The Weed Wire: Purple Loosestrife
Purple Loosestrife on the lower Yaquina River
History and Native Range: Lythrum salicaria is an herbaceous perennial which grows natively in temperate regions of Europe, Asia, northern Africa, and eastern Australia. As of the present day, it has been introduced to southern Africa, North America, and South America. Introduction in North America began on the East Coast in the early 1800s, as an ornamental garden plant or as forage for European honeybees, which are highly attracted to the plant. The earliest recorded occurrence in Oregon was in 1952, in Polk County. It has now spread through most of the state, with distribution concentrated in the Willamette Valley, and occurrences still relatively sparse on the Coast.
Purple loosestrife prefers wet environments, and is typically found in wetlands, roadside ditches, retention ponds, or on river edges.
Identification: During the blooming season (June to September), purple loosestrife visually stands out in the landscape due to its tall flower spikes. Individual flowers are small and are usually purple, but may be pink or in rare cases, white. The stems are square and leaves are lance-shaped and oppositely arranged. Though purple loosestrife can resemble the native fireweed, the preferred habitats of each plant are usually quite different, with purple loosestrife preferring wet areas, and fireweed more often being found in dry habitats.
Close inspection of the flower spikes can help differentiate the two species. Additionally, fireweed leaves are alternately arranged, not oppositely like purple loosestrife’s.
Left: Fireweed (native) | Right: Purple Loosestrife
Photo credit: PenobscotNation.org
The Problem Here: With each plant producing up to 2.7 million tiny seeds annually, small infestations can rapidly become massive. These dust-like seeds mainly spread via wind or by water in the wet habitats they usually colonize. While distribution within Lincoln County is currently limited, regions where the plant has been present for longer have seen it completely overtake entire marshes, pushing out sensitive native plant and animal species. As a high-priority species in our Early Detection, Rapid Response (EDRR) program, Lincoln SWCD may be able to offer free chemical treatment for purple loosestrife. Please contact info@lincolnswcd.org for help in identification and determining if your property is eligible for this program.
Manual Control: Digging up purple loosestrife is one of the more effective means of control, although care must be taken to ensure all root fragments are removed from the soil to prevent resprouting. If digging up all loosestrife plants is not possible within the year, deadheading the flower spikes before they go to seed will help curb the spread of infestations. Take care to bag any removed material and dispose of in landfill garbage.
Herbicides: Chemical control can be very effective in controlling purple loosestrife infestations at nearly any time of year, but care must be taken when exercising these methods due to the plant’s tendency to occur in sensitive aquatic habitats. Treatment should be carried out by an applicator with an Aquatic Pesticide Applicator license, and in accordance with label directions. In particular, glyphosate, imazapyr, or triclopyr have been found to be most effective in controlling purple loosestrife.
Biological Control: Four biological control agents are currently established in Oregon: two beetle species (G. calmariensis and G. pusilla), a root weevil, and a seed weevil. However, biological control is typically used on large, landscape-scale infestations and would not likely be practical within Lincoln County.
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