Plants that are well adapted to this region won't be hurt by the frigid weather we have had. If the low temperatures continue, however, we will lose flowers on early-flowering trees such as magnolias and cherries.
Permit me to do some horticultural forecasting and to prophesy how several of my favorite more-tender plants might fare through the rest of the winter. All hardiness zones I refer to are from the Department of Agriculture's Plant Hardiness Zone Map. We are on the colder side of Zone 7, with an average annual minimum temperature of 0 to 5 degrees. (The zone numbers get lower as the expected weather gets colder, so just to the north of us is Zone 6.) The map is on the Web at www.usna.usda.gov/Hardzone/ushzmap.html.
* Camellia. This Asian native is one of the showier flowering, broadleaf evergreens semi-hardy to this region. Hardy species will winter well. The sasanqua camellias are commonly fall-flowering, and the Japanese varieties bloom from late winter to spring. There is mixed data as to which is the hardier species, but because the Ackerman hybrids were bred for hardiness here at the National Arboretum, they are comfortably Zone 6 and 7 plants and hardy to 0 degrees, or colder with protection. A list of Ackerman hybrids is available on the Internet at members.aol.com/camsoc1/ACKERMANCOLDHARDYVARIETIES.htm.
* Chindo Viburnum. This fine specimen is a conversation piece for its showy, shiny evergreen leaves. Its lure compelled me to design these Zone 8 plants as background shrubs on a courtyard wall in Georgetown and to grace the tall entry stoop of a house in Bethesda with them. The tops of the plants might die back in Georgetown. The shrubs in Bethesda might lose their foliage but will survive and grow from the roots or leaf out from branches that didn't freeze to the point of suffering tissue damage.
* Crapemyrtle. In the 1970s and earlier, I would have given these a thumbs down for surviving winter in this region. But ever since Donald Egolf of the National Arboretum introduced an impressive array of tough hybrids in the 1970s, I have come to count on this tree to flower from summer into fall in full sun. A very few of the hardy Egolf crapemyrtle varieties are Tonto, a fuchsia, and Zuni, a medium lavender. They stand five to 12 feet tall. Apalachee, light lavender, and Sioux, dark pink, grow 13 to 20 feet. Choctaw, bright pink, and Natchez, white, are magnificent trees 20 to 35 feet tall. Check with the Arboretum or your local garden center to learn names of others.
* Hybrid tea roses. The most temperamental of the roses, they don't like extreme temperatures. This cold snap will probably result in a few more blackened canes than usual. Prune them in spring. If the cold temperatures continue, more damage is likely if you did not cover them. Many northern gardeners cover canes with topsoil in fall and uncover them in spring, before growth begins. The Montreal Botanic Garden, where it gets bitterly cold and snowy, covers its rose garden with a vinyl-backed foam on metal frames, and the plants suffer virtually no winter damage. If all your canes die from winter kill and new growth comes only from the roots below the graft scar, the plant should be discarded and replaced.
* Nandina. This member of the barberry family has evergreen foliage that turns red in winter and almost matches the long racemes of brilliant red berries that form in fall and are excellent for use in cut flower arrangements. The berries will persist, but the leaves will drop if they are in an exposed site. If they are growing in a protected location with a wall or shrubs surrounding them and screening the prevailing winds, nandinas might not die back. Should yours defoliate, it will re-foliate by July. Even when nandinas are killed to the ground in the harshest of winters, they grow back from the roots.
* X Fatshedera lizei. If you are trying to grow this interesting, large-leafed, climbing woody shrub, this was not the best year to begin. This inter-genetic hybrid is very shade tolerant and would be a great trial plant for a woodland garden. The eight-inch-long leaves would be evergreen in a milder winter. If you have an established specimen, and the bitter weather continues, an unprotected plant could freeze back to its roots. It should be root-hardy enough to grow new foliage.
* Rosemary. We are on the northern edge of the region where rosemary survives winter. In bitterly cold weather, this woody evergreen will die. Most are Zone 7 and 8 plants. They struggle in colder or warmer climates. Unless we have several more bitterly cold weather snaps, though, a well-established rosemary in a sunny, protected area should weather the winter in fine shape. A newly planted one would struggle. If you lose the one you are growing, replace it in spring with a hybrid named Arp. It is considered the most cold tolerant, to Zone 6, of all rosemary hybrids.
* Hyssop. If you have a showy flowering hyssop hybrid, such as Agastache "Pink Panther" or "Tutti Frutti," you could lose it, because it is more a southwestern or Mexican plant, and the moisture and cold bode poorly for it. The more common North American anise hyssop, such as A. foeniculum, and Asian species A. rugosa are tough perennials that won't be deterred by several more cold snaps. They will be back next year to display their tubular orange, rose, red or violet flowers, exceptionally attractive to hummingbirds, and foliage that smells so strongly of licorice that people in your garden will return to brush and sniff regularly.
To help protect tender shrubs for the rest of the season, during a warm spell, arrange stakes around shrubs; drive them into the ground, and attach fabric to them. Use three or four four- to six-foot-long stakes made of 2-by-2-inch oak stakes or half-inch-thick steel bars. Attach burlap or landscape fabric to the stakes to protect plants from freezing, drying winds, especially in temperatures below 15 degrees.
For more information on trees and shrubs that grow in warm climates, zones 7 to 11, the ultimate text is "Dirr's Trees and Shrubs for Warm Climates: An Illustrated Encyclopedia," by Michael A. Dirr (Timber Press, $69.95). It's a must for the serious gardener who works with marginally hardy plants.
Correction: In the Jan. 18 column, the instructions for pruning blue hydrangeas in January were incorrect. The statement should have been: "The blue hydrangea (H. macrophylla) should only be pruned, if necessary, right after flowering. If you prune them in January you will remove most or all of the flowering buds for the coming year."
Joel M. Lerner is president of Environmental Design in Capitol View Park, Md. His e-mail address is email@example.com; his Web page is at www.gardenlerner.com.