Beautiful Invaders

Beautiful Invaders header image
Researchers are using new bugs, biology and trickiness to fight an invading green army.

A new sort of land war is raging across Oregon. Invasive plants that landed like aliens from the outer spaces of Asia, Europe and South America are taking over entire ecosystems.

Many of them are pretty; some are poison to people and animals. Others crowd out crops or wildlife habitat. All share the same fatal flaw that has landed them officially on the list of 105 least-wanted plants in Oregon: they don’t grow well with others. With no natural enemies and few climatic or geographic controls, these plants can aggressively crowd out native flora, potentially unraveling the dense tapestry of plants and wildlife into the ecological equivalent of a parking lot.

Scotch broom, for example, was brought to the Pacific Northwest by early settlers homesick for the flowers of their native British Isles. Scotch broom rapidly spread into forests and woodlands throughout western Washington and Oregon, where it now occupies millions of acres.

“Some people may look at the golden hillsides resulting from Scotch broom and think it’s a pretty wildflower that belongs in Oregon,” said Tim Butler, supervisor of the Oregon Department of Agriculture’s Noxious Weed Program. That’s the farthest thing from the truth. Scotch broom costs the state about $47 million each year by its impact on natural resources, particularly on timber production.

photo of scotch broom along highway

Scotch broom has been transported throughout the temperate world, where it has few natural enemies to keep it in check. Photo: Lynn Ketchum

“Controlling invasive weeds is a real white-hat issue,” said Steve Radosevich, an Oregon State University Extension Service weed ecologist who has 30 years of experience in weed research. Farmers, ranchers, environmentalists and landowners all form a united front against these invaders.

On a five-acre test plot in the former Camp Adair military base north of Corvallis, Radosevich and a research assistant have targeted one of the state’s thorniest and most aggressive agricultural runaways, Himalayan blackberry. They discovered that by mowing the crowns (the point where the canes join at the base), disking them up and raking them out, they have reclaimed much of the thicket. It is a simple, affordable treatment using standard farm equipment that has kept the blackberries at bay even three years later, according to Radosevich. Such controls target and exploit the plant’s weakness: remove the crowns, and you’ve knocked back the plant.

“The landowner could get goats or sheep in here the year after treatment to keep down the resprouting canes,” Radosevich said.

The war on weeds has led researchers to look for weaknesses in many of Oregon’s least-wanted invasive plants. A famous example is the successful control of tansy ragwort using its natural enemies imported from Europe. Peter McEvoy, an entomologist with OSU’s Department of Botany and Plant Pathology, helped spearhead the biological control of tansy ragwort 20 years ago. Now, he says, there are seventy insects and pathogens providing a measure of control on thirty different weeds.

“The public gets back about $15 in benefits for every dollar spent when a noxious weed is reunited with its natural enemy,” said ODA entomologist Eric Coombs, who was integral in helping to bring tansy ragwort under control.

Take, for example, purple loosestrife, whose thick stands clog waterways and reduce water quality, stream flow and habitat for waterfowl. The best hope for biological control of this European native lies with two leaf beetles, a root-feeding weevil and a seed capsule weevil. Since their introduction a few years ago, the insects have eaten their way through 90 to 95 percent of the purple loosestrife at the Baskett Slough National Wildlife Refuge west of Salem.

While the reduction of purple loosestrife is promising, McEvoy said it doesn’t necessarily mean an instant return of native vegetation and associated vitality. Reed canary grass — a noxious weed introduced to Oregon in the early 20th century — has spread into some areas where the loosestrife declined.

photo of Jed Colquhoun photo of purple loosestrife

OSU weed scientist Jed Colquhoun considers all points of entry for weeds, including birdseed and wildflower mixes that might contain seeds of the next weed invasion. Photo: Lynn Ketchum


A beautiful barbarian introduced from Europe, purple loosestrife clogs waterways throughout Oregon and elbows out a variety of native plants. Photo: Rodney G. Lym, North Dakota State University

Weeds infest all parts of Oregon, from city parks to suburban backyards to crop fields and rangelands. Many of the beautiful invaders — purple loosestrife, Japanese knotweed, butterfly bush — began as garden specimens, imported for their beauty in the landscape.

