Purple loosestrife

About

Purple loosestrife is a showy Eurasian wetland perennial, once sold as a garden plant, that chokes marshes, ditches and shorelines. One plant can make millions of seeds, and even 'sterile' garden cultivars cross with wild plants to make seed.

Also called loosestrife

How to identify it

Look for

Stiff, square (four- to six-sided) stems grow up to 10 feet tall from a woody taproot, with 30 to 50 stems on older plants. Smooth, lance-shaped leaves up to 4 inches long sit opposite or in whorls. From July to September it carries tall spikes of pink-purple flowers with five or six petals.

Look-alikes

Native fireweed has round stems and alternate leaves, and Douglas' spirea also has alternate leaves. Native blue vervain can also be mistaken for it.

Where it grows

Wet soil and shallow water: marshes, pond and lake edges, ditches and wet gardens.

When

It flowers from early July into September, and seed ripens from late July on.

How it spreads

By seed (up to 2.7 million per plant a year) carried by water, wind, animals and equipment; seeds can lie dormant for years.

Most common in

Where it is regulated

Listed as a prohibited noxious weed in Minnesota and a B-rated noxious weed in Oregon.

What to do, gentlest first

Start at the top. Weed killers are a last step; always read and follow the label.

1. Don't plant it; grow natives instead Prevent

   Don't buy or plant loosestrife or its cultivars, and check wildflower seed mixes. Minnesota DNR suggests swamp milkweed, cardinal flower, Joe-Pye weed, blue vervain or blazing star instead. Report new wild patches.

   Planting it is illegal in some states, including Minnesota.

2. Cut flower spikes and dig out plants By hand

   Cut off flower spikes before seed forms and dig out young plants with the whole root. Bag everything for the trash.

   Never compost it. Clean boots and tools so seeds don't travel. Work near public water may need a permit.

3. Galerucella leaf beetles Natural enemies

   Leaf-eating Galerucella beetles are used for large stands (about an acre or more). Ask your state DNR or Extension.

4. Spot-spray triclopyr or glyphosate Conventional spray

   Oregon State lists triclopyr (spares most grasses), glyphosate and imazapyr for spot treatment.

   Read and follow the label. Many lawn weed killers also kill garden plants, so keep spray and drift off beds and vegetables. Near water, only aquatic-labeled products are allowed, and they often require a licensed applicator.

Uses, wildlife and pets

For wildlife

Poor habitat for wildlife compared with the native wetland plants it replaces (Minnesota DNR).

Pets

Not listed by the ASPCA.