My Soil Is Crap, Part II

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.

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

“Have you ever seen the rain?”

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.

Leave your lawn alone!

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.

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.

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.

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.

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.

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.

To (direct) sow, or not to sow, that is the question: whether ’tis nobler in the garden to transplant

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:

  1. 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
  2. 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. 
  3. 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?

  1. 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.
  2. 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. 
  3. 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. 

Chart showing how to start vegetables transplant vs direct sow: Transplant: Broccoli, Brussles sprouts, cabbage, Chinese cabbage, Cauliflower, Eggplant, Leeks, Lettuce, head; peppers; tomatoes. Direct sow: asparagus; beets; beans, bush; beans, pole; beans, lima; carrots; chard, swiss; collards, kale; cucumbers; kohlrabi; lettuce, baby salad; muskmelons; mustard; okra; onion, bulbing; radish; potatoes; southern pea (cowpeas); spinach; squash, summer; squash, winter; sweet corn; sweet potato; pumpkins; rutabaga; radish; turnips; watermelon. VCE and Master gardener logo at the bottom with link to publication (also included in post text)
This info is good for many areas for spring planting, but climate and planting time can change options for some gardeners
Source: Virginia Cooperative Extension Master Gardener Facebook

My Soil is Crap

My Soil is Crap! Or is it?

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.

Where to find spring frost dates and the mystery of Southeastern frost trends

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,

  1. Go to the top under Single Station and choose “First/Last Dates.”
  2. Under Options Selection choose:
    1. your preferred output, (Graph, table or CSV)
    1. Year range (POR is period of record, which will vary depending on which station you choose)
    1. Under Criteria set minimum temperature at less than or equal to 32 F (or another threshold for a special crop)
    1. Period beginning (for spring frost dates, usually July or August)
    1. Pair results (for spring frost dates, usually by Calendar year)
  3. 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.
  4.  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!

The contrarian rosarian–debunking rose mythology

Roses are perhaps the most frequently cultivated landscape plant across America. Rose gardens are common to parks, landscapes, botanical gardens and for homeowners. Everyone seems to have an opinion about rose culture and there are numerous clubs and societies to support the hobby of rose growing. This week I am in the midst of pruning my rose fertilizer study here in Santa Paula California. I have 240 roses of eight varieties and my thoughts are on roses now, so I offer this blog to dispel some of the myths about rose horticulture.

Myth I–Roses are difficult and require a lot of pesticides

Roses grow well in California soils. A selection of varieties here in Santa Paula CA

Most roses grow easily in most soils in most places. Roses tolerate environmental extremes very well. They grow in many climates and tolerate below freezing temperatures during winter dormancy and high temperatures during summer. Current rose varieties have been developed through breeding of wild rose types. Floribundas, hybrid T roses, grandifloras, shrub or landscape roses, climbing roses and dwarf roses offer the enthusiast a variety of forms and functions in the Rosa genus. In the early 19th century Empress Josephine of France gave rose development a great boost in her own garden at Malmaison. Her patronage of rose research led to the development of thousands of varieties in Europe and later in the United States. The genetics of garden roses is now quite diverse. Because of the diversity of roses some grow better than others, some are highly disease resistant some are very susceptible. Like all plants, roses develop various kinds of diseases and attract pests. Because they are grown commonly in gardens there are many rose pesticides available for use. In my decade of rose research growing hundreds of roses, I have never used pesticides to maintain them. Susceptible varieties could be treated with pesticides or gardeners can chose to avoid varieties that host pests and focus on ones that are not so afflicted. With so many varieties available to gardeners there will be strong varieties and weak ones, pest prone and healthy. The variety you select will determine the necessity for pest control. Many many roses are relatively pest free and grow well without any treatments.

Myth II Roses Require lots of irrigation

The idea that roses need more water than other landscape plants is a horticultural misnomer. In the Central Valley of California roses are grown for production to consumer markets and they typically are furrow irrigated once every eight days in the growing season. Even during triple digit weather, they are held to this schedule without damage.

