Q. We bought our house seven years ago and have a 12-foot-tall American holly (Ilex opaca) that has never had fruit since we moved in. Would it help to fertilize it? If so, what do I use? Also, many of the leaves are disfigured -- not eaten, but they look blistered. What causes this?
A. Either it is a male holly and it will never get berries, or it is a female holly that does not have a male nearby to produce pollen to fertilize the flowers.
Plants that have male and female flowers on separate plants are known as dioecious. Your holly falls into this category, along with red maple (Acer rubrum), fringe tree (Chionanthus spp.), gingko (Ginkgo biloba), Kentucky coffee tree (Gymnocladus dioicus), junipers (Juniperus spp.), spicebush (Lindera spp.), bayberry (Myrica pensylvanica), yew (Taxus spp.) and many others.
Although having a female plant is desirable in the case of your holly, there are other plants that produce messy or smelly fruits. In those cases, male plants are preferred. For example, female ginkgo trees produce fruits that have a putrid smell, and female Kentucky coffee trees produce sharp messy pods that are a nuisance to clean up.
When your holly blooms next spring, examine the blossoms. Female hollies have a small green fruit (actually the ovary) in the center of the flower; male hollies have only stamens, the structures that produce pollen. Once you figure out the sex of your holly, you will have to buy it a mate of the opposite sex. You should shop for your holly's mate when it is in bloom so that you can examine the flowers rather than relying on the plant tag at the nursery. Tags can get switched, and it would be disappointing to bring home the wrong holly. It is important that your holly and its prospective mate are the same species -- Ilex opaca -- and are in bloom at the same time. They should be planted in reasonable proximity to each other but do not have to be side-by-side because holly flowers are pollinated by different species of bees, including honeybees.
The damage to the foliage is probably caused by the native holly leafminer (Phytomyza ilicicola), a small insect that lays its eggs between the layers of the newly emerging leaves. The larvae hatch out and develop and feed inside the leaf, creating the damage you are seeing. To protect your holly against this damage, make an insecticide application as new growth begins in May.
BioNeem (azadirachtin); Sevin (carbaryl); pyrethrins and piperonyl butoxide; and Captain Jack's Deadbug Brew (spinosad) are all labeled to control leaf miner on American holly. Avoid spraying when the hollies are blooming because bees and other pollinators will be working the flowers and could be killed by the application. Although this damage is unsightly, it is not life-threatening. You may opt to live with it.
While fertilizing will not help produce fruit, it is not a bad idea to collect a soil sample and submit it to Penn State's Agricultural Analytical Laboratory to make sure that the soil pH and fertility levels are in the preferred range for hollies. It is difficult to guess what nutrients or pH adjustments may be needed, so it is best to fertilize on the basis of soil test results.
Soil test kits are available at your local Penn State Extension office. In Allegheny County, kits for home gardeners come with complete instructions for taking a representative sample and understanding your soil test results. They are $12 for the first kit, and $9 for additional kits purchased at the same time. The fee covers the cost of basic soil testing. Your only other expense is the postage to mail the sample. Make checks payable to Penn State Extension, and send them to: Soil Test Kits, Penn State Extension, 400 N. Lexington St., Pittsburgh, PA 15208.
Send questions to Sandy Feather by email at firstname.lastname@example.org or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh, PA 15208.