Purple loosestrife, a beautiful but aggressive invader, arrived in eastern North America in the early 1800s. Plants were brought to North America by settlers for their flower gardens, and seeds were present in the ballast holds of European ships that used soil to weigh down the vessels for stability on the ocean. Since it was introduced, purple loosestrife has spread westward and can be found across much of Canada and the United States.

Characteristics

Flower: Each flower spike is made up of many individual flowers. Individual flowers are small and have five or six pink-purple petals surrounding small, yellow centers. Purple loosestrife generally flowers from late June to early September and require pollination by insects or birds.

Seed Capsule: As flowers begin to drop off, capsules containing many tiny seeds appear in their place. Depending on where you live, plants may go to seed as early as late July.

Seed: Each mature plant can have more than thirty flowering stems which can produce up to 2.7 million seeds annually. As tiny as grains of sand, seeds are easily spread by water, wind, wildlife and humans. Germination can occur the following season and in many environmental conditions. Seeds are hardy and can lay dormant in the seed bank for several years before sprouting. Ornamental "seedless" cultivars have been shown to produce viable seeds when fertilized with pollen from naturalized populations.

Leaves: Leaves are downy, with smooth edges. They are usually arranged opposite each other in pairs which alternate down the stalk at 90° angles, however, they may also appear in groups of three.

Stalk: Stalks are square, five or six-sided, woody, as tall as 2 m (6.5 ft) with several stalks on mature plants.

Perennial Rootstock: Mature plants can reproduce vegetatively with underground stems that can spread at a rate of 25 cm (9.8 in) each year. On mature plants, rootstocks are extensive and can send out up to 30 to 50 shoots, creating a dense web which chokes out other plant life.

Distribution

Purple loosestrife is now found throughout most of North America with the exception of Mexico, Florida and northern Canada. Currently, the area of greatest concentration and impact has been in southern Ontario, Quebec, New Brunswick and Nova Scotia and adjacent areas in the northeastern United States. The spread of this plant occurs primarily on disturbed soils of recently built roads, canals, railway lines and cultivated areas. It appears purple loosestrife thrives in areas of populated places with connecting roadways and degraded and disturbed wetland habitats. Purple loosestrife can also invade healthy wetland habitats and since seeds can be spread by water, it can spread within the watershed. Most of the isolated incidents of purple loosestrife have likely resulted from intentional introductions or escapes from ornamental gardens.

 Download Purple Loosestrife distribution map

Impact

Purple loosestrife is a very hardy perennial which can rapidly degrade wetlands, diminishing their value for wildlife habitat. Wetlands are the most biologically diverse, productive component of our ecosystem. Hundreds of species of plants, birds, mammals, reptiles, insects, fish and amphibians rely on healthy wetland habitat for their survival. When purple loosestrife gets a foothold, it forms a dense monoculture, replacing native plant species and the habitat where fish and wildlife feed, seek shelter, reproduce and rear young.

Since purple loosestrife can invade drier sites, concern is increasing as the plant becomes more common on agricultural land, encroaching on farmers' crops and pasture land. The spread of purple loosestrife also has a direct economic impact when plants clog irrigation or drainage ditches on farmlands or cause degradation and loss of forage value of lowland pastures. An estimated 190,000 hectares of wetlands, marshes, pastures and riparian meadows are affected in North America each year, with an economic impact of millions of dollars.

Prevention

One of the simplest prevention measures one can take is to be sure that purple loosestrife (or one of the many "sterile" cultivars) is not growing in your garden and that you don't intentionally or unintentionally plant it (some wildflower seed mixes may contain purple loosestrife so be sure to read the label). The best time to control purple loosestrife is in June, July and early August when it is in flower and easy to recognize before it goes to seed. Hand-digging young plants can be done in an area with a small infestation. Cutting the flowers stalks before they go to seed also ensures that seeds will not produce future plants. Proper disposal of the plant to ensure that seeds do not contaminate other areas is important. This can be done by putting plants in plastic bags that will remain intact at the landfill site. Plants can also be incinerated. Chemical control (herbicide) is another form of control but should only be used on individual plants, in dry, upland areas and on your own property.

For large infestations, the most effective method has been biological control through the release of insects that are the plant's natural enemy in its native habitat. Two beetle species: Galerucella pusilla and Galerucella calmariensis eat the leaves and new shoot growth which seriously affects growth and seed production of the plant. These beetles will not eradicate purple loosestrife, however, they will act to significantly reduce plant numbers and reduce seed production and therefore, the spread. The O.F.A.H. in partnership with Ontario Beetles and the Ministry of Natural Resources co-ordinated a beetle release program in Northern Ontario in 2003 and will be conducting 80 releases in Eastern Ontario in 2004, with new the partners Ontario Wildlife Habitat Fund and Ducks Unlimited Canada.

Many organizations throughout North America have taken action to control the spread of purple loosestrife. National wildlife services, state/provincial natural resource and environment agencies, universities, nursery trades associations, and conservation and community organizations have responded to the purple loosestrife invasion by raising awareness of the threat posed by this invasive plant, and how to prevent its spread. OFAH programs such as Project Purple and The Biological Control of Purple Loosestrife  are examples of such actions that are being taken in Ontario and are available for the public and other conservation groups to get involved.