University of Vermont
Department of Plant and Soil Science
Anytime News Article
IS IT TRUE, OR NOT?
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
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
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 (www.nepdn.org).
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