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.

SUPER Thriving Lettuce?

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

  1. 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
  2. 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
  3. Chalker-Scott, Linda. (2019) The Efficacy and Environmental Consequences of Kelp-Based Garden Products.
  4. 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

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!

To mulch or not to mulch? It shouldn’t even be a question.

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.

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).

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.

Holy Hydroponic Houseplants, Batman!: Can you grow houseplants without soil? Yes!

Just when you thought you got the hang of growing houseplants in potting soil (or if you’re a doting plant parent, a special homemade mix someone on the internet told you to use) comes a new trend – hydroponic houseplants!  Or, “semi-hydroponic” to use the more technical term that is used when describing the trend.  How do you grow houseplants semi-hydroponically?  Do they grow this way?  But first, maybe we should ask the question – why? 

Why grow semi-hydroponically? 

I think for most casual houseplant growers, this method is attractive because it is a challenge.  Something new to try after you’ve mastered growing houseplants the old-fashioned way. And quite possibly a pandemic project to provide a distraction after being cooped up in the house for months on end.   But are there benefits to growing houseplants this way?  Turns out, there are some.    

Many articles you find on the subject state that semi-hydroponic houseplant growth can be beneficial for those who struggle with chronic over- or under- watering.  The media used for semi-hydroponics is a big, porous puffed clay stone called hydroton or LECA (Lightweight Expanded Clay Aggregate, of course we have to have an acronym!).  It is used in some hydroponic vegetable (and other plant) production systems.  The large pore spaces it creates and the wicking action it does in the container helps keep a balance of air and water for the roots. The #1 leading cause of death among houseplants is overwatering – it creates a lack of air in the potting media, the roots lack oxygen (called hypoxia), and are either damaged or die.  This can also make it easier for fungal infections to cause root rots.  But how can you stop from overwatering plants if you’re growing them in water?  We’ll talk about that in a bit when we talk about the “how to”. 

There are some houseplants, like epiphytes, that might also benefit from having a media that isn’t like soil.  Plants that are used to growing on tree bark, or in rocky environments in their native habitat that might actually perform better in a media that is a large, rough pebble that does kind of resemble the texture or tree bark or stones.  There are lots of tropical houseplants that also grow in areas with high levels of large particulate organic matter like chunks of wood and bark.  Plants from boggy environments that have high water requirements or grow in a more “mossy” type soil might also benefit. 

One other application of this method is for propagation of cuttings.  Many houseplant growers like to propagate cuttings in water, but this often isn’t the best practice because the water can be depleted of oxygen (causing hypoxia and rot) or become spoiled or soured (and cause infections).  Most horticulturalists will recommend propagation in a light media like seed starting mix, perlite, or sand.  But keeping water consistent without overwatering is difficult in this situation, and media can also be a vector for disease.  The air space and wicking action of the LECA media used for semi-hydroponics can help keep cuttings hydrated without the issues of water propagation.  This method is commonly done in clear class containers, so there’s the added benefit of being able to see root growth to monitor progress. 

How do you grow semi-hydroponically?

Of course, in this short article we won’t be able to cover every detail, so if this is something you’re interested in trying, I’d suggest some self-study.  I’ll be covering some of the basics, but there’s a lot more to learn. 

The Kratky Method - Grow Food The Passive Hydroponic Way (Step by Step  Guide) | Trees.com
Kratky hydroponic method Photo: UpstartFarmers

First, this method somewhat resembles one of the simplest forms of hydroponic production that lots of home hydroponic gardeners use, called the Kratky method.  In this passive hydroponic method, a plant is suspended above a water-based nutrient solution.  At first planting, the nutrient solution is right below the plant, close enough for a few inches of the roots to touch the solution.  As the plant grows, the roots elongate and the nutrient solution level is reduced to keep just a few inches of the roots submerged.  This allows the roots to take up solution, but the space between the plant and the solution allows a majority of the roots to be surrounded by air to avoid the issues of hypoxia. 

In semi-hydroponic houseplant growing, a container (usually clear glass, at least for beginners) is filled with the LECA media and the plant’s roots are distributed through the media.  (The media should be washed and soaked in water first, to remove dust and allow it to hydrate.)  It is easier if it is a young plant or recent propagation so you don’t have too many roots to deal with (you may need to root prune larger plants).  Smaller plants will also withstand the shock of going to this system, especially if they’re moving from a potting soil media.  (Note: Clear glass container + nutrient solution + light = algae. Be prepared to clean up the algae from time to time.)

