Poisonous Plants Found in Yards, Gardens, and Nearby

A number of plants commonly found in urban areas are poisonous. Some are naturally occurring, some are cultivated. Two of the more common and potentially deadly plants are the water hemlock and its close relative, the poison hemlock. Both plants can grow up to eight feet tall, and have small white flowers that grow in clusters called umbels, meaning the flower stems come together at one point like the ribs of an umbrella. The flowers are similar in appearance to dill. The leaves are fern-like and the stems are hollow. All parts of these plants are highly poisonous. Several human deaths occur each year when water hemlock or poison hemlock plants are mistaken for the edible cow parsnip. To control hemlock, dig the taproot or cut the plant back at ground level, taking care not to touch
your mouth when handling the plant. Water hemlock is a perennial, meaning it grows from rootstock for several years. Control of this plant can be achieved more easily using the herbicide glyphosate. Glyphosate is relatively safe for humans and breaks down quickly in the environment. Remember, though, that it will kill any plant it touches, including desirable plants and trees, so apply it very carefully. 

Another fairly common poisonous plant in Montana is the silverleaf nightshade. This plant is also a perennial, and is vinelike with trailing or climbing stems up to ten feet long. It has star-shaped purple flowers with five petals that turn backward and a yellow center. The blossoms are about one inch in diameter. Berries are formed that look like small tomatoes, first green, then orange or red. Control of the nightshades can often be achieved by hoeing or digging because usually only a few plants are present on a site. If herbicide applications are needed, again, a glyphosate is a good choice, if applied carefully. 

Domestic yard and garden plants that can be toxic include the marsh marigold, chokecherry leaves and pits, lily of the valley, daffodils, iris roots, and autumn crocus. These plants are not deadly, simply take care when handling them and be aware of the potential toxicity. 

References: 

Pacific Northwest Weed Control Handbooks. 1997. Extension Services of Oregon State University. Corvallis, OR 97331-0817. 

Thomson, W.T. 1996. A Guide to Lawn, Garden, and Home Pest Control Products. Thomson Pubs., Fresno, CA 93791. 121 pp. 

Weed Control Guide. 1997. Meister Pub. Co. Willoughby, OH 44094-5992. 538 pp. 

Whitson, T., et al. 1991. Weeds of the West. Pioneer Printing, Jackson Wyoming. 630 pp. 

Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997