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Winning the weed wars

If there’s one thing gardeners can be sure of, it’s the fact that 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.

A brief look at weed seed production and survival tactics provides some clues as to why weeds continue to be an on-going problem for most home gardeners. Almost without exception, weeds produce copious amounts of seeds that can remain viable in the soil for years. In fact, a single Redroot Pigweed plant, which is fairly common in our area, can produce a whopping 117,400 seeds in just one growing season! Other examples of prolific weed seed production include: Lambsquarter – 72,450 seeds per plant, Purslane – 52,300 seeds per plant and broadleaf Plantain – 36,150 seeds per plant.

Prolific seed production is only part of the story, however.

Weed seeds are also notorious for their ability to remain dormant in the soil until the right environmental conditions favor germination. In fact, weed seed viability experiments conducted at the University of Michigan found Curly Dock seeds were still viable after 70 years of burial in sterile sub-soil. A single lambsquarter plant will produce 72,000 seeds that will remain viable for an average of 40 years. Pigweed seeds on the average will survive in the soil for at least 10 years. Common dandelions are not nearly as prolific with each plant producing a mere 15,000 seeds which remain viable for just six years.

The weed seeds that are currently germinating may well have been deposited by weeds allowed to go to seed decades ago. 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.

Gardeners lose ground when weeds are allowed to go to seed at any time or if they are introduced unintentionally. Manure and other uncomposted materials are good organic matter additions to the garden, but they can also contain weed seeds. Late-season weeds can be a major source of new weed seed deposits. By mid- to late summer, when it’s too hot to work in the garden or we go on vacation, weeds can take over and go to seed unnoticed. Some weeds have the botanical characteristic of flowering when days get shorter in the fall. This day-length response results in weeds that quickly go to seed soon after they germinate, producing seeds on very small and young plants. For example, a spring germinating pigweed will get very large by the time seed heads form, while a late-summer germinating pigweed will form a seed head when the plant is much smaller.

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.

One of the most effective methods of controlling weeds in gardens is by using mulches. In addition to weed control, mulches help maintain soil moisture. Mulches also keep soil temperatures cool and keep vegetables cleaner. Examples of organic mulch materials include wood chips, sawdust, lawn clippings and leaves. A layer of organic mulch from one to six inches deep may be needed in order to effectively control weeds.

Fabric mulch (sometimes called “weed barrier” fabric) is highly effective in preventing the establishment of annual weeds. It is permeable to air and water, but acts as a physical barrier against germinating weed seedling shoots and roots. Though designed primarily for use under bark chips in ornamental plantings, these fabrics can also be sued in vegetable gardens.

This column is printed as a service of the Washington State University Extension Office at Elma. Donald D. Tapio is the regional gardening and farming specialist for Grays Harbor and Pacific counties. He can be reached at (360) 482-2934 or tapiod@wsu.edu.