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More Weeds Than Ever
Exotic Invasives Are Adding to Our Pulling Chores

By Adrian Higgins
Washington Post Staff Writer
Thursday, October 12, 2006

There is a certain urgency these days to Laura Beaty's weeding patrols. The hillside garden behind her McLean home has become a perfect habitat for the plant world's version of the Asian snakehead fish -- a take-no-prisoners ground cover called Japanese stilt grass.

It spreads quickly, though it can be beaten back with persistent effort. At this time of year, however, it sends up antennae loaded with seeds. The key to containing stilt grass, Beaty said, is to remove it before the seed ripens and falls in the autumn. Otherwise, it will sprout again next spring with a vengeance. "One plant is going to put out a hundred seeds or more," said Beaty. "But it comes out easily. Bag it, seal it, nuke it."

Gardeners have always lived with weeds and have come to accept weeding as the single most constant chore in a well-tended garden. And yet the more we pull, the more exotic weeds we seem to be battling.

"Yes, it's getting worse," said C. Colston Burrell, a garden designer, author and native-plant specialist in Free Union, Va. "The plants are continuing to reproduce, and there are more invasive plants showing up." Some people believe that climate change has extended the range of these invaders and that elevated carbon dioxide levels have made woody vines, well represented in the gallery of invasive exotics, more aggressive.

"Japanese honeysuckle will probably become much more serious with global warming, like lots of vines," said Brenda Skarphol, a horticulturist at Green Spring Gardens Park in Alexandria, where she has been battling it.

Japanese stilt grass, which first came to the United States in the early 20th century as packing material for ceramics from Asia, is now a pest in 20 states and the District.

"It's an amazing problem for natural resource managers," said Carole Bergmann, forest ecologist for the Maryland-National Capital Park and Planning Commission. "Twelve years ago people didn't really know what it was" in suburban Maryland's parks, she said. "Five years from that it really started to take over, and now there's not one park that doesn't have it up or down the trails."

It is easy to see why stilt grass is so stealthy. Fine-bladed and low-growing, it blends in well in the dappled shade of the woodland floor, its preferred habitat. But it just keeps running and rooting and soon it smothers other ground covers. This is particularly vexing for Beaty, a member of the Virginia Native Plant Society who has worked hard to make her hilly, 1.2-acre woodland garden a haven for indigenous flora. She and her husband, Patrick Berry, have lived in their contemporary home for 19 years. Beaty has been battling the stilt grass for most of that time, and has seen it spread from the moist stream-bank areas, where it gets a foothold, to dry soil and areas of full sunlight.

It is mostly gone from Beaty's landscape, due to constant weeding and a weed-pulling frenzy late last month in advance of seed set. In its absence, native perennials and shrubs are flourishing, including a grasslike sedge, goldenrod and the white wood aster, which is now in bloom. But at a neighbor's property, the entire lawn consists of Japanese stilt grass, which is also known by its botanical name, microstegium, as well as Nepalese browntop. It dies in the winter, turning a straw brown, but by then it has seeded the ground for the following year.

You can suppress populations with pre-emergent herbicides, used much as you would to kill crabgrass in early spring, but Beaty is averse to using chemicals close to her stream.

Stilt grass and other invaders differ by garden. What may be a real pest in one neighborhood may be unknown in another. But merely being in the garden in the fall, raking leaves, planting bulbs and mulching, gives opportunities to see which weeds have taken hold and need pulling. Beyond the ubiquitous pests such as henbit (newly sprouted), wire grass and wood sorrel, gardeners are likely to encounter invasive vines such as English ivy, Oriental bittersweet, Asian wisterias and porcelain berry. Another pest is garlic mustard, which comes up in the spring and can soon take over untended lots.

If some of these relatively new invaders seem familiar, it's because many once were used ornamentally, and some still are. English ivy, for example, is innocuous in its immature form, but when it climbs a tree or fence, it morphs into a plant that sets loads of white blooms and black berries spread widely by birds. Plants that seem to be mainstream ornamentals, including burning bush (winged euonymus), Japanese barberry and vinca, are assailed by native-plant advocates.

As troublesome as these may be in the home garden, they can transform natural areas that are not intensively weeded.

Burrell, author of the new Brooklyn Botanic Garden guide "Native Alternatives to Invasive Plants," said long-established invasive ornamentals are so bad that they are "beyond discussion" and include the ailanthus, paulownia and mulberry trees, shrub honeysuckles, privet and the multiflora roses, Russian olive, winged euonymus, purple loosestrife and Japanese barberries. The newer pests include that universal street and yard tree of the 1980s, the Bradford pear (as well as lesser known varieties of the callery pear).

Burrell is worried about the spread of jetbead, the linden viburnum and a common shade and street tree, the goldenrain tree.

"Another one emerging is nandina, especially farther south," he said. "The jury's out on whether it's going to be a real menace or just an annoyance."

How does something that has been around for years, such as the goldenrain tree, suddenly become a problem? A bird drops 20 seeds over several years, one grew "and five years later it fruits heavily," Burrell said.

"It appears that if someone would go in and get that little population of whatever when it's very small, that would control it," said Mary Pat Rowan, head of the D.C. chapter of the Maryland Native Plant Society. Sound advice for the wilderness. And for the garden.

As Laura Beaty discovered, "They get into a niche, and they really like it."

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