A (dilute) nutrient solution is added to the container.  The roots should not be submerged in the solution, but rather it should be added to a level where it will wick up through the media to surround the roots.  The basic rule of thumb is to fill the container about 1/3 of solution, but if the container is exceptionally large or the roots are very small, you may need to fill it higher to make sure the media around the roots stays hydrated. 

This nutrient solution is one of the trickier parts.  You can use a general all-purpose hydroponic nutrient mix, available in lots of garden centers now or online.  You can also try some of the general houseplant fertilizers or ones specific to whatever houseplant you’re trying to grow.  You’ll want one with micronutrients as well as the macronutrients like N-P-K – since we’re growing without soil or an organic matter based media you’re going to have to supply all of the plant’s nutritional needs.  You’ll want to mix the solution between ¼ – ½ the recommended strength – you’ll need to see what works for you and your plant.  And then you’ll want to pH balance the water to create the right environment for the plant and make sure that nutrients are available for uptake.  The pH range for most plants is between 6.0 and 7.0 (aim for 6.5), unless you have one with specific needs.  For this you’ll either need pH test strips or a meter (which you can now get for less than $20 online) and some acidic and basic solutions to adjust pH (you can use some household items like vinegar to do this, but your best bet would be solutions specifically prepared for adjusting hydroponic or aquarium pH levels commonly referred to as “pH up” and “pH down”).  This pH adjustment is a lot easier (and maybe unnecessary) if you start with distilled or reverse osmosis water (or if you have a really good water filter that removes dissolved solids).  The pH levels and dissolved solids in some tap water makes it hard to adjust (my water here in Nebraska is very basic because it is very heavy due to high calcium levels, which also throws off the nutrient balance). Rainwater or melted snow can also work (though may not be pH balanced). 

You want to keep the solution topped off so that the media stays sufficiently moist. As with hydroponic production, plants pull nutrients out of the solution at different rates, so you can get build-up of some nutrient salts over time that could result in poor growth and even toxicity.  To avoid this, every few weeks (or more often if your plant is a heavy drinker) you might need to perform a flush, where you drain off the nutrient solution, give a quick rinse with tap water, and start over with fresh nutrient solution. 

More experienced growers might graduate to using this method in containers other than clear glass.  This adds a level of challenge, since you can’t automatically assess the level of nutrient solution by visual inspection.  The use of self-watering pots that have net pot or hole-y insert pots are commonly used for this.  Or you can buy net pot or orchid pot plastic inserts to use in any non-porous container you desire.  Growing in net pots can make the flushing process easier, since you can just pop it out of the container and run tap water through it.  Otherwise, you’ll have to find a way to pour the tap water out of the container or

completely remove the plant and wash the media. 

What can I grow semi-hydroponically?

Well, you can try with a lot of different plants.  I don’t know that there’s a list of plants out there for do’s and don’ts, but there are a few good candidates to try. Most tropical houseplants are good candidates. I’ve seen lots of articles on orchids, and I just recently put a rescue phalaenopsis in semi-hydroponics.  Other epiphytes like holiday cacti and bromeliads are also good candidates – think of things that like to grow on trees/treebark. Hoya, which are all the rage in houseplant circles, are also candidates due to their mostly epiphytic habits.  Lots of tropicals like Monstera, Philodendron, and Pothos also do well in this system.

Things that probably won’t do the best in this system are ones that don’t like to have “wet feet” – I’m thinking mostly desert cacti and succulents. But some of the LECA lovers that I talked to said that some succulents, like “string of pearls” and other strings of things (hearts, dolphins, turtles, etc) do grow well. But if we take a look at their natural habitat, where they grow over rocky outcroppings, it makes sense.

There isn’t really an exhaustive list, so you might want to experiment if you’re wanting to try it out. As long as it isn’t an expensive plant (and there are lots of expensive houseplants out there), a little experimentation can help you find the plants that would work best for you and your situation. 

In conclusion…..

Growing houseplants semi-hydroponically isn’t for everyone.  Getting everything just right can have a learning curve, especially if you weren’t great in chemistry class.  But, it can be a fun way to challenge yourself and may also benefit your plants in the right situations.  It is becoming so common that the materials are getting easier to find – many garden centers now carry the LECA and hydroponic supplies, you can always order them online, and you can even find small bags of the LECA/hydroton in the ever expanding houseplant section at IKEA (of all places, if you’re lucky to have one).  So if you’re up for a challenge, give it a try!  You might find a fun new way to grow houseplants….or a new way to kill houseplants!  But the fun will be in the trying. 

