"Dioecious" and "monoecious" are terms that refer to plant reproduction. They are adjectives used in horticultural descriptions. Dioecious describes a plant group that includes distinct male and female plants. Monoecious describes a single plant that bears both male and female flowers. The pronunciation for the two words is dahy-EE-shuhs and muh-NEE-shuhs.
The Prefixes Tell 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, some plants of the species have only the male reproductive organs, or stamens, while other plants of the species have only female reproductive parts, or pistils. In other words, the plant species in question has distinct male and female members. With monoecious species, each plant has some flowers with stamens and some flowers with pistils.
When the plants in question are dioecious plants, 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, you need to supply a 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. Still, it is better to be able to tell a male holly apart from a female holly (or any other dioecious plant) on your own, just in case the garden center where you shop makes a mistake in labeling or if you have some existing plants on your property that should bear fruit but don't.
A monoecious plant can reproduce (that is, bloom and set seed) all on its own and does not need a partner. For example, if you are buying a red-twig dogwood shrub (Cornus alba) for its berries, then you only have to buy one plant.
Unisexual Flowers vs. Bisexual Flowers
Dioecious and monoecious plants actually have something in common, in that they both bear unisexual flowers. This means that each bloom has only male or female reproductive parts. With dioecious plants, the male and female blossoms appear on separate plants. With monoecious plants, each plant has both male and female flowers.
By contrast, some plants have blooms that are bisexual. Each individual bloom has both male and female parts. The process of reproduction occurs right within the individual flowers. These are sometimes referred to as "perfect" flowers, because they are self-sufficient. In a sense, we can say that this kind of plant takes the independence of the monoecious plant one step further. The pollination process is extremely self-contained. It is not just contained within the scope of a single plant, but within a single flower. An example of a plant with perfect flowers is the lily (Lilium).
Short List of Dioecious Plants
Most of the landscape plants home gardeners grow either are monoecious or they produce bisexual flowers, so you don't have to give them much thought in terms of reproduction. With dioecious plants, you have to know what you're dealing with to ensure the females that will bear berries or seeds. In addition to holly, here are some common dioecious plants you might want in your landscape:
- American bittersweet vines (Celastrus scandens)
- Aspen trees (Populus tremuloides)
- Bayberry shrubs (Myrica pensylvanica)
- Ginkgo biloba trees (scientific and common name are the same; the males are less messy)
- Juniper shrubs (Juniperus)
- Ornamental kiwi vines (Actinidia kolomikta)
- Mulberry trees (Morus alba)
- Pussy willow bushes (Salix discolor)
- Swamp tupelo trees (Nyssa sylvatica)
- Sweetfern shrubs (Comptonia peregrina)
- White ash trees (Fraxinus americana; do not grow the males if you are an allergy-sufferer)
- Yew shrubs (Taxus)