The concept of a tree being strictly “male” or “female” is a simple way to describe a complex biological reality, but it only applies to a minority of tree species. Plant reproductive strategies vary widely, ranging from individuals that function as a single sex to those that possess both sexual structures simultaneously. Understanding these systems requires observing where the pollen-producing organs (stamens) and the seed-producing organs (pistils) are located on the plant. This arrangement determines how a tree reproduces and has implications for horticulture and the urban environment.
Trees with Separate Sexes
A small group of tree species maintains a strict separation of the sexes, meaning an individual plant is either entirely male or entirely female for its entire life. Botanists refer to this arrangement as dioecious, a term derived from Greek meaning “two houses.” This reproductive strategy promotes outcrossing, which is the fertilization of one plant by another, ensuring greater genetic diversity in the offspring.
The male tree generates pollen, and it will never produce fruit or seed. Conversely, the female tree produces the ovules that develop into seeds and fruit after successful pollination, but it cannot produce pollen. This distinction is seen in species like Holly, where only the female plants develop the familiar red berries, provided a nearby male plant supplies the necessary pollen.
A well-known example is the Ginkgo, where the sex of the tree carries a major practical consequence. Female Ginkgo trees produce a soft, fruit-like structure that, when ripe, emits a highly unpleasant, rancid odor due to the butyric acid it contains. Male Ginkgo trees, which produce only pollen cones and no messy fruit, are often preferred and intentionally planted in urban landscapes to avoid this nuisance. Other species exhibiting this separate-sex system include Date Palms, Willows, and Eastern Redcedar.
Trees with Combined Sexes
The majority of tree species do not maintain separate sexes, instead carrying both male and female reproductive structures on a single individual. This strategy is divided into two categories, both representing a more self-contained reproductive system. The first, known as monoecious, describes trees that bear distinct male flowers and distinct female flowers on the same plant.
In monoecious species, the male flowers typically appear as catkins or cones that house the pollen, while the female flowers are separate structures that contain the ovules. This reproductive separation on the same plant is common in many familiar trees, including Oaks, Pines, and Walnuts. In some monoecious species, such as Corn, the male flowers are located higher on the plant (the tassel) than the female flowers (the ear), a positioning that helps reduce the chances of self-pollination.
The second category is the hermaphroditic, or bisexual, tree, which is characterized by having flowers that contain both male and female parts within a single bloom. These flowers are called “perfect” flowers because they possess both the stamen (pollen producer) and the pistil (ovule receiver) together. Many common fruit trees, such as Apple, Cherry, and Pear, employ this system, with the flower structures facilitating self-pollination.
Identifying Tree Sex and Its Practical Importance
A layperson can determine a tree’s reproductive status by observing its mature structures, particularly the presence or absence of fruit and seeds. If a tree consistently produces fruit or seeds, it is a female of a dioecious species, or it belongs to a monoecious or hermaphroditic species. However, if a mature tree never produces fruit, it is likely a male of a dioecious species, such as a male Holly or Ginkgo.
The practical knowledge of a tree’s sex is important for successful horticulture and urban planning. For dioecious species like the American Holly, a grower must ensure both a male and a female plant are in relatively close proximity to guarantee the female plant produces the characteristic berries. Without the pollen from a male counterpart, the female plant will not set fruit.
A second consideration relates to public health, particularly for allergy sufferers. Male trees are the sole source of allergenic pollen, producing vast quantities of light, wind-borne particles necessary for reproduction. A historical preference for planting male-only clones in city parks and streets—chosen because they do not drop messy fruit—has inadvertently contributed to higher seasonal pollen counts in many urban areas.