Can you tell which one got Epsom salts? No. there is no difference between roses grown with applied magnesium sulfate vs those not receiving the treatment.

Myth III Roses require rose specific fertilizers

Roses need the same mineral element as other plants. There is no evidence that increased magnesium (Epsom Salts) benefits roses in any way. Prescriptive fertilization is not appropriate for rose culture or any landscape setting. Fertilizers should be applied on the basis of soils tests that determine the necessity of minerals that may be missing from the soil.

Rose varieties respond widely to field conditions. In the same field some varieties consistently thrive and others grow poorly. Rose varieties have variable vigor, tolerance of soil conditions and pest resistance.

Myth IV Prune rose canes at 45 degrees that is with angled Cuts

There are many pruning strategies for roses. One of the most consistent myths is that roses should be pruned with angled cuts so water is shed away from the cut end. There is no scientific basis for this and therefore it is not recommended. Pruning back to an outward facing bud is a good idea as it maintains a less tangled rose canopy and helps to promote a more organized architecture in the shrub. Various sources recommend more or less severe winter pruning for roses. Our research shows that the less severely you prune major canes the more flowers that will result. Severe pruning did not increase rose flower quality or quantity. The best rose shrubs (most flowers) are pruned to maintain their shape and reduce tangle while maintaining shrub size.  I almost forgot–Don’t seal pruning wounds made to rose canes.  Leave cuts to dry.

Myth V Mounding soil around the base of roses should be done every winter

Some rose experts, especially in places with cold climates have advocated mulching with manure or soil over the crown of the rose before freezing winter temperatures set in. Most rose varieties survive the cold winters without this treatment if snow is present. If temperatures fall rapidly without snow, a covering of leaves or straw may be helpful.

Myth VI Grafted roses are better than non-grafted roses

The recent advent of landscape or shrub roses has proven that this myth is incorrect. Non-grafted roses have the advantage of not producing annoying suckers that need to be removed frequently as on some grafted varieties. Many of the landscape roses growing on their own roots are more disease resistant, more vigorous, and produce more flowers consistently than their grafted counterparts. Not all scions are perfectly compatible with their rootstocks so some grafted roses are less vigorous due to graft incompatibility.

Roses are easy to grow once they are established. In recent years, I have had trouble with roses purchased from garden centers that would not grow when planted out. This may be because the plants were held too long in storage before coming to market. It is also imperative when first planting roses to frequently sprinkle the canes to avoid them drying out. Desiccation is a common killer of freshly harvested roses. Once buds “pop” and shoots emerge, culture can continue as with any garden plant providing appropriate moisture as needed. Fertilization should follow recommendations of your soils analysis.

Reference:

Downer, A.J., A.D. Howell, and J. Karlik. 2015. Effect of pruning on eight landscape rose cultivars grown outdoors. Acta Horticulturae 1064:253-255.

Why Fresh is Best—when it comes to mulch?

Fresh wood chips!

One of the most misunderstood gardening practices is mulching. There is much mulch misinformation in horticulture books, web pages and even extension leaflets. First,what is Mulch? Mulch is any substance the covers the soil surface. Mulch can be inorganic (rock), hydrocarbon (plastic) or carbon based (chips, bark etc.) While any material applied to the soil surface could be considered mulch, the benefits of mulching especially to woody plants are greatest from fresh woody chippings of tree trimmings–so called “arborist chips” applied fresh—not composted. Annual plants such as vegetable plants are often mulched as well but usually with materials that rapidly break down such as straw or some mixtures of shavings and manures. These materials are easily incorporated later when the next crop is planted. For woody plants such as trees and shrubs, mulches that persist for a longer time are desirable. Plastic mulches used in agriculture are not suited to shade trees or other landscape uses nor are landscape fabrics. Each of these deteriorate into landscape trash rapidly and do not benefit soils under the mulch layer. Stone mulches while used extensively in the South west US are not as beneficial to soils as arborist chips.

