Do you pour Coca Cola on your cucumbers? What about shampoo on your salvia? Have you drowned slugs with suds...beer, that is?

Are these left-field ideas gardening myths, or do some of the unorthodox potions and tricks really work in the garden?

“It depends,” says Dr. Jeff Gillman, University of Minnesota extension horticulturist. “We’ve tested some of the crazy concepts and they do work, but some of the others are a total waste of money and time.”

Here are some insights from Gillman from a recent presentation titled “Dispelling and validating urban gardening myths.”

Soil moisture-retention products. Many companies market hydrogels as soil additives to help soil retain moisture, particularly in containers. The first patents for hydrogels – which resemble pebbles made of jello – were issued in 1967. University of Minnesota tests showed hydrogels to be rarely effective. Although Consumer Reports recommended sponges as a replacement for hydrogels, Gillman said sponges were not effective in University tests.

High-phosphorus fertilizers. The theory is that high-phosphorus fertilizers, such as 0-52-0, can increase root development and, as a result, plant vigor and fruit production. Based on a hydroponic study comparing phosphorus levels, Gillman said high-level applications of P do not lead to increased root growth and may even inhibit plant performance. Results were measured in both hydroponic and container studies, and supplemented with a thorough literature search.

Soil mycorrhizae. Mycorrhizae are beneficial soil bacteria present in most gardening soils. Some companies market supplemental mycoorhizae to help “mine the soil.” University studies, according to Gillman, suggest that supplemental mycorrhizae are generally not necessary since they already exist in most gardening soils.

Root-ball pruning. When trees, shrubs and other woody plants are purchased at a garden center and discovered to be root-bound when removed from the container, does root pruning help the plant get established in the soil and prevent growth problems later on? A common planting technique, supported in the past by University horticulturists, as been to use a knife to cut vertical slashes through the bound, circling roots before planting.

A University study subjected root-bound potted lindens and willows to various treatments and then measured tree growth after two years. Pot-bound roots were left “as is” and planted, “butterflied” (root ball cut in half with axe or spade), scored vertically from top to bottom on four sides, and “teased” (roots pulled away from root ball with fingers before planting).

None of the four treatments resulted in a measurable growth advantage, according to Gillman. One possible conclusion, he says, is “once a plant becomes pot-bound it stays pot-bound.”

Slug treatments. Slugs are 1-inch long mollusks that delight in eating the leaves of hostas and other plants, especially at night on plants growing in moist soils.

A common gardening myth is that crushed egg shells sprinkled on the ground around susceptible plants will deter slugs because the soft skin of slugs cannot survive the crawl across egg shells. In Gillman’s research, slugs had no problem crawling over egg shells.

A follow-up study using diatomaceous earth showed improved results against slugs. Gillman advises using a diatomaceous-earth product from a garden center rather than products designed for swimming pools because the garden product will have the sharpest edges.

What about beer traps for slugs? The technique, which involves attracting and drowning slugs in stale beer, is effective as long as the saucer or container of beer is recessed in the soil at ground level. Slugs, in other words, will not climb up a container to drink the beer...they have to fall into it.

Homemade “witches brews.” Various potions have been concocted and promoted for controlling black spot and powdery mildew diseases on roses and other plants. In University tests, treatments with chlorothalonil and baking soda were both effective in preventing powdery mildew, and chlorothalonil was also effective in preventing black spot. Compost tea may have contributed nutrients to the plant but did not protect against disease.

Another substance that provided both black spot and powdery mildew protection, Gillman said, was mouthwash containing ethanol when mixed 1:3 with water but the mix did lead to some leaf burn.

Vinegar as herbicide. University trials showed that vinegar definitely has herbicidal properties but full-strength vinegar can also burn the plant being protected. Vinegar applied undiluted will kill all weeds. An 80% vinegar solution provided moderate weed control while a 50% vinegar solution was not effective in controlling weed growth. An 80% vinegar solution should not be applied to the plant being protected as it will burn the plant, says Gillman.

So what’s the bottom line on gardening myths? It appears, based on research, that some of the myths have validity while many others don’t work. Asked why the myths got started and continue to lure susceptible gardeners, Gillman said many propagators of myths have discovered they can make money promoting bogus products or spreading mythical information.