There are approximately 65 ash species (Fraxinus), which are primarily deciduous trees of the north temperate zone. Of the trees in the family Oleaceae, ashes are the only trees with native North American members. All 16 are found in the United States, 7 in Mexico, and 4 in Canada.White ash, F. americana, is found throughout the eastern states from the Mississippi drainage up to Ontario to Nova Scotia. Green (or red) ash, F. pennsylvanica, extends further across the Northern Plains to the Rocky Mountains and Saskatchewan. Oregon ash, F. latifolia, is found from the Pacific Northwest to central California. European ash, F. excelsior, is found throughout mid-Europe.

White ash is a large, moderately long-lived tree (up to 200 years), moderately shade-tolerant. It grows up to 120 feet, with a straight trunk extending into the upper crown. Roots are shallow to moderately deep. Leaves are pinnately opposite, with five to nine leaflets (usually seven), which are 2.5 to 6 inches long, oval to lance-shaped, tapering on both ends. Samara (winged fruit) are dispersed by wind and water, and are single, in comparison to the double fruit of maples. Ashes are valuable timber trees for interiors and furniture, as well as used in landscaping and as street trees. Although relatively free of major insect and disease pests, Aceria fraxinuvorus mites may attack male flowers, causing galls that persist into the winter.

With the exception of F. cuspidata and F. ornus, which have strongly fragrant white flowers similar to privet (Ligustrum spp.), ashes are wind-pollinated. F. americana and F. pennsylvanica have male and female flowers on separate trees. Whereas male trees pollinate every year, female trees flower and fruit every 3 to 5 years. Pollination begins as early as January in California and the southwestern and south-central states, primarily in April in the remainder of the United States, and May in Canada. The importance of ashes in hay fever is variable. Thought to be of questionable risk in Canada, Wodehouse felt they were "notorious for the large amounts of pollen shed" and "a real menace to hay fever sufferers."Other reports supported this view. European ash has either perfect flowers, or male and female flowers on separate trees or the same tree. It may pollinate as early as March, persisting through June in parts of Switzerland and Germany. It is considered a moderate producer of hay fever, and skin test positivity in allergic patients is approximately 11%.

Adapted from: Annals of Allergy, Richard Weber, MD, March 2003.

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