Garden Loosestrife – August 2017 Weed of the Month

Garden loosestrife infestation at Marymoor Park on Lake Sammamish.

What plant is so tough that it can outcompete common cattail (Typha latifolia), purple loosestrife (Lythrum salicaria), and even Himalayan blackberry (Rubus armeniacus) along King County’s shores? Garden loosestrife (Lysimachia vulgaris), a Class B noxious weed from Eurasia, has managed this impressive takedown. First recorded in Washington in 1978 (on King County’s Lake Washington), garden loosestrife also occurs throughout the Northeast and in the western states of Colorado, Montana, and Oregon. But only in two states, Washington and Connecticut, is the plant disruptive enough to be designated a “noxious weed.” This may be because the colder winters in some of these states help to keep garden loosestrife populations in check. Whatever the reason, King County’s garden loosestrife is a beast of a plant, and it’s really hard to get rid of.

Weed Specialist Karen Peterson shows off one of King County’s unusually massive garden loosestrife plants.

Garden loosestrife is a perennial plant that grows in wet areas such as river and stream banks, lakeshores, and wetlands. It usually grows 3-6 feet tall—sometimes reaching 10 feet—with round, erect stems covered in soft hairs. (In its native range, garden loosestrife tops out at about 4-5 feet) Its 3-5 inch long, ovate leaves occur in whorls of 3 (sometimes 2 or 4) and have hairy undersides. Showy, yellow, 5-petaled primrose-like flowers appear in clusters at stem ends between July and September. Flower bases are ringed by green sepals with distinct orange-brown edges.

The plant reproduces mostly via red rhizomes that can spread up to ten feet in open water. It also produces seeds in egg-shaped capsules, each of which has a few seeds.

Garden loosestrife has two major look-alikes: purple loosestrife (Lythrum salicaria) and yellow loosestrife (Lysimachia punctata). The first, purple loosestrife, is easier to identify. Despite its similar name, purple loosestrife belongs to a different family  than garden loosestrife. The plant has square stems with lance- to oblong-shaped, smooth-edged leaves. As the name implies, its flowers are purple or magenta, appearing clustered in tall, dense spikes. Purple loosestrife spreads through seed, producing up to 2.5 million seeds per plant, as well as via roots and vegetative growth.

Yellow loosestrife (Lysimachia punctata) is a more closely related species that can be harder to tell apart from garden loosestrife. However, its yellow flowers appear all along the flower stem, not just at stem ends, beginning their bloom in mid-June. At around 4 ft. tall, it’s also shorter than garden loosestrife. Last, its petals are more pointed than garden loosestrife’s, which are somewhat rounded.

As we mention above, garden loosestrife can be very difficult to control. For the most part, digging is only practical for individual plants or small infestations. If you do use this method, make sure to dig out as much of the root system as possible—the plant breaks easily, leaving the roots behind to resprout. Mowing can help to slow a population’s growth, especially its spread by seed, but won’t eradicate it. For flowering mature plants, you can also cut the plants at the base to prevent seeding.

With all three methods, make sure to bag the plants—being careful not to disperse any seeds—and throw all fragments in the trash, not the compost. They can reroot if you leave them behind. Covering an infestation of seedlings with a landscaping fabric, black plastic, or cardboard and six inches of mulch can also help to slow growth and seed dispersal, but it won’t kill the roots of mature plants, and it isn’t possible in areas that flood. If you use this method, be sure to monitor the covering for gaps and check the edges for spreading rhizomes.

For large infestations, you may have to turn to an herbicide treatment. A number of herbicides are effective to varying degrees on garden loosestrife. Our program’s Aquatic Weed Specialist, Ben Peterson, is currently collaborating with Washington State University’s Tim Miller on a study about the effectiveness of various herbicides and herbicide combinations on garden loosestrife. You can find their recent results in this presentation. You can also find information on herbicide treatment, as well as ID and control methods, at the following pages:

Garden Loosestrife Weed Alert

Garden Loosestrife Best Management Practices

These days, garden loosestrife infests the shorelines of Lake Washington, Lake Sammamish, and the Sammamish River, with smaller populations on Lake Burien, Rutherford Slough, the lower Snoqualmie River, the Raging River, and a few other locations. The invasions are especially worrisome because the plants can clog shallow waterways and displace native vegetation, reducing habitat for waterfowl and fish—including several important salmon species.

Rutherford Slough’s garden loosestrife infestation as of July 21, 2017. The infestations on the right, southeast portion of the slough are much smaller than those on the left.

Rutherford Slough, near the Snoqualmie River in Fall City, has been a particular focus area for our program. We first controlled the site in 2002, when garden loosestrife formed an 11 acre monoculture in the slough. Because garden loosestrife is less of a problem in other states, we found little advice on how to eradicate it. Starting in 2003, Rutherford Slough became our trial area for control methods. In 2007, our program received a grant from the Washington Department of Ecology to control garden loosestrife along the Snoqualmie River, thinking the plant had spread into the river from Rutherford Slough. The weed ended up being much more widespread, and harder to kill, than we’d expected. We’ve since made a big dent in the southeast part of the slough, which we’ve treated almost every year for the past eight years, as the below before and after photos show:

rutherford slough_2003
Rutherford Slough in 2003. The yellow streaks are garden loosestrife.
rutherford slough_2014
The same location in 2014.

But we continue to battle the infestation, especially in the northwest half of the slough, where control has been intermittent for external reasons. “It’s popping,” says Weed Specialist Sawyard Glise, who joined Ben Peterson for a survey of the slough earlier this month. Apart from the heavy garden loosestrife infestation and reed canarygrass (Phalaris arundinacea), the slough has remarkably few invasive plants. But this weed hangs on. We’re still looking for the perfect way to eradicate it.

LYSVUL stolons
Rutherford Slough’s garden loosestrife infestation has been partially controlled, but we’re still trying to find the best way to eradicate the plant.

As always, if you have any questions about garden loosestrife identification or control, feel free to call us at 206-477-WEED (206-477-9333) or email us at We look forward to hearing from you!

Ben & Sayward
Weed Specialists Sayward Glise and Ben Peterson surveying Rutherford Slough.





  1. I’ve seen a house in Sahalee that I think has this Garden Loosestrife growing in their front yard, what should do about that?

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