University of Vermont
Department of Plant and Soil Science

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Dr. Leonard Perry, Horticulture Professor Emeritus
University of Vermont

Is it true that grass clippings will lead to thatch, or that watering during the day burns leaves, or that paint over tree wounds prevents disease?  These are some of the products and practices, often called myths but perhaps better termed misinformation, to watch for in your gardening reading and recommendations from friends. Such myths usually aren’t intended to deceive, but rather are based on incomplete knowledge, often from many years ago.
Many believe grass clippings should be removed after mowing, otherwise they will cause thatch to build up.  This is a layer of undecomposed living and dead stems, leaves and roots above the soil but below the tops of a lawn.  Some is good but an inch or more can keep water, nutrients and air from the roots.  Surface rooting can result, with the grass susceptible to drought, or diseases can form there.  While poor mowing—too infrequent leaving too much behind—can add to thatch, it is formed by other practices.   Thatch results from more grass growing than can be broken down by soil organisms.  Reduce this such as through less fertility, and protect the organisms such as through proper watering and minimal use of pesticides, and thatch often can be avoided.
If watering during the day caused leaves to “burn” or turn brown, then farmers and gardeners would be in trouble after daytime rains!  If you put a magnifying glass over a leaf, you’ll see that nothing happens if it is right on the leaf.  Only when you raise it to a certain height, focusing the sun’s rays, does it start to heat up an object.  This myth likely arose from watering plants with water high in salts—one cause of leaf burn.
For years, and you’ll still see in many references, the recommendation after pruning large cuts was to cover them with a special tree paint.  This was believed to seal out moisture and disease, similar to how we treat our own wounds.  Actually, especially in hot climates, these coatings may crack and allow moisture and disease to enter.  Trapped inside, they cause rots.  Such paints inhibit the growth of callus too, which is the growth the plant produces to seal over such wounds. 
You may see ads, or products, touting vitamin B1 (thiamine) to prevent transplant shock or to stimulate new growth after planting.  This came from a study in the 1930’s that showed such effects from pea roots cut off and placed in a culture medium in the lab.  Thiamine is produced in plant leaves, and found in roots, but subsequent studies with whole plants of mums, some trees and some vegetables failed to show any effects.  Effects in products with thiamine come from other ingredients such as rooting hormones and fertilizer nutrients.
Isn’t it a good idea when planting a tree or shrub to amend the backfill soil with organic matter such as peat moss, or, if the soil is really poor to completely replace it?  This was thought the case for years, but some recent studies have shown this is not really effective.  Water movement can be slowed, and build up in the bottom of the planting hole, when soil of one texture meets that of another.  If the soil is really poor, and that in the planting hole has been replaced with good soil, the roots will likely stay in the hole and never create a normal root system.  So plant without amending the soil, and if it is poor, choose a plant that will at least tolerate such soils.
Speaking of planting, it is not true that you should dig a hole deeper than the existing root ball or pot size.  Replacing loose soil under the weight of such a root mass will result in the roots and plant settling, the trunk being partly below ground.  Trunks and stems of plants should remain above ground where they can get air, and wont rot.
Related to this is the practice of heavy mulching around trees, believed by some to control weeds.  This works if the bark mulch is pulled a few inches away from the trunk, not piled up against it resembling a volcano and giving rise to the term “volcano mulching”.  Just as planting too deeply can smother the bark, so can such mulching.
Several beliefs about using pesticides are myths and not true.  When using garden chemicals, even fertilizers, it is not true that if some is good then more is better.  Often the result is damage to the desirable plants, called “phytotoxicity.”  Too much pesticide or fertilizer can wash into and pollute watersheds.  Too much, even fertilizer, can kill beneficial organisms.
It’s also not true that non-selective herbicides only kill herbaceous plants.  Weed killers for broad-leaved plants, such as dandelions, can kill your broad-leaved ornamentals just as well.  Keep in mind if applying such to lawns that tree roots often extend far out from the plants and can take up such herbicides.
It’s also a myth, or false, that if you have leftover pesticides you can dilute them and so safely dump them down drains and sewers.  Dispose of according to the container directions, directions from your local waste disposal center, or use up properly.
If you see a problem with plants, should you go right to the pesticides to stop it?  This is false, as there may be suitable non-chemical options, your plants may be able to tolerate the pest or problem, or the problem might not even be caused by a pest.  To properly identify the problem and cause, which may turn out to be cultural or environmental, check with your local trained garden store professionals, or state university plant diagnostic clinic (  

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