Hemp nettle is native to Europe and northwestern Asia but has been introduced into North American and is a problem in the cool areas of the United States. It is considered invasive in Wisconsin. An annual in the mint family, it grows well in disturbed areas such as fields and waste areas and can diminish crop production when it becomes established in cultivated fields or home vegetable gardens.
Branching stems are square in cross section and grow 1-3 feet tall from a densely branched root system. They are hairy and swollen just below the point where the leaves are attached. The leaves are opposite, simple, toothed and hairy. The two-lipped flowers are produced in terminal spikes in summer. They are ½ inch across and may be white, pink or lavender. The seeds ripen in the fall and are the only means of reproduction.
The best control is to make sure seeds do not form by shearing off the tops of the plants as they flower. Young seedlings are easy to hand pull but large plants are difficult because of the extensive root system. Since the plant is an annual and reproduces only by seed, removing the flowers before seed production will eliminate it. In extreme cases non-selective herbicides like Round-up or selective broadleaf herbicides can be used.