Purple Loosestrife

Purple Loosestrife

Lythrum salicaria

A summer drive through the Boquet watershed usually reveals some wet ditches and marshes marked with the reddish-purple spikes of purple loosestrife. When you get up close the spike of flowers is attractive: good color and a tight head of well formed flowers. But for all its beauty it is an alien species. It invades wet areas, crowds out native plants, and makes it difficult or impossible for wetland animals to survive.

Purple loosestrife arrived in the New World around 1830. It was first found around eastern seaports and is thought to have arrived as a stowaway in the ballast the sailing ships used on voyages from Europe. During the next hundred years it slowly spread across the northern United States and southern Canada to the west coast. Because of its showy flowers people noticed it as it came into new areas. In some cases it was helped along by gardeners desirous of its blooms. Initially it didn't seem to crowd out other species. It was just there, along with myriads of other immigrating species.

This changed in the 1930's. Some populations of purple loosestrife located along the Saint Lawrence River suddenly became much more "invasive." Instead of coexisting in wetlands with other native species, they began crowding them out. No one is certain why this change occurred. Was it a mutation in some individual plant that enhanced its competitiveness? Or was it an accidental introduction of a new strain from Eurasia?

Whatever the reason, this new form of purple loosestrife began to spread rapidly. It displaced the original form and spread to areas where the original form never penetrated. This brought about many different problems: Purple Loosestrife

Purple loosestrife causes all these problems. Why? There are many characteristics in its biology that allow it be such an aggressive invader:

  1. In its homeland in Eurasia purple loosestrife is adapted to live in disturbed soil. In North America, prior to the European immigration, there was little disturbed soil and few native plants evolved the ability to grow under this condition. Once it arrived, purple loosestrife had a clear field.
  2. It has a high reproductive rate. Each flower on the spike produces a capsule with roughly 100 seeds. Each spike might produce 1000 capsules, so figure 100,000 seeds per flower head. Then each mature plant has about 30 flower heads. A reasonable estimate seems to be about 2 to 3 million seeds per plant ! If you think of a marsh full of these plants, the number of seeds they might produce is mind numbing. In addition the seeds disperse well, survive for a long time, and germinate well.
  3. An additional advantage of purple loosestrife is that when it immigrated to the New World, it left behind the bugs and diseases it had to cope with in its native land. In Europe purple loosestrife is not a nuisance and this may be largely due to the other creatures that keep its numbers down. Again, it had a clear field in the New World.

We've seen that purple loosestrife is a pest and some of the reasons why this is so. But what can we do about it? Most biologists studying the problem recommend a two pronged strategy:

In areas where purple loosestrife is just beginning to encroach, hand picking is an effective method of control if the stems are pulled before the flowers begin to set seed. As it begins to flower purple loosestrife is easy to spot and train volunteers in its recognition. If the stems of a plant are pulled for a few seasons the rootstock dies and the plants can be eradicated from the area.

In areas where it is already well established this won't work. It would work in theory, but the number of plants overwhelms any reasonable effort. A number of different strategies have been tried to control the plants:

Burning and/or Mowing: In small areas this can work, but it requires several years to be effective as the plants sprout back from the underground rootstocks. It is expensive and the machinery needed for mowing can tear up the wetland making it difficult for native species to become re-established.

Water Level Manipulation: Some aquatic weeds can be controlled by raising and lowering the water level in the wetlands. Purple loosestrife, on the other hand, seems to enjoy this. Not an option.

Herbicides: At present the herbicide of choice seems to be Round-Up. It works, although it may take several applications to kill the rootstocks. Unfortunately it also eradicates many of the other plants you might want to have around. Although the manufacturer claims it is short-lived in the soil and has little effect on animals, there have been few studies on the type of long term effects an ecologist might be concerned about.

Biological Control: Introducing a predator, parasite, or pathogen in order to reduce the damage caused by the pest species is known as biological control. In theory it ought to be the best possible solution. It is, sometimes. But there can be practical problems. Finding effective control agents can be difficult. It may be difficult or expensive to raise adequate numbers for release into the wild. The control agents may not survive or reproduce in these areas into which they are being introduced. The biological control agent may die out and need to be reintroduced continually. It is also possible the control agents will "jump" to other species and cause collateral damage.

In Europe, where it originated, purple loosestrife does not cause problems. This suggests it may be susceptible to biological control. Two species of leaf eating beetles (Galerucella spp) and a species of stem boring beetle (Hylobius sp) have been imported and are undergoing field tests. Results so far are very positive. In most of the areas where the Galerucella beetles were released the purple loosestrife has been much reduced and the beetles are spreading beyond the release areas.

Integrated Pest Management (IPM): IPM stresses the use of all of the above techniques, each one of which may be appropriate under certain circumstances. The goal is not to eradicate the pest completely, but to reduce its impact to a reasonable level. For purple loosestrife, biological control may be the predominant long-term strategy, but we will need to combine it with hand pulling in areas of low density and with other methods under certain circumstances.

Purple loosestrife is an alien species and has been called a "bad" plant. And certainly we would like to control it and ameliorate its effect on our wetlands. But consider for a moment this plant and others which have become established in our area:

Many of the plants with which we are familiar are aliens. A list of the common wildflowers we see along roadsides or in meadows is almost entirely made up of aliens: ox-eye daisy, vetch, yarrow, colt's foot, black-eyed susan, and on and on. But these have all been here for a long time and have become integrated into the local concept of what our ecosystem should be. Perhaps they might have caused problems in the past, but they don't now. Over the years they have probably acquired a following of pests and predators that keep them in check and prevent them from becoming a nuisance. Had not the new aggressive strain showed up in the 1930's, this would have been purple loosestrife's story. This natural balance, the result of coevolution between plants and their predators, is what we are trying to mimic by the introduction of biological control species. The distinction between a familiar roadside plant and an aggressively invading one is a matter of degree. It depends both on the biology of the situation and on our perception of what our ecosystems should be like.

Purple Loosestrife

Here is a link to a web site with more information about purple loosestrife and to one with more information about its biological control.

For more information on plant species considered invasive in New York State see the Invasive Plant Council of New York State website.

Plants Boquet River