The more the public recognizes that some pretty plants have ugly – noxious – habits, the better, said Jed Colquhoun, an OSU Extension weed specialist in the crop and soil science department. Recently, he has been speaking to public groups about the dangers of noxious weeds – sometimes with surprising results.

“I was giving a talk on noxious weeds and the audience was very surprised that some noxious weeds began their ‘careers’ as garden ornamentals,” he said. The group admired the beautiful small broomrape, loosestrife and even water hemlock. And who can blame them? What gardener doesn’t like a plant that establishes well, requires little care or fertilizer and grows almost anywhere? When does a gardener decide that a pretty, hardy perennial has become a problem?

And so it is with the rise and fall of lilac-like butterfly bush, Buddleja davidii. A few years ago, garden clubs and nature advocates touted the virtues of planting the pretty butterfly bush as a way of attracting … butterflies. That was before the native Chinese variety of Buddleja thickly infested the North Fork of the Willamette River near Oakridge and the Coquille River near the coast, spreading to almost every county in western Oregon and Washington.

Last year, the ODA added Buddleja to its list of class B noxious weeds, targeted for control everywhere in Oregon except peoples’ yards, where gardeners now can grow different horticultural varieties of butterfly bush. James Altland, a nursery crops researcher at OSU’s North Willamette Research and Extension Center in Aurora, and graduate assistant Julie Ream are investigating whether these cultivated varieties are better sharers of the land than their aggressive parent.

Not all introduced weeds came by invitation; many came as stowaways. Downy brome, commonly called cheatgrass, hitchhiked in from the Mediterranean a century ago and has since turned huge swaths of rangeland into single-species wastelands. The spread of downy brome is contributing to the loss of habitat for a threatened bird of the western range, the sage grouse.

photo of sage grouse photo of Carol Mallory-Smith

Downy brome is contributing to the loss of habitat for a threatened bird of the western range, the sage grouse. Photo courtesy of U.S. Fish and Wildlife Service


Carol Mallory-Smith, an OSU weed biologist, tests wheat as a foil against small broomrape to discourage infestation of the weed in the seed crop of red clover. Photo: Lynn Ketchum

Downy brome represents one of the biggest challenges to eastern Oregon farmers and ranchers, according to Corey Ransom, an OSU assistant professor in weed control research at the Malheur Experiment Station in Ontario. Although the grass provides some forage for grazing animals early in the season, the quality drops as the grass matures and the seedheads become sharp and irritating to tender mouth tissues. Cheatgrass is especially problematic in dryland fields of winter wheat, where its roots can extend three to four feet and greedily suck moisture from the soil. A severe cheatgrass infestation in winter wheat can lower grain yields by up to 90 percent.

Estimating the real-world cost of noxious weeds is a challenge, according to Carol Mallory-Smith, a weed biologist with Oregon’s Agricultural Experiment Station. Mallory-Smith studies herbicide resistance in weeds such as jointed goatgrass, which infests wheat fields in eastern Oregon. Winter wheat’s “evil twin,” jointed goatgrass is a clever mimic that grows in wheat fields, contaminating wheat in the Great Plains and Pacific Northwest and costing U.S. wheat farmers $45 million in losses each year. So farmers were keenly interested in an herbicide-resistant wheat variety that enabled herbicide treatment of fields that would target jointed goatgrass and would not harm the wheat. Unfortunately, jointed goatgrass and wheat can produce weedy hybrids with the potential to be herbicide resistant as well.

Increasingly, weed control in crops involves an integrated strategy that targets specific vulnerabilities of the weed.

Consider small broomrape, an exotic-looking flower stalk whose roots have been sucking the life out of the Willamette Valley’s seed industry, leaching nutrients from the root systems of red clover. Mallory-Smith explains the current control strategy, which involves chemical control and a bit of trickery. “We’re growing wheat as a ‘false host’ that will stimulate germination of weed seed, but does not allow the weed plant to develop. It reduces the seed in the soil over time.”

