Weeds are invasive plants with a rapidly spreading formation that overtakes other plants. Some types of weeds produce flowers that gardeners cultivate. Many species of broadleaved weeds have square stems. Most species belonging to the plant family Lamiaceae, or Mint, develop square stems. Square-stem weeds come in a wide assortment of plants grown as perennials, biennials and summer and winter annuals.
Devil's beggarticks (Bidens frondosa) is an annual summer weed indigenous to North America. This Asteraceae family member has tall, reddish, square stems that reach heights of 72 inches. During the fall, devil's beggarticks produces small yellow blossoms with barb-covered seeds. Common hemp nettle (Galeopsis tetrahit) is a member of the Lamiaceae plant family. This square-stemmed summer annual weed grows up to 36 inches high. The light green leaves have a serrated edge and hairy veins. Purple flowers that resemble small snapdragons develop on the common hemp nettle.
Henbit (Lamium amplexicaule) is a common winter annual turf weed from the Mint family. This native of Europe, Asia and Northern Africa is an established plant in eastern regions of the United States. A low-growing plant with green to purple square stems, henbit develops pink to purple spring flowers. Cleavers (Galium aparine) is an herbaceous annual weed that originates from North America, Europe and Asia. This Rubiaceae plant family member has an unkempt appearance with whorling leaves covered in sticky hairs. The square stems of the cleaver plant choke crops with their climbing form.
Wild teasel (Dipsacus fullonum) is a tall biennial weed endemic to Europe, Asia and North Africa; however, it's also well-established in the United States. This 36- to 72-inch plant has a square stem covered in prickles and wide, dark green lance-shaped leaves. The lavender-coloured flowers have cone-shaped and spiny bracts. European stinging nettle (Urtica dioica) is a 2- to 3-foot-tall biennial and perennial herbaceous weed. This member of the Urticaceae plant family has square stems and foliage shrouded by hollow stinging hairs. These hairs emit an acid that sends waves of pain throughout the area of contact. During July through August, the European stinging nettle blooms with green to brown flowers.
Goutweed (Aegopodium podagraria) is a perennial herbaceous weed with a mat-like formation and a medium size, and is used as a ground cover. This square-stemmed plant has 1-inch-long leaves that form from basal crowns. This Apiaceae family plant develops flowers with long stalks during the early summer. American stinging nettle (Urtica dioica ssp. Gracilis) is a perennial species of the Urticaceae plant family with a square stem. This upright herbaceous weed grows from 1-½ to 6 feet in height with sword-shaped, toothed leaves. The small cream-coloured flowers of the American stinging nettle blossom from June through September.