Special thanks to:

  • Anni Moira
  • Sydney Tillotson Sehi
  • Suzi Sellner
  • Tiffany Caldwell
  • Shelbi Sorrell
  • Maggie Pope

Sources:

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 seasonal climate forecasts aren’t always accurate

Do you use predictions of seasonal climate to plan your garden work? Or are you frustrated because they don’t seem to be very useful? I’ve been getting a lot of complaints this year about how bad the climate forecast for winter was because what we have seen so far has not matched the predictions in many parts of the country. Let me take a few minutes to explain how they are made and what you can learn from them.

First, let me specify that I am not talking about long-range climate forecasts for 50 years down the road. Nor am I talking about weather forecasts for the next week. I am talking about the forecasts that cover the period from about 15 days to 3 months, which climatologists call the “seasonal to sub-seasonal forecasts”. These are the kinds of forecasts that say “Winter is likely to be warmer and drier than normal” or “Get ready for a big warm-up in the next month.” They can be useful in planning garden work a few weeks ahead, but they come with caveats.

“Glory of the Snow” in the snow. Taken by User:Ruhrfisch April 2006, Commons Wikimedia.

Unlike weather forecasts, there are only a few models that predict climate in the monthly to seasonal time period. That is because we can’t just run the weather models out four to twelve weeks and expect to get anything like real weather. The weather models are built to handle short time steps and detailed information about temperatures, rainfall, and all the other factors that make up your daily weather, and to do it fast enough that you can actually use the forecast to decide when to wear your raincoat. They are useful out to about a week, but then their accuracy starts to break down because there are too many things going on around the globe to capture accurately over time, and so the short-term models tend to drift away from reality the farther from “now” you get. Models for monthly to seasonal climate tend to be based not on dynamical atmospheres like weather models but on statistics.

La Nina causes the jet stream to move northward and to weaken over the eastern Pacific. During La Nina winters, the South sees warmer and drier conditions than usual. The North and Canada tend to be wetter and colder. Source: https://oceanservice.noaa.gov/facts/ninonina.html)

NOAA’s Climate Prediction Center (https://www.cpc.ncep.noaa.gov/) is the biggest provider of seasonal forecasts, although there are a few others out there. This year we are in a La Niña, and so most of the seasonal forecasts have been based on that affecting our climate this winter. I won’t discuss La Niña here today (that is a topic for a future post, perhaps) but you can read a good general description at https://oceanservice.noaa.gov/facts/ninonina.html. The basic patterns of La Niña affect the temperature and precipitation across the United States in fairly predictable ways, and you can use statistics to show these patterns. You can see some examples of how La Niña has affected past winters at https://www.weather.gov/mhx/ensoninaanomalies. This year, the primary predictors of the winter climate have been the La Niña and the persistent trend that we are seeing towards warming temperatures due to greenhouse warming. From a statistical standpoint, it made great sense to predict that this winter would be warmer and drier than normal in the southern US and colder and wetter than normal in the north, because that is statistically the most likely pattern to expect in a La Niña winter, even when the climate is trending warmer over time.

So why did it not work this year? Because statistics can’t account for rare events that don’t follow the expected patterns. At the end of 2020 the atmosphere over the North Pole experienced a Sudden Stratospheric Warming (SSW), which means that the atmosphere about 10 miles above the North Pole suddenly got much warmer than usual. That messed up the usual distribution of temperatures in the Northern Hemisphere and helped push the really cold air to the south. It also pushed the winter storm track far south of where it usually occurs, making this a very wet winter in the Southeast, which is not what we expected! My farmers are not happy, but at least it means less likelihood of drought this summer. You can read more about the SSW at https://climate.gov/news-features/blogs/enso/sudden-stratospheric-warming-and-polar-vortex-early-2021. It might happen only once every ten years, or the cold air might just get pushed in a different direction next time, missing you and your winter garden altogether. Since the models are based on statistics, they will always show the most likely pattern, and instead we might experience winter that happens just once in ten years. Not so different that being the lucky person who gets rained on when the National Weather Service predicts just a 10 percent chance of precipitation!

