How close do male and female hollies have to be planted to produce fruit?
Barb Lowe, Monroe, MI
Dioecious holly plants produce either male or female flowers. The female plants bear fruit but only if a male is present for pollination.
Photo/Illustration: Allison Starcher
Contributing editor Steve Silk replies: Unlike the vast majority of plants, which are capable of self-fertilization and, thus, setting fruit or seed all by themselves, dioecious plants are sexually distinct, bearing either all male flowers or all female flowers. Only about 5 percent of the world’s plant species are dioecious. The best-known in this group are hollies (Ilex spp. and cvs.), which require both male (staminate) plants and female (pistillate) plants to produce their shiny red berries.
Berries are borne by the female plants, but only if they’ve consorted with a male. You won’t get berries on, say, a female winterberry (Ilex verticillata) without the presence of a male pollinator, such as the studly ‘Southern Gentleman’ or ‘Jim Dandy’. Furthermore, male and female plants must be the same species. Ilex verticillata, for example, cannot pollinate an Ilex opaca. And you must use plants that flower at about the same time. That mid- to late-June-flowering pollinator ‘Southern Gentleman’ won’t be an effective suitor for I. verticillata ‘Afterglow’, which flowers in late May or early June.
Finally, it’s worth pointing out that not all hollies are dioecious. Some, such as Ilex cornuta, are parthenocarpic, or capable of producing berries without fertilization. Most parthenocarpic hollies are not as fruitful as their dioecious sisters.
So you know you need male and female plants, but how many females can a single male service, and how close must the male be to his harem to get good fruit set? In the wild, male and female plants are distributed fairly evenly, but in the more artificial world of the garden, creating a fruitful population is left to the gardener. I usually plant at a ratio of about one male for every three to five females for a good berry turnout. Because most dioecious plants are wind-pollinated, the closer you can get the males to the females, the better. Ideally you might situate a few males at the back of a large, massed planting. But failing that, try to plant males within 25 feet of the females.
To ensure optimal results when working with hollies or dioecious plants of any genus, consult a nurseryman or a good garden center. Most catalogs that offer dioecious plants include useful information for insuring good berry set.