Dioecious Plants
by Dr. Robert Black, Consumer Horticultural Specialist

Many people do not realize that there are male and female plants in some plant families. Such plants are called dioecious. If we break the word down into its component parts we find that di means two and oecious, from the Greek oikos, means house. Therefore, when this term is used in reference to plants it means that the sexes are in separate houses (plants).

The separation of the sexes in different plants have advantages and disadvantages. One of the advantages is that for species that produce objectionable fruit, as is the case with the ginkgo tree, the planting of only male trees will eliminate this problem. The chief disadvantage of dioecious plants is that it is very difficult to tell which is male or female unless they bear flowers or fruits. Hollies are grown primarily for berries and since only the female produces berries, you would want to be certain you planted the female. However, there must be a male tree near the female or no fruit is produced. Generally, one male tree to ten females is adequate to insure pollination and good set of berries.

Other plants grown in which the male and female flowers are on separate plants include date palm, tree or heaver, some poplars, sago palm, persimmon, spinach and asparagus.

The date palm represents the earliest recognized example of the dioecious condition. As early as 3500 B.C., the farmers of Mesopotamia recognized that there were male and female trees and practiced artificial pollination by bringing clusters of male flowers in contact with female flowers. The methods of bringing about fruit production in the date palm today differ little from those practiced in ancient times. Male trees to provide pollen are grown with female trees, which bear the fruit.

In Arabic lands, the cutting of male trees was practiced when war was being raged between tribes. The opposition would sneak into their enemy's palm grove and destroy the male trees. As a result no fruit would be produced. Since dates were a stable item of the diet, starvation was induced. The reason for removing only the male tree was that only a few were needed for a large palm grove. It did not require as much time or effort to remove the male trees as it would have required to remove the more numerously planted females.

Propagation of dioecious trees is achieved vegetatively by planting offshoots of palms and by cuttings from hollies. The sex of the plant is determined in advance, as the vegetatively propagated plant always reproduced the parent type. Some dioecious plants are grown only from seed, as in the case with asparagus and spinach. Vegetative propagation of asparagus and spinach is not practical and with both the male and female vegetative parts are equally edible.

Many fruit trees have perfect flowers (contain both male and female parts) yet do not set fruit with their own pollen. They require pollen from another variety. Varieties such as these are called "self-unfruitful." Many apple, pear and plum varieties are considered to be "self-unfruitful" and require two or more varieties in the planting. To assure adequate pollination, the different varieties should have the same blooming period.