- An exotic USA plant, purple loosesrife is highly invasive.loosestrife on a meadow image by Maria Brzostowska from Fotolia.com
Seeing the term "exotic" applied to plants might bring rare tropical orchids or rainforest trees to mind. Exotic, however, means something entirely different to the Missouri Department of Conservation. In its view, exotic plants are those not native to U.S. soil. Introduced by accident or intentionally to the United States, some of these plants are popular and well-behaved ornamentals. Others have spread unchecked to choke out native vegetation, clog waterways and deprive U.S. wildlife of food and shelter.
- Tree-of-heaven (Ailanthus altissima) came to the United States from China in the early 19th century. Introduced to feed silk worms for the silk-making industry in New York, it has since spread across the country. It's now considered an invasive weed. This remarkably adaptive tree can sprout between concrete sidewalk cracks, according to the Missouri Botanical Garden. This talent made it the inspiration for Betty Smith's 1943 novel, "A Tree Grows in Brooklyn." Its easily breaking branches have divided, compound green foliage resembling that of the sumac.
Tolerating temperatures as low as minus 30 degrees Fahrenheit, tree-of-heaven has clusters of greenish flowers in June and July, followed by brownish-red seedpods in early fall. All parts of the tree, however, emit an unappealing odor. The tree spreads rapidly by self-sowing and through root suckers, even in infertile soil and full shade.
- Purple loosestrife (Lythrum salicaria), also hardy to minus 30 F, is a perennial from the wetlands of Europe and Asia. Possibly arriving with contaminated seed from those areas, it has been naturalizing across the United States since the early 19th century. Standing 2- to 4-feet high, purple loosestrife is an appealing plant with tall spikes of brilliant, magenta flowers lasting from May until September.
Its beauty, however, fails to compensate for its aggressive habit. Purple loosestrife has now invaded countless ponds, marshes and other wet areas. The plant spreads easily by seeds or roots to displace native vegetation that feeds and shelters wildlife. It's now classified as a noxious weed in over 20 states, according to the USDA's Animal and Plant Health Inspection Service.
- Not all exotic plants in the United States are invasive. Many, like Japanese spirea (Spirea japonica), are now popular ornamental garden plants. This low-growing, deciduous, rose-family shrub provides profuse clusters of pink flowers against mounds of green foliage from late spring until July, according to the Missouri Botanical Garden. Butterflies flock to the blooms. In the right conditions, its leaves become red or orange in autumn. Hardy to USDA plant hardiness zone 4, the shrub tolerates almost any well-drained soil. It performs best in full sun.