'Berry' important holly bush question

Q. Do all holly bushes have berries? — Julie, Goshen.

A. Unlike most plants, which are capable of self-fertilization and setting fruit or seed all by themselves, dioecious plants are different, bearing either all male flowers or all female flowers on the plant. Only about 5 percent of the world’s plant species are dioecious. The best-known in this group are hollies (Ilex spp.) which require both male (staminate) plants and female (pistillate) plants to produce their berries. Therefore, only female holly bushes will produce berries. Males do not.

In the home landscape, however, knowing how to tell the difference between a male and female holly bush is important. The best way to determine the sex of holly plants is by examining the flowers, which are located between the leaf and branch joint. Male blooms contain four insignificant petals with obvious extended stamens with pollen-laden anthers. Male holly flower centers are vacant. Female plant flowers display four or five small petals. Insignificant sepals stand around a distended pistil that houses the plant's ovary.

Furthermore, male and female plants must be the same species. Ilex verticillata, for example, cannot pollinate an Ilex opaca. You must also use plants that flower at about the same time.

Although some plants may be tagged in the garden center either male or female, this is not always the case. Therefore, it is oftentimes up to you to determine the difference. If the plant is a named cultivar you can look it up online to see if it is a male or female. It is nearly impossible to distinguish the male and female holly bush prior to blooming. If you find a plant with berries, it’s safe to say that it is female.

Now that you know you need male and female plants, how many females can a single male pollinate, and how close must the male be to the female plants to get good fruit set? In the wild, male and female plants are distributed fairly evenly, but in the garden, creating a fruitful population is left to the gardener. 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. 

About giant root borer beetles

We have had many samples of large beetles brought into the diagnostic lab.

The giant root borer beetle is a Prionus species and member of the longhorn beetle family, as evidenced by their unusually long antennae. The prionus beetle can grow up to 3 inches and is the largest North American long-horned beetle. The widest range of plant hosts occurs with P. californicus (California prionus), which feeds on roots of woody plants including serviceberry, sumac, pines, fir, Douglas-fir, stone fruits, oak, willows and poplars/cottonwoods.

While they look scary, adult beetles do not bite or harm humans. Adults usually emerge in July and August. Adults live only 5 to 7 days. Just enough time to mate. After mating females insert eggs (300 to 500) into soil, using the extensible ovipositor at the tip of her abdomen, depositing the eggs in small groups a few centimeters below ground. Upon egg hatch, the larvae move to plant roots where they feed and grow for a period of 3 to 5 years before becoming full grown. During that long time they feed and feed and feed on live, dying and decomposing trees, shrubs and woody vines. They can be quite destructive in their voracious activities. In fact, the grubs can cause fatal damage to a tree's root system. Pupation occurs within a soil chamber dug out near the roots.

— Debbie Lester is community horticulture educator at Cornell Cooperative Extension Orange County.