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.