What is purple loosestrife?

Purple loosestrife is a tall wetland perennial from Europe and Asia that arrived in North America in the 1800s and has since spread through marshes, ditches and shorelines. Learning purple loosestrife identification is worth it because a single plant produces huge numbers of tiny seeds that move on water and mud, and dense stands can push out the native cattails, sedges and wildflowers that wildlife depends on.

It’s a Class B noxious weed in Pennsylvania, so it can’t be sold or planted, even though some older garden varieties were once marketed as sterile.

Purple loosestrife identification: what to look for

In bloom, purple loosestrife is hard to miss. Before and after, look closely at the stem and leaves.

• Flowers: dense, tall spikes of magenta to purple-pink flowers, each with five to seven crinkled petals. The display lasts weeks in mid to late summer.
• Stems: square, or sometimes with up to six sides, 2 to 7 feet tall, often with many stems coming from one crown.
• Leaves: narrow, smooth-edged and stalkless, set in opposite pairs or in whorls of three around the stem.
• Roots: a thick, branching taproot that gets woody with age.
• Winter: dried brown stalks and seed capsules remain standing.

Where it grows in Pennsylvania

Purple loosestrife grows in wet ground all over Pennsylvania. Penn State notes it’s common in roadside ditches and in wetlands built during road construction. You’ll also see it along river and creek banks, pond and lake edges, wet meadows and stormwater basins.

Around central PA homes, it often turns up on the soggy edge of a farm pond, in a drainage swale, in a ditch along the road frontage or in a low spot that stays wet most of the year. It prefers full sun but tolerates some shade, and it can survive on drier ground once a stand is established.

Because the seed is so small, it travels in mud stuck to boots, boat trailers, mowers and excavating equipment. That’s why new plants often appear along a freshly dug ditch or the edge of a new stormwater pond a year or two after the work is done.

Purple loosestrife look-alikes

Several native plants have purple or pink flower spikes. Fireweed and blazing stars have alternate leaves rather than paired or whorled leaves. Blue vervain shares the square stem and opposite leaves, but its leaves are stalked and toothed and its flowers are small and blue-violet.

Native swamp loosestrife and winged loosestrife grow in the same places but have far fewer, scattered flowers. If you’re not certain, check the leaf arrangement and stalks before pulling anything near water, since native wetland plants are worth keeping.

How to get rid of purple loosestrife

For a few plants on your property, you can make real progress yourself. Large stands in wetlands need a planned, multi-year approach.

Hand pulling and digging. Young plants pull out best when the soil is saturated. Use a garden fork to loosen older plants and remove the whole root crown, since pieces left behind can regrow. Bag the plants, especially if they’re flowering, so seed doesn’t scatter.

Cutting. If you can’t dig, cut the stems to the ground at the start of flowering. This won’t kill the plant, but it stops seed for the year.

Biological control. The PA Department of Agriculture and USDA have released leaf-eating Galerucella beetles at selected sites. They work slowly and results vary, so this isn’t a homeowner tool, but it does help on large wetlands.

Chemical control. Penn State lists spot treatment with glyphosate or triclopyr products labeled for aquatic sites, applied at the bud to early-bloom stage. Glyphosate kills any plant it touches; triclopyr is easier on grasses, sedges and rushes.

• Read and follow the product label. The label is the law, and only products labeled for aquatic or wetland sites may be used near water.
• In Pennsylvania, applying herbicide in standing water or saturated soil requires an applicator certified in the aquatic category and a permit from the PA Fish and Boat Commission. Treating during dry spells when no water is present avoids that, but check the rules first.
• Applying herbicide for hire in PA requires a PA Department of Agriculture pesticide applicator certification, so wetland jobs belong with a licensed vegetation-management company.

Keeping it from coming back

Seed can sprout for several years after the last plant is gone, and nearby stands can reseed your site. Check the area every summer and pull new plants while they’re small.

Encourage native wetland plants to fill the gaps. If the loosestrife is in a soggy spot in your yard that shouldn’t be wet, the real fix may be correcting the drainage; our French drain and drainage page explains how that works. Natural wetlands and stream banks are protected, so don’t drain or fill them without checking permit requirements.

Questions we get about purple loosestrife

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