"Dioecious" and "monoecious" are terms that refer to plant reproduction. They are adjectives used in horticultural descriptions. The former means that the plant group so described is actually made up of distinct male and female plants. By contrast, the latter term is applied when a single plant bears both male and female flowers.
It is easy to remember the difference between these two words if you recognize the Greek prefixes of the respective terms, namely, di- and mono-, as is explained below.
The pronunciation for the two words is dahy-EE-shuhs and muh-Nee-shuhs.
It's All Greek to Me: the Prefixes Tell the Story
Let's say the male reproductive organs (stamens) of a plant species appear on certain plants of that species, but not on others. Instead, those others (and only those) bear the corresponding female reproductive parts (pistils). In other words, the plant species in question has distinct male and female members. The plants in that species are said, then, to be "dioecious." The di- prefix indicates "two" in Greek (as in two separate plants being needed for reproduction, in this case). And that is the easiest way to remember the meaning of the word.
What does that mean in practical terms? It means that you must have at least one corresponding male plant growing in or around your landscaping for the fruit-bearing female plants to be pollinated, when the plants in question are dioecious plants.
Holly shrubs (Ilex) are an example of such plants. To get good berry production from a 'Blue Princess' holly shrub, for example, 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 will clearly label the plants, so that, when you buy them, you will not be confused as to whether you are buying a female or a male cultivar.
Still, it is better to be able to tell a male holly apart from a female holly on your own, just in case the garden center where you shop makes a mistake in labeling.
One Is the Loneliest Number
But you do not have to be concerned with providing a partner in the case of "monoecious" plants. They bear both male and female flowers on the same plant. The mono- prefix indicates "one" in Greek, which is a simple way to remember the difference between "dioecious" and "monoecious" plants. A monoecious plant can reproduce (that is, bloom and set seed) all on its own. It does not need a partner. This makes life much easier (and less expensive) for gardeners wishing to enjoy berry production on a bush. For example, if you are buying a red-twig dogwood shrub (Cornus alba) for its berries, then you only have to buy just one plant. You do not have to worry about selecting a female plant, then finding and buying a male pollinator, in addition, to partner up with her.
Another Distinction: Plants With Unisexual Flowers vs. Plants With Bisexual Flowers
You have learned the difference between dioecious and monoecious plants above, but these two classes actually have something in common, botanically, too: namely, the flowers that they bear are considered "unisexual" flowers.
The difference is that, with the dioecious type, those blossoms appear on separate plants, whereas they appear on the same plant in the case of the monoecious type. Nevertheless, either way, we are talking about flowers of one distinct sex that exist separately from flowers of the opposite sex.
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 types of landscape plants that we grow either are monoecious or produce bisexual flowers. This is convenient for us growers, since we do not have to worry about supplying a pollinator. As a result, there is no need to compile a list of such plants. Dioecious plants, however, are another matter: It requires more work on our part to grow females that will bear berries or seeds. And before we can even take that step, we must become aware that we are, in fact, dealing with a plant that needs a mate. Thus the need for the short list of dioecious plants that follows (in addition to the holly already mentioned):
- American bittersweet vines (Celastrus scandens)
- Aspen trees (Populus tremuloides)
- Bayberry shrubs (Myrica pensylvanica)
- Ginkgo biloba trees (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)