If you have ever stopped to pick flowers along the roadside, taken them home, put them in a vase and when they died, thrown them out, then you might be helping to spread invasive species.
Roadsides are disturbed and stressed environments, the ideal spot for non-native species to take hold. Natives, coping as they are with the challenges of the many predators – macroscopic and microscopic – that impeded their growth and reproduction, are at a competitive disadvantage against introduced species. They have been placed in a new ecosystem where nothing recognizes them as food. If they can cope with the stresses of the physical environment, then they are off and running.
A very beautiful, pervasive and noxious example of an invasive plant is the purple loosestrife (Lythrum salicaria). Purple loosestrife is native from Great Britain to Japan and south to northern India. It was introduced to the United States in the 19th century. It is still sold in many states as an ornamental, but Wisconsin, Minnesota, Illinois have prohibited its sale. It is found in every state except Florida.
The earliest introductions may have been through seeds in ships’ ballast and caught up in imported wool. It was noted in estuaries and canals of northeastern United States in the early 19th century, but was much written about in horticultural texts as a popular addition to ornamental gardens in both Europe and the United States.
Once it had a foothold along the east coast it penetrated inland along the canals that were being built during first half of the 19th century and also it followed the westward advance of the clearing that accompanied agriculture and European settlement.
In the early 20th century it was imported for use by beekeepers; it flowers profusely between June and September and is an excellent source of nectar.
Efforts to physically remove purple loosestrife have been largely unsuccessful. In some wildlife refuges where it has come to form monotypic stands, crowding out native species and providing substandard browse to herbivorous animals, volunteers efforts have been organized to physically remove the plants by simply digging them up or carting them away. It is, though, something like bailing out the ocean.
Cornell University is looking for natural predators to introduce that reduce the density of the species if not eradicate it. Bernd Blossey has found that the Galerucella beetle feeds on purple loosestrife foliage. It was introduced to rangeland in Washington state in 1995 and three years later had made serious inroads to the loosestrife population. It would take several years, however, to eradicate the plants because they are able to store energy in their roots and grow back through several years of defoliation.
Galerucella calmariensis and G. pusilla are native to Europe, where they feed only on purple loosestrife. They show no interest whatsoever in native loosestrife species and, when their introduction to North America is complete, are expected to reduced purple loosestrife by 90 percent over 90 percent of its range.
The lesser celandine (Ranunculus ficaria) is another introduced plant that becomes invasive. At present there are no biological controls available to limit its numbers. Like purple loosestrife, it is attractive. Its chief detrimental effect is to prevent native spring wildflowers from emerging; it forms such an extensive and dense mat of vegetation that it actually blocks other plants from poking through.
This introduced plant forms a lush green mat of vegetation as early as March in the northeastern states. It flowers through late March and April. By May the foliage has died back, leaving large bare areas in lawns that it has invaded. It spreads vegetatively; just beneath the soil surface is a dense network of tubers. All of these must be removed in order to rid an area of the plant.
Lesser celandine was introduced as an ornamental and is still commercially available.
Many invasive plants are as attractive as purple loosestrife and lesser celandine and may prove tempting to add to a garden. Many non-native thistles are invasive, as are orange daylilies. And it is not only herbaceous plants that can be invasive; aquatic plants, vines, shrubs and even trees can move in and take over acreage.
Consult the following websites (and more) before bringing any species into your yard: