down quickly.) Accumulation of thatch is said to lessen lawn resilience and increase disease, but this appears to be a classic CCC (correlation conflated to causation) error. I’ve seen nothing in the literature to suggest that thatch causes these problems. Instead, I see evidence that thatch is yet one more negative result of poor lawn management. Removing thatch, without addressing the CAUSE of thatch, is an exercise in futility.
Lawn with thatch layer
Natural grassland
Look at these two images of grass-covered soil: one is a typical lawn, and the other is a natural grassland. There are no roots extending below the “thatch” layer in the lawn, while grassland soils support deep and extensive root systems. The problem with the lawn is that the system is not well aerated, meaning that the grass roots are shallow and contribute to the buildup of thatch. Lack of aeration also inhibits a robust community of microbes, which are necessary to decompose the organic material that makes up thatch.
If you have standing water on your lawn, there is no oxygen in the soil beneath.
So, lack of poor oxygen and water movement between the grass layer and the underlying soil creates a dead zone in that soil, with life restricted to those few inches of soil where oxygen and water can penetrate. Thatch accumulates and underlying roots from nearby trees and shrubs are forced upwards into the lawn to obtain water and oxygen. This is where lawn maintenance companies promise to fix the problem through core aeration or verticutting.
It’s not goose poop. It’s core aeration.
Vertical mowers look impressive but do they work?
Popularly recommended gadgets and gizmos
Does core aeration and verticutting improve home lawns?
While there is scant research on home lawns, the results are fairly uniform: core aeration does not reduce thatch accumulation and does not improve grass coverage. Verticutting can decrease thatch slightly but decreases grass coverage and reduces turf quality. Several quotes from published research stand out:
“All cultivation practices [which included core aeration and verticutting] resulted in some quality loss at various times during the spring transition period compared to the control.”
“Thus, under homelawn conditions, core aeration and vertical mowing should only be used if a specific problem exists and not as routine practices to prevent thatch accumulation.”
“After two years, no treatments consistently reduced thatch accumulation compared to the non-cultivated control.”
There is no published research, anywhere, that supports these techniques in maintaining healthy home lawns. So, it’s time to stop using these heavily promoted products and practices and instead focus on why lawns accumulate thatch in the first place.
It’s all about the oxygen!
There’s no question that lawns can be heavily compacted, but it’s not because grasses can’t tolerate foot traffic. Think about those hundreds of thousands of bison that use to roam the Great Plains grasslands. Even modern cattle ranching, done sustainably, does not damage pastureland by compacting the soil. There’s something else going on in home lawns that creates compacted conditions and the cascade of negative effects that follow; it’s improper soil preparation and management.
Pastureland dosen’t become compacted despite the significant pressure cattle exert on the soil.
When sod is laid for home lawns, several inches of compost are tilled into the soil bed. The tilled soil is then flattened with a roller, and then a layer of sand is applied. Then the sod (which consists of grass and growing media and a mat of some sort) is arranged. And voilà! You have a turfed landscape that more closely resembles a five-layer dessert than a functional grassland. Those layered materials restrict the movement of water and oxygen, and this restricts root growth into the underlying native soil. Not only do these barriers create a shallowly-rooted turf, they compound the problem by stimulating ethylene gas production in grass, further inhibiting root growth. To top it off, the anaerobic conditions in the lower layers restrict microbial decomposition. As decomposition and root growth slow, thatch accumulates. And homeowners despair.
All of these practices are bad for soil structure and function, and over time will result in an unhealthy lawn.
So, thatch serves as a warning sign that soil conditions are poor – and any attempts to permanently remove thatch without addressing poor soil preparation and management are going to fail. Possible corrective actions to improve soil structure and function are beyond the scope of this column; over the years we’ve had blog posts touching on this topic and I encourage readers to explore our blog archives.
Most experienced gardeners will tell you what should be
started indoors (or purchased) as transplants and what should be direct sown
into the garden, but this can often be confusing for new gardeners. Add to the confusion the fact that some plants
have a gray area when it comes to what is best, sometimes it depends on the
time of year, and sometimes it depends on where you are as to whether what is
possible. So if you forget to start your
favorite tomato or begonia indoors in time to transplant, do you have
options? Let’s explore!
Why start transplants, anyway?
This is a good question.
