Urticaceae is a small family in Montana, with only 4 species. Some are annual herbs, others are perennials, then die back each year and sprout up new growth from an underground root network. Any large (3 ft tall or more) nettle encountered is certain to be a form of Urtica dioica (Stinging Nettle). Worldwide, the family consists of about 40 genera and 1000 species, widely distributed but mainly tropical and subtropical.
In Urticaceae, the plant grows as a large main stem, the leaves are in opposite pairs, and the flowers concentrate in clusters from the leaf axils. The flowers do not have petals; male flowers have a 4-lobed calyx and 4 stamens - the female flowers are either 4-lobed or 2-lobed and have a pistil that produces a single seed.
Most species have stinging hairs on the leaves and stems. The hairs contain a skin-reactive poison that can cause severe stinging in the affected area that can last for days.