Most gardeners will agree, no matter what they plant, weeds are sure to germinate and grow. What is difficult to understand however, is how weeds can be so prolific in garden soils that have been carefully weeded for years.
It’s easy to understand how weeds continue to appear, despite control efforts, by taking a look at weed seed production and survival rates. Each pigweed plant that goes to seed, for example, produces just short of 118,000 seeds — which on the average will survive in the soil for at least 10 years.
A single lambsquarter plant will produce 72,000 seeds that will remain viable for an average of 40 years. Common dandelions are not nearly as prolific with each plant producing a mere 15,000 seeds that remain viable for just 6 years. Weed scientists frequently refer to existing weed seeds in the soil as the “Weed Seed Bank.”
Virtually every garden soil has a weed seed bank with varying weed species and seed numbers. With every tilling of the soil, a percentage of the dormant weed seeds are exposed to the soil surface, where they promptly germinate and grow.
A relatively new concept for controlling weeds relies on using plants that produce toxic chemicals that prevent plant growth. Most home gardeners are probably most familiar with this phenomena as it pertains to the inability to grow plants beneath black walnut trees. Since the time of ancient Rome, gardeners have observed that walnut trees are toxic to many other plants. Botanists refer to this characteristic as allelopathy.
The use of allelopathy for the control of weeds is at the cutting edge of environmentally sound weed management. Researchers at the University of Michigan have found that cereal rye is extremely effective in controlling many common garden weeds. In an experimental plot, rye was planted as a cover crop in the fall. In the following spring, the rye was mowed down or sprayed with a short-term non-selective herbicide (Roundup). Large seed vegetables such as beans and peas were then planted through the mulch.
As the rye decomposed, toxins were released that inhibited the growth of a number of common garden weeds, including redroot pigweed, purslane, foxtail and ragweed. The rye not only produced the usual benefits of a mulch with increased soil moisture, greater microbial activity and buffered soil temperature, but also provided biochemical weed control.
Home gardeners can easily implement allelopathic principles in their own gardens by planting a fall cover crop of rye and using the residue as a mulch following the procedure developed at the University of Michigan.
Another option would be to grow a cover crop of rye in the back area of the garden, mow it monthly, and spread the clippings between rows of vegetables. Aisles between trees in home orchards might also be planted with rye and kept mowed.
Although a mulch of cereal rye may not control all weeds, it will certainly help to reduce overall weed populations.
Donald Tapio is a WSU Extension regional specialist emeritus. He may be reached at tapiod@wsu.edu.
