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Q&A with Bob Polomski 4

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Extension Consumer Horticulturist, Clemson University

My hostas have become crowded and need to be divided. Is spring a good time to separate them?

—E.R., Rolla, MO

Answer: Yes. Hostas can be divided as soon as the “noses,” or new shoots, begin to emerge. Although they can be divided later in the season, the risk of plant injury or loss increases.

The simplest way to obtain a division is to use a sharp knife or spade to cut out a slice from the parent plant. Fill in the hole that results with additional soil, and further divide or simply replant the removed piece elsewhere.

When dividing more than one clump of hostas, it is worth taking the time to disinfect your tools between clumps with a mix of one part chlorine bleach to nine parts water. This will reduce the chance of spreading infection.

A more disruptive form of hosta division is to dig up the entire plant. With a spading fork or spade, pry up the clump, lift it out of the ground, and shake off the loose soil. Rinsing off the clump with water may be necessary to help you to clearly see the location of offsets. Then, with a sharp knife, cut the clump into pieces, each one with at least one root-bearing shoot. Particularly large clumps may require the use of an axe, or two spading forks wielded back to back, to split the pieces into a manageable size.

Although more drastic, this technique of lifting an entire plant does offer an opportunity to improve the soil beneath the parent plant with organic matter and other desired amendments before replanting.

How far south can lilacs be grown? I live in southern Florida (Zone 9) but I have seen lilacs in southern Georgia, where it's also quite hot and humid. Do they really need a cold winter to bloom?

—J.V., Florida, by e-mail

Answer: Robert Bowden, of the Harry P. Leu Gardens in Orlando, Florida (Zone 9), has tried a number of lilac cultivars unsuccessfully, including a series of California-bred lilacs known as the Descanso hybrids. These were developed at Descanso Gardens, and were bred specifically to thrive in the dry, hot environment of southern California. Los Angeles County shares your same USDA Hardiness Zone, but there the lilacs experience lower nighttime temperatures. The Descanso hybrids planted in the Southeast, according to reports, have been shortlived and highly susceptible to powdery mildew.

Gardeners in Zone 8, however, can try growing the Japanese tree lilac (Syringa reticulata ‘Ivory Silk’), Peking lilac (S. reticulata subsp. pekinensis), cutleaf lilac (S. xlaciniata), early-flowering lilac (S. xhyacinthiflora), and S. pubescens subsp. patula ‘Miss Kim’.

Another possibility is ‘Betsy Ross’, a recent National Arboretum release. This white, early-flowering lilac has the characteristics of French lilacs, with the added bonus of heat tolerance from one of the parents, S. oblata var. donaldii. Dr. Margaret Pooler, research geneticist at the National Arboretum, hopes to release one or two other heat-tolerant varieties in the next several years.

I am looking for a beautiful flowering vine that I saw on the streets of Comfort, Texas. The locals call it queen's crown, but I can't find a plant by that name anywhere.

—J.K., Texas, by e-mail

Answer: Queen's crown, queen's wreath, coral vine, or chain of love are all names used for Antigonon leptopus. This member of the Polygonaceae, or buckwheat family, is native to the tropical forests of Mexico and Central America. It is a tuberous perennial hardy in USDA Zones 8b–11, and is also cultivated as an indoor plant. Heat-loving and extremely vigorous, it can climb to heights of 30 feet or more by securing itself with tendrils. The arrow-shaped leaves are accompanied by drooping racemes of beautiful, bright pink flowers from summer through fall.

Queen's crown is well suited to full-sun locations on fences, pergolas, and arbors. It makes a useful screen that provides privacy and shade on the southern or western exposures of a porch, patio, or terrace, though some pruning may be necessary to keep this rampant grower within bounds. Available varieties include ‘Album” (white) and ‘Baja Red” (bright rose pink).

A word of caution, however: the vine re-seeds itself readily and has become an invasive, noxious weed in Florida and Hawaii, and on several other Pacific islands. Its growth is so vigorous that it was reportedly used during World War II to conceal anti-aircraft batteries.

When is the best time to lime a vegetable garden?

—N.A., New Hampshire, by e-mail

Answer: Agricultural limestone is inexpensive and can be applied at any time of year. However, you may find that it is most convenient to apply it in the spring when you're preparing the soil for planting. It's important to maintain a soil pH in the range between 5.5 and 6.5, because this is the level at which nutrients are most available. Besides increasing the availability of nutrients, an appropriate soil pH improves the environment for earthworms and beneficial microorganisms. The precise amount of limestone to apply will depend on how acid your soil is. The best way to determine this is to send a sample for professional testing by your state's extension service or other soil-testing lab.

Limestone can be either calcitic or dolomitic. Calcitic limestone is calcium carbonate, while dolomitic limestone is a mixture of calcium and magnesium carbonates. Both will reduce soil acidity, but dolomitic limestone is a better choice for magnesium-deficient soils.

Since limestone is relatively insoluble in water, use a finely ground form that can be mixed in the soil to react quickly. Pulverized limestone has a fine, talcum-powder-like consistency that makes it difficult to spread with a rotary or drop spreader. For ease of handling and application, you can use the more expensive pelletized limestone; however, pelletized lime will take longer to react in the soil than pulverized limestone.

Lime moves slowly downward in the soil profile, so incorporate the required amount of limestone into the top four to six inches of soil in your vegetable garden. As soon as moisture is present, the lime will begin to react.

Can you tell me when spireas should be pruned?

—T.C., Milford, DE

Answer: The genus Spiraea contains some 70 species, most of them Asian. Whether to prune these multistemmed shrubs of the rose family before or after bloom depends on whether a given species flowers on old or new growth.

Those species that flower in spring bloom on old wood. Premature pruning of these when the plant is dormant will remove many of the flower buds, thus reducing the spring display. Spring-blooming spireas include double Reeves spirea (S. cantoniensis ‘Lanceata’), Nippon spirea (S. nipponica), bridal-wreath spirea (S. prunifolia), baby's breath spirea (S. thunbergii), and Vanhoutte spirea (S. xvanhouttei). These should all be pruned shortly after they have finished flowering.

Summer-blooming spireas, by contrast, flower on new wood. These include the Japanese spirea (S. japonica) and all its many varieties, including the Japanese white spireas (S.j. var. albiflora), and Billiard spirea (S. xbilliardii). These spireas should all be pruned in late winter before bud break in the spring.

Spring bloomers should be pruned by cutting back one-fifth to one-third of the oldest branches to the ground. The older branches produce fewer flowers than younger shoots, and removing them eliminates overcrowding and creates room for new growth. Also, the increased sunlight penetration encourages side shoots to develop within the center of the shrub to create a full appearance.

Long, lanky branches can be shortened by making a cut where there is a side branch or bud oriented away from the center of the shrub. Also, head back a few of the branches to outside buds to encourage branching from below the cut. Finally, deadhead any remaining branches back to a pair of side shoots. As you prune, try to maintain the plant's naturally graceful habit.

In the case of the summer-blooming spireas, the same principles apply, although you can also cut the entire plant down to a third of its height to encourage the production of many young shoots that will bloom later in the season. Alternatively, you can maintain an informal framework by not cutting back so heavily, and by staggering your pruning cuts.