"Dioecious" and "monoecious" are terms that refer to plant reproduction. 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 I explain 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 (staminate flowers) 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 (pistillate flowers). 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 are needed for reproduction, in this case) -- and that is how you can remember the meaning of the word.
What does that mean in practical terms? It means that you must have at least one 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 are an example of such plants. To get the kind of berry production from a 'Blue Princess' holly that you see in my photo, you need to supply a male cultivar to do the pollinating. This naturally raises the question of how to tell the genders apart, which I answer in my FAQ, How Can I Tell a Male Holly Apart From a Female Holly Shrub?
One Is the Loneliest Number
By contrast, you do not have to worry about 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, although, frankly, I do not see the fun in doing it that way.
Another Distinction: Plants With Unisexual Flowers vs. Plants With Bisexual Flowers
I have highlighted the difference between dioecious and monoecious plants above, but these two classes actually have something in common, botanically, too: namely, the flowers 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. The reproduction occurs within the individual flowers. 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 self-contained -- not just within the scope of a single plant, but within a single flower.
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 flower. 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 (which I have restricted to plants that I cover in detail on this website; click the links to learn more about them):
- American bittersweet vines
- Aspen trees
- Bayberry shrubs
- Ginkgo biloba trees (the males are less messy)
- Holly shrubs
- Juniper shrubs
- Kiwi vines
- Mulberry trees
- Pussy willow bushes
- Swamp tupelo trees
- Sweetfern shrubs
- White ash trees (do not grow the males if you are an allergy-sufferer)
- Yew shrubs