Where it is common in central North America, hemp pollen has been reported to cause severe pollinosis in mid-summer to late-summer.
This annual course herb has large palmately divided leaves (with parts diverging from a common base, like the fingers of a hand) with 5-9 leaflets. The plant can grow to be 12 feet tall. Plants are topped by greenish female flowers, or long spikes of male flowers on separate plants. This plant is native to Asia but has been cultivated elsewhere for fiber to make rope, paper, clothing and other products. Favorite habitats are moist fields, pastures, roadsides, and waste places. This weed is fairly common in the wild. Flowering occurs usually from June to August in northcentral locations, and September to October in northeast area. It dispenses large amounts of pollen and is wind pollinated. Illegal uses of this plant are to produce the drug marijuana. The types of cannabis used for rope-making and as an illegal drug are both members of the same species, but different varieties.
The pollen grains are oblate spheroidal; the amb rounded triangular to circular with 2-4 pores. The sexine is thin and granular and somewhat thickened at the pores.
Grains are typically 23-28 micrometers.
The shaded areas on the map indicates where the genus has been observed in the United States.
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