Landowners, weed ecologists and landscapers are working together to identify and control the noxious weeds in Oregon. A new Internet-based network, called WeedMapper, was developed by OSU and ODA scientists to allow rapid reporting of noxious weeds across the state.

Early detection and rapid response are critical in weed control. It was an ODA Noxious Weed program staffer who first detected the arrival of kudzu, the “scourge of the South,” outside of Portland. The carpet-like foliage of this sweet-smelling vine can advance 60 feet a year. Kudzu immediately made the state noxious weed “A” list as a kill-everywhere entry. Public involvement in finding a second site in 2000 and a third in 2001 has helped the ODA in its “search and remove” efforts to bring kudzu under control, for now.

In some Oregon cities, early intervention and public involvement already are proving effective. For example, volunteers with the Forest Park Ivy Removal Project in Portland descend every week on the park as a no-Ivy League to remove smothering English ivy, or that “botanical barbarian,” as they call it. Their motto: “Leave no vine behind.”

When it comes to noxious weeds in general, researchers are hoping similar sentiments regarding noxious weeds will take root — and spread.

Oregon's least wanted

Researchers at Oregon’s Agricultural Experiment Station are working with state and federal officials to combat some of the most invasive noxious weeds in Oregon. Among the state’s “Least Wanted” weeds are, from left to right: butterfly bush, whose unnamed non-horticultural varieties spread aggressively; puncturevine, whose sharp-thorned seedpods attach to tires or feet; spotted knapweed, which is poisonous to livestock and can even be toxic to the touch; medusahead, which has reduced grazing capacity up to 70 percent in some eastern Oregon ranges.

photos of invasive noxious weeds

To learn more about weeds in Oregon:

A Vanquished Invader

photo of cinnabar moth larva on tansy

It took insects from Europe, such as the cinnabar moth larva, to control the European import tansy ragwort. Photo: Ken Kingsley

Peter McEvoy speaks of tansy ragwort like a veteran general who knows his enemy well. McEvoy, an entomologist with Oregon State University’s Department of Botany and Plant Pathology, helped to rein in tansy ragwort, one of Oregon’s first identified noxious weeds.

A pretty European native, with daisy-like yellow flowers and lacy leaves, tansy ragwort first showed up near Portland in 1922. It soon became a bane to landowners all over the state, especially to dairy owners and cattle ranchers. Although its alkaloid content made it bitter to grazing livestock, a relatively small amount could cause fatal liver failure in cattle, pigs, horses, goats and deer. By mid-century, the weed racked up $5 million in crop and livestock losses each year.

Tansy ragwort (officially called Senecio jacobaea) grows up to six feet tall. Its taproot digs deep and spreads wide, and each plant can produce up to 150,000 seeds that remain viable for up to 16 years. In 1975, the state legislature added tansy ragwort to the state’s list of unwanted weeds behind ragweed, bringing the total number on the list to two.

Searching for tansy ragwort’s natural enemies in Eurasia during the 1960s and 1970s, researchers found and imported three of its Eurasian insect enemies: the cinnabar moth, the golden ragwort flea beetle and the ragwort seed fly.

Propagated and released in Oregon, these three insects ate through tansy ragwort’s grip on the state’s pasture land. Within five years, the weed was reduced significantly. The flea beetle was particularly effective and continues to keep the weed in check.

But the story is not over. The cinnabar moth, introduced to control tansy, now seems to have found a taste for a North American native plant.

photo of Peter McEvoy

OSU entomologist Peter McEvoy continues to monitor the effect of cinnabar moths on native plants such as arrow leaf groundsel.

“The cinnabar moth arrived when threats to native plants did not receive much public or scientific scrutiny,” McEvoy said. Now a three-year survey conducted across 25 sites in western Oregon determined that cinnabar moths have been munching on arrow leaf groundsel, a native wildflower found principally in the mountains and occasionally on the coast. Scientists had thought that the places where arrow leaf groundsel grew were too cold for the cinnabar moth. But incremental climate change may raise temperatures enough to allow the cinnabar moth to thrive at higher elevations.

Although the state hasn’t reclassified the cinnabar moth as a pest, the story serves as a cautionary footnote in the tansy ragwort success story.

Published in: Ecosystems