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One of the worst features of weeds is their speed of travel across bare ground. Many gardeners have been frustrated by the flush of green across a freshly weeded garden. Clean it off, turn the soil over, and within a week hundreds of weed seeds will germinate, to the annoyance--and worse--of the gardener. What are some methods to deal with summer weeds?
Persistence and method will combine to help with annual weed control. The main enemy is the production of seeds--that's the primary weapon of the annual weed. If the plant can be prevented from producing seeds, by some method of weed birth control, a major continuing problem can be reduced, even though weeds will never be eliminated.
Weeds, like most flowering plants, have basically two possible life cycles, annual or perennial. (There are also biennials, with a two year life cycle, but they can be managed as annual weeds are.) Knowing the life cycle of the plant is important in developing appropriate control measures. Today we'll discuss annual weeds and their management, not their extermination, because they won't be exterminated!
Annual weeds sprout, grow to maturity, go to seed, and die out in one year. Annual weeds are relatively easy to pull up, and don't leave persistent roots behind. Especially as infant weeds, they respond to scuffling with a hoe. But they get even by producing thousands of seeds and scattering themselves across the landscape for future plants. Common annuals that may germinate in cool weather and also persist through mild winters in western Washington are shepherds purse (Capsella bursa-pastoris), annual bluegrass (Poa annua), peppercress (Cardamine oligosperma), and common chickweed (Stellaria media). Winter weather doesn't stop weeds in western Washington, so weed management is a year-round process. This time of year the summer weeds are forming seed heads and the ground is full of last year's seeds.
Just to highlight what insidious and nasty habits annual weeds have, one writer said of common chickweed that it "...has many characteristics of the 'ideal' weed. It is an annual that very quickly grows from seed to seed-producer, and produces seeds continuously while conditions permit. The requirements for germination vary from seed to seed, ensuring that seeds do not all germinate at once, which would make the species vulnerable to weed control methods." (Ronald Taylor, Northwest Weeds.) In other words, chickweed is darned hard to get rid of.
Many bothersome weeds were introduced to North America as food sources, and it's possible to add young tender chickweed plants, peppercress, or shepherds purse to salads. Eating them, while interesting, doesn't provide a reliable method of weed control. Munching on weeds does have psychological and ecological value, for it reminds us that every plant we eat or use was developed from a wild-occurring plant. (Be sure to identify the weed before sampling it.) No scientist has ever created a food plant in a laboratory, but many have worked to enhance the edible and useful characteristics of thousands of wild plants. We owe our lives to weeds.
That fact may not be very comforting when looking at an overgrown landscape. Two problems confront gardeners when controlling annual weeds. The seeds persist for a long time in soil, and as noted with the chickweed, come up at irregular intervals. Seeds are produced in prodigious quantities and are distributed by wind, birds and animals, and the plant's own ability to expel and propel them.
All weeds, no matter what their life cycles, are easier to control as small, immature plants. The first method of control is mechanical scuffling of the soil to kill newly-emerged plants. If weeds are to be controlled by some form of hoeing, the gardener needs to observe and hoe as often as needed to keep the emerging weeds down before they go to seed. A weekly weed walk through the garden with a scuffling tool in hand can reduce time and effort later. The old saying, "One year's seeds, seven years' weeds" reflects the persistence of weed seeds.
Annual weeds--and all weeds--tend to hide out under plants or disguise themselves as garden ornamentals. Lift plant edges and look closely for sneaky seedlings. In addition to hoeing, another method of controlling annual weeds is to smother them, which removes chances for the hidden seeds to get to light and thus germinate.
Using ground cover plants in a garden is a good way to reduce weed problems. A well-established stand of low perennial plants will shade out weeds. Rock rose (Cistus species and cultivars), works well as a weed control plant in sunny, dry places and has attractive late spring blooms. But ground covers must be weeded as they fill in, and it may take three years of persistent care before their branches offer substantial weed protection.
Don't leave ground bare. Cover it after weeding. Covering the ground with 2 to 3 inches of any organic mulch, such as compost, leaves, aged sawdust, or commercial compost like Cedar Grove Compost (Or Tagro, or Groco), will help prevent the thousands of annual weed seedlings from coming up. It's also possible to use one of the weed prevention geotextiles, (sold as landscape fabric.) These are made of a woven synthetic fiber that allows water and air to penetrate but won't allow light to the weeds. Put mulch on top of these textiles for best appearance. They last for years if not torn by careless digging---a patch under a garden path in my garden has lasted 5 years.
These permeable textiles work more to the advantage of ornamental plants and soil health than solid black plastic does. Black plastic doesn't allow air or water penetration. Soil under it dries out.
What about herbicides for annual weeds? Herbicides are weed killers, pesticides that are often relied on for weed management in home gardens. It's not ecologically sensible to put a systemic herbicide that moves into the plant for its killing effect (like glyphosate, sold as Round-Up) on an annual weed that is going to die out at the end of the season. Indeed, some annual weeds treated this way with systemics will die but will produce their seeds while dying, thus defeating the purpose of getting rid of the weed. What about spreading an herbicide on the ground to keep seeds from germinating? Many products are sold for this purpose. It's important to consider the use of the area before selecting one of the herbicides called "pre-emergent," named for the fact that they keep seeds from coming up. These products work, but they have to be used very carefully. Some are best avoided because they essentially sterilize the ground for a year or more. (Check the labels on products with common names such as monoborchlorate and prometon--these are soil sterilants, and aren't recommended by WSU specialists because they can leach and pollute water.) Many a gardener has found out the sorrow of treating a piece of land for weeds and then discovering nothing at all will grow there.
Probably the most common product sold for pre-emergent control is dichlobenil, with the brand names Casoron or Norosac, and others. Use this product carefully. It's only labeled for use around plants that have woody root systems and have been established in the ground for at least one growing season (6 months or more.) It can't be used where the gardener intends to plant, or has planted, annuals, perennial flowers, vegetables, bulbs, or new woody shrubs or trees. If over-applied, dichlobenil will kill desirable plants. If pre-emergents are chosen, use them sparingly, cautiously, and in combination with other weed control methods such as mulch. Check labels carefully before using any pesticide.
Learn to identify weeds--it's essential for good control. A useful book is Ronald Taylor's Northwest Weeds, Missoula Montana, 1990. The best methods for dealing with annual weeds all involve persistence---hoeing, mowing, shading out with crops or ground covers, covering the ground with geotextiles or mulch or both, and keeping up the vigilance. It's a constant effort.
Hortsense: Managing plant problems with Integrated Pest Management
