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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. 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 or 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). Cold weather doesn't stop weeds in western Washington, so weed management is a year-round process.
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 .)
Many bothersome weeds were introduced to North America as food sources, and it's possible to add young tender dandelions, chickweed, 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. 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.
The scattered annual seeds will germinate and new plants appear whenever the soil is dug or disturbed. Many gardeners have been frustrated by the flush of green across a newly-raked 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.
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, if not eliminated.
We'll never eliminate weed incursions from gardens. But 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, any of the Cistus species and cultivars, work 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. 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, 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 textiles work more to the advantage of ornamental plants than solid black plastic does. Black plastic doesn't allow air or water penetration, and these absences can affect plant root health. Chemicals? Next week we'll discuss chemical weed killers.
Hortsense: Managing plant problems with Integrated Pest Management
