On a misty summer morning, ecologist Christy Brigham sinks down to the sand at Point Mugu State Park, part of the patchwork of federal, state and private lands in Los Angeles County's Santa Monica Mountains. She watches a darkling beetle forage among rare dune plants -- lacy, lavender sand verbenas and beach primroses, which resemble large buttercups. When Brigham came to this area eight years ago to work for the National Park Service, she thought she'd become an expert on plants like these, part of the region's unique Mediterranean-climate flora. But instead, she's spent most of her time dealing with common plants, many of them fugitives from local gardens and nurseries. She points out a thicket of fountain grass (Pennisetum setaceum) nodding its blond, tufted seed-heads in the breeze. It's already overtaken some of the dune plants and is closing in on more. Fountain grass, an invasive from North Africa that became popular in California gardens in the 1990s, is "very drought-tolerant and can grow in a lot of different habitats," Brigham explains. "I've seen it expand massively. It's just everywhere."

Indeed, in recent years, fountain grass has elbowed its way onto thousands of acres in Southern California, cropping up in a startling array of places -- along the coast and in the deserts, on hillsides, in streambeds, on rocky slopes -- even in cracks in urban alleys. Brigham notes that in infested areas, "we have found few native plants, fewer animals." The plant is also highly flammable. And fountain grass has plenty of company: The nonprofit California Invasive Plant Council has identified some 100 ornamental plants, introduced through gardens or deliberately planted for erosion control, running amok in California alone. There are forests of non-native tree-of-heaven, thickets of castorbean and Spanish broom, groves of Mexican fan palms and thousands of acres of bamboo-like arundo.

In addition to creating dense monocultures, some invasive plants slurp up precious river water, cover the gravel beds fish need for spawning, and push already-rare species to the brink of extinction. Yet despite the region's long experience in waging an expensive and Sisyphean battle against the likes of tamarisk, arundo and other big, bad weeds, many nurseries still sell known runaways, in part because of lax regulations and limited government resources. And it's hard to convince state agencies and the industry that a pretty garden plant is a problem. "It has to get where it's hurting people somehow -- visibly ecologically, economically, or causing fires -- to get the public's attention," says Ed Northam, a weed biologist with the University of Arizona Cooperative Extension. "But when you're at that point, it's a little late."

Western states have been at war with weeds for more than a century. State weed programs, however, tend to focus on plants that hamper agriculture and ranching. Rogue garden plants, typically perennials that farmers can easily vanquish through tilling, primarily threaten wildlands -- less-familiar ground for state ag agencies. And many states are reluctant to impinge on their horticultural industries. When California's Department of Food and Agriculture banned the sale of a handful of the state's worst invasive plants a few years ago, it declined to include invasive pampas grass, which plays a considerable part in the state's $2.7 billion wholesale growing industry. Instead, it prohibited jubata grass, a pampas relative that's seldom sold or grown in the state. "CDFA's argument was that there was going to be a lot of resistance," says Joseph DiTomaso, co-author of Weeds of California and Other Western States. "So they went with all the easy ones." The agency responds that state agriculture code prevents it from banning a wildland weed if doing so is "detrimental to agriculture." The detriment in this case, according to spokesman Steve Lyle, was economic loss to the cut-flower industry, which grows pampas plumes.

Oregon has shown more moxie. The state banned the sale of butterfly bush (Buddleja davidii) in 2004 because it invades logged forests, pastureland and riverbanks. Last year, it prohibited the sale of all plants commonly sold as English ivy (Hedera helix, Hedera hibernica). Ivy now smothers the ground throughout the greater Portland area, impeding the growth of native sword ferns and wildflowers such as snowberry and weighing down the trees it climbs, making them more likely to topple in storms. Quashing an ivy infestation and restoring native plants costs as much as $10,000 an acre, says Jonathan Soll, stewardship manager with Metro, a tri-county governmental agency that manages parkland. Even so, it took nearly two decades of surveys and advocacy to get the ivies pulled from nursery shelves. "We had to document that it wasn't just a city phenomenon," he says. "We had to show it was moving and was going to keep moving until it wreaked havoc throughout forests in Oregon."