English ivy is also overwhelming parkland in western Washington, especially around Seattle.  Although state law allows  counties  to  force  landowners   to eradicate infestations of four ivy cultivars as well as butterfly bush, it's still legal to buy the plants.

Less-densely populated states, whose legislatures are packed with folks from rural, agricultural counties, tend to be more vigilant about invasives, according to DiTomaso, who cites the policies of Idaho and Montana. For the last 20 years, Montana has run a well-financed weed-awareness campaign that employs newspaper and television ads as well as billboards. The state recently banned Russian olive trees, which push out native cottonwoods. But the horticulture industry there is tiny, and most of the weeds aren't escapees from gardens: They snuck in with desirable crops or were deliberately introduced as forage.

And with the economy still limping, the situation elsewhere isn't likely to improve soon. Dire state finances have crippled Arizona's nascent efforts and threaten programs in other states. "We put what limited resources we have to functions that will most benefit the agricultural community," says Arizona quarantine program coordinator Brian McGrew. "Noxious weeds usually take a back seat in relation to other plant pests and diseases." California is proposing to eliminate its weed programs entirely, and Washington may do away with its Invasive Species Council, which coordinates the state's response to exotic pests.

In eastern L.A. County, sustainable landscaping expert Drew Ready meanders among San Gabriel Nursery customers toting blushing azaleas and budding sweet brooms in red wagons. He points out pretty violet-hued periwinkle (Vinca major), freeway ice plant (Carpobrotus edulis) and two other species that Ready's employer, the Los Angeles and San Gabriel Rivers Watershed Council, lists as invasive. "Certainly a lot of the wholesale growers have horticultural experience, understand the issue of invasive plants and aren't taking the problem seriously enough," he says. "At the nursery level, I would guess it's mostly just ignorance of the extreme impact that some of these plants are having in the wildlands."

With states struggling -- and failing -- to rein in runaway plants, some conservationists have tried to get growers and nurseries involved. In 2001, horticulture industry and botanic garden leaders joined invasive plant experts at the Missouri Botanical Garden to hammer out a voluntary code of conduct on invasive plants. Known as the St. Louis Declaration, it featured a pledge to phase out the worst offenders.  But a decade later, only a few in the trade have taken the issue to heart, especially since many businesses in these tough times are focused on simply surviving.

"I liken the attitudes on invasive plants to those about global warming," says Craig Regelbrugge, vice president of research with the American Nursery and Landscape Association. "Some people refuse to believe there's an issue. Some see the problem but have different ideas on solutions." Many growers oppose state bans -- officially quarantines -- because they already primarily ship weedy plants only to areas where they aren't invasive. Others wonder why they should stop selling plants that states haven't bothered to prohibit. And many invasives threaten only specific habitats: In Southern California, for example, ivies mostly menace riparian areas. "I don't think English ivy should be banned in California," says John Schoustra, past president of the Nursery Growers Association of California. "It's one of the very few things that can (grow on) the north side of a 30-story building, and take bums sleeping on it (and) hundreds of little dogs peeing on it."

Seeking alternatives to bans, a few plant breeders and growers are developing sterile cultivars and less invasive hybrids of problem plants. So far, the results aren't reassuring. Dwarf pampas grass was thought to be sterile -- until some nursery plants produced viable seeds. Newer types of ice plant are thought to be less invasive, but some are cropping up in wetlands. (Scientists say there's frequently a lag time: A plant may lie low for a while -- sometimes decades -- before it starts spreading aggressively.) California nurseries now sell a sterile red cultivar of fountain grass, but a pilot study at the University of California, Riverside, found that a red version grown near invasive green ones would cross-pollinate and produce seed. Likewise, sweet broom, which the horticultural trade doesn't consider invasive, appears to be cross-pollinating with California populations of an invasive shrub called French broom. This intermingling is troubling, as populations of invasive plants with more genetic variability may be more difficult to control, and more likely to adapt to and thrive in new environments.

Perhaps the most significant voluntary effort to address the issue to date is California Horticultural Invasives Prevention (Cal-HIP), a consortium of nursery people, weed experts and conservationists that has focused on developing consensus, supporting scientific research and disseminating educational materials. It's developing a third-party certification program that would award an "environmentally safe" label to nurseries that don't sell known invasive plants.

Because of variations in climate and soils, exotic plants that threaten one region can be harmless in others, so the federal government is unlikely to take the lead on invasive plants currently sold in nurseries. But the U.S. Department of Agriculture's Animal and Plant Health Inspection Service is proposing, for the first time, to subject new garden plant imports to risk analyses before they can be brought into the country. Inspectors would consider a plant's potential to become invasive in the U.S., and either prohibit risky species or subject them to restrictions.

In the meantime, though, ornamental weeds have become so prevalent in Southern California that they can easily be mistaken for local flora. "A lot of people think fennel is native, that there should be a heinous fennel forest," says ecologist Brigham. "They don't know that there actually should be a wildflower meadow with beautiful little violets and little fairy lanterns." Intent on showing off California's native beauty, Brigham takes me hiking up a stretch of the Santa Monica Mountains' Chumash Trail. Tall spikes of creamy yucca blossoms and spires of lavender-colored sage blossoms mingle with golden monkey flowers and tiny constellations of goldenstars. It's a landscape equal to anything a garden designer could dream up. But Brigham has to return to work eradicating weeds. I hike on alone and discover something she's missed -- a young fountain grass plant. Just a small spurt, but raring to grow.