Why do we take the time and energy to start seeds indoors, or the added
expense of buying vegetable or annual transplants? There are a few good reasons:
Germination temperatures. Many of the plants
that we traditionally start as transplants require minimum soil temperatures of
around 60F and have optimum germination between 70F and 80F. Waiting for soils to reach these temperatures,
especially in cooler climates, can really shorten the growing season. Vegetable
temperatures, via UNL Extension
Extending the growing season. Related to germination temperatures,
starting transplants for warm season crops before soil temperatures warm up and
before the weather is suitable for planting can have a plant ready to go once those
weather conditions are ideal. This can
give you a head start of weeks or months over direct sowing.
Ideal growing conditions. Seedlings for many vegetable and annual
crops are quite tender and dainty when they first start out and any changes in
temperature, water, or even sunlight can cause damage. This is even more important as spring weather
is becoming a bit more unpredictable as the climate changes, where temperatures
can drop suddenly and the weather can go from rainy to dry (or snowy) at the
drop of a hat (he writes as the temps drop to the 30s and 40s from the 70s the
previous week and some parts of the state are receiving 6+ inches of snow in
late April).
What about direct seeding?
Ease.
Many gardeners, especially newbies, find it a lot easier and less
intimidating to just hop out to the garden and plop seeds in the soil versus
staring seeds indoors. Of course, buying
transplants is equally as easy, but that does limit the variety you have available
to plant.
Cost effectiveness. Only needing a pack of seeds (or saved seeds)
is typically much cheaper than buying transplants or buying the equipment than
starting seeds indoors. This allows for
much better cost effectiveness for gardeners.
Some things don’t transplant well. Root crops, like radishes, carrots, and beets
don’t transplant well because damaging that tiny little root in any way as you
transplant can damage the actual harvestable portion of the crop and result in
much lower produce quality (or even loss).
Additionally, some plants don’t like to have their roots disturbed, even
when they’re tiny little transplants.
Cilantro and zinnias, for example, don’t do well with root disturbance
so if you do want to transplant them you’ll need to start them in large enough
containers so that you don’t have to repot them, and then plant them carefully
as to not disturb the roots.
So sow, or not to sow?
How do I know?
This is a good question. Oftentimes we can take a look at
the seed packet and know, but sometimes we don’t have that packet or maybe we
want to fudge a little with what we read on the packet. So what is possible, and what is “best
practice”?
A newly transplanted pepper, getting a start for the season
Using some of the information we discussed previously about soil temps and growing season, most of those warm season crops you plant that take a while to grow from seed, like tomatoes, peppers, and eggplants should be started as transplants, especially for folks in cooler climates (like most of the US). Same for those summer annuals (if you absolutely MUST grow annuals, I know some people love them and some loathe them). In warmer or topical areas, you may be able to direct sow these crops, but they may still do better as transplants.
Some of the warm season crops, like beans, corn, cucumbers,
squash, and pumpkins can be started indoors and transplanted, but it isn’t
necessarily needed. These crops
typically grow much more quickly from seed and the seedlings are a bit hardier. We also typically grow some of these plants
in much larger quantities, making them take up more space for indoor starting
and resulting in a bit more work to transplant versus sow. Therefore, it is usually easier to direct sow
these crops, but there could be situations (like overcoming weed pressure in
the garden or if you have a really short growing season or low soil temps)
where you might want to start them indoors.
What about cool season plants? Sometimes the answer to this one is – “it
depends.” Lots of the leafy greens,
like lettuce and spinach, and those aforementioned root crops can be direct
sown into the garden well before the last frost date. If you have a soil thermometer, or a nearby
weather station with soil temp probes, keeping an eye for when soil temps get
into germination range can signal when to direct sow outdoors. The leafy greens
can be started as transplants, but figure out the optimum soil temperature for
gemination – for some, like spinach, it may be way cooler than your indoor
temperatures can get (unless you keep your house around 45 degrees). For the Cole crops like broccoli, cauliflower,
and cabbage, transplants should be started for spring planting, since they
still require warmer (75ish degrees) temperatures for germination. However, if you’re sowing them for fall crops
you can possibly direct sow them if other conditions, like water availability and
low weed pressure, will support good growth in the garden.
There are several resources, like this graphic from Virginia Cooperative Extension, that can help you out. But keep in mind that certain situations may make other options possible. For example, this graphic is for spring planting, so some of the items, like the Cole crops, may have options for direct sowing for fall cropping depending on where you’re located and your local climate.
