Here's the situation: You've had one holly shrub in your yard, and it has never produced berries; you learn, subsequently, that holly shrubs are dioecious and need a mate, so you correctly reason that the lack of berry production must be attributed to one of two possible causes:
- You have a male holly shrub.
- You have a female holly shrub, but its flowers haven't been pollinated.
So now you have to determine what gender your holly shrub is, then go out shopping for either a male holly or a female holly, so that your holly will have a mate for reproduction. Question is, though, without having the crutch of holly berries (a sure indication of the female gender in holly shrubs) to rely on for identification, how can you tell a male holly from a female holly?
The answer lies in the flowers, as male holly flowers are different from female holly flowers (of course, this limits identification to those periods when that particular type of holly comes into bloom).
Both male hollies and female hollies bear flowers that have 4 petals -- so that's no help in identifying them. However, take a closer look at the flowers, specifically, at what protrudes from their centers. Male holly flowers have 4 yellow stamens; each female holly flower has a green ovary (that's the green "bump").
For a picture of male holly flowers, please see my photo (above right). For a picture of female holly flowers, please click "More Images" under the photo, which will open up a mini-photo gallery; the second photo in the gallery shows female holly flowers.