"Dioecious" and "monoecious" are terms that refer to plant reproduction in horticultural descriptions. "Dioecious" describes a plant group in which individual plants have either male and female parts, but not both. "Monoecious" describes the group species in which each plant bears both male and female flowers. Yet another category is the group in which each flower contains both male and female parts, known as "bisexual" or "hermaphroditic" species.
The Prefix Tells the Story
An easy way to remember the meanings of dioecious and monoecious is to look to the Greek prefixes di, which means two, and mono, which means one. With dioecious species, each plant is either a male or female member. Some plants of the species have only male reproductive organs, or stamens, while other plants of the species have only female reproductive parts or pistils.
With monoecious species, each plant has some flowers with stamens and other flowers with pistils—both types of flowers are found on the same plant.
Finally, in the hermaphroditic or bisexual group, each flower is identical, containing both male stamens and female pistils.
When the plants are dioecious, you must have at least one corresponding male plant growing in or around your landscaping for the fruit-bearing female plants to be pollinated. For example, holly shrubs (Ilex) are dioecious plants. To get good berry production from a 'Blue Princess' holly shrub, therefore, you need to supply a nearby male cultivar to do the pollinating.
This naturally raises the question of how to tell the genders apart. Quality garden centers clearly label their dioecious plants so you know whether you're buying a female or a male cultivar. However, you can learn to tell a male holly apart from a female holly (or any other dioecious plant) on your own. Look for stamens, usually loaded with pollen, that indicate a male plant.
Unisexual Flowers vs. Bisexual Flowers
Dioecious and monoecious plants have something in common, in that they both bear unisexual flowers with only male or female reproductive parts. If the plant is dioecious, the male and female blossoms appear on separate plants. If the plant is monoecious, each plant has both male and female flowers.
A third category of plants have blooms that are bisexual or hermaphroditic, with each flower containing both male and female parts. The process of reproduction occurs right within the individual flowers. These are sometimes referred to as "perfect" flowers because the pollination process can potentially be self-contained within a single flower. An example of a plant with perfect flowers is the lily (Lilium). By some definitions, hermaphroditic plants with perfect flowers are considered to be part of the monoecious group.
Many, but not all hermaphroditic, bisexual flowers are self-pollinating, but others require cross-pollination from hermaphroditic flowers on other plants.
Short List of Dioecious Plants
Most of the landscape plants home gardeners grow are either monoecious (both male and female flowers on the same plant) or bisexual, with each flower containing both male and female parts), so you don't have to give them much thought in terms of reproduction. With dioecious plants, on the other hand, you must have both male and female plants to ensure the females that will bear berries or seeds. Only about 5 percent of all plants fall into this category. In addition to holly, here are some common dioecious plants you might want in your landscape:
- American bittersweet vine (Celastrus scandens)
- Aspen tree (Populus tremuloides)
- Bayberry shrub (Myrica pensylvanica)
- Ginkgo biloba tree (scientific and common name are the same)
- Juniper shrub (Juniperus)
- Ornamental kiwi vine (Actinidia kolomikta)
- Spinach (Spinacia oleracea)
- Pussy willow bush (Salix discolor)
- Swamp tupelo tree (Nyssa sylvatica)
- Asparagus (Asparagus officinalis)
- White ash tree (Fraxinus americana)
- Yew shrub (Taxus spp.)
Short List of Monoecious Plants
Other garden plants and landscape trees and shrubs are monoecious, containing separate male and female flowers on the same plant, making them self-pollinating. The group of plants that have distinctly separate male and female flowers on the same plant is thought to comprise only about 5 percent of all plants. A small sampling of such plants includes:
- Birch (Betula spp.)
- Oak (Quercus spp.)
- Spruce (Picea spp.)
- Pine (Pinus spp.)
- Squash (Cubito pepo)
- Walnut (Juglans spp.)
- Hazel (Corylus avellana)
- Cucumber (Cucumis sativus)
- Corn (Zea mays)
- Pumpkins (Curbita spp)
- Watermelon (Citrillus lunatus)
- Fig (Ficus spp.)
Short List of Hermaphroditic, Bisexual Plants
Hermaphroditic plants with perfect flowers (containing both male and female parts) can be considered a specific subset of the monoecious group. This is a very large group, including most native trees and shrubs as well as most vegetables not found on the dioecious and monoecious lists. By some estimates, fully 90 percent of all plants fall into this category. These plants can potentially self-pollinate, but the possible problems posed by continuous inbreeding are often avoided because these species have evolved so that male and female parts mature at different times. Thus, many bisexual plants cross-pollinate rather than self-pollinate.
A small list of common examples of plants with hermaphroditic, bisexual flowers include:
- Apple trees (Malus spp.)
- Pear trees (Pyrus spp..)
- Cherry trees (Prunus spp.)
- Roses (Rosa spp. )
- Olive trees (Olea europaea)
- English daisies (Bellis perennis)
- Tomatoes (Solanum lycopersicum)
- Bell pepper (Capsicum annuum)
- Beans (Phaseolus spp.)
- Peas (Pisum sativum)
- Cabbages (Brassicacaea)
- Strawberries (Fragaria x ananassa)
Botanical Terminology: Flowers, Houses, and Sexual Reproduction. Iowa State University Extension and Outreach.