Over several years of teaching basic soil science to arborists, master gardeners and students something started to coalesce into a trend. If I ask my students do they have “good” soil, many say no. I have heard Master Gardeners complain their soil is terrible or that a certain soil is bad in some way. People form opinions about soil based on its color, texture, odor, or even how plants grow in it (perhaps the most diagnostic quality). So how do you know if your soil is “crap”? Soil is a combination of physical, chemical and biological properties not all of which are obvious from a casual examination. Soil is infinitely variable depending on how it was formed and what has happened to it. Many soils are fragile and their growing properties can easily be harmed.
Soil forms from its parent material or rocks that weather over time to form smaller and smaller particles
Soil Formation
To understand soil you need to understand how it forms. Soils are often depositional, forming as particles are deposited in place from wind, or water or other weathering factors. Deep soils form from the alluvium as water washes particles down from mountains. Terraces along streams also form soil deposits when they overflow the stream bed. Almost all soils form from rocks that are referred to as the parent material. The kind of rocks that form the parent material determine the minerals that will dominate that soil. Exotic soils like serpentine soil contain large amounts of magnesium but lack calcium. Soils can be young (not deep or fine textured) or very old (deep clays). One of first things gardeners should seek to find out is if they have “native” soil or are gardening on fill. Soils are also modified by climate especially rainfall. High rainfall areas have leached soils, are usually forested, and have acid soil reaction (pH). Arid soils usually have excess salts, and tend toward being alkaline. Understanding soil formation helps to understand what kind of soil you have and how to utilize it best for your garden.
Residential landscapes are often on fill soils with various textures and interfaces. Here decomposing granite surface soils cover the actual clay loam textures underneath. Soils can vary significantly on the same property requiring multiple tests and actions for their treatment.
Fill is not Soil
One of first things gardeners should discover is if they have “native” soil or are gardening on fill. Fill around homes and cities is not soil in the natural sense. Fill soil is not formed in a natural process, it will not have the predictable qualities of soils and may be extremely variable even on a single property. Soil maps are available from your cooperative extension office and on line from the NRCS (https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm). The web soil survey is a map of naturally occurring soil types. Soils are described in detail and understanding your soil type will inform its ability to grow plants, hold water and minerals, etc.
Soil Physical Properties
No matter which soil you have, gardeners will want to know what to do to make it better for growing their plants. The physical characteristics of soil are important for gardeners to understand. Soil texture is described by analyzing the content of various particle sizes. Sands are composed of large particles silts have intermediate size particles and clays contain the finest particles. Soils texture is the relative content of sand, silt and clay particles and are described by their content of these particles such as a “clay loam” Pure loams are relatively rare because they have equal measures of sand silt and clay and are considered the most arable soil textures. A clay loam has more clay than the other particle sizes but enough to still be considered a loam. Textural classes are described by the soil triangle. You can diagnose your soil texture by using a ribbon test where you feel the soil and analyze its qualities. A laboratory can separate the particles and give an exact analysis. Soil texture affects horticulture directly as it determines drainage characteristics, moisture content and mineral holding capabilities.
Soil Chemical Qualities
One of the most defining chemical qualities of soil for gardeners is nutrient content. Minerals or elements in soils are highly variable based on soil age, their formation processes and the parent material from which they developed. Fine textured soils have more mineral nutrients and storage capacity than coarse textured soils. Sands tend to be hungry for plant nutrients and clays are usually rich in nutrients. This is because as particle size decreases the electrical properties of soil become more negative in charge and tend to retain positively charged mineral nutrients. You can estimate nutrient content by seeing how plants grow in a given soil without fertilization. If weeds are abundant and happy, the soil may contain adequate amounts of the 18 different elements necessary for plant growth. The only way to accurately know the nutrient content of a soil is to have it analyzed in a soils lab. There are other blogs at this site that tell you how to take a soil sample. Never fertilize a soil that already grows plants well as you will be polluting surface waters and contaminating streams with excess fertilizer elements that can leach or run off.
A well structured soil has water-stable aggregates, pore spaces, roots, hyphae, organic matter etc. This kind of soil is the product of a robust soil food web.
