Q&A with Bob Polomski 12

Extension Consumer Horticulturist, Clemson University

I'm not sure when I should fertilize my shrubs and trees. I used to think that spring was best, but I see people fertilizing in summer, and recently I was told that fall is the best time. What do you suggest? –R.T., Fountain Head, MD

Answer: As it turns out, late summer and early fall are the best times to fertilize shrubs and trees. At this point in the year, woody plants make the most efficient use of the fertilizer's nutrients, which are absorbed when the roots are actively growing and shoot growth has ceased. Research has shown that early-spring growth depends almost exclusively on nutrients that were absorbed and stored the previous year.

Spring is the second best time, after new shoot growth has ceased and the leaves have reached mature size.

The concern used to be that late-season applications of fertilizer would trigger new growth that might not survive the winter. This has proven not to be an issue in the case of plants such as conifers and most hardwoods. However, with species that are characterized by indeterminant growth—multiple flushes of shoot growth during the growing season, as seen in apples and crape myrtles—you will need to use caution in determining the amount of fertilizer to apply in the fall. For indeterminate-growth species, split applications of fertilizer—one in early summer and the other in the fall—are probably best.

Last spring, my fescue lawn was riddled with chickweed and henbit. Is there something I can do now to prevent this from happening again? –A.P., Eudora, GA

Answer: Chickweed (Stellaria media) and henbit (Lamium amplexicaule) are both winter annuals. Their seeds germinate in the fall and the resulting plants overwinter, often as rosettes. They resume growth in the spring, flower, produce seeds, and die.

The best way to control winter annual weeds is to maintain a healthy, dense lawn that will outcompete the germinating weed seeds for light, water, and nutrients. Fertilizing your lawn and overseeding thin areas will reduce the likelihood of invasions.

You can also apply preemergent herbicides, which will prevent winter annual seeds from germinating. Benefin (Balan), isoxaben (Gallery), pendimethalin (Pre-M), and prodiamine (Barricade) are examples of preemergent herbicides that control a wide variety of winter annuals. They should be applied with a fertilizer spreader, applying one-half of the recommended rate in one direction and the other half at right angles to the first.

The timing of application, however, is critical. To be effective, these herbicides must be applied two or three weeks before the winter annual seeds begin sprouting. As a guide, you should wait until the nighttime temperatures fall to 55° to 60°F for several consecutive nights. Because these herbicides prevent seeds from sprouting, their use should not be combined with lawn reseeding. Generally you must wait 45 to 60 days after an application before you can sow grass seed. Also, some preemergent herbicides cannot be applied to newly seeded areas until the lawn has been mowed at least three times.

Do you know of any varieties of citrus plants that I can safely grow outdoors in my USDA Zone 8 garden? –C.Y., Calera, AL

Answer: Raising citrus outside of the traditional citrus-growing regions is made difficult both by minimum winter temperatures and by fluctuations in temperature that interfere with citrus achieving maximum winter hardiness. However, a recent four-year evaluation of citrus cultivars in Savannah, Georgia, when the winter temperatures dipped to between 13° and 18°F, suggests several possibilities. (See “Field Evaluation of Cold Hardy Citrus in Coastal Georgia,” by M. Rieger et al., Hort Technology, vol. 13, no. 3, 2003.) Among the survivors were the mandarins ‘Owari‘ (grafted on sour-orange rootstock) and ‘Changsha’ (own-rooted), the citrange-quat ‘Mr. Johns Longevity‘ (both own-rooted and grafted on ‘Carrizo’ citrange root-stock), and the orangequat ‘Nippon’. These plants experienced some defoliation and minor shoot dieback, but they fruited consistently on an annual basis.

To improve any citrus plant's chance of survival, plant it in a well-drained location in full sun, preferably at the top of a south-facing slope with protection from prevailing winter winds. Planting in raised beds will improve drainage and will encourage the soil to warm up more quickly. Young trees are less cold tolerant than mature trees, so care during the first three growing seasons after planting is critical for their long-term survival.

Keep the tree well watered, and avoid fertilizing between August 1 and February 15 to avoid encouraging growth that can be injured by frosts. For more information about growing citrus and other tender plants, visit the Web site of the Southeastern Palm and Exotic Plant Society (www.speps.net), where “members are devoted to growing hardy palms and subtropical plants in Zones 7,8, and 9 in the Southeast.”

I have discovered tiny holes in my dry beans. What's eating them and how can I stop it? –S.V., Davenport, IA

Answer: Your dry beans are infested with bean weevils (Acanthoscelides obtectus), a common pest of beans, peas, black-eyed peas, and lentils. The adult is a small, 1/8-inch-long snout beetle that is olive brown with darker brown and gray mottling on its wing covers. Either in the field or in storage, the adult female lays eggs in the developing pods or loosely among the beans. The eggs hatch in one to three weeks and the hairy white grubs begin to tunnel and feed on the seeds. A single bean can support up to a dozen larvae. The wrinkled, 1/8-inch-long brown-headed larvae feed for two to six weeks before they mature and pupate. After one to three weeks, the adults emerge from their pupal cases and exit by chewing round, telltale holes through the seed coat. Six or more generations can occur in a year, although only one or two occur in the field and the rest in warm storage conditions.

To destroy weevils, place the harvested seed in the freezer for at least four days. If you are planning to use your seed for planting, be sure that it is completely dried before you freeze it. A fully dried bean will splinter into pieces when hit with a hammer. Placing your beans in an electric dehydrator at a temperature between 95° and 100° F for eight hours should ensure that the seed is sufficiently dry to freeze safely. After freezing, store your beans in dry, airtight containers. H