Purple loosestrife - Biology and control

Purple loosestrife, one of Nebraska's official noxious weeds.

Purple loosestrife (Lythrum salicoria) was introduced to North America from Europe in the 1800s and has since slowly invaded wetlands and waterways across the Midwest including Nebraska. It is estimated that about 12,000 acres of Nebraska’s wetlands are infested with purple loosestrife, mostly along the main rivers. It has been identified as one of the state’s official noxious weeds, necessitating that landowners work together to stop the spread of this highly competitive weed.

When wetlands are taken over by loosestrife, the natural habitat is lost and the productivity of native plant and animal communities is severely reduced. Similarly, purple loosestrife growing in irrigation canals, ditches, stream banks and reservoirs can clog these waterways and limit water available for ag production. Purple loosestrife can also reduce recreational lands.

A perfect plant

Purple loosestrife can colonize and thrive easily because it is a prolific seed producer and has a strong perennial root system (rhizome). Each plant can produce up to 2 million seeds in one season. Seeds can be carried far away by water, wind and some birds and can remain viable for many years. The rhizome grows well in marshy soils and can help the spread of the species if washed away by the river water.

Not many birds, fish or animals feed on purple loosestrife. They feed, however, on other plant species that grow around purple loosestrife. By doing this, indirectly, the wildlife population “eats themselves out of house and home.” As native vegetation gets consumed, more space is created for purple loosestrife to spread and produce new plants.

Life cycle

In general, purple loosestrife can grow 3-9 feet tall with several, square stalks per plant. Leaves, which grow on opposite sides of the stalk, are thin and sharply pointed with the base rounded or heart-shaped. In Nebraska, it will flower from July to September. Flowers can range from red to rose-purple in color and are arranged on 1- to 3-foot long spikes. The fruit is a small oblong capsule with two valves containing many small seeds. Each spike has many capsules and can produce up to one hundred thousand small seeds. The tiny, light seeds are readily moved by wind. Seeds are also extremely viable and will easily germinate when exposed on bare soil. Root system is very strong and when mature, the root branches become thick and woody.

Control

Purple loosestrife has no natural enemies nor other plant competitors here in North America and it has been hard to stop its spread to new areas. A single control measure can’t provide long-term, sustainable, management; however, if the control practices are integrated in a systematic manner, significant advances can be achieved. The biggest challenge is how to stop the spread of the current 12,000 acres of infested wetland across Nebraska.

Control methods must be based on an integrated management approach which includes: 1) prevention and education; 2) manual control; 3) cutting; 4) chemical control; and 5) biological control.

Prevention and education

Public education needs to be a major element of any plan to control the spread of purple loosestrife. Landowners and others have accidentally contributed to the spread of this noxious weed. In one incident several duck-hunters reported that they used plants with purple flowers to build their duck-blinds and to camouflage their boat. Unintentionally, they may have spread purple loosestrife up and down the riverbanks.

Manual control

Pulling and digging plants can be effective for small areas. Pulling is most effective on plants that are one to two years old. Loosestrife spreads vegetatively from stems, therefore, regeneration from discarded plants is likely. They should be dried and burned.

If plant pulling is not feasible, flower head removal helps reduce the spread of the seeds. Simply cut the heads in July and August (before the flower sets seeds). Seed formation starts at the bottom of the flower and progresses to the tip. Before cutting the seed head off, check to see that no ripe seeds are present.

Cutting can actually spread loosestrife if the cuts are not removed because the cut stalk portions can sprout. Therefore all cuts must be removed and burned. Make sure that all plant parts are in a carton or protected site so that they can dry completely without danger of being spread by wind, water, human or animal activity.

See next week's CropWatch for information on integrating chemical and biological control measures.

Stevan Knezevic
Extension Weeds Specialist
Haskell Ag Lab, Northeast REC

Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.

Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.

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