If you're an avid gardener - or even if you're not - you've probably come across more than one invasive plant this year. That's because, in spite of attempts to control them, invasive plants are growing by leaps and bounds in our yards, vacant lots, city parks and open spaces.
An invasive species is a species that is non-native to the environment under consideration, and whose introduction is likely to cause harm to the environment, economy, or human health. There are dozens of invasive plants in the Willamette Valley and Oregon. If you would like to help safeguard our forests and open spaces, avoid purchasing, planting or growing them.
Here's a short primer on some of the most common, how to recognize them, and recommended control strategies.
Knotweed invades rivers and creek banks, permanently displacing native vegetation and reducing fish habitat. If it's growing somewhere on your property, this plant is fairly easy to spot. It features tall bamboo-like, green or reddish stems with smooth-edged, oval to heart-shaped bright green leaves, 1 to 10 inches wide. There are currently no biological controls available.
This plant has a huge and vigorous root system, so it's easiest to eradicate if caught early. Some watershed councils offer free knotweed control.
Himalayan blackberry tends to out-compete native vegetation because it spreads by roots, stems and seeds. It is difficult to control. Stems are long, arched and thick, up to 3/4 inch diameter, and covered with prickles. Leaves are alternate, compound and evergreen, and flowers are white or pink, up to 1 inch across. Remove or kill the root crowns and roots, and plant native seeds or vegetation to reoccupy the site. Safe application of herbicide is another alternative.
This large, arching woody shrub is 10-15 feet tall, with large lance-shaped leaves and large pyramidal-shaped clusters of purple flowers that resemble lilac. Butterfly bush is difficult to control, but if there are only a few small plants, pulling and digging are effective. For larger or more numerous plants, consult your local OSU Extension Service office.
This prolific invader reproduces from seed and root, crowding out other vegetation and creating a monoculture. It's an easy plant to spot growing along Oregon roads and highways, but for those unacquainted with it, it features dark green angled stems (somewhat star-shaped in cross section) with pea-like pale yellow, bright yellow or maroon red flowers from March to June. Pulling them out by hand can be effective with smaller plants; it can be difficult to get all the roots. Safe use of herbicides is an option.
It looks harmless but this plant is very aggressive and hard to eradicate. I know, because a plant showed up in my yard one year, has taken repeated seasons of pulling, and still shows up again in the spring. This is a low-growing perennial that spreads. Leaves are divided into 3 lobes, which are deeply cleft, and flowers are composed of five to ten bright shiny yellow petals up to an inch in diameter with several yellow stamens in the middle. Your best bet is to dig it out with a sharp trowel or fork-type tool; try to remove all of the runners. But beware, incomplete digging may increase creeping buttercup because it can sprout from nodes along stem and root fragments. Try to pull up any seedlings before they flower or establish runners.
This woody vine with lobed, dark green leaves is an aggressive, invasive, introduced species with no natural controls. Ivy transforms natural areas into monocultures which do not provide habitat for indigenous wildlife. It's been popular with gardeners in the past because it grows quickly, suppresses other plants and doesn't need much to grow. However, these very qualities are major reasons for its devastating impact. Control methods include hand pulling, cutting, and safe application of herbicides.
Canada Thistle, Bull Thistle
These large, erect plants with prickly foliage and flower heads of purple, pink or white are not shade tolerant and can be found on a wide variety of sites, including home gardens. Hand pull using thick gloves, and control small patches before they spread. Reseed disturbed areas immediately with desired species.
An erect biennial or short-lived perennial, Tansy Ragwort is toxic to cattle and horses. It grows 1 1/2 to 6' tall, with dark green, deeply lobed leaves and yellow, multi-rayed flowers. Hand pulling prior to flowering is effective. There are several effective biological controls, including cinnabar moth. Check the Oregon Department of Agriculture's website for more information.
Good plant choices: some alternatives to consider
There are literally hundreds of invasive plants that could show up in your local park or garden anytime. If you've already found invasive plants on your property, remove them and wait until they're dead to add them to your mulch pile.
Then, replace with some native or non-invasive varieties.
Instead of scotch broom, try Tall Oregon grape or Forsythia. As a replacement for Butterfly bush, why not plant a native like Blue blossom, or an attractive Weigela variety? Dogwood or willow could make a nice replacement for knotweed. Salmonberry is a good native plant substitute for Himalayan blackberry.
A word about firewood and invasive pests
Invasive species councils in Oregon and Washington strongly recommend not moving firewood long distances, as it can harbor invasive insects and diseases that can infest and kill native trees. Firewood-related pests in the Pacific Northwest include Emerald ash borer, gypsy moth and Asian longhorned beetle. They can't move far on their own, but when people move firewood they can jump hundreds of miles. House Bill 2122 was passed in 2011 and prohibits transport or sale of firewood in Oregon unless the wood is from Oregon, Idaho or Washington or has been heat-treated to kill insects.
To learn more
We're only just scratching the surface here when it comes to identifying invasive plants and pests found in our yards and communities and ways to control or eradicate them.
For more information, please visit Oregon's Invasive Species Council website, at http://www.oregon.gov/OISC. If you live in the Portland-metro area, you might like to learn more about the work of the No Ivy League: www.portlandonline.com/parks/index.cfm?c=47820
Cynthia Orlando has a degree in forest management and is a certified arborist with the Oregon Department of Forestry.