Why use mulches anyway? Mulches support healthy tree and woody plant growth in landscapes around the world. They increase soil organic matter, the diversity and functionality of the soil food web (particularly saprophytic fungi), support mycorrhizal partners of woody plants, supply nutrients and suppress weeds. Thick mulch layers increase root development, and help to suppress soil borne plant pathogens. The breakdown of woody mulches on the soil surface encourages development of soil structure, increased water infiltration, water holding capacity, and nutrient holding capacity of underlying soil layers. Well mulched trees and shrubs grow healthfully without fertilization.

So why not mulch with compost? Compost is not suited for use as a mulch around trees and shrubs. Compost is often screened and is of fine texture. Fine texture presents a few problems. Fine compost will make hydraulic conductivity with soil and allow for water to evaporate through the compost/soil interface. Thus the moisture savings we see under arborist chips will not be the same under compost. Compost is also able to allow weeds to germinate in it so the weed suppression effects of a mulch will also be lost. Composts applied as mulch can make an interface between the soil surface and the mulch layer which should always be avoided as it will impede water movement through the interface.

Another important reason for not mulching with compost is that compost is poor nutritionally for soil microbes. Composts have most of their active or labile carbon burned away during the composting process by the rapid respiration of microbes. The compost is turned aerated and kept moist until the process stops at this point it has some level of maturity. It won’t reheat when turned. The microbes have consumed most of the available carbon for their own growth and respiration in the compost pile, none of this remains for microbes in the landscape. Fresh arborists chips are full of labile carbon. When laid over the soil surface spores of fungi invade and they begin to uses this carbon for their own growth as an energy sources. Placing fresh wood chips on the soil surface is feeding the soil microbiology at the soil-mulch interface. In time (a few years) these processes go deeper in the soil and begin to feed the soil food web beneath the mulch layer. The diversity of fungi increases, mycorrhizae begin to transfer mulch nutrients to their woody hosts and pathogens are destroyed by enzymes that leach from the fungi infested wood chips. While composts supply minerals (all that is left of the feedstock after composting) they can’t supply the labile carbon as a source for microbes. Fresh arborists chips do all this and are thus the best mulch for woody plants.

Fungi eventually invade fresh mulches releasing nutrients and enzymes to underlying soils

There has been some concern lately for using mulches that are recycled as yardwastes. This concerns me as well because gardeners may be disposing of dead plants in their greenwaste cans. In theory, pathogens could be coming through the greenwaste stream to gardeners. Getting tree chips is best because there is little likelihood for soil borne pathogens since the materials are chipped branches. There is some possibility of wilt diseases (Verticillium spp.) surviving in arborists chips but little research has established that the pathogen can infect especially if the chips are stockpiled for a short time. In my own research we showed that pathogens, weeds an insects had very short survival times in stockpiled (not turned) piles of greenwaste. There is very little chance of pathogens coming to your garden from arborist chips and the benefits to your soil and perennial plants are worth the effort to get a “chip drop” from your local tree care company.

Pathogens buried in fresh yardwaste do not survive for very long

Literature

Chalker-Scott, L. 2007. Impact of Mulches on Landscape Plants and the Environment — A review. J. Environ. Hort. 25(4) 239-249.

Chalker-Scott, L., and A. J. Downer 2020. Soil Myth Busting for Extension Educators: Reviewing the Literature on Soil Nutrition. J. of the NACAA 13(2): https://www.nacaa.com/journal/index.php?jid=1134&fbclid=IwAR0cPfBl3V-3car-RPeEmlqzwW8bPEOPgND07xMTNgCOa5GkuSWtdD5WzF8

Downer, A.J., and B.A. Faber. 2019. Mulches for Landscapes UCANR publication #8672

Downer, A.J., D. Crohn, B. Faber, O. Daugovish, J.O. Becker, J.A. Menge, and M. J. Mochizuki. 2008. Survival of plant pathogens in static piles of ground green waste. Phytopathology 98: 574-554.

