The terms "male" and "female," though used to describe trees, fit as awkwardly as someone else's shoes. Although the flowers of trees need to be fertilized with pollen to produce fruit, the geography of plant sexuality is less rigid than that traditionally described in humans.
GIrl Flowers and Boy Flowers
The flowers that trees bear either function as female or male. Female flowers contain ovaries that develop into fruit while male flowers bear pollen that fertilizes the female flowers. Some trees bear flowers of only one sex; those trees are sometimes called male or female. Many trees, however, bear flowers of both sexes. The terms used to describe trees are "dioecious," which refers to a tree that has either male or female flowers, and "monoecious," which describes a tree that has both male and female flowers.
If a kind of tree is dioecious, then distinguishing a male tree from a female tree is a matter of watching the trees carefully. Male trees have male flowers, which produce pollen. Female trees have female flowers that produce fruit. Many kinds of backyard trees are dioecious; they include white ash (Fraxinus americana), which is hardy in U.S. Department of Agriculture plant hardiness zones 3 through 9, ginkgo (Ginkgo biloba, USDA zones 4 through 9), and boxelder (Acer negundo, USDA zones 2 through 10). If you are allergic to pollen and a tree makes you sneeze, it is a male.
Secondary Sexual Features
Once you have established that a tree species is dioecious, look for other distinctions between males and females of the species. For example, the common persimmon tree (Diospyros virginiana, USDA zones 6 through 10) is dioecious; male persimmon trees have small flowers in clusters while females have large, single blossoms. The tips of male flowers on the cedar of Lebanon (Cedrus libani, USDA zones 6 through 9) look dirty during pollen season. Researchers at Weill Cornell Medical College in Qatar learned to identify male and female date trees by their genetic differences, which is extremely useful because date trees are difficult to tell apart before they fruit. The pygmy date palm tree (Phoenix roebelenii) is hardy in USDA zones 10 through 11.
Eastern white pine (Pinus strobus, USDA zones 3 through 8) is an example of a monoecious tree. A pine tree can produce pine cones without another member of its species nearby. It is not optimal for a tree to fertilize itself, though. So monoecious trees have different methods of discouraging self-fertilization. Those methods include physical separation, with the female flowers on top of the trees and males below.
Tree sexual reproduction is much more nuanced, however. Some trees have flowers that contain both male and female parts, and some flowers have sexual parts that don't function or that function only occasionally. Likewise, some trees change sex from season to season while others change once or twice during their lifetime. In short, tree gender is a complex and ever-evolving phenomenon that biologists continue to study.