A perennial weed that is perhaps most notably known for the skin irritation that this weed causes when contacted. The hairs and spines on the leaves and stems of this weed release formic acid when touched, which gives a burning or stinging sensation to humans. Stinging nettle is primarily a weed of landscapes, orchards, pastures, and roadsides. This weed reproduces by seed and rhizomes, which are underground stems that are capable or generating new plants. Stinging nettle is found throughout most of the United States. The leaves are arranged oppositely along the stem, occurring on petioles. Leaves are egg-shaped to lanceolate in outline with serrated or toothed leaf margins. Mature leaves are mostly without hairs, except for the long hairs capable of 'stinging' humans that occur on the lower leaf surface. Younger leaves usually have both short hairs and the longer 'stinging' hairs on the upper leaf surfaces. Long, pointed stipules occur in the area between the stems and leaf petioles.
Stinging Nettle: Urtica dioica - Virginia Tech, https://oak.ppws.vt.edu/~flessner/weedguide/urtdi.htm (accessed January 24, 2017).