Yellow-flag iris, recently added to Oregon's noxious weed list. Photo by Nancy Loewenstein, Auburn University, Bugwood.org
CORVALLIS, Ore. - Dreams of a garden filled with banks of bright flowers all spring and summer comfort many gardeners through drab winter months. But in gardening as in life, be careful what you wish for.
The beauty of ornamentals, perennials and shrubs grown for their attractive foliage and flowers, has enticed traveling gardeners throughout history to carry some of them home. And more recently, with modern travel and commerce, plants have been imported and sold in parts of the world where they never grew before.
In their new surroundings, free from their normal predators, competitors, and environmental constraints, some of these lovely new arrivals become unruly and nearly impossible to control, says native plant specialist Linda McMahan of the Oregon State University Extension Service.
A large percentage of plants that are now considered "invasive" were originally introduced for horticulture. A study by a University of Washington investigator found that of 235 non-native woody plants that had escaped cultivation, 82 percent had been introduced for horticultural purposes. The Washington State Noxious Weed Control Board estimates that 50 percent of the plants on the state's noxious weed list are escaped ornamentals.
In fact, McMahan explains, the very qualities that might make some non-native ornamentals attractive for your garden – vigor, the ability to spread rapidly or reseed and survive our summer dry periods – are the very qualities that may allow them to escape and out-compete the native plants where you live. That's when they join the growing list of "invasive ornamentals" and start showing up on state noxious weed lists instead of in gardening catalogs.
"Invasiveness is highly regional," McMahan said. "Plants that are invasive in one area can be very well-behaved in another. There are even differences in Oregon, depending on the region."
One of the most familiar non-native invasive plants in the Pacific Northwest is Scotch broom (Cytisus scoparius). Native to Europe and northern Africa, it was imported as an ornamental some time in the early 20th century. It produces large numbers of hard-coated seeds, which can survive for decades, traveling by wind, water, animals or human traffic. Eventually, the seeds settle and germinate into seedlings, establishing new colonies.
Scotch broom now covers large tracts of open areas and shows no signs of slowing down. Left undisturbed, it can grow into a dense stand more than six feet tall with trunks a few inches in diameter. These evergreen stands crowd out grasses and other native vegetation, destroying habitat for grazing wildlife, such as elk. Miles of highway and square miles of grasslands are now densely packed with its intense yellow blooms in the late spring.
Some invasive plants are still favorites of unwary gardeners, who continue to cultivate them. Even after they are recognized as invasive, it may take years to regulate their sale. And gardeners continue the age-old tradition of trading and sharing their favorite plants with each other.
Other ornamentals that have become invasive include English holly (Ilex aquifolium), and sweet fennel (Foeniculum vulgare). Although they are not on the Oregon noxious weed list, both dwarf periwinkle (Vinca minor) and Verbena bonariensis are both capable of taking over your garden and areas beyond.
Unfortunately, non-native invasive ornamentals continue to be introduced and escape into the wild. One of the most recent arrivals in Oregon is Paterson's curse (Echium plantagineum). The Oregon Department of Agriculture believes it was introduced in a wildflower seed mix in 2003. With blue-gray foliage and pretty blue to pink flowers that bloom all summer, Paterson's curse looks harmless. But it's poisonous to livestock and extremely invasive. Australia, where it is rampant, is losing an estimated $30 million annually because of it. To see pictures of it, go to http://www.oregon.gov/ODA/PLANT/WEEDS/profile_pcurse.shtml.
There are some simple ways all gardeners can be responsible and help combat invasive plants.
First, do some homework. Know what you're buying or accepting from a friend. Look up new plants on the web or in gardening references. Become familiar with your state's noxious weed list. In Oregon you can find it at http://www.oregon.gov/ODA/PLANT/WEEDS/statelist2.shtml.
Don't buy or share plants on these lists. If you have them already in your garden, consider replacing them with a better alternative. Look into crinkle leaf creeper (Rubus calycinoides) as a good replacement for English ivy, especially in sun or part-shade. Try Ceanothus or red flowering current (Ribes sanguineum) as a colorful blooming shrub.
At the very least, control any potentially invasive plants you have. Dead-head them promptly before seeds can form. Keep them from spreading beyond a strictly defined and monitored area. Don't compost them. The seeds of many invasive plants are not killed by composting.
"As we move more toward water-wise gardening, we need to be extremely careful about the new plants we use," McMahan cautions. "Some of our favorites may yet become problematic."
For more information, you can download a booklet that describes over a dozen invasive ornamentals and better alternatives for them at http://www.invasivespeciescoalition.org/GardenPlants/Garden%20Wise.
OSU Extension has many publications on invasive species and weeds in Oregon. Go to our online catalog at: http://extension.oregonstate.edu/catalog/details.php?search=weeds.