Biological Qualities of Soil
The most elusive quality of soil is the biological quality. Soils are ecosystems of organisms. Much has been written about the soil food web and it is a critical part of how soils and plants interact. While we can see worms and small arthropods; bacteria, fungi and nematodes are not visible. It is difficult to visually assess soil biology. However there are some indicators. “Healthy” soils are often well structured. Soil structure is a physical description of the way soils form aggregates, clumps and clods. Well structured soils have abundant pore spaces, bits of organic matter, and have distinct clods or clumps. Often these clods are water-stable, that is, if you put a soil clod in a jar of water it will not dissolve. This is an easy test you can make of your soil. Place a clod in water and leave it there over night if it dissolves it is not a water-stable aggregate. Water stable aggregates from from the action of soil microorganisms that bind soil particles with polymers as well as the hyphae of fungi which connect particles together.
Soil Carbon Drives Soil Biology
Healthy soils have more carbon in them then soils that are not biologically active. Organic matter is an important part of soil and is added as litter or mulch breaks down and by plants themselves as they deposit carbon through exudates and associations with microorganisms. Plants can add as much as 20% of their carbon captured through photosynthesis into soil through root exudates and microbial association. Carbon is food for microbes and an essential component of a healthy soil. Soil with large amounts of organic matter are dark in color (but so are many low OM clays so don’t be fooled). Again the only way to know exactly how much organic matter is in soil is by a soil test. A detailed soil organism analysis may not help you that much because it is difficult to assign specific roles to groups of organisms living in soil. If we provide organic matter (fresh wood chip mulches in perennial plantings) the food web will grow to utilize it and we do not need to worry about who is using the carbon.
A bio-assay of three soils (2 cups each) planted with radish and carrot. From top left to bottom right: clay loam; silt loam and potting medium
Despite all these factors soils are still a bit magical. Even with soil surveys, and soil analyses you really can’t tell if a soil will grow well until you try to do so. In my University class I am having my students do a simple bio-assay (growing seeds in soils) The assignment was to grow radish and carrots in three different soils, hoping that some would show up signs of damping off disease. I did the experiment as well. My seedlings were grown in a silt loam, a clay loam and a potting medium. The soil-based differences are very visible. The clay loam grew the largest seedlings. Bio assays such as this are helpful to see what the growing qualities of soil are. They don’t tell the entire story but they are very interesting for comparative purposes. Bio assays are great to do before you purchase soil for raised beds or if you are gardening in a new soil that you don’t know much about. In the next blog I will touch on how, when, and why soils should be modified to enhance your garden.
The Garden Professors have previously written about the ubiquitous garden center product, SUPERthrive, here and here. The manufacturer claims a plethora of beneficial uses for SUPERthrive —everything from Christmas tree care to turf to hydroponics. They claim SUPERthrive will “revive stressed plants and produce abundant yields” and that it “encourages the natural building blocks that plants make for themselves when under the best conditions” thus “fortifying growth from the inside out,” but I know of no body of rigorous, peer-reviewed literature to support any of those claims (1, 2, 3, 4). In fact, I’m not entirely sure what those claims really mean, but I’m encouraged on their website and bottle to use it on every plant, every time I water, to receive these amazing benefits!
A test case
The hydroponics claim intrigued me because during the winter
months I grow plants hydroponically under lights. One of the benefits the
manufacturer claims is “restores plant vigor” and “works with all hydroponics
systems.” As a plant scientist, and knowing something about the ingredients, I
was skeptical to say the least, but I thought that if SUPERthrive was going to
show any beneficial effect it would surely be in hydroponics since that is a
more uniform environment than outdoors. So, I shelled out my $11 for 2 oz (the
things we do for science!) and set off to design a simple experiment.
The hypothesis
A typical experiment like this starts with what we call the
null hypothesis (denoted “H0”). The null hypothesis is defined prior to the
experiment and often states that we think there will be no difference between
the treatment and control. In this case, my null hypothesis is that the
SUPERthrive treatment will have no effect on the mean fresh weight of the
harvested lettuce relative to the control lettuce. Note that I haven’t made any
hypotheses about other parameters that might be important, e.g., flavor,
compactness, number of leaves, color, disease incidence, survival rate, etc.
For this experiment I am interested in only one thing: total harvested weight
as a signifier of healthier plants.
