Two horticultural professionals took some of the most popular garden myths into the university laboratory to prove or disprove the accuracy of these myths. Dr. Linda Chalker-Scott from Washington State University and Dr. Jeff Gilliam from the University of Minnesota tested these myths under controlled conditions to determine if they really work.
The Myth: Adding bone meal to the planting hole will stimulate root growth for trees and shrubs.
The Facts: Among all of the soil amendments on the market today, bone meal seems to be the magic elixir to encourage plant development. Website after website, and article after article we are told that bone meal stimulates root growth, will reduce transplant shock and is generally a "fix all" for all plant problems. I'm sorry to report that bone meal is primarily calcium and phosphorus, two elements which are usually adequate in most soils. The NPK of bone meal can vary but is usually in the range of 0-12-0 or 3-20-0. Both calcium and phosphorus are required for plant growth but both and especially phosphorus can cause problems if the concentrations are at high level.
Neither of these two elements nor any mineral will "stimulate" plant growth.
The most plausible explanation as to how this myth got started is the effect that a phosphorus fertilizer has on mycorrhizae. When plants are grown in low phosphorus environments they emit organic acids from their root tips. These acids allow mycorrhizal fungi to penetrate the roots and form the networks that assist roots in absorbing water and nutrients. If the phosphorus levels are too high the roots do not emit the acid and the mycorrhizal networks do not form. This forces the plant to put more effort into root growth to compensate for the lack of mycorrhizae.
Seedlings and transplants are less efficient in absorbing water and nutrients from the soil and are more likely to experience transplant shock than plants where mycorrhizae are present.
So remember: Bone meal supplies high levels of phosphorus and calcium which are usually adequate in most soils. DON'T GUESS, GET A SOIL TEST. Phosphorus from bone meal or other sources does not stimulate plant growth. High levels of phosphorus from bone meal or other sources will inhibit growth of mycorrhizal fungi. Without these fungi, plants must put additions resources into root growth at the expense of other tissues and functions.
The Myth: Newspaper and cardboard sheet mulches are an excellent way to retain moisture and deter weed growth in the garden.
The Facts: In their quest to create a more sustainable landscape and reduce the use of chemical herbicides gardeners have been focusing on organic mulches. If the mulches are from a recycled material all the better. The less that goes to a landfill is much better for the environment. Newspaper--whether shredded or in full sheets--has been proven to work very well in conserving moisture and reducing weeds growth in row crops.
Cardboard, on the other hand, has not been so successful. In fact, it has shown to be havens for pests and termites were found to prefer cardboard over wood chips as a food source.
Both newspaper and cardboard were found to be less effective than other mulches (e.g. wood chips or shredded bark). Also, newspapers and cardboard can be dislodged by high wind if not completely covered. Both of these mulches can induce anaerobic activity (which means without oxygen) if used on wet or poorly drained soils. When wet, the layers of paper are compacted and create a barrier to water and gas exchange in the soil.
There are many other choices of organic mulches available, in fact many cities will provide wood chips at no charge to residents. Shredded leaves or grass clippings are also excellent choices in lieu of newspaper or cardboard.
The Myth: Hydrogel products (Soil Moist, Hydrosorb, Watersorb etc.) will conserve and release water to plants)
The Facts: Hydrogels were first patented by two employees of Union Carbide Company in 1967. Hydrogels are crystals about the size of large grains of sugar; when water is absorbed they look like large chunks of clear jello. They are mixed into the planting medium whether it be containers or directly into the soil at planting. The theory is that they will absorb moisture and release it to the roots during dry periods. The crystals can hold 600 times their weight in water, however this retained moisture is not necessarily transferred to plants.
In the lab several brands of hydrogels were used in the testing. The plants were grown in 1-gallon containers and watered when the medium was less than 80% saturated. After all of the plants matured to the same size watering was stopped and the plants were allowed to dry out. Each plant was tested at regular intervals to determine moisture content.
The results showed that none of the hydrogels kept the plants supplied with water any better than the plant that had nothing added to the soil. One product (Hydrosorb) actually stunted the growth to the plants to which it was applied. Experiments conducted at other universities showed similar results on these products.
There is one application where hydrogels proved helpful: when plants were removed from the soil bare root to another site the roots were dipped into moist crystals. The crystals clung to the roots and prevented them from drying out. Hydrogel crystals are on the pricy side, the tests concluded that even though the crystals absorb a lot of water, the water is not necessarily taken up by the plants roots. It sounds to me as if these are probably a waste of money.
The Myth Vinegar is an effective weed killer
The Facts Vinegar is acetic acid. There has not been much research done on the effectiveness of vinegar as an herbicide.
For the tests various strengths of vinegar were used from 100% to several diluted solutions. For the test white vinegar at a 5% acidity level was used.
Plants sprayed with 100% straight vinegar turned brown within 2 days. Plants sprayed with a 50% diluted rate had significant damage but probably not enough to satisfy most gardeners' expectations. Plants sprayed with a 20% rate had a little damage from which the weeds recovered from quickly. Vinegar is a contact poison, which means it kills what it touches but doesn't affect nearby plants. By not coming into contact with the roots the sample plants roots quickly sent up new growth.
If poured directly onto the soil it will kill the roots but it takes a lot of vinegar to accomplish this. There are many other ways to control weeds both organic and chemically. I favor preventing weeds before they get a foothold. 3-4 inches of organic mulch will help to prevent weed growth and conserve water at the same.
In closing, I expect some of you might disagree with some of these findings, or have experienced different results. However remember that this research was done in reputable university labs under controlled conditions and each of us is free to draw our own conclusions.
About Paul Rodman
Paul Rodman has been gardening for over 45 years. He is an Advanced Master Gardener, and American Rose Society Consulting Rosarian. He is President Emertius of the Western Wayne County Master Gardener Association in Wayne County, Michigan. He currently serves as the greenhouse chairman of this group. Rodman has amassed over 5500 volunteer hours in the Master Gardener program.
Rodman is the garden columnist for The News Herald newspaper, in Southgate, Michigan. He has also written for the Organic Gardening.com web site.
He is a certified Master Canner and has taught classes on Home Food Preserving for 7 years.
He has lectured on various gardening topics throughout southeastern Michigan.
His favorite pastime is teaching children about gardening. For the past several years he has conducted classes for second grade students teaching them about subjects ranging from vermi-composting to propagation.