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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