A main threat to wildflowers is the invasion of nonnative plant species. nonnative or "exotic" plants have been transported from a different continent or even a different region of the same state. They may have been brought to a new area purposely by humans or transported naturally by wind, water currents, birds, or roving animals.
Many nonnative plants will not survive outside of their usual habitats. Some coexist with indigenous flora, but others are aggressive species that can upset regional ecosystems. Because they have not evolved to suit the area, they may crowd out neighbors by hogging soil nutrients, water, and sunlight. Their spread can cause native species to die out, which reduces the floral diversity and wreaks havoc with the local food chain.
A hundred native species have become extinct in Pennsylvania over the past 50 years. Nonnative populations may also grow abnormally large because local predators do not recognize them and so do not eat them. Pennsylvania is wrestling with the invasion of lesser celandine, an exotic plant in the buttercup family.
"For the last 20 years it has been taking over," says Ann Rhoads, director of botany at the Morris Arboretum of the University of Pennsylvania in Philadelphia. "It is competing with the trout lily and other native wildflowers." Even Bowman's Hill Wildflower Preserve in New Hope, Pa., cannot keep the weed in check. Lesser celandine spreads through parts of the preserve in thick mats of green that sprout bright yellow flowers in spring.
"People want to know, 'Where can we get that beautiful buttercup?' " says Matt Palmer, a naturalist at the preserve. "It can be frustrating when the best show florally is a nonnative."
Some states, especially in the agricultural Midwest, have noxious-plant laws that make it illegal to grow nonnatives. In the last decade, native-plant gardening has become more popular. An increasing number of gardening books on native plants cater to those who are concerned with preserving the environment in its natural state. Even if conservation is not a gardener's primary concern, the books point out that native plants thrive without fertilizer or a lot of care.
"There is a native plant for every location and condition," says Barbara Pryor of the New England Wildflower Society in Framingham, Mass. "We have native ferns growing in the middle of the parking lot."