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Gardeners' nemesis: invasive plants


Advice for dealing with intruders in the garden lawn

by: COURTESY OF GLENN MILLER - Canada thistle may be pretty but it's invasive With time and effort, invasive species can be controlled in your yard or garden.

Experienced, conscientious gardeners in Oregon know that nonnative, invasive plants are a topic of conversation that never seems to go away. That's because invasive plants not only affect composition and plant mix in our gardens and landscapes, they also incur large economic and environmental consequences in our forests and natural areas.

Fortunately, every gardener can do something to safeguard the things we love about Oregon living, such as our abundant forests and wildlife, healthy lakes and streams, our urban parks and working ranches. Avoiding the purchasing, planting and growing of invasive plants is a very good first step.

What are some of the most troublesome garden invaders these days, and the best steps to take toward their eradication?


Scotch broom plants are large, multi-branching woody shrubs with erect green stems, yellow "pea flowers" and legumes (pods); plants can reach up to 10 feet tall. Stems are dark green and strongly angled with five ridges. In Oregon, Scotch Broom is attributed to losses totaling $47 million annually due to decreased timber production.

Researchers are hopeful that introduction of biological control agents including a beetle, a sand weevil and a twig miner will reduce the spread of brooms in the Willamette Valley. They’re also exploring control strategies using a mite that creates galls on the plants.


Canada, Bull, Scotch, Musk and Italian thistle all fall under this category. These plants are erect herbs with prickly foliage and pink, purple or white flower heads from June through August. Plants are 1-7 feet tall, and leaf bases extend nearly all the way down between stems as spiny "wings."

Care should be taken to verify thistles are exotic invaders, not natives, as Oregon has more than 15 native thistle species. Native thistles are important to wildlife such as pollinators, and are generally not considered problem plants.

Hand-pull thistles using thick gloves, and control small patches before they spread. Then, re-seed disturbed areas immediately with desired species.

Researchers and entomologists are hopeful that stem gall fly can help curb the spread of Canada thistle.

Tree of heaven

This invasive tree, also called sumac, is of concern in Oregon — especially southwest Oregon and the Columbia Gorge — and in disturbed urban areas. It’s a tall tree reaching 80 feet in height, with alternate, large (1-4 ft. long) compound leaves and gray bark. Tree of heaven bears showy clusters of small, yellowish-green flowers in June, and reproduces by seed and by vigorous re-sprouting, especially in response to breakage or cutting. Before attempting control, make sure you aren’t mistaking it for ash or walnut.

Young seedlings are fairly easy to pull out; take care to remove all root fragments. Elimination by repeatedly cutting trees down to the ground requires diligence. Systemic herbicides are sometimes called for when working to control this persistent landscape invader.

English ivy

By now most Oregon gardeners know that given time, English ivy encourages root rot in your trees, can harbor rats, will climb and smother your trees, increases fire hazard, and makes the tree canopy heavier and more susceptible to wind-throw during winter storms. This vigorous woody perennial can creep in the understory of your trees, or, form erect, shrubby stems with diamond-shaped leaves.

How to get rid of it? Portland's "No Ivy League" uses a technique called girdling. Using loppers or a pruning saw, cut through each ivy vine clinging to the trunk at ankle height, and at shoulder height. Then, being careful not to hurt the bark, strip all ivy away from the tree between the two cuts. Afterwards, recheck the girdled area for any thin vines hiding beneath bark. Chop, mince and dispose of all cut vines.

Finally, leave a "lifesaver" of ivy-free ground that extends in a 6-foot circumference from the tree. Consider enlisting the help of your neighbors and throw a hand-pulling “party.” It may take awhile, but given time and patience, cutting English ivy back, as well as applications of Roundup during winter months, are all effective strategies.

Japanese knotweed

Japanese knotweed spreads primarily by broken stem fragments that are easily dispersed by flowing water, and unfortunately, these days it’s often spotted growing in urban yards — even, as this author can attest, the urban neighborhoods of cities like Portland, Eugene, or Salem. This plant is fairly easy to spot, featuring tall bamboo-like green or reddish stems with smooth-edged, oval to heart-shaped bright green leaves one to 10 inches wide.

“Knotweed is almost impossible to control by pulling because of its complex root system,” says Bruce Alber, of Wilber-Ellis, a San Francisco-based, family-owned agribusiness. Alber recommends spraying in spring or early fall with glyphosate or a combination of glyphosate and Imizapyr. “Be willing to use all tools,” he adds.

Oregon State University is working on getting a biological control agent approved for Japanese knotweed. Some watershed councils offer free knotweed control.

A few last words of advice: read up on invasive plants in Oregon so you can become better at identifying them. Get to know your gardening-loving neighbors and find out what invasive plants they’ve seen in the neighborhood. Also, hop on the web and see what resources are available. You’ll be doing your yard, neighborhood and community a world of good.

For more information:

Oregon Department of Agriculture’s Noxious Weed Control website:


GardenSmart Oregon – free publication from Oregon Dept of Agriculture:


Oregon Invasive Species Council


PNW Weed Management Handbook


No Ivy League


Cynthia Orlando has a degree in forest management and is a certified arborist with the Oregon Department of Forestry