Ash trees (Fraxinus spp.) often go unnoticed in the landscape, although many ash species decorate the urban, suburban and natural landscapes of the United States. Several species are native to North America and grow in U.S. Department of Agriculture plant hardiness zones 3 through 11, although great variances occur depending on the ash species. In general, these graceful trees are either male or female, although some ash trees have flowers of both sex while others will produce female flowers on certain branches one year and male flowers the next. Therefore, distinguishing between male and female trees is sometimes difficult, although there are a few tell-tale signs.
Look at the tips of branches or twigs in early spring just after the buds have opened and before the leaves emerge. Male ash flowers are smaller than female flowers and appear as small, spiny tufts on either side of the tip of the twig. Female flowers are angled backward, similar to open wings, from the tip of the twig. Flowers of both sexes are small and inconspicuous, but they are still visible if you know when to look. They are greenish in color and have small, soft, purple spikes -- remnants from the reddish-purple buds -- emerging along the edges.
Look for the appearance of seeds, known as keys or samaras, in winter when the leaves are gone. The seeds remain on the tree throughout winter and then release in spring. The seeds are very similar to those of maples and are affectionately known as helicopters to anyone who has had the pleasure of seeing them twirl in the wind to their final resting spot. If your ash has seeds, it's a female, not a male, or it is a tree of both sexes.
Look for the presence of ash galls, small galls which form due to infestations of a tiny eriophyid mite, Eriophyes fraxinivorus. These small mites feed only on male ash flowers. Galls are the result of the feeding; the ash tree begins to grow tissue around the feeding mites, which eventually turns from green to brown. The galls resemble green, 1/2- to 1-inch tumorous growths on the flowers at the beginning stages. The galls remain on the tree for up to two years and eventually fall off. An ash in full leaf may hide the galls well; galls are easiest to spot after the ash drops its leaves in autumn.