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I'm talking about plants like Purple Loosestrife (Lythrum salicaria), the striking purple wetland plant that is overrunning New England and the Midwest, to the great detriment of local wildlife whose former habitat and food sources have become one big inedible purple mat. Not so long ago this plant was offered for sale in catalogues and it is gorgeous. Some seed savers may still have it. Don't intentionally grow this plant. If you see it in Washington State, please report it to the your County Weed Control Board . This is one enemy our Washington wetlands can do without.
Another introduced plant we don't ever want is the infamous Kudzu ("The Plant That Ate The South") which has been spotted as near as the Willamette Valley in Oregon. All of the crafty basket-makers supplying Martha Stewart have not been able to make a dent in the kudzu growing in Virginia alone (even though it can be used for crafts.) Western Washington shares the same climate zone as some of kudzu's favorite territory, and we could be overrun too. After all, look how well Scotch Broom (Cytisus scoparius) and Japanese Knotweed (Polygonum cuspidatum) have done here. I think the long sea voyage just made them stronger.
Europe has sent us the lovely pink Knapweeds (Centaurea x pratensis, Centaurea biebersteinii and Centaurea diffusa) to delight our eyes as they take over our grazing lands. From Asia, we have the Himalayan Blackberry, with root clumps larger than my teenager's tennis shoes. The weed battle will continue for generations, as we pry our gardens out of the grip of these terrorists. Shoulder your weapons, gardeners.
The questions is what do you do when you've got these weeds invading your borders, flower beds, and lawn. There are a number of strategies that, when used together, can actually kill off those brigands. Start with the mechanical means-cutting, pulling and smothering.
One way is to clean all plants off the area. Removing all vegetation and tilling the soil for a complete growing season ("fallowing") is one technique used by farmers. It's harder for a suburban gardener to look at bare earth for a growing season. I suggest clearing the vegetation and putting down a thick mulch, which is probably more realistic in smaller gardens. I use multiple sheets of newspaper-10 deep or more-covered with wood chips. This will decay and disappear over two to three years without any work on my part.
Any organic mulch, such as compost or sawdust, will work over the newspapers, but you must have both layers. Newspapers by themselves don't look good and don't stay in place. Other gardeners smother weeds with landscape fabric, carpet, or black plastic. I prefer newspaper since it is cheaper, readily available and has proven itself effective. I hate black plastic because it won't biodegrade and the scruffy-looking scraps are around forever. In any event, smothering is step one, but you can't stop there. It is a good way to prepare an area for planting next season.
Weeds will creep through the newspaper and mulch like infiltrating spies. At this point I either hand pull them or I paint them with a selective herbicide, depending on the weed. Persistence in weeding definitely pays, especially with annual weeds, but the worst deep-rooted perennial weeds will not die in my lifetime through pulling alone. If, like me, you can't wait for eternity, you may want to use a chemical weeder (herbicide).
CAUTION--Don't open a bottle of herbicide until you have READ THE LABEL and know what it will kill and how to apply it. Each brand is slightly different in its use but there are two basic kinds-the non-systemics that kill through contact only and the systemics that move through the plant tissues and kill the roots. Some are extremely toxic and persist in the soil for long periods while others are more benign.
If I must use an herbicide, I usually take a middle road and reach for a glyphosate (such as Roundup) but I don't use a sprayer any more because I have damaged too many good plants through spray drift. I mix up a very small amount according to the package directions and I paint it on to the leaves and stems using a disposable sponge paintbrush. I always wear disposable gloves and eye protection as well. I am careful not to let the chemical drip off my sponge on to desirable plants-glyphosate will kill a peony or a lawn grass just as fast as it takes out herb robert or morning glory. It's non-selective.
Other choices of herbicides exist, and the one you choose needs to be appropriate for the weed-management situation you face. trip to the garden store for herbicide always results in tough decisions. If you find the choice of herbicide confusing, please ask an expert. Master Gardener clinics with helpful handouts can be found in many garden centers and plant nurseries around the region. You can also call the WSU Master Gardener Hotline (206-296-3440 for King County, 253-798-7170 for Pierce County and 425-357-6010 for Snohomish County) or your local WSU Extension office. For specific weed info on the Web try www.wa.gov/agr/weedboard, our State Weed Board .
United we stand! You don't need to battle the Beautiful Deceivers alone.
Hortsense: Managing plant problems with Integrated Pest Management
