Audubon Living
Alien Landing
After a gardener discovers a plague of exotic invasive plants overrunning his yard, he decides to take matters into his own hands.
By Howard Youth

“The birds love the strange lilac and deep-blue wild grapes growing in our backyard,” I proudly told my friend Marilyn. I thought she would be pleased. After all, she was the one who handed me my first pair of binoculars, when I was seven. But Marilyn was looking back at me as if I had swallowed poison. “You don’t have wild grape,” she said. “That’s porcelain berry. I have a heck of a time trying to keep it off our property. It definitely doesn’t belong.”

Marilyn was right. Those berries that the robins, mockingbirds, catbirds, and cedar waxwings gulped down come from an Asian vine that delighted ornamental gardeners in the late 1800s but now bedevils conservationists trying to preserve wildlife habitat. Once the birds have digested the berries their seeds will rain down in droppings upon neighboring gardens and parks. That’s why porcelain berry blankets forest edges from Michigan to New England and at least as far south as North Carolina. Soon I learned that porcelain berry wasn’t my only problem: The fruit-rich bramble in our back corner was not native raspberry but imported wineberry. Worse, I found the feathery flowered “sumac” that I’d left to grow was actually a tree of heaven, otherwise known as ailanthus—a plant more associated with the netherworld.

The trouble is these plants fall under the menacing category of invasive exotics—species introduced from foreign lands that aggressively proliferate with reckless abandon in their new environs. And even though they sometimes provide food for birds and wildlife in the short term, they cannot replace the value of multi-layered natural habitats, says Steve Kress, Audubon’s vice-president for bird conservation and author of The Audubon Society Guide to Attracting Birds. “When an invasive exotic plant spreads over the landscape, it can provide a bounty when it is in fruit. But after the fruit passes, that habitat is barren the rest of the year,” he says. “By contrast, a diverse habitat of native plants provides birds with nutrition year-round.”

What I discovered in my backyard is just a small sampling of the invasive exotic plants marching across the continent and gobbling up large areas of wilderness, posing the second-biggest threat to biodiversity after habitat loss. So far at least 50,000 alien plants and animals have hopped across U.S. borders, contributing to the perils faced by roughly 42 percent of the nation’s threatened and endangered species. Last year alone the National Park Service spent nearly $11 million eradicating alien plants on 75,000 acres of infiltrated parklands. Nationwide such weeds are creeping across roughly 1.7 million acres of wildlife habitat every year, according to researchers at Cornell University. The environmental damages and losses in agriculture, forestry, and other parts of the economy from alien plants and animals cost the United States $120 billion annually—nearly five times the 2007 budget of the U.S. Department of Energy.

These bad actors also compound the challenges facing more than one-third of the species on the Audubon WatchList—a compilation of birds that are declining or threatened because of habitat loss. For instance, the common reed, or phragmites, replaces diverse, native salt marsh plants with thick stands that are far less hospitable to seaside sparrows; yellow starthistle causes the eviction of long-billed curlews and other nesters from once-productive grasslands; and smooth cordgrass, an Atlantic and Gulf Coast plant now established along the West Coast, overtakes native marsh vegetation and mudflats critically important to migratory shorebirds.

Global warming is redrawing the battle lines. All plants are primarily restricted in range by the minimum winter temperatures they can tolerate. Even vicious kudzu—the bane of the Southeast—has its limits: minus-4 degrees Fahrenheit. But these temperature boundaries are moving north. The change has been so substantial that in 2006 the National Arbor Day Foundation had to revise the 1990 USDA hardiness zone map, bumping up most parts of the United States at least one zone. (For the updated map, click here.) As a result, gardeners, farmers, and land managers will need to bolster their defenses. “We expect to see major and more rapid shifts in invasive plant ranges than we have in the past,” says David Wolfe, a professor in the department of horticulture at Cornell University.

What’s more, Wolfe says many of the fastest-growing weeds and invasive species may be better able to take advantage of increasing carbon dioxide in the atmosphere. One alarming example is poison ivy, which, while native to many parts of North America, is likely to boost calamine lotion sales all over. Researchers at Duke University published a study in 2006 revealing that increasing the carbon dioxide in an intact forest ecosystem is like putting poison ivy, the plant world’s Galactus, on steroids: Not only does its growth rate and girth go crazy, it also produces a mega-dose of the allergen that makes people itch.

Todd Forrest, vice-president for horticulture and living collections at the New York Botanical Garden, is starting to see some disturbing trends in the 50-acre old-growth urban jungle he manages. Where hemlocks and sugar maples once predominated, both are now in trouble. “The trees are stressed by the climatic changes—not just the warmth but drought followed by deluge,” he says. As a result, Forrest believes the trees are more vulnerable to introduced pests. Meanwhile, hundreds of volunteers have worked alongside the garden’s staff to remove invasive exotics such as Amur corktree, a space-hogging specimen named for its spongy, golden-colored bark; Amur honeysuckle, a shrub that creates a canopy so dense it can cloak a woodland in darkness; and Japanese honeysuckle, a surly evergreen vine that crowds out seedlings.

