Since the above question is asked so very often and in so many forms, I have concluded that a great deal of confusion exists, and a simple, brief explanation is needed. The following is what I tell someone asking these questions. Since the questioners are usually not scientists, botanists, nor taxonomists, the answers are couched in non-technical terms. In a few cases, the technical term has also been included (in parenthesis).
Hollies have two different kinds of flowers, which can be described as male flowers (providing pollen) and female flowers (which later produce the berries). Unlike many plants, male holly flowers are borne on male trees, and female flowers are carried on female trees. Plants that have this characteristic are said to be “dioecious”. Although hollies are perhaps the best example of this condition, there are a number of other plants that share this distinction, for instance, asparagus, willow, ginko and the white fringe tree.
Here in Zone 6, some hollies begin blooming in March, while others wait for April, May, or even early June. However, their lovely white (sometimes pink, sometimes yellow) flowers are so small as to go undetected by many homeowners who can be quite insistent in stating “my hollies never bloom”! I counter by asking whether they have ever noticed the honeybees swarming around their hollies. Although the aroma given off by holly blossoms is very faint, the bees and other insects are quick to detect it and come to collect the nectar.
Most male holly blossoms unfold four white petals to reveal an equal number of tiny white horns (filaments) atop which can be observed the pollen sacks (anthers) which contain the sticky yellow pollen. Male trees carry huge clusters of these blossoms, which are a major attraction for bees and other insects. In collecting the nectar, these visitors become well coated with the pollen. Female holly flowers open to reveal what appears to be a small green berry in the center of the flower. Although they also have a set of horns, these are strictly decorative and produce no pollen. The little green berry displays a white, brown, or black spot (stigma) near its center which is the receptor for the pollen, delivered by a visiting bee or other insect. Female trees generally have a much smaller number of flowers than male hollies.
Once the female blossom is pollinated, it very quickly drops its petals and horns, and the berry grows rapidly to its full size. It then waits for fall to make the striking change from green to red (or yellow, orange, white, black). If it does not receive pollen, the female blossom of most hollies soon dies and falls off the host tree, berry and all.
In a few hollies (especially Chinese hollies, Ilex (cornuta), the berry is retained and grows, even though not pollinated. Such hollies are said to be “parthenocarpic” or sterile, as are such fruits as Navel oranges, bananas and seedless grapes. Perhaps the most famous parthenocarpic holly is Ilex “cornuta” & “Burfordi”, which can be found completely loaded with berries with no pollen source for miles. Many plant catalogs advertise self-pollinating hollies. This is incorrect. What they mean is “parthenocarpic hollies”, or they might call them hollies that have berries without pollination.
The best pollinator for any holly is a male of the same species. This does not require, however, that you have a pollinator for every species and hybrid in your collection. Although many holly species can provide pollen to other holly species, and skilled propagators have succeeded in crossing most holly species, there are some practical limits to the type of male holly that will consistently produce a display of berries on female hollies. The limits I have observed or found in the literature are included in the accompanying table.
Some years the hollies will be loaded with berries while other years there are few holly berries to be seen.
1. Frozen blossoms or berries.
Late frosts can freeze holly blossoms or newly formed berries. Since Chinese hollies are so early to bloom, they are quite often damaged by late frosts, but some years even late-blooming Ilex opaca blossoms are frozen.
2. Summer drought.
When water is short the berries are first to suffer. They can be seen to develop wrinkles and if not watered immediately, begin to fall off the bush.
3. Late fertilization.
If the spring fertilization is delayed, there will be little energy to put into blooming – few blooms/few berries.
4. Biennial cycling.
If the cedar waxwings don’t get around to clearing your holly of old berries, there is little energy to go into blooming and few sites from which blooms can arise. Every other year, the berries on my Ilexx attenuata “Foster #2″ stay all summer due to the highly efficient protection provided by the resident pair of mockingbirds (1 mocker = 100 wax-wings!). The mockingbirds apparently like holly berries for dessert, about 5 per day, whereas waxwings can completely strip a holly in a matter of minutes.
5. Unsynchronized bloom periods.
Since the holly bloom period covers several months, some species have completed this process before other species begin. Also, although rare, there have been cases documented where the males and females of the same species are observed to bloom at different times.