By Robert Cox, Horticulture Agent, Colorado State University Cooperative Extension
Many consumers assume that products on the store shelf must have been tested to prove their claims. Certainly, fertilizers have to meet nutrient content requirements, and pesticides are rigorously tested for safety before EPA registration.
For some other garden products, however, no such testing is required before sale to the public.
A good example is vitamin B1 (thiamine), often sold to "prevent transplant shock" and "stimulate new root growth" when planting trees, shrubs, roses and other plants. A study in the 1930's provided the basis for such claims. Pea roots cut off from the plant were placed in a culture medium in the laboratory.
The researchers knew that thiamine was normally found in roots, so they put thiamine in the culture medium and found that root growth did occur. Vitamin B1 is manufactured in 0lant leaves and sent to the roots, but if roots are cut off and placed in a petri plate, vitamin B1 stimulates growth of the roots when it saturates the culture medium.
Planting trees in a soil environment, however, is vastly different from a laboratory culture. Most important, gardeners aren't in the habit of cutting off the root system when planting. Several studies using intact mums, apple trees, orange trees, pine, tomato, beans, pepper, corn, pear, watermelon and squash have failed to demonstrate that vitamin B1 treatments provide any type of growth response.
Some "root stimulator" products contain a rooting hormone and fertilizer along with vitamin B1. These materials may increase rooting and growth, not the vitamin B1.
The bottom line: While root stimulator products are not necessary for transplant success, if you do use one, make sure it contains a rooting hormone and fertilizer rather than just vitamin B1. The vitamin B1 is for marketing purposes rather than actual effect.
Sulfur is claimed to "reduce alkalinity." When applied to our soils, sulfur must be oxidized by soil bacteria to the sulfate form; then sulfate reacting with water forms sulfuric acid.
In our soils, the bacteria responsible for sulfur oxidation are sparse, so the reaction may take many months or years. If sulfate is formed, it just reacts with the lime (calcium) usually prevalent in our soil to form gypsum (calcium sulfate). The bottom line: Don't spend a lot of money on it unless a soil test shows that your soil has low lime levels.
Gypsum is claimed to "break up and loosen clay soils." Again, in the Front Range area of high calcium (calcareous) soil, this is a local myth. Gypsum (calcium sulfate) added to clay soils having high sodium replaces the sodium with calcium, a much more desirable soil condition. The sodium is than leached out of the soil with water.
Locally, however, clay soils already are high in calcium. High sodium soils are rare along the Front Range. Adding calcium to a soil that does not need it is a waste of money. Additional calcium in the form of gypsum, a salt, will only make soils more saline.
Wound dressing for pruning cuts have been shown not only to be unnecessary, but many actually inhibit callus growth over the cut. Tars, emulsions, asphalts and waxes can dry and crack, especially in Colorado's climate. When water gets behind the crack, disease may be promoted rather than prevented.
The best treatment of a pruning cut is not treatment at all. Many people expect to treat tree wounds just like they would treat cuts in the human body--with a dressing. the public expects to see tree wound treated in some way, usually with a black "sealer." As a result, one city tree crew, knowing that dressings are not helpful but also aware of public expectation, applies a thin coat of black spray paint to pruning cuts.
The Myth of Day Watering
Still showing up in some popular garden literature is the notion that "day-watering can burn plants." The notion says that sunlight is "magnified by the water drop on the leaf to cause a leaf burn.
Anyone who ever burned ants using a magnifying glass and the sun knows that the magnifying glass did not burn the ant if it were placed directly on the ant. Rather, it had to be held a distance (focal distance) from the ant to concentrate the sun's rays enough to burn the ant.
If this notion were true, all gardeners would cover all their plants prior to every rainstorm.
Farmers would not be able to prevent widespread "leafburn" after rain clouds gave way to sunshine. The root of this notion may have come from the effects of applying poor-quality water high in dissolved salts. As water drops evaporated from leaves,the salts left behind could cause a leaf burn.
These are but a few of many claims and examples of conventional wisdom offered to the gardening public.
Ever since gardens were planted, observations and anecdotal claims have been offered to improve garden success. Some of these may be myths in Colorado but good advice in other areas of the country. Be cautious of label and advertising claims for garden products and skeptical of what you hear--and read!
Photograph courtesy of Judy Sedbrook.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010