After the data is collected and analyzed, we decide whether
to accept or reject the H0 by running an appropriate statistical test. If there
is no statistically significant difference, then we cannot reject the H0—that
is, we accept the H0 that there is no difference between treatment and
control. If there is a statistically significant difference between
treatment and control, then we say we reject the H0 and conclude that
the treatment did have an effect. Keep in mind, sometimes no difference
between treatment and control is a good thing, e.g., in toxicity studies.
Experimental design
With my skeptical spectacles on, I set up my experiment to test my hypothesis. I made a six-gallon batch of hydroponics nutrients suitable for leafy greens. I split the batch in half and added SUPERthrive, per the manufacturer’s dilution recommendation, to one of the three-gallon aliquots as the treatment. I then divided the control and SUPERthrive treatment each into six individual, identical, two-quart containers. I thus had six independent replicates of a treatment and a control. (See Figure 1 below for a schematic of the experimental design.)
Figure 1. Outline of experimental design
To further avoid any experimenter bias, I had my wife assign
numbers randomly to each container, record which were SUPERthrive treatment and
which were untreated control, and then re-sort all the containers. I had no
idea which containers contained which nutrient mix. I did not open the “secret
decoder envelope” until after all measurements were complete!
Figure 2. Identical 2 quart containers randomized on day 1 in the hydroponics solutions. This kind of hydroponics is called “Kratky” or passive. Enough nutrient solution is supplied at the beginning to last the plant for its entire life-cycle.
Into each of the 12 containers I placed a 12-day-old lettuce seedling, taking care to select plants that were of equal size and leaf number. The containers were then placed under my lights (cool white T8 fluorescent) for the remainder of the experiment. I rotated the rows of plants several times to try to control for any edge effects in my grow area. After 30 days in the containers, I harvested and weighed each plant.
Figure 3. Plants after 30 days of growth.
What did my experiment show?
The graph below is a box and whisker plot that shows the spread of the data and the mean for each group in grams of harvested fresh weight of the plants (roots were removed). In my experiment, the SUPERthrive treatment showed a clear drop in harvested fresh weight! In fact, the heaviest SUPERthrive plant weighed less than the smallest control plant, and the SUPERthrive set was much more variable in harvested weight. These results surprised me a bit.
Figure 4. Box and whisker plot of lettuce plant fresh weight. Master Blend: Master Blend nutrients; Master Blend + ST: Master Blend nutrients plus SUPERthrive (0.9 ml/gal.)
A standard statistical test (Student’s T-test, unpaired,
two-tailed) was performed to show that that there was in fact a statistically significant
difference (p<<0.01) between the two groups. Thus, we can reject the H0
(remember our null hypothesis is that there will be no treatment effect) and
conclude that there is a difference between treatment and control harvested
weights, with the treatment mean plant weight being significantly smaller than
the control mean plant weight.
What can we make of this experiment?
Well, we need to keep in mind a few things.
1) Six replicates is a very small sample size; this could be
a spurious, unlucky result. There is always some distribution of growth rate,
even in a uniform genotype. Did I get unlucky and happen to put six plants that
would always be on the smaller end of that distribution into SUPERthrive?
2) After analyzing the data, I discovered that four of the SUPERthrive
plants ended up in the same row and were the smallest heads in the experiment
(sometimes you flip a coin and get four heads in a row!). Could this be the
reason for the unexpected results? The other two treated plants were in the
other two rows, but neither was as large as the smallest control plant.
3) I do not have a perfectly controlled environment like one
would find in a lab or even in a larger growing facility. However, something
marketed with such aggressive claims of amazing plant health benefits and vigor
should give a noticeable effect under a variety of imperfect, real-world
conditions, such as those one would find in a home garden situation, don’t you
think?
4) Perhaps my plants were already growing at their maximum
potential and there was nothing for SUPERthrive to “improve.” Afterall,
hydroponics indoors is already a relatively stress-free environment, as the
SUPERthrive manufacturer also points out. Then what do they think their product
is improving in hydroponics? Would I have seen an effect under less-than-ideal
or more stressful conditions then? This could certainly form the basis of other
testable hypotheses.
Conclusions
What I think we can conclude is that in this experiment,
with this genotype of lettuce, and under these hydroponics conditions and environment,
SUPERthrive had no positive effect whatsoever and may have even had a negative
effect. Under other conditions would one see a positive effect? Possibly. Would
different plants or genotypes respond to the SUPERthrive differently? Possibly.
