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Gardening hangovers, Part 5: Scotch and Spanish broom

April 23, 2010 |  7:05 am
Invasive_Broom One of those gorgeous winter days when the interplay of light, mist and silvery white ceonothus makes the Santa Monica Mountains sparkle, National Park Service ecologist Christy Brigham steps onto the Backbone Trail.

She points out the skeletons of shrubs she’s been trying to kill: fragrant, yellow-flowered Spanish broom (Spartium junceum). The plant is mild-mannered in its homeland. But once abroad, away from the diseases and insects that used to keep it in check, this broom can become plant-zilla, bulking up quickly and out-competing indigenous plants.

It spits out seeds — they explode out of drying pods — that can fly or roll quite a distance. Hikers also tramp them into the wilderness. “Any place you get a disturbance — a gopher mound, a fire — they will grow into these big bushes,” says Brigham.

The shrub is also colonizing parts of the San Bernardino Mountains that burned in 2003.

Spanish and Scotch brooms were planted, years ago, along many of California’s mountain roads to stabilize soils. They’re also sold in nurseries.

California nurseries aren’t allowed to stock some of the worst weeds, especially those that threaten agriculture. But, according to Doug Johnson of the nonprofit California Invasive Plant Council, “State law actually prevents plants currently in the nursery trade from being banned.”

The U.S. may soon require tighter screening of new horticultural imports. However, Johnson says, it’s unlikely to place new restrictions on plants already in home gardens, “so it’s important that we develop voluntary measures.”

One such effort is California Horticultural Invasives Prevention, a consortium that urges the nurseries and gardeners to avoid invasive plants.

Azusa-based Monrovia Growers, a member of Cal-HIP, has replaced many runaway plants with less aggressive alternatives. The company’s computer system won’t allow salespeople to ship invasive plants to regions where they threaten wildlands, according to Nicholas Staddon, director of new plants. Staddon keeps a wary eye on new imports: “I’ve learned to look for certain traits in plants that could mean they’ll become invasive.”

But Monrovia offers Spanish broom and Mexican fan palm on its website, which doesn’t identify them as potentially weedy. And other companies continue to sell about 30 of the state’s worst weeds.

“The issue has been bewildering for the industry,” says Craig Regelbrugge, a vice president at the American Nursery and Landscape Assn., because some invasive plants vary regionally and “the science is fast evolving.”

Scientists know that some species of broom, pampas grass, and ivy are invasive, but which of their cultivars threaten wildlands is unclear. Research suggests pampas grass cultivars that seem sterile in nurseries are nevertheless contributing pollen to feral populations, helping them spread. Some weeds are also hybridizing in the wild, making identification difficult.

Gray areas and divergent perspectives cloud consensus efforts. For instance, Staddon argues some of the especially useful runaways should be allowed in “highly populated areas where they will never have the opportunity to become invasive.” 

“It’s ridiculous to think nurseries would ask every customer how urban they are and base their sales on our response,” says invasive plant educator Drew Ready, “It just won’t happen.”

He estimates some 5 million California homes are within a few miles of a wildland, creek or park.

So land managers such as Brigham could be weeding indefinitely, and scrambling to pay for it. “If we can prevent people from planting these weeds, that’s money the Park Service can spend on other things — interpretative and education programs — things that people are excited about.”

To find alternatives to invasive garden plants, check these websites: plantright.org, cal-ipc.org, and weedwatch.org.

-- Ilsa Setziol

Photo: Scotch broom. Credit: Los Angeles Times

Recent and Related:

Gardening hangovers, Part 4: Acacia

Gardening hangovers, Part 3: fountain grass

Gardening hangovers, Part 2: Mexican fan palms

Gardening hangovers, Part 1: periwinkle

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