You might deck your halls with boughs of homegrown holly, but unless you planned ahead, those boughs could lack red berries. And that leads us to some frank talk about sex.
A holly berry, like any other fruit, is a mature ovary, home for a seed or seeds. Seeds are what stimulate development of any fruit, but seeds themselves can't get started without sex.
Sex for a plant happens when male pollen lands on the female part of a flower, called the stigma, and then grows a pollen tube down the style, which is attached to the stigma, to reach and fertilize an egg. The product of this union is a seed, the development of which induces the surrounding floral part to swell to become a fruit.
WHY HOLLY SEX IS IMPORTANT
Why all this concern with holly's sex life? You probably didn't have similar concerns about this summer's tomatoes; you planted whatever varieties you wanted, and then reaped plenty of swollen ovaries ... er, fruits ... and, incidentally, seeds.
Holly is different because its pollen is borne on flowers that are strictly male and its eggs are contained in flowers that are strictly female. Each tomato flower, in contrast, has both male and female parts, so can take care of itself. Similarly self-sufficient are rose flowers, apple flowers, sunflowers and the flowers of many other plants.
Plants like holly that have single-sex flowers are known botanically as "imperfect" flowers. They also include many nut trees.
HOLLY KEEP THE SEXES SEPARATE
Holly goes one step further, with whole plants being either male or female. Such a situation encourages species diversity by mandating cross-pollination among different plants. Nut trees achieve the same effect with biochemical or physical barriers, or different bloom times that prevent male flowers from pollinating female flowers on the same plant. Even some perfect flowers, such as apple blossoms, have biochemical or physical barriers preventing self-pollination.
So the upshot is that you need an all-male holly tree if you are going to deck your halls with (berried) boughs from your all-female holly tree. A male plant -- all leaves and no berries -- is not as showy as a female, but it only takes one to help a half-dozen or so females bear fruit. The males, like the females, do bear flowers, but neither male nor female holly flowers are worth a second look unless you want to peer closely to determine the sex of the plant.
No need, perhaps, to plant a male holly just to get your female to yield berries. Suitable pollen could conceivably come from wild or neighbors' trees (perhaps a male you kindly offered to plant in your neighbor's yard, heh heh).
Making things even more complicated for those trying to bring about berries, hollies are not all that promiscuous. A few different species supply us with berried boughs -- notably American holly, English holly and Meserve holly -- but generally, each stays faithful to its own species. (An exception is English holly, which can pollinate Meserve holly, a hybrid offspring of the English species.) And some males cannot adequately pollinate some females in the same species because bloom times do not overlap.
If you need a male, breeders have come up with a number of superior varieties, their gender obvious from their names: for example, Blue Prince and Blue Boy Meserve, and Jersey Knight American. These males, as you might guess, are particularly good mates for the varieties named, respectively, Blue Princess, Blue Girl and Jersey Princess.