90-day temperature departure from normal. Source: https://hprcc.unl.edu/maps.php?map=ACISClimateMaps

The good news is that we are getting better at these sub-seasonal to seasonal predictions, and we can expect to see improvements in the future as computers become more powerful and we have more experience looking at these periods. But for now, statistical models will continue to control the predictions at these intermediate periods, and we will continue to see the occasional miss when an unusual weather event occurs.

The dirt on rock dust

One of the newer “miracle products” targeted to gardeners is rock dust. Rock dust (also called rock flour or rock mineral powder)  is exactly what it sounds like. It is a byproduct of quarry work and is generally a finely pulverized material that resembles silt. It’s heavily promoted as a way to provide macro- and micronutrients to your soils and plants. Is it worth adding to your gardens?

Rock crushing at a quarry

First, it’s worth acknowledging that repurposing an industry byproduct is always preferable to throwing it away. Fortunately, the last few years have yielded some peer-reviewed research that we can use to make informed recommendations.

What’s in rock dust?

Obviously, the mineral content of rock dust is dependent on the rocks used to make it. This means the mineral content varies considerably, but in general rock dusts contain:

  • Large amounts of silicon, aluminum, and sometimes iron
  • Lesser amounts of calcium, copper, magnesium, manganese, potassium, sulfur, and zinc.
  • Potentially toxic levels of aluminum, arsenic, cadmium, chromium, copper, lead, nickel, and sodium.

I’ve added some tables from a few research articles that analyzed their rock dust mineral content below. Note the high silcon, aluminum, and iron content. (LOI = loss on ignition, meaning some materials were burned off during analysis.)

How is rock dust used as a mineral source?

Rock dusts must be solubilized to release minerals. There are some criteria that can speed mineral release:

  • Decreasing the particle size of rock dust.
  • Blending the rock dust with nutrient-rich organic matter like manure. This provides an acidified environment for mineral solubilization.

When is it beneficial to use rock dust?

There are documented benefits to using rock dusts – but only in agricultural production systems:

  • Rock dusts can contribute minerals to nutrient depleted soils, such as agricultural soils that have been overworked for decades.
  • Organic farmers can use specific rock dusts to supply micronutrients, rather than commercial fertilizers which are not certified for organic crop production.
  • Cereal crops – members of the grass family – require silica as a micronutrient (though silica is rarely if ever deficient in field conditions).

What’s the bottom line for gardeners?

As one article states, “…there is a potential for using [rock flour]…where there is a lack of these nutrients and where conventional chemical fertilizers are either not available or not desired.”

And how do you know if you have a lack of a certain nutrient? Why, by having your soil tested, of course! There is no point in adding anything to your soil unless something is missing. It is MUCH harder to treat a nutrient toxicity than to add a deficient nutrient. Iif  a soil test reveals a lack of a particular nutrient,  a carefully chosen product could supply this mineral. But you would have to know what else was being supplied and possibly creating a mineral toxicity.

At this point, there is no evidence to suggest that rock dusts are of any value to a home garden or landscape.  And adding these products can easily contribute to aluminum and heavy metal toxicities. I would never add it to this soil, for instance, as it already has excessively high aluminum levels.

Aluminum is already at potentially toxic levels in this soil. No need to add more.

This blog is full of great ideas on how to manage your soil naturally, sustainably, and safely. Rock dusts are just the latest garden product with lots of marketing but little benefit.

Houseplant Hubub: The rage about variegation

It is no secret that houseplants are hot right now.  Interest was growing before the pandemic, especially with millennials and younger folks.  Then the pandemic hit.  Houseplant interest skyrocketed since people were stuck at home and wanted to bring a little bit of nature indoors to make their spaces a little more cozy for 24/7 habitation. 

This has caused the demand, and price, of many houseplants to increase, especially if they are on the rarer side.  One thing that increases the price of many plants is when a variegated version of a standard plant has been developed. 

My reading nook/houseplant oasis

Just as an example, after posting a photo of my “reading nook/houseplant oasis” in my home office I was informed that variegated form of a Monstera deliciosa vine that I had was the highly sought M. deliciosa “Albo-Variegata” cultivar, usually referred to as a Monstera albo, or just Albo.  Folks were reaching out to buy cuttings right and left.  I ended up selling 5 single leaf/node cuttings over one weekend and made $675 in the process.  That’s right, $675!  The most variegated of the leaves sold for $200, and that was actually a bargain price.  The garden writer for the local paper, the Omaha World Herald, even picked up the story and shared it as a focus on the four new houseplant shops that have popped up in the city over the last few months.

