In 2001, University of Washington Urban Horticulture Professor Sarah Reichard and her husband bought a house on Crown Hill, just a few minutes from where they lived on Phinney Ridge. The new house's most attractive feature was its half acre of land, dominated by a lush ravine, where the previous owner clearly had spent some time and money on landscaping. "I sometimes think we are insane for getting this property but I wanted a big garden," says Reichard. The problem was not what the previous owner had planted but the invasive plants that had taken over the ravine.
Clematis crept up massive bigleaf maple stumps like the gnarled green fingers of some subterranean beast. Himalayan blackberry, with its spiny branches that seem to double in length overnight, choked out native shrubs and herb Robert, a stinky, little lavender-flowered plant, sprouted in teeming masses that prevented native wildflowers from growing. When another botanist visited, he even discovered an invasive that no one had ever seen in North America. "We have spent and still spend every weekend pulling up invasives. It's funny. I have been studying these plants for years but now it's personal," exclaims Reichard, an assistant professor of urban horticulture in the College of Forest Resources.
She is part of a growing movement of scientists who are studying how invasive plant species impact native systems. These scientists have discovered a disturbing fact - after habitat destruction, our invasive plants, animals and fungi cuse more harm to natives than pollution, disease or overharvesting. A 1999 study placed the annual economic cost of invasive plants in gardens, natural areas and agricultural lands in the United States at $35 billion.
Cheatgrass - One of the 10 Worst Invaders in the Pacific Northwest. Photo courtesy the Nature Conservancy.
Ecologists have not totaled up the annual cost of invasives in Washington state but the threat is pervasive. English ivy in city parks strangles and kills native alders, Douglas fir, and maples, in addition to providing ideal habitat for rats. Invasive Scotch broom and Russian knapweed (Centaurea repens) have helped push the golden paintbrush (Castilleja levisecta) and Washington polemonium (Polemonium pectinatum) on to the state's threatened and endangered species list. Some invaders, such as St. John's wort, are poisonous to livestock. Others reduce property values (Reichard believes she got her property at a good deal because of the invasives). In addition, herbicides and pesticides used against invasives end up in streams and threaten wildlife.
People have been collecting and transporting plants worldwide for thousands of years. European ornamentals appeared in the new world as early as 1631. Wealthy Englishmen sponsored explorers to scour the empire's colonies in search of rare and unusual plants. Thomas Jefferson once wrote that "The greatest service which can be rendered any country is to add a useful plant to its culture." One dubious plant that he helped introduce, at least in the east, was Scotch broom, which he grew at Monticello.
More recently and closer to home, Dan Hinkley, '85, has been a leader in plant exploration and introduction. Nowhere is this better seen than at Heronswood, his nursery near Kingston. You feel transported back to the Permian Era, when all the continents were joined together. In one patch grow black-violet Chinese irises. In another, a Costa Rican giant with six-foot-wide leaves and yet another garden features plants from South Africa, Turkey and Korea. The shades of green alone would fill the largest box of Crayola crayons and the different shapes and colors of wildflowers feel like a kaleidoscope. You can clearly understand why people want exotic plants in their garden.
Unlike some in the horticulture trade, however, Hinkley makes his buyers aware of the risks of bringing in plants from other countries. His catalog contains a concise, thoughtful essay on being aware of the dangers of invasives. He also has Reichard review his plant list and red-flag those plants with a serious potential for invasion. Hinkley adds, "I love plants and it is a satisfying experience to offer a rich palette of plants to those who enjoy the process of gardening. It makes our lives more interesting and the world more understandable. I believe with thought plant introductions can be a safe and respectful process."
Hinkley represents a new breed of horticulturalist. Nurseries, botanical gardens and individuals have historically driven the introduction of new species. One study found that 82 percent of the 235 woody plants indentified as invasive originated in the horticulture trade. For example, legendary plant propagator Luther Burbank introduced the Himalayan blackberry from Europe - not Asia - in 1885. Capt. Walter Colquhoun Grant, who recieved seeds from the British consul in Hawaii, planted Scotch broom on his 30-acre farm on Vancouver Island in 1850. Not to be outdone, the United States Department of Agriculture can claim responsibility for bringing over kudzu and tamarisk, both of which rank high on a recent list of Washington state's worst invaders.
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Sidebar: The 10 Worst Invaders in the Pacifc Northwest