Purple Loosestrife

(Lythrum salicaria)

Where is Purple Loosestrife (PL) from and how did it get here?

PL is a wetland plant native to Europe and Asia. It was introduced to North America in the 1800s, likely carried over either unintentionally via its very small seeds, or intentionally as an ornamental.

How do I identify PL?

Depending on the age and maturity, PL plants can be 2-7 feet tall. Each plant can have up to 50 stems. Stems are usually square with opposite leaves. Flowers grow close to the stem with 5-6 purple/pink petals. PL blooms late in the summer, usually August-September. PL’s seeds are smaller than a pin prick, and each plant can produce millions of seeds that easily float in the water, and can stick to boots and other gear without owner’s knowing.

Please contact our Woods and Water Team if you think you have found PL.

*PL is most often confused with fireweed. Fireweed does not have a square stem, and its flowers grow further away from the stalks.

How does PL affect Wisconsin wetlands?

PL is an aggressive invader. It replaces native vegetation, and forms dense, impenetrable mats that are not suitable as cover, food, or nesting sites for many wetland critters. It can also disrupt water flow.

What can be done once PL enters a system?

PL can be difficult to control due to its high seed production. Each plant is capable of producing millions of pinhead-sized seeds, and can therefore establish a large seed bank within the soil. Herbicides are an effective means of killing PL, but sites must be revisited each year to herbicide re-sprouts. Permits are usually required to apply herbicides in aquatic areas.

PL can also be removed manually. Care must be taken to dig up all parts of the plant, including roots. This is best done earlier in the growing season before the stems get too woody, and before the plant starts to produce seeds.
Spread of PL can be slowed by cutting flower heads from PL plants so that they do not produce seeds. Sites usually must be revisited a few times each season for new flower blooms.

PL can also be kept in check with the use of "Cella beetles," which eat only purple loosestrife. Please visit the WDNR's webpage on cella beetles if you are interested in learning more. The Discovery Center also raises cella beetles to be released onto PL populations. Contact our Woods and Water Team if you are interested in touring our beetle rearing area during the summer.