Q: I’ve heard that holly plants are either male or female. Can you explain this?
A: Holly plants are indeed either male or female. The term for this is ‘dioecious.’ The female holly needs pollen from the male holly’s flowers to fertilize her flowers to produce fruit. (An occasional ‘perfect’ flower, i.e. with all the reproductive parts, may produce a fruit.) The most common evergreen hollies we grow are the hybrid ‘blue’ hollies, developed by Mrs. Meserve in NYS. They are hardy to zone 5 and prefer an east or northeast exposure. This winter, my ‘Blue Princess’ holly experienced leaf browning to an extent unprecedented in 20 years. However, vigorous new growth is emerging from the dormant buds under the bark. If you are uncertain whether a branch is dead, scratch the bark gently and look for the green that will indicate life. Dormant buds take time to develop, however, so don’t give up hope too soon.
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| common winterberry Ilex verticillata Fruit(s) Photo by Rob Routledge |
Other woody species may also be dioecious. Bayberry, spicebush, and fringetree are three dioecious native shrubs. While their fruits are very attractive to birds, it is difficult to buy a shrub of known sex. Winterberry is a native deciduous holly with red fruits on leafless stems in late fall and winter, often in wet areas. On a recent garden trip in southeast PA, we observed winterberry in bloom. It was quite easy to distinguish the sex of the flowers – no hand lens needed. A quick visit to the nursery may be in order, if the potted shrubs are mature enough to flower.
So what is the purpose of the dioecious condition? Well, just like with people, it insures that genetic recombination happens, rather than self-pollination. The male and female plants are genetically different in many more ways than their sex. Genetic recombination of traits is an evolutionary safeguard. As growing conditions vary, some plants will be better adapted than others. Vegetative reproduction may be more efficient, but it is riskier when growing conditions change.
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| lily Lilium spp. Flower(s) Photo by Lesley Ingram |
Plant sex is much more varied than animal sex. Plants may have ‘perfect’ flowers with all the reproductive parts, or they may have separate male and female flowers, but on the same plant.
There are also other mechanisms to prevent self-pollination. Male and female flower parts may mature at different times, but even if they mature at the same time, self-pollination may be prevented. Apples, for instance, need not just another apple tree, but a different apple tree, for pollination to occur. Not all apple or pear varieties pollinate one another, either. Always check the pollination requirements before selecting fruit plants.
Ask a Gardener appears weekly in The Journal during the growing season. For answers to other garden, lawn, landscape and pest questions, call Cooperative Extension at 607-272-2292 or email: growline1@gmail.com. This article was written by Patricia Curran, horticulture program manager at Cornell Cooperative Extension of Tompkins County.
