University of Vermont Extension
Department of Plant and Soil Science
Fall
News
Article
HORTICULTURAL
MYTHS
Dr.
Leonard Perry, Extension Professor
University of Vermont
Myths
may be a polite way of referring to statements or beliefs that are
believed
true, but are not accurate or totally true.
The internet has made possible the easy proliferation of such
misinformation. Two of these myths
relating to household products used in landscapes, often in attempts to
be
sustainable, concern Epsom salts and baking soda.
The
common household ingredient Epsom salts (magnesium sulfate) has been
widely
recommended for many garden uses including to control pests, to deter
slugs and
voles, to make plants grow bushier, to increase chlorophyll (the green
in
leaves) production, to help seeds germinate, to produce more flowers,
and to
improve uptake of some nutrients.
Recommendations also are seen on how much to regularly feed plants from
trees and shrubs to houseplants, and in particular tomatoes.
The
reality, if one examines the research on this chemical back to early in
the
last century, is that it does correct and has been effectively used for
magnesium deficiency in soils or plants.
Other than this, it is generally a myth that applying Epsom salts will
fulfill the often-seen claims to otherwise healthy plants.
Some
soils are naturally low in magnesium, such as those light and sandy, or
even
clay soils if intensively cropped. Most
research, and applications of Epsom salts, has been on intensive
cropping
systems of fruits and vegetables that can deplete magnesium.
Magnesium may be deficient on pastures, and
may cause disorders in grazing cattle if deficient.
Nutrient
chemistry in plants can be complex, an example relating to
magnesium. An excess of potassium (the third number in a
fertilizer formula) can make magnesium less available to roots.
So under this situation, extra Epsom salts
won't help. Reducing the potassium, or
increasing nitrogen, may be all that's needed to make the existing
magnesium
available if a plant is deficient. This
illustrates why it is so important to do soil tests.
One
claim is that Epsom salts won't accumulate in, and so won't hurt, the
soil. What isn't mentioned is that it
will be leached, and so potentially pollute water, instead. Epsom
salts are highly soluble, and if
applied in excess can leach into water (49 percent in one study), the
pollution
negating its benefits. Other less soluble forms of magnesium
supplements often
are better. This is especially true on
soils under high rainfall or irrigation.
Another
common household ingredient, baking soda (sodium bicarbonate), is
claimed and
believed by many to kill fungi on plants and to reduce diseases.
The reality is that baking soda (by the way
it's not legally labeled for use as a fungicide) helps prevent rather
than kill
diseases. The baking soda acts by making
surfaces more alkaline-- not conducive for growth of fungi. So it
is technically fungistatic (prevents
fungi growing) rather than fungicidal (kills fungi).
If
some is good, more is not better, as an excess even of baking soda can
cause
problems on fruits and leaves. In
addition to providing better prevention of diseases, higher
concentrations can
cause discoloration of some fruits (as little as a 2 percent solution)
and
interveinal chlorosis on leaves (as little as a one percent solution).
Baking
soda, while showing great results in lab studies, has been less
consistent in
field trials. Outdoors, where it has been
tried on powdery mildew (including my own studies on garden phlox in
Vermont),
it appears more effective consistently when combined with
horticultural oil. Other compounds, such as the
related potassium bicarbonate (which is in some
legally registered fungicides), often are more
effective.
In a couple of scientific studies on roses,
this combination of baking soda and horticultural oil was effective on
powdery
mildew on roses but not blackspot -- a use often seen. Siting
roses (as well as other plants)
properly with good air movement and well-drained soil, keeping from
stress through proper watering, and
using a coarse organic mulch such as wood chips, may provide adequate
to good
control of leaf diseases such as black spot.
Another
use of baking soda is to control postharvest diseases on fruits.
Even here, it is more effective when combined
with yeast organisms that prevent diseases from growing.
More
on these horticultural myths as well as many others, and better
alternatives,
can be found on the website of Washington State University Extension
Horticulturist and Associate Professor Linda Chalker-Scott
(www.informedgardener.com). Tips she
provides for evaluating popular or "gray" (questionable validity or
basis in science) recommendations include asking the questions:
-- Does the author have reputable ties, such as to in
academia or a scientific institution?
-- Does the recommendation come from information or
sources that can be verified?
-- Does the recommendation refrain from, and is not
tied to, selling a product?
-- Does the article appeal to reason rather than
emotion?
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