by Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor,
Puyallup Research and Extension Center, Washington State University
The Myth: “Garden plants do not become invasive.”
We’re all familiar with weeds in our landscapes: Calystegia sepium (hedge bindweed), Equisetum arvense (horsetail), Taraxacum officinale (dandelion), and Cirsium arvense and C. vulgare (Canadian and bull thistle) are but a few of the weeds we battle in Pacific Northwest gardens. Larger herbaceous and woody perennials such as Hedera helix (English ivy), Ulex europaeus (gorse), Cytisus scoparius (Scots broom), Rubus discolor (Himalayan blackberry), and Polygonum cuspidatum (Japanese knotweed) are ubiquitous in parks and along roadsides. These species have cost countless hours of labor and gallons of herbicide in our quest to restore impacted landscapes to a more natural and diverse state.
Of course we avoid this scenario in our own home landscapes; few of us deliberately purchase weeds. Instead, as buyers or sellers we choose ornamental plants that are both attractive and tolerant of hostile environmental conditions. We desire plants that establish quickly, flower profusely, and spread so densely that weeds can’t penetrate. These characteristics allow us to minimize labor and use of pesticides, creating what appears to be a sustainable landscape. Isn’t this a desirable outcome?
Not all of our beloved garden plants behave themselves. The adjectives that epitomize some of our favorite choices – like “fast-spreading”, “self-sowing”, and “tolerates poor soil” – are also indicators of potential invasiveness. Some herbaceous weeds, and all of the woody ones listed above, were deliberately introduced to this country. Look at the common and Latin names; many of them indicate the country of origin. At one time they were seen as beneficial additions to our landscape in terms of ground cover (ivy), natural fences (gorse and Japanese knotweed), attractive flowers (Scots broom) or abundant fruit production (Himalayan blackberry). Unfortunately, these exotic visitors wore out their welcome by leaving the confines of the garden and spreading to other landscapes, particularly sensitive natural areas. In our remnant natural areas, these invaders quickly establish and eliminate many of the native flora, indirectly eliminating wildlife that depends on a diversity of plant life.
To address this issue on a national scale, individuals from diverse fields have collaborated to draft voluntary Codes of Conduct* for government agencies, nursery professionals, landscape architects, botanic gardens and arboreta, and the gardening public. This effort is being supported by a broad array of organizations representing the nursery industry (American Nursery & Landscape Association), botanical gardens (American Association of Botanical Gardens and Arboreta), landscape architects (American Society of Landscape Architects), scientists (USDA Agricultural Research Service), and conservationists (Center for Plant Conservation). Through self-governance and self-regulation, these codes would encourage behaviors that detect and prevent future introductions of invasive ornamental species to North America.
The Bottom Line
Here are some suggested ways that nursery professionals and the gardening public could participate in
regional and national efforts to reduce the effect of invasive plants:
• Phase out existing stocks of regionally invasive species
• Purchase and promote non-invasive, environmentally safe plant species
• Remove invasive species from your land and replace them with non-invasive species suited to
site conditions and usage
• Work with neighbors or volunteer at botanical gardens and natural areas to eliminate
populations of invasive plants
• Visit the Center for Plant Conservation to learn more about preventing plant invasions.
For more of Dr. Chalker-Scott’s series on Horticultural Myths, visit The Informed Gardener.
Image of Buddleia davidii courtesy of the Washington State Noxious Weed Control Board.
*Thanks to Dr. Sarah Reichard (firstname.lastname@example.org)
for information on the voluntary Codes of Conduct.