Had my plant not had the variegation that made it an albo, each of those cutting would have been worth a few dollars apiece.  So what makes some plants variegated and others not?  Sometimes the variegation is the standard form found “in nature” and sometimes it is a cultivar or variety that has been bred or discovered by chance.  Let’s take a look at all the ways that a plant can get that variegation, whether it is standard or rare. 

Chimeric variegation

My Monstera albo that caused the hubub

This is a common form of variegation and the one responsible for the variegation of my Monstera.  In this form, a genetic mutation in some cells changes that cell’s ability to produce chlorophyll.  It may reduce chlorophyll production, resulting in yellowish or silver coloration, or eliminate chlorophyll altogether, resulting in white coloration. 

The name chimeric or chimeral is based on the fact that the plant displays two (or more) chromosomal patterns on one plant.  In Greek mythology, a Chimera is a frightening fire-breathing female monster with the head of a lion, body of a goat, and the tail of a serpent. 

Image result for chimera
An ancient chimera statue

This variegation can be stable, where the pattern persists throughout the plant.  Or it may be unstable, where it is random on certain leaves and parts of the plant can revert back to the standard green form.  These plants can also produce leaves that are almost totally white, which usually results in a leaf that will die since it can’t photosynthesize. 

This type of variegation also means that cutting or propagations may or may not be “true” to the pattern.  It can be random.  For my Monstera, the presence of white striping in or around the node that will become the new plant is the important marker for whether the new plant will be variegated or not. 

One common chimeric houseplant is the plant formerly known as Sanseveria, now a Dracena (Snake plant or mother-in-law’s tongue). Many of the different color patterns on some of the cultivars are due to cuttings taken from different parts of the “original” natural type that display different colors on them.

Viral Variegation

Image result for tulip mania
Viral variegation that was all the rage in Tulip Mania

While beautiful, this variegation will often reduce the productivity of plants if not kill them outright.  There aren’t a lot of houseplants that have this variegation, but some Hosta cultivars do.  Probably the most famous case of viral variegation is the Tulip Mania during the Dutch Golden Age (in the 1600s).  Prices of tulips skyrocketed and people were buying them as investments (maybe like the current houseplant craze, or GameStop stocks, or bitcoins).  Unfortunately, as the virus reproduced plants kept getting weaker and weaker.  Eventually the tulip market collapsed and lots of people went broke.  Let’s hope that doesn’t happen with the houseplant market….at least with my fancy Monstera. 

Natural Variegation

Natural variegation on Tradescantia

This type of variation occurs when the patterns or colors of the variegation are written into the DNA of the whole plant.  It will occur regularly throughout the entire plant, not randomly on some parts as in chimeric or viral variegation.  This variegation is passed through cuttings and usually through sexual reproduction from seeds as well, though different variations may pop up that cause a more desirable or rare cultivar. 

Common houseplants such as Tradescantia, Maranta (prayer plant), and many more common plants have this type of variegation. 

Blister, bubble, or Reflective Variegation

Reflective variegation on Phildodendron ‘Birkin’ following veins in the leaf

This type of variegation occurs when there is an air pocket or bubble between the lower layer of tissue and epidermis, or skin, of the leaf.  The lower level typically has green pigmentation from chlorophyll and the epidermis does not, resulting in a pattern that is usually white, silver, or yellowish though other colors could appear.  This pattern can be blotchy or splotchy like in some types of Pothos and Pepperomia.  It can also occur along the veins of some plants, resulting in white or silver veins on green leaves, as in some Alocacia, Anthurium, and Philodendron varieties. 

In conclusion…..

Even if you don’t have an expensive plant hiding in the corner, houseplants can add lots of fun and color to your living spaces.  And sometimes, your houseplant obsession can even pay for itself.  Online swap and sale groups have houseplant afficionados swapping and selling cuttings and plants all over the place.  So enjoy your plants….and maybe you’ll find a cash cow hiding in the corner.  Don’t mind me….I’m just over here propagating more Albos to fill up my “mad money” jar. 

Sources

Variegation mutants and mechanisms of chloroplast biogenesis

Variegated Indoor Plants: The Science Behind The Latest Houseplant Trend

Chimeras and Variegation: Patterns of Deceit

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