We must always be careful of over-extrapolating both positive and
negative results from a single experiment.
But, because the individual ingredients have not been shown
to provide any beneficial effect, and no plausible mode of action is given by
the manufacturer for their broad general claims, we should remain highly
skeptical. As pointed out in the previous post, the SUPERthrive manufacturer
has certainly had plenty of time to scientifically demonstrate efficacy of
their product, since they proclaim to be “always ahead in science.”
Because the results showed a clear and unexpected negative effect,
the experiment surely needs to be repeated. Repetition is a central tenet of
science. I hope to share additional results with you in a post later this
spring—after all, I have a whole bottle of SUPERthrive and we love salad!
References
Banks, Jon & Percival, Glynn. (2012) Evaluation of Biostimulants to Control Guignardia Leaf Blotch (Guignardia aesculi) of Horsechestnut and Black Spot (Diplocarpon rosae) of Roses. Arboriculture & Urban Forestry. 38(6): 258–261
Banks, Jon & Percival, Glynn. (2014) Failure of Foliar-Applied Biostimulants to Enhance Drought and Salt Tolerance in Urban Trees. Arboriculture & Urban Forestry 40(2): 78–83
Chalker-Scott, Linda. (2019) The Efficacy and Environmental Consequences of Kelp-Based Garden Products.
Yakhin Oleg I., Lubyanov Aleksandr A., Yakhin Ildus A., Brown Patrick H. (2017) Biostimulants in Plant Science: A Global Perspective. Front. Plant Sci., 7:249
Most gardeners this time of year are thinking about the last frost dates for their locations and how soon they can get out into their garden plots. Here in the Southeast, many areas have already passed their last frost or will soon, while in other parts of the country, it may be many weeks before the threat of frost is over. In this week’s column, I want to describe a way to get frost dates for your location and discuss the mystery of why the date of the last spring frost is getting later in the Southeast in spite of temperatures that are rising across the country.
Frosted kale. Source: Tracey from North Brookfield, Massachusetts, USA, Commons Wikimedia.
Resources for finding your frost date
There are many places that you can go to find information on the average date of the last spring frost. Many gardening guides publish them, and John Porter had an excellent discussion of last frost and planting dates a year ago, including a number of sources of information and a map for the continental United States.
You can also look at frost dates for individual locations using xmACIS, an online free database that allows you to list yearly last spring and first fall frost dates and the growing season length. This database contains observations taken by National Weather Service cooperative observers and is incorporated into the NOAA 30-year averages (normals) that John mentioned in his posting. You might find it helpful to see not only the average but also the variability from one year to the next at whatever station is closest to you. (Here is a quick reference sheet for xmACIS.) Of course, there are other places to get this data in a variety of formats, but xmACIS is quick and easy and works for the whole country, which is an advantage for all our readers.
To access data near you,
Go to the top under Single Station and choose “First/Last Dates.”
Under Options Selection choose:
your preferred output, (Graph, table or CSV)
Year range (POR is period of record, which will vary depending on which station you choose)
Under Criteria set minimum temperature at less than or equal to 32 F (or another threshold for a special crop)
Period beginning (for spring frost dates, usually July or August)
Pair results (for spring frost dates, usually by Calendar year)
Under Station Selection, you can find a station by ID if you know it, by choosing from the list or searching by zip code. Or change your CWA (National Weather Service County Warning Area) to your local region and available stations in that area will be listed. A map of the CWAs is shown below. Pick the station that is closest to you to get the best data for your location.
Hit “Go” and you will get a list of the yearly last and first frosts of the growing season. The average date is at the bottom.
National Weather Service office identifiers.
Climate change and frost dates
With increasing temperatures due to global warming, you might wonder how these frost dates are changing over time. As temperatures get warmer, you might expect that the average date of last spring frost would be getting earlier in the year over time and the average date of first fall frost would be getting later. And this is generally true in most of the US, with the exception I will discuss in a minute.
I did some work with Melissa Griffin of the South Carolina State Climate Office in the past, and we determined that a 1-degree F rise in average temperature over time corresponded roughly to a 1-week increase in the length of the growing season. That is an important statistic for farmers, who plan what to plant depending in part on how long the growing season is. If the temperature in the US goes up 4 F by the year 2100, then we can expect that the growing season would increase by generally four weeks or one month, although that will vary from place to place.
