In our quest to be great gardeners, it is important to learn how to distinguish evidence based best practices from common garden myths. In this week’s “Ask a Master Gardener”, Susie Everding discusses how to differentiate fact from fiction and tackles three common garden myths.

First, a word about evidence…

There is plenty of gardening information available from books and the internet, but just because it appears in writing, or is an anecdote from a friend, doesn’t necessarily make it true. Professionals who research soils, plants, insects and other facets of gardening will conduct controlled experiments to test ideas and practices, and will publish those results in peer-reviewed scientific journals. Many of these are available to the public as “open source” publications. So ask yourself, what evidence exists for the methods I use in my own garden? Search out credible sources of information (see reference list below) or perhaps consult with a Master Gardener!

Susie has had her own experiences with the myths she discussed, but in her training to become a Master Gardener has researched and debunked several of the practices she formerly embraced!

Are Epsom salts good for plants?

There are numerous purported benefits to using epsom salts in the home garden. These include everything from improved germination of seeds, more chlorophyll production, bushier plants with more flowers, pest deterrence and more. But when Susie delved further into the subject, this is what she found.

Epsom salts are hydrated magnesium sulfate. Magnesium (Mg) is one of several secondary macronutrients found in soil and is needed by plants for chlorophyll development and photosynthesis. It is true that some soils may be lacking in Mg, particularly when certain crops are intensively farmed, and in sandy or acidic soils. However, before attempting to remedy any possible nutrient deficiency or imbalance, a soil test should always be done first. A list of soil testing labs is found at the end of this article. 

Do epsom salts promote seed germination? Susie notes that most seeds contain within them all the requirements for germination when exposed to the correct light, moisture and temperature conditions, so epsom salts are not useful in speeding up germination.

Likewise there is no evidence that epsom salts will result in bushier plants with more chlorophyll or flowers, if your soil has adequate Mg and other required elements. Nor is there any evidence that epsom salts have a deterrent effect on insects, slugs, rabbits or other garden creatures. And finally, because epsom salts are highly soluble, it has been claimed that it will not persist in garden soils. However,  a nutrient applied in amounts beyond what is needed by the plant may cause reduced absorption of other elements (e.g. calcium)

The bottom line: Do not use epsom salts in the garden unless a professional soil test indicates that your soil is low in magnesium.

Does wood chip mulch “rob” the soil of nitrogen?

Various types of mulch are useful in the garden as a means of decreasing weeds, retaining moisture, regulating soil temperature, reducing erosion and adding nutrients back into the soil. Mulches are laid on top of the earth, rather than worked into it.

Wood chips are one such mulch, but some have claimed that because wood chips are a high carbon material, this will “tie up” or “rob” the soil of nitrogen. As micro-organisms start to decompose the wood chips, they do use up nitrogen as fuel and will cause a temporary and localized reduction in nitrogen. However, because wood chips are placed on top of the soil, rather than mixed in, the deficiency will only occur at a thin (several mm) interface between the wood chips and the soil. As decomposition occurs, nutrients including nitrogen will be added back into the soil.

The bottom line: Wood chips are a great mulch on top of your soil!

Do mycorrhizal fungi products enhance plant performance?

In nature and in your garden, most plant roots have a mutually beneficial, or symbiotic, relationship with naturally occurring fungi in the soil. Mycorrhizal fungi are a fine filament like network growing in association with plant roots. The fungus obtains nutrients from the plant and in return, increases the surface area of the root that can take up nutrients, such as phosphorus. Thus mycorrhizal fungi are a win-win !

Your garden already contains many species of mycorrhizal fungi native to your area. Some are specific to certain types of plants.

Soil supplements such as “Myke” contain mycorrhizal fungi, as do many commercially available potting mixes. The manufactured mycorrhizal fungi in these products are limited to just a few types, which may or not be the ones that your plants will benefit from. In addition, as living organisms they are heat sensitive, and if exposed to adverse conditions, may not be viable when they reach you. Mycorrhizal fungus products will also have an “use by” date on them, after which the fungi may no longer be alive. While the evidence is not yet available regarding addition of a “foreign” inoculum to your garden, thought should be given to any unintended consequences of this introduction.

Mycorrhizal fungi may be a useful addition in degraded or depleted soils such as those found in greenhouses. However, when establishing a new vegetable garden, it may be just as helpful to sprinkle a handful of native soil. Susie also recommends regular addition of compost and minimizing disturbance of soils (i.e. avoid rototilling and double digging) to encourage mycorrhizal growth.

Summary

When adopting practices in your garden, Susie cites advice from Linda Chalker-Scott, from Washington State University, who suggests two questions to ask:

• does the rationale behind the practice make sense given our current scientific understanding?
• does the practice actually have a significant effect?

As with many things, our knowledge is always evolving so it is important to critically evaluate what you read and hear, and to seek credible advice to assess whether a recommendation offered is a practice you wish to adopt, or may in fact be a garden myth!

References

Joe Gardener, Scientific Method in the Home Garden (from minute 43 to 46:30)

Open source publications from the University of Guelph. Look for the words Open Access, and the orange open lock icon.

Miracle, myth or marketing – Epsom salts. Linda Chalker-Scott, PhD.

Miracle, myth or marketing – Wood chip mulch: Landscape boon or bane?. Linda Chalker-Scott, PhD

Using arborist wood chips as a garden mulch WSU Extension Fact Sheet

Sheldrake, M. 2020. Entangled Life: How fungi make our worlds, change our minds and shape our futures. Random House, NY.

Soil Testing Labs (Ontario Ministry of Agriculture, Food and Rural affairs)

More about horticultural myths:
Dr. Linda Chalker-Scott, Washington State University extension fact sheets:

Another good (Canadian) site is Robert Pavlis “Garden Myths” website: