This is the perfect time of year to spot the perennial pest Purple Loosestrife (Lythrum salicaria). Its tall purple spikes are conspicuous from July through September. This introduced plant is usually found in open sunny locations around wetlands or other bodies of water, or in disturbed areas like roadsides.
You might find the color attractive, but beware. A single mature plant can produce an estimated 250,000 to 2,000,000 seeds in one year. Seed survival is also high – 60 to 70%. That is why it is important to work to eradicate it before it gains a foothold. Purple Loosestrife is capable of overrunning wetlands thousands of acres in size. It can fill in open water habitat. It will eventually choke out native plants like cattails, sedges and ferns that are important sources of food, nesting material and cover for wildlife. Wetlands specialists like the State endangered American Bittern and Bog Turtle usually disappear from wetlands dominated by Purple Loosestrife.
The first line of defense with small groups of young plants is hand pulling. Ease it out of the soil using a garden fork. Since it is hard to get the entire taproot in a single shot, so you will probably have to go back at least 3 years in a row for sprouts. Bag all plant parts and throw them in the trash or burn them. (Composting may not destroy the seeds, and woody parts take a long time to decompose.) Because they do not usually flower until they are 3-5 years old, it can be hard to find young plants. For those that are flowering, cutting off the stalks will reduce seed production. Do it as early as possible before the seeds start to set (capsules turn brown.) Since they begin producing seeds once the petals start to drop from the bottom of the spike, bend the spike over a plastic bag and cut it off into the bag. (Make sure the bag does not have holes in it, and brush off your clothes and equipment to avoid spreading seeds elsewhere.) Mowing older plants several times a year before they bloom will slow invasion.
If you are into “better living through chemistry,” pesticides that contain glyphosate will kill Purple Loosestrife. On dry land, a 1.0-1.5% solution of Roundup at bloom works. You only need to wet about 25% of the plant. Rodeo is approved for use around water. However, if the weed is growing in standing water at the time you want to treat it, you need a permit from the CT Dept. of Environmental Protection. To get one, submit an application and a fee of $100. (The application form is available online at www.ct.gov/dep but is hard to find – see www.ourbetter.nature.com/pl.htm for the exact address, or call  424-3369). The permit is good for the growing season. According to Brad Robinson, a Supervising Environmental Analyst with the Pesticide Management Program, you should allow 1-5 weeks to receive the permit (especially if you are applying during their busy season, which is early spring.) “Make sure you apply the pesticide according to the directions, and don’t zap everything else around it,” says Robinson. The "cut and paint" method targets the problem plant and reduces pesticide use. Cut large plants six inches off the ground. Then paint all freshly cut stems within 5-15 minutes of cutting (using a sponge or paint brush) with a 20-30% solution of glyphosate.
The most promising and cost-effective control for large stands is biological, according to Donna Ellis, an Extension Educator at UCONN. In 1996, CT began introducing several species of beetles that depend on Purple Loosestrife and do not threaten native plants. These specialized insects are not expected to eradicate the plant, can significantly reduce Purple Loosestrife populations, which will allow native plants to rebound.
There are no sterile varieties of Purple Loosestrife. The Connecticut Invasive Plants Working Group encourages the use of non-invasive alternatives, particularly when planting near parks, natural areas, or other minimally managed habitats. If you like purple spiky flowers, try the native Liatris species instead, which go by common names such as “Blazing Star” or “Spiked Gayfeather.” Ironweed (Vernonia spp.) has a similarly colored flower.
There are two other non-native plants called “Loosestrife,” even though they are not in the same family as Purple Loosestrife. Yellow Loosestrife (Lysimachia vulgaris) is classified as “potentially invasive” because it can escape into natural areas and reproduce. Gooseneck Loosestrife (Lysimachia clethroides), which sports white, crooked spikes, is not listed. Gooseneck Loosestrife can be a problem in managed landscapes because it spreads rapidly in moist locations in full sun.