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Purple loosestrife (Lythrum salicaria), probably brought to North America to adorn gardens, moved out of gardens and has infested hundreds of thousands of acres of wetland. Brilliantly colored, it rims water edges throughout the Puget Sound. The garden relatives of Lythrum salicaria, one of which is botanically named Lythrum virgatum (cultivars Morden's Pink or Morden's Gleam), are also prohibited by local and state noxious weed control programs.
Purple loosestrife is easy to spot in July and August. The beauty of its color silhouetted against water often impels people to reach for cameras. The plants reach 4 to 6 feet in height, bearing large showy flowers in prominent spikes, with a distinctive pinky/purple color. Long, slender leaves are arranged in opposite pairs on a 4-sided square stem. (Run your hand along the stem and note the square shape. This square shape is common to many plants, including the edible herb, mint.) Several web sites have good photos of this plant. Check in with either of these addresses:
http://splash.metrokc.gov/wlr/LANDS/weeds/weedid.htm
http://ohioline.ag.ohio-state.edu/b866/b866_8.html
Why would an escaped garden plant threaten wetlands? Purple loosestrife is a weed of lake margins, stream edges, and wetlands. Purple loosestrife plants produce no food. Even though individual plants can generate prodigious amounts of seeds, up to 300,000 viable seeds per plant, the seeds don't feed waterfowl or other wildlife. They are good for nothing. They cause choked waterways, crowding out native vegetation and reducing wildlife food. Here's a weed nicknamed "beautiful killer" by weed control experts in Minnesota and the Canadian plains. It kills by depriving wildlife of nutritious food, not by being directly poisonous.
Gardeners may become confused about purple loosestrife when they see photographs of English or European gardens with it featured as a summer ornamental, not as a noxious weed. A very common plant, Lythrum 'Morden's Purple' cannot be sold in Washington State nurseries and garden centers because it may hybridize with the Lythrum salicaria to create an even larger problem. If you see this plant in a national catalog, don't buy it. If your garden already contains 'Morden's Pink,' or 'Morden's Gleam,' yank it out, especially if your property is close to water, even small waterways.
To manage small infestations, start with hand-pulling. If the plants are only 1 to 2 years old, and the roots have reached only about 1 foot down, dig thoroughly to remove all the root mass. Bag and tie the plants for disposal; do not compost them.
If the plants are older than 1 to 2 years, it's nearly impossible to remove them with hand-pulling alone. Fragments of roots left in the ground will re-sprout, so be sure to check the same area in the following two years.
Cutting off plants can slow down the invasions. Cut the flowers after they start blooming but before they set seed. If the plant has already gone to seed, slide a bag over the top of it to cover the seeds, then cut off the stalk. Again, do not release the seeds or place any plant parts in a compost pile.
Smothering the plants with black plastic can reduce their vigor and may kill plants. This obviously works best in a small stand of young plants. Cover the stand with black plastic, held down firmly with rocks. A mulch on top of the plastic will improve its appearance but has no effect on the loosestrife. It's the light exclusion that slows the plant down. Continue to monitor the infestation over several years.
Weed killers can be used as part of a loosestrife management program, but because it often grows in or near water, be very careful when proceeding. Do not apply any herbicides in or near water without consulting a licensed Aquatic Pesticide Specialist in Washington State. It's not legal for anyone but a licensed specialist to apply pesticides to water. Your local noxious weed control board can help with more information on this subject.
These plants: purple loosestrife (Lythrum salicaria), garden loosestrife (Lythrum virgatum), and yellow loosestrife (Lysimachia vulgaris) are also on the noxious weed lists for Pierce County (253-872-2972), Snohomish County (Sunny Gorman, message number 360-422-6083) and Skagit County (360-336-9430). For more information, contact local Noxious Weed Boards in those counties.
If you see an infestation within King County, report it to the King County Noxious Weed Board. They need assistance gathering data on the locations of medium size or even small infestations of both purple loosestrife and the garden variety, (Lythrum virgatum.) Larger infestations in King County are well known, but all small ones need to be reported. The King County Weed Board is also gathering information on yellow loosestrife, Lysimachia vulgaris, which has large spikes of vivid yellow flowers. That one colonizes the same areas as purple loosestrife and is quite visible in places like the damper areas of Marymoor Park. Your input can help. Call 206-296-0290 to inform the Board in King County.
Hortsense: Managing plant problems with Integrated Pest Management
