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Saturday, July 14, 2001

Q. We have large, overgrown rhododendrons in front of our house. They have grown to the point where they are covering the windows. They are healthy, beautiful plants, and I do not want to remove them entirely. Is it possible to cut them back by 4 or 5 feet without hurting them? When is the best time to prune them?

A. Rhododendrons are tolerant of the kind of severe pruning you are contemplating. Although you normally prune them when they finish blooming, severe pruning is best done in late March. You will sacrifice next year's flowers. Rhododendrons bloom on old wood, which means that they set flower buds for the following year as soon as they finish blooming. If you prune after the buds are formed, you remove the following season's flowers. The type of severe pruning you want to do is stressful to the plant, and is best done before they bloom. That way, the rhododendrons can put their energy into regrowth and recovery rather than into flowering.

Be sure to use hand pruners and limb loppers rather than hedge shears to maintain the lovely natural shape of the plants. Prune back to outward-facing side branches or buds, rather than cutting in the middle of the stems. This allows you to direct the future growth of the plants. By pruning to outward-facing side branches and/or buds, you avoid having the plant grow back in on itself. Directing the growth out permits good light penetration and air circulation in the middle of the plant, which is important to the overall health of your rhododendrons.

Q. I am interested in growing red currants, but I recall there was a ban on planting them because they are the alternate host for white pine blister rust. Can you tell me if that ban is still in effect? I do not know of any white pines close to our property where we want to grow them.

A. According to Bob Henry, supervisor of the Bureau of Plant Industry, Region IV, Pennsylvania Department of Agriculture, white pine blister rust is no longer on the state or federal quarantine list. The ban on currants (Ribes spp.) was lifted in 1966.

	Send questions to Sandy Feather by e-mail at slf9@psu.edu or by regular mail c/o Penn State Cooperative Extension, 400 N. Lexington St., Pittsburgh 15208. Due to volume of mail, personal replies cannot be provided.

Black currants are the most susceptible, and if you did have white pines growing nearby, red or white currants would be a better choice since they are less susceptible. Two good varieties of red currants are Red Lake and Wilder. Both are high-yielding with good quality fruit.

White pine blister rust is a fungal disease that has a complex life cycle. It completes some stages on susceptible five-needled pines and other stages on gooseberries or currants. Both hosts must be present within about 300 yards of each other for the fungus to survive and reproduce. It causes branch cankers and dieback on the pines while generally causing minimal damage to gooseberries and currants. Eradicating and prohibiting the planting of Ribes species near five-needled pines was considered the primary means of control for many years after the disease was introduced in the early 1900s.

Q. I have seen advertising for citronella plants that repel mosquitoes. Is there anything to this?

A. These so-called "citronella" plants are scented geraniums (Pelargonium spp.). They have strongly citrus-scented leaves. The fragrance is particularly evident whenever you brush against the leaves. Their flowers tend to be insignificant compared to their more ornamental relatives.

I am not aware of any research that documents their mosquito-repelling qualities. However, they are a beautiful and interesting addition to the garden, particularly when they are grown in containers strategically placed where you will brush the foliage and release their fragrance. Scented geraniums come in many flavors: lemon, lime, nutmeg, pine, peppermint and rose, to name a few. They also make good houseplants.