Gardeners can do their part to stem the incursion. In my yard I wage a constant battle against reemerging sprouts from taproots I missed, plus the thousands of tiny plants that pop up where birds strafe our yard with their exotic seed-filled droppings. My strategy is “identification, control, and vigilance.” I know it sounds like a police department motto, but battling invasives is like fighting crime: You can never let your guard down. Many days I wipe my sweaty brow and feel grateful to own just a small patch of property by the weed-necklaced Washington Beltway. In nearby Rock Creek Park, thick twining ropes of porcelain berry, mile-a-minute, English ivy, Oriental bittersweet, and other unwelcome vines smother the river birches, tulip poplars, sycamores, and silver and red maples, often causing their limbs to snap in stormy weather. Dense clumps of exotic bamboo, Amur honeysuckle, Japanese stilt grass, and Japanese barberry now stand where spicebush, blueberry, native azalea, and other shrubs and herbs once prevailed.

“Forty of our 41 most problematic plants in Rock Creek Park are garden plants,” says Sue Salmons, the liaison for the National Capital Region Exotic Plant Management Team, which oversees 14 national parks in Maryland, Virginia, West Virginia, and the District of Columbia, where Rock Creek Park is located. One of her biggest nuisances is ailanthus. This Chinese native thrives in sunny vacant lots and on hillsides and forest edges such as those lining many U.S. highways. First planted as an ornamental in Philadelphia in 1784, it found its way, through Chinese immigrants, to the West Coast during the Gold Rush in the mid-1800s.

This fast-growing plant dominates open areas that have been scarred by development, and it is tough to control in many of the 42 states where it resides. The roots run unseen just below the soil, anchoring saplings that pop up like Whac-a-Moles at a carnival stall. Many believe that ailanthus roots produce chemicals that inhibit other plants from growing nearby.

After correctly identifying my 15-foot-tall tree of heaven, I girdled the trunk. When the tree continued to thrive, I sawed it down. Wrong move. Cutting these devilish weeds spurs growth in stump sprouts and root suckers. My effort erased the trunk from view, but the roots were having a party down below. I started digging up eight-foot-long fleshy spider roots that sprang from the stump and sprouted tiny trees of heaven 10 feet away. When I tried to prod them from the soil, the pale, brittle anchors snapped, emitting an annoying nutty odor that urged me to dig more and break a cardinal rule of native plant gardening—minimizing soil disturbance, which often invites more exotic weeds to take hold.

Whether reaching for the sky or tunneling underground, it’s no wonder this plant drives some placid environmentalists to grab chemical tanks, axes, and sprayers. Marc Imlay, vice president of the Maryland Native Plant Society and the native plant restoration volunteer coordinator with the Maryland chapter of the Sierra Club and the Anacostia Watershed Society, is one of these. He rallies volunteers called “weed warriors” who eliminate invasive plants in nearby parks. Imlay and his volunteer brigade do their best not to spray native vegetation or wet areas, but sometimes they have to “hack and squirt” large ailanthus trees with a concentrated solution of the herbicide triclopyr, which he says is biodegradable.

Call me paranoid, but I’ll do anything to avoid spraying chemicals on my property. I’m afraid of health effects and what chemicals might do to wildlife. So after meeting Imlay at an exotic-plants conference in Philadelphia, I grilled him on how a homeowner (yours truly) with a yard the size of a postage stamp might eradicate just one tall ailanthus without herbicides. “Well, if you’re really prepared to work at it over five years, you can do it,” he told me. “It’s a bit of a commitment. It’s not feasible for most of us.” I vowed to try, but I hoped it wouldn’t require that I dig up my entire hillside to accomplish my goal.

At the conference, I learned that some state agencies, including the Pennsylvania Department of Conservation and Natural Resources and the Delaware Department of Agriculture, now publish booklets advocating native or at least non-invasive alternatives to the most popular invasive plants. For example, nice eastern substitutes to Buddleja, or butterfly bush, are native butterflyweed and Virginia sweetspire. Virginia creeper or trumpet creeper are fine replacements for English ivy, porcelain berry, and other climbers.

I also learned that the “brown gold” we manufacture in our backyards may be a storehouse for exotic weeds-in-waiting. Even the best compost heap m ay not cook all the seeds and cuttings thrown in. That’s why Salmons cautions park neighbors not to dump yard waste into any natural area, let alone besieged Rock Creek Park. At least in your yard you can keep track of where plant cuttings and compost go and watch for emerging invaders.

Removing exotic weeds requires two commodities often in short supply—patience and persistence. Every day during the growing season I go into my garden and come back with fists or arms full of invaders. This ongoing effort creates more room for natives, which I buy at a growing number of specialized nurseries and native plant sales held by local groups. Now my garden is brimming with spicebush, Solomon’s seal, arrowwood viburnum, pawpaw, and redbud. Our sassafras and Virginia creeper just came out of the blue, likely souvenirs dropped in bird bombs. My hope is that the birds will find our yard a woody simulation of good habitat and will dine here frequently all year long, on the right foods.

Even the ailanthus is gone—for now. I finally managed to dig up and yank out the monster stump and roots. But next year I expect a flurry of ailanthus sprouts to reveal the ones that eluded me. This time I’ll be ready.

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Howard Youth has written about nature, wildlife, and birds for a variety of publications, including National Wildlife, The Washington Post, Bird Watcher’s Digest, and Wildlife Conservation.

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Video Spotlight: Park Invaders
Nonnative plant species are intruding on our national parks. Filmmaker Liz Smith (naturefilm.montana.edu) reveals what it takes to fight these invaders.