Downer, A.J., J.A. Menge, and E Pond. 2001. Association of cellulytic enzyme activities in eucalyptus mulches with biological control of Phytophthora cinnamomi Rands. Phytopathology: 91 847-855

Downer, J. and D. Hodel. 2001. The effect of mulching and turfgrass on growth and establishment of Syagrus romanzoffiana (Cham.) Becc., Washingtonia robusta H.Wendl. and Archontophoenix cunninhamiana (H.Wendl.)H. Wendl. & Drude in the landscape. Scientia Horticulturae: 87:85-92

The weather where you are

Greetings from Athens, GA! I am happy to join the group of contributors to the Garden Professors blog. My name is Pam Knox, and I am an agricultural climatologist in Extension at the University of Georgia as well as the Director of the UGA Weather Network and a former State Climatologist from Wisconsin. While I don’t claim to be an expert in gardening, I do know a thing or two about how weather and climate affect plants and hope to share some of that expertise with you over time. You can learn a little more about me from my bio on the blog page.

Source: Merritt Melancon, University of Georgia College of Agricultural and Environmental Sciences

If you really like learning more about weather, climate, and agriculture, you are welcome to visit my own blog page, “On the CASE—Climate and Agriculture in the SouthEast” at https://site.extension.uga.edu/climate/, where I post almost daily about stories that have caught my eye as well as climate summaries and outlooks for the southeastern US. I plan to post on the Garden Professors blog here about once a month and am happy to answer questions at any time at pknox@uga.edu.

A simple way to compare temperatures around your yard

For my first post, I thought I would talk a little bit more about the weather in your yard and how you can learn more about it. As gardeners, you probably spend more time in your yards than I usually do, and so you have noticed that the climate of your yard or field can vary quite a bit from one spot to another. We call that “microclimate” and if you search this blog for that term, you will find several articles about microclimates in previous years, so I won’t spend a lot of time on that here.

Source: toby everard / Blaen y Cwm in a frost pocket / CC BY-SA 2.0

One easy and inexpensive way to measure how temperature varies across your domain is to use an infrared thermometer to spot-check the temperature at a variety of locations. These thermometers are used a lot now to check forehead temperatures in the age of COVID, but they are also used by HVAC technicians to check heating and air conditioning, for example. You can find inexpensive ones selling for less than $20 online, and many hardware stores have them, too. You will be amazed how much difference there is in temperature between sunny and shady locations! Don’t forget to try it at night too to see how much tree canopy can affect night-time temperatures. Of course, if you want a more systematic and scientific approach, you can follow Linda Chalker-Scott’s experience using multiple min-max thermometers as described in http://gardenprofessors.com/microclimate-follow-up/.

Infrared thermometer. Source: LuckyLouie, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported.

CoCoRaHS: Precipitation measurements by citizen scientists

One of the many things I do is to serve as a regional coordinator for CoCoRaHS, short for Community Collaborative Rain, Hail, and Snow network. This is a group of dedicated citizen scientists who take daily rainfall measurements and report them online via computer or smartphone as part of a nationwide (and now international) network of precipitation observers. Theses observations are used by the National Weather Service, drought monitors, water supply managers, and others to document local variations in rainfall at a much denser scale than other available observing networks. I am sure that some of the readers of this blog are already contributing! You can learn more about the network and how to sign up at https://www.cocorahs.org/. Please keep in mind that they do require the use of a particular scientific rain gauge, so a hardware store gauge is not likely to have the degree of accuracy that is needed to participate. A list of inexpensive vendors (costs start around $40 plus shipping) can be found on their site in the right column. By measuring precipitation at your house, you are not only monitoring your own conditions but contributing to our knowledge of water availability around the US and beyond.

One version of the standard CoCoRaHS precipitation gauge. Source: Lamartin, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported.

