All species are wind-pollinated and shed large quantities of pollen. Cross-reactivity to ragweed is known. Iva is a significant allergenic plant, particularly in the southern U.S.
Iva are annuals or tall perennials. Eleven species are native to North America. Plants grow 1 to 6.5 feet tall in wet conditions along the Atlantic and Gulf Coastal states and in the western U.S. One species, burweed marshelder (I. Xanthifolia) looks similar to giant ragweed, but the leaves have no deep lobes. Another species, rough marshelder (I. ciliata) is similar in height, but has longer flowering structures with a rough appearance. Dune and coastal marshelders are native to coastal areas in the east and southeast US. Greenish flowers occur at the top of the stem and in small, upper leaf-like structures. Flowering occurs from June to October.
Pollen grains of Iva are oblate-spheroidal to prolate-spheroidal; the amb triangular and 3-4 colporate. The sexine is thick, tectate and covered with short spines. They are of the Dicoria type.
The pollen grains are 18-30 microns in diameter, depending on the species.
The shaded areas on the map indicates where the genus has been observed in the United States.
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