On Gardening: Noxious plants to avoid in the garden

By Linda Stephens-Urbaniak

As gardeners, we look to our gardens as not only a place for rest but also a place to satisfy the quest for discovery. Almost every passionate gardener has lusted after a plant that has captured our fancy. The downside to this quest is the introduction of plants that not only thrive here, but thrive too well.

The introduction of exotic plants can have disastrous results. For instance, ivy and holly are taking over our gardens and our forests. Purple loosestrife and the lovely but invasive yellow iris, Iris pseudacorus, crowd out native marsh dwellers. These invasive species and many others have been introduced intentionally or accidentally in wildflower seed mixes or as garden plants.

King County has developed, in coordination with Washington state, a list of noxious weeds that is adjusted each year. The definition of a noxious weed by the Washington State Noxious Weed Board is a plant that is "non-native, highly destructive, invasive, competitive and difficult to control or eliminate." The county list is divided into Class A, Class B and Class C weeds. It also has a Noxious Weeds of Concern list. If a plant in your yard is found to be on the A list, eradication is required by Washington state law. If you fail to do so, the county may do so or contract for their control at your expense.

Included on the A list are garden favorites of the past like Galega officinalis (goatsrue), Heracleum mantegazzianum (giant hogweed), Isatis tinctoria (dyer's woad) and Salvia sclarea (clary sage) as well as others like Alliaria petiolata (garlic mustard) and Pueraria montana var. lobata, known as the invasive weed kudzu. Washington law states that: "It is prohibited to transport, buy, sell or offer for sale, or distribute all plants or plant parts, seeds in packets, blends or wildflower mixes' of any of the plants on the A list, and many on the B list."

Class B weeds are required by law to be controlled and contained. They should not be purchased or planted, and if they are on your property ,you should take steps to eradicate them. State law requires that they must be prevented from going to seed, so they must be dead-headed (removal of flowers and/or seed heads). The most common garden favorites of the past are Buddleia davidii (butterfly bush), Cytisus scoparius (Scotch broom), Impatiens glandulifera (wild impatiens or policeman's helmet) Lysimachia vulgaris (garden loosestrife) and Lythrum salicaria (purple loosestrife).

The Class C weeds also require control and containment and include Artemisia absinthium (wormwood), Hedera helix and Hedra hibernica (ivy, several cultivars) Hypericum perforatum and common St. Johnswort.

Then there are the Noxious Weeds of Concern. Favorites such as Geranium robertianum (Herb Robert or Stinky Bob), Polygonum (knotweed, several species) all have been introduced as desirable plants only to later reveal their dark side as they have invaded natural areas.

Knowing that it would take millions of dollars to remove ivy from our parks indicates that it would be expensive and difficult to remove in our gardens and common sense dictates that ivy, or any of these that are allowed but of concern, will be hard to control if planted. There are many other desirable plants that can be chosen to duplicate their performance in the garden.

There are other non-native weeds that aren't on the State list but that the King County Weed Board recognizes as being invasive. These "obnoxious" weeds include Ilex aquifolium (English holly), Prunus laurocerasus (English Laurel) Rubus discolor (Himalayan blackberry), Rubus laciniatus (evergreen blackberry) and Solanum dulcamara (bitter nightshade). These are plants to avoid when planning new gardens.

For more information, contact the State Weed Board at or King County at

We encourage an open exchange of ideas on this story's topic, but we ask you to follow our guidelines for respecting community standards. Personal attacks, inappropriate language, and off-topic comments may be removed, and comment privileges revoked, per our Terms of Use. Please see our FAQ if you have questions or concerns about using Facebook to comment.
blog comments powered by Disqus

Read the Oct 26
Green Edition

Browse the print edition page by page, including stories and ads.

Browse the archives.

Friends to Follow

View All Updates