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Q&A with Bob Polomski 11

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Extension Consumer Horticulturist, Clemson University

Every year I bring my potted rosemary indoors, but I have never succeeded in keeping it alive. By midwinter it sheds its leaves and dies. Is there any trick to keeping rosemary over the winter?

—W.L., St. Albans, VT

Answer: You are not alone. Rosemary (Rosmarinus officinalis) is a notoriously challenging herb to overwinter indoors. Rosemary needs at least four hours of direct sunlight a day or 12 hours of strong artificial light. It also likes cool nighttime temperatures that do not exceed 55°F, and prefers higher humidity levels than other herbs do.

You must avoid both over- and underwa-tering rosemary. Allow the potting medium to become moderately dry, but never let the leaves wilt. Then, water the medium thoroughly until water seeps out of the drainage holes.

Overwintering your rosemary in a clay pot offers the advantage of allowing water to freely evaporate from the entire root ball, which can reduce the occurrence of root rot, but the medium is likely to dry out more rapidly. Other herbs will revive from a serious wilt. Not rosemary.

For those gardeners who live in areas warmer than USDA Zone 6, the best way to overwinter rosemary is to leave it where it is growing outdoors.

Every year I purchase a large number of bare-root plants by mail order, and inevitably they arrive earlier than I can plant them outdoors. I just don't have enough room in my refrigerator to store them all. Is there a better way to hold the plants until I can get them into the ground?

—R.T., Denver, CO

Answer: All shipments of bareroot plants should be unpacked immediately. Check for any damage and be sure that the roots are moist.

If you cannot plant your order immediately, store the dormant plants where the temperature is between 33 and 40°F. This can be in the refrigerator, root cellar, or other cool location. Keeping the plants wrapped in plastic will help to maintain high humidity levels and moist roots. You may need to add additional water to the peat moss, excelsior, or shredded newspaper that is commonly used as packing material.

Small numbers of plants, especially those that have broken dormancy, can be potted up indoors and then moved outdoors when conditions are favorable. Soak the roots in a bucket of water for half an hour before potting them up.

Fred McGourty, a Connecticut nurseryman and author of The Perennial Gardener, has at times resorted to using a large, shallow, plastic children's wading pool filled with moist soilless mix to accommodate large orders of plants. He places the pool in a cold garage that's protected from wind and frost until it's time to move the plants to their home outdoors.

Can you suggest a few houseplants that are suitable for growing in a north-facing window?

—C.R., Avalon, PA

Answer: A northern exposure receives the lowest light levels of any window in the house. As a consequence, foliage plants will perform better than flowering plants, which require higher light levels in order to bloom. There are limits even for foliage plants, however. You can't expect any plant to grow where there is insufficient light to read comfortably. The good news is that plants in a north-facing window will require less water and fertilizer and will benefit from cooler temperatures.

Among the candidates for a north-facing window are Aglaonema species and hybrids, such as silver evergreen (A. commutatum) and Chinese evergreen (A. modestum); cast iron plant (Aspidistra elatior); dumbcane (Dracaena spp.); golden pothos (Epipremnum aureum); English ivies (Hedera helix); prayer plant (Maranta leuconeura); Swiss cheese plant (Monstera deliciosa); watermelon peperomia (P. argyreia); philo-dendrons; snake plant (Sansevieria trifasciata); peace lily (Spathiphyllum wallisii); and arrowhead vine (Syngonium podophyllum).

Ferns that tolerate low light and low humidity include hen and chicken fern (Asplenium bulbiferum), bird's nest fern (A. nidus), holly fern (Cyrtomium falcatum), and brake ferns (Pteris spp.).

Finally, the mistletoe fig (Ficus deltoidea) and fiddleleaf fig (Ficus lyrata), ponytail palm (Nolina recurvata), and lady palm (Rhapis excelsa) should all do well in conditions of reduced light.

How can I get my potatoes to form good, vigorous sprouts prior to planting, instead of long, fragile ones?

—E.M., Maples, WI

Answer: The secret is to expose the seed pieces to warmth and light before planting. This practice is called chitting, or green-sprouting, and is common in Europe, especially in the United Kingdom. Not only are the resulting sprouts compact rather than etiolated, but chitted potatoes can go on to produce more stems and tubers per hill than unchitted seed. It is a particularly useful technique for regions such as yours that have a short growing season.

About six weeks prior to planting, place whole seed potatoes that are about the size of a hen's egg in flats, shallow boxes, or egg cartons. Lay them in a single layer with their blunt “rose” or bud ends oriented upwards. The eyes are concentrated at this end. The opposite end, or “heel,” is where the tuber was attached to the potato plant.

Set the seed potatoes in a warm (70°F) but dark location until the eyes begin to grow. Then move the potatoes to bright direct or indirect sunlight. The light will keep the developing sprouts short, thick, and green. If you chit them outdoors, bring them inside when there is a threat of frost.

Larger tubers can be cut into block-shaped pieces containing one to three eyes (about one and a half to three ounces in weight). They can be presprouted after they have been cut and the tissue has healed or suberized (formed a corky protective layer).

The ideal chitted piece of seed will have stocky sprouts less than an inch long. These should be planted a couple of inches deep with the shoot pointing up.

Can you suggest a few winter-flowering trees for my USDA Zone 7a landscape?

—D.P., Salisbury, MD

Answer: Trees that bloom in the winter are an uncommon delight. Many have intensely fragrant blooms, and they offer food for winter-flying pollinators. However, one consequence of blooming in winter is the risk of losing the flowers to freezing temperatures. Nevertheless, for the gardener this is a chance worth taking.

The winter-flowering cherry (Prunus subhirtella ‘Autumnalis’) blooms sporadically from fall through winter on mild days, with a larger show in the early spring. Its semidouble white flowers are tinged with pink. There are also some 300 varieties of Japanese flowering apricot (Prunus mume), which bears single or double flowers in white through pink to red. Hard freezes will kill flowers and swelling buds, but with its profuse number of buds, those that have yet to open usually replace flowers lost to cold.

Cold-hardy camellias—such as ‘Autumn Spirit’, an early-blooming C. sasanqua with deep pink flowers—are another option. ‘Red Jade’, a C. japonica with light red semidouble flowers, blooms in January or February and lasts into March.

Chinese witch hazel (Hamamelis mollis) has fragrant yellow flowers in late winter or early spring. Paperbush (Edgeworthia chrysantha) is a shrub that can be trained into a small tree. It bears extremely fragrant creamy yellow flowers in midwinter.

The Cornelian cherry (Cornus mas), a small, shrubby, deciduous tree, bears one-inch, round clusters of small, yellow, fragrant flowers in very early spring, before the leaves appear. They mature into dark red fruits in late summer.

In late winter to early spring Stachyurus praecox (a deciduous shrub to small tree) produces small, yellow, bell-shaped flowers in two- to three-inch-long racemes before the leaves appear.

Finally, ‘Okame” cherry (Prunus xincamp ‘Okame’) has clear pink flowers in late winter to very early spring.