I am looking forward to interacting with you all in the months ahead, and please feel free to contact me if you have specific weather or climate questions.

Pruning Paints Debunked

When my turn comes up to blog for the Garden Professor site I like to reflect on the horticulture in my own gardens and orchard. Right now I am focused on pruning my old apple and stone fruit orchard. It has suffered bear attacks, drought, and mismanagement before we arrived in 2018. The previous owners were very aware of the need to treat pruning cuts large and small. The remnants of tree wound dressings are found all through our orchard and range from white latex paint to silicone caulk. Unfortunately there has never been good research evidence to support pruning paint use. Despite the lack of any published evidence, for their usefulness, pruning paints are still available in garden centers and there are no end of do it yourself preparations that gardeners continue to use on pruning wounds.

Wound dressings did not protect this apple branch from decay fungi

So why paint the cuts on your fruit trees after pruning? One idea is to keep the surface protected from infection by pathogens. Plant pathogenic fungi and bacteria can cause disease that may lead to blight, cankers, or wood decay.

Laetiporus gilbersonii (chicken of the woods) is a common brown rot wood decay fungus that destroys cellulose in wood.

Wounds are often implicated in pathogenesis or disease development. Many horticulturists believed that wound dressings provide a barrier to entry of pathogens and insects. Fruit trees are easily decayed by a number of fungi which cause white and brown rots in their wood. Wood decay organisms enter through wounds created when branches break from excessive fruit loads or when pruning wounds expose heartwood or significant amounts of sapwood. So painting cuts became a very common practice advocated by gardening columns and various books over the last century.

Wound dressings used in Ukraine for many years on this shade trees did not stop decay fungi from fruiting under the wound dressing! Photo courtesy Igor Signer, Kiev, Ukraine

Wood contains cells that store starch. Here, parenchyma cells in the wood ray tissues have been stained purple to show their starch content. Fungi that invade wood use this stored energy to grow, invade and degrade wood. Fungi invade both the heartwood (non-living) and the living, water transporting sapwood. Sap-rotters typically lead to the decline in tree vigor and canopy density.

Over one hundred years ago Howe (1915) recognized that pruning paints did not help wounds to close, in fact, they retarded the development of callus wood especially in peaches. Howe called into question the necessity of using wound dressings at all. Still the use of wound dressings has prevailed to this day.

Shigo and Shortle (1981) showed that wound dressings do not prevent decay nor do they promote wound closure. If the poor pruning practices that harm trees are abandoned, then wound dressings are unnecessary (never mind that they don’t work). Shigo often maintained that tree genetics determine the extent of decay forming in a given species. His work conclusively showed that flush cuts would lead to more decay than cuts that were made outside the branch collar or bark ridge.

Expanding foam? As far as I know there is no research on expanding foam but lots of anecdotes and observations of how it is often used to fill tree cavities. Filling cavities with cement to prevent or limit decay is a practice that subsided some decades ago and is generally not recommended as part of modern arboricultural practice. By the time decay has caused a cavity it is usually well entrenched in the wood of a tree and is not controlled by filling in the void. The best way to limit decay in trees is to prune them frequently so cuts are never large and the tree (fruit or shade) develops a strong structure that is unlikely to fail.

Literature:

Chalker-Scott, L., and A.J. Downer 2018. Garden Myth Busting for Extension Educators: Reviewing the Literature on Landscape Tree. Journal of the NACCA 11:(2) https://www.nacaa.com/journal/index.php?jid=885

Howe, G.H. 1915. Effect of various dressings on pruning wounds of fruit trees. New York Agricultural Experiment Station, Geneva, N.Y. Bulletin No 396.

Shigo, A.L and W. C Shortle. 1983. Wound dressings: Results of studies over 13 ykears. J. or Arboriculture 9(10): 317-329.

Shigo, A.L. 1984. Tree Decay and Pruning. Arboricultural J. 8:1-12.