Southeast frost date mystery
In most places in the US, the date of last spring frost is getting earlier in the year, as expected. But there is one regional exception, and that is the Southeast, especially in Georgia and to a lesser extent, Alabama. You can see this in the once-again public EPA climate change page.
It is not clear why this trend towards a later spring frost date is occurring in the Southeast. One theory is that perhaps a local weather phenomenon we call “the wedge” is changing due to alterations in weather patterns across the region as the global temperature increases. “The Wedge” is a thin, dense layer of cold air which moves southeast along the eastern edge of the Appalachian Mountains, bringing cold air and cloudy conditions to that region.
A group of University of Georgia students and I looked at this “wedge theory” in 2020. We tried to identify where the wedge of cold air was most likely to be occurring in the Spring and correlate those areas with changes in frost date. So far, the results have been inconclusive. More research will be needed to figure out why this odd pattern is occurring now and whether it will continue in the future.
Air temperature on February 20, 2019, from the University of Georgia Weather Network, showing the cold “wedge” of air in northeastern Georgia.
Implications for home gardeners
Knowing your average spring frost date can be an important brake on most gardeners’ eagerness to get back out in the garden in spring. Who hasn’t wanted to start planting on the first warm and sunny day? But if you know that more frosts are likely based on the local climate, you may be willing to wait to get started until your plants are safe from cold damage. Then the real growing season can begin!
There’s wood chip mulch peeking out of all of our landscape beds
One of the popular arguments against mulching landscape and garden soils is that mulch delays soil warming and thus retards plant growth. Given that a well-chosen mulch will moderate temperature extremes – both hot and cold – is this an argument supported with evidence? In today’s post, I’m reporting the data I collected in visiting various parts of my home landscape and gardens and measuring soil temperatures.
My trusty soil thermometer
For measurements, I used a soil thermometer placed at the
same depth in every soil tested. This required movement of mulch if mulch was
present, so that thermometers were inserted completely into the soil. These
thermometers read the entire length of the probe, so readings represent the
average temperature in the top 5” of soil. I took close-up photos of each of
the areas tested. I took 5 measurements for each location.
Bare mound in the sun
Bare soil in the sun
Bare soil in the shade
Turf in the sun
Turf in the shade
Newest mulch in bed
Older mulch in bed
Unmulched soil in bed
Examples of the readings collected in different locations of our landscape
Our evening temperatures have been near or below freezing, but the past several days have been sunny and the air temperatures are well into the 50F range. On March 17, it was 68F at 2 pm in the sun, though it was 27F that morning. The morning after (March 18), it was 35F.
There are several interesting trends to see on the
box-and-whisker graph:
The variation of soil temperatures is most extreme in unprotected soils
Mulched raised beds have the most consistent
temperatures, with no differences seen at any time or in any location measured.
Unmulched soil mounds have extreme changes,
mirroring air temperatures.
Bare soil in beds under sunny conditions have extreme
changes mirroring air temperatures, but not as great as that in raised beds.
They are warmest during the day and coldest during the night.
Bare soil in beds under shaded conditions are
the coldest soils during the day and even colder at night.
Soil under living mulch (turf) and beds with varying
depths of wood chip are cooler during the day than bare soil in sunny conditions,
but warmer at night.
Bare soil in beds that were newly mulched are much
warmer than bare soils not near mulched areas.
The soil temperature under turf or in beds at
least partially mulched did not change at night (data not shown on graph).
Raised beds – mulch only right now
Mounded soil. Weeds were removed several months ago.
Mulched beds
Turfed area, exposed to sun and shade
Examples of our sample sites
Extreme temperature swings can result in the death of germinating
seeds, seedlings, expanding buds, and other tissues that aren’t cold hardy. This
is especially true of tissues near the soil surface, where temperature are
colder than they are at increased depths. Unprotected soil mounds show huge daily
vacillations; comparative raised structures under mulch are cooler during the
day but warmer at night. And bare soil in the shade is colder than any mulched
soils. Consistency is important for young tissues, as they have few protections
against environmental extremes.
What my little experiment demonstrates is what mulch research
has consistently shown: appropriate mulch materials will moderate soil
temperature extremes due to air temperature fluctuations. Just because a bare soil
is 55F in the daytime doesn’t mean it won’t be 35F at night.
