Exotic plant species are those that people transport from their native habitats to new ones in which they flourish, according to the California Parks Department. They become invasive, taking over the habitat of native plants. Free of their regular predators, they have a competitive advantage and often spread unchecked. They deprive wildlife of food and shelter. Two hundred of California's plants are now invasive exotics--with many of them growing in Sacramento Valley, according to the California Invasive Plant Council's database.
Tree of Heaven
Tree of heaven (Ailanthus altissima), native to northern China, is invasive in Sacramento, throughout California and in 29 other American states. Rapidly growing up to 80 feet high and 60 feet wide, it thrives even in poor soil, full shade and urban pollution. Trees spread to form thickets by self-seeding and through root suckers, cautions the Missouri Botanical Garden. Weak wood makes tree of heaven a hazard in high winds.
All parts of the tree emit an unpleasant aroma. In June and July, it has greenish flowers that produce winged seeds (samaras) in September. Plants will sprout even through pavement, where their dropped twigs, leaves and seeds can be unsightly. Individual trees compensate for their short lives by spreading quickly.
Water Hyacinth (Eichhornia crassipes) is a perennial native to South America. In Sacramento, with relatively mild winters, it has invaded the aquatic systems. Standing up to 9 inches high and 18 inches wide, water hyacinth is a free-floating, water-garden ornamental. Plants have basal rosettes of thick, glossy green leaves. Thick leaf stalks (petioles) serve as floats. Rapidly spreading, water hyacinth has showy spikes of lavender blooms from June to September. Dangling roots provide shelter for fish. Water hyacinths have a "high" invasive rating from the California Invasive Plants Council. It has had a severe impact on the native plants and animals of the Sacramento area.
Another plant with a "high" invasive rating, perennial purple loosetrife (Lythrum salicaria) is native to Europe and warm Asian regions. The Missouri Botanical Garden cautions that this plant numbers among the World Conservation Union's 100 Worst Invasive Species. Between 2 and 4 feet high and wide, purple loosestrife thrives in swamps, marshes, wet meadows and even roadside ditches.
While its May-to-September 18-inch spikes of magenta flowers make a dramatic show, this species self-seeds with such ease that it chokes out all other vegetation and eliminates wildlife shelter. Planting purple loosestrife is now illegal in many areas. Where the plant is established, shearing off the dead flower heads before they produce seeds may control its spread.