Purple Loosestrife

Purple loosestrife is a beautiful perennial plant that has a dark side. It has the ability to invade natural wetlands and displace native plants such as cattails and sedges. It also affects agriculture by becoming a nuisance in moist rangelands and in crops planted in flood plains. Some states have passed legislation officially designating purple loosestrife (Lythrum salicaria) as a noxious weed, and thus restricting its sale and distribution. Unfortunately, Lythrum salicaria cannot be distinguished from the non-agressive Lythrum virgatum and many states broadened their regulations to include all cultivars and nonnative species of Lythrum so that their previous restrictions on purple loosestrife could be enforced.

Nurserymen and commercial growers have protested these regulations claiming that several of their cultivars were sterile, and that the broad ban on loosestrife would hurt their businesses. Some gardeners supported the nurserymen's viewpoint saying that they have never seen any seedling development, but other gardeners have had different experiences.

Purple loosestrife has a specialized method of reproduction. Purple loosestrife has three types of flowers, with only one type of flower per individual plant. These trimorphic flowers each differ by having pistils of varying lengths. Some are short (3 to 5 mm), medium (7 to 8 mm), or long (9 to 12 mm). Each type of flower also has two different whorls of stamens that vary in length, but are never the same length as the pistil or each other. These can be observed easily without magnification. Seed-set is favored when a pistil receives pollen from stamens of similar length ( legitimate crosses); if pollination occurs in a different manner (illegitimate crosses), seed-set is suppressed. This reproductive barrier is called self-incompatibility, or SI. The natural selective advantage of having three flower types, as opposed to two is that all individuals will have two-thirds of the population available for successful outcrossing, instead of only one-half, thus increasing the odds of each flower bearing a maximum seed-set.

Drs. Neil O. Anderson and Peter D. Ascher in the Department of Horticultural Science at the University of Minnesota, have conducted thorough investigations on the fertility of purple loosestrife and several of the cultivars of Lythrum. There is enough evidence to conclude that no purple loosestrife cultivar is sterile. Most cultivars are SI, but they produce large quantities of seed when used as either male or female parents in crosses. In several cases, the cultivars are as fertile as the L. salicaria populations.

Although purple loosestrife is an attractive plant and banning its sale may result in lost revenue by some perennial growers, it can be a noxious weed. Every plant has the potential to produce 1.5 million seeds annually. These are easily dispersed and can remain viable for at least 5 years. Bees and wasps are effective pollinators of loosestrife, and provide the means for legitimate crossing, perhaps even for isolated plantings.

The U.S. Fish and Wildlife Service and the Department of Natural Resources began an aggressive program to control purple loosestrife about 15 years ago. Their early control methods included the physical removal of plants, mowing, regulating water levels in marshes, and the application of herbicides registered for use in wetlands. The results from these methods were disappointing. Researchers are still looking for ways to effectively control purple loosestrife. For the past 5 years, Dr. Richard Malecki of the U.S. Fish and Wildlife Service and participants in the DNR have been investigating biological control methods. Six herbivorous insects from Europe have shown potential: two leaf-feeding beetles, one root-feeding weevil, and three flower-feeding insects. Some of these insects have been released in infested areas, and others are being reared in insectaries for future releases.

The information in this article was taken from the May 1995 issue of Greenhouse Grower. It was written by Dr. Robert Parker and Dr. John Weaver from the University of New Hampshire, Durham, New Hampshire.

This article originally appeared in the May 5, 1995 issue, p. 55.


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