Spring to Fall
Nettle is considered a moderate source of allergy, both in terms of skin tests and amount of exposure to the pollen in the air.
Nettles are annuals and perennials with upright, fibrous stems from 2-7 feet tall. The leaves are opposite, stipulate, oval to lance-shaped and reach up to 5 inches long and 3 inches wide. Few to many stinging hairs cover the stems, leaf stems and major veins. Greenish male and female flowers with no petals are located in clusters in the leaf axils on the same plant or separate plants. Nettles are found in damp, rich soil along roadsides, waste places, and wooded areas. Stinging nettle (U. dioica), weak nettle (U. chamaedryoides), and dwarf nettle (U. urens) are representative species. Skin contact with stinging nettle hairs releases histamine and acetylcholine that cause welts or inflammation, which is not an allergic reaction. Flowering can occur during winter months to late fall (usually during the summer to autumn period), depending on the climatic region of North America. Pollen is shed as puffs into the air, and superficially resembles fungal spores, for which they may be mistaken. All species appear to be wind-pollinated.
Pollen grains are oblate-spheroidal to spheroidal, and are small with a rounded-triangular amb. The grains are 2-4 porate and the pores are circular.
Pollen grains are usually 12-15 micrometers.
The shaded areas on the map indicates where the genus has been observed in the United States.
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