Q&A with Bob Polomski 30

I have a six-foot African milk tree that has grown so tall that it is continually toppling over. Would it be OK to prune off some of the upper branches that are making it top-heavy?

—R.U., Brookfield, MA

Answer: Dealing with overgrown cacti and succulents does present a problem. Most columnar forms respond poorly to pruning. Some will send out lateral branches from a topping cut, but many will simply be badly scarred from the procedure. Furthermore, topping a columnar specimen destroys its natural form. The best that can be said for the procedure is that you can root the severed top and give the parent plant away.

Although it is often sold as a cactus, your African milk tree is Euphorbia trigona, a deciduous succulent with small spatulate leaves. Pairs of spines do run the length of its three to four angled stems, but lack the areoles that characterize cacti. With time, African milk trees can become top-heavy, especially when grown in pots, and the clusters of upright stems are sometimes so dense that the spines scar adjoining ones.

Cutting off some of these branches will reduce the top-heaviness and generate propagating material. Use a sharp knife or razor blade to minimize scarring. Be careful not to touch the milky latex, which can irritate skin. To stanch the flow of sap, put the cut end under running water. Afterwards, let the cutting sit in a dry, shaded airy location for a week, so that the surfaces will callus.

To root cuttings, use a mixture of equal parts peat moss and sand. Keep the cuttings out of bright sunlight at a temperature of 60 to 70°F until they have rooted and resumed growth. As for the parent plant, consider donating it to a botanical garden, university, or other institution that can give it the space it deserves.

When we bought our house, we acquired several crowded boxwoods that form an impenetrable hedge five to six feet high and wide around the house. Is there any way to reduce their size?

—T.K., Franklin, VA

Answer: Like most broad-leaved evergreens, boxwoods will produce new growth from latent buds on branches and trunks. However, boxwoods have a very slow rate of growth, so drastically reducing the overall size of your boxwood hedge would be difficult.

An alternative strategy might be to “shovel prune”—that is, transplant— a number of the specimens, thus reducing their number. Then, when you are satisfied with the spacing, you can begin reducing the size of the remaining shrubs. In late winter before new growth emerges, remove any dead, diseased, or damaged branches. Then make thinning cuts to open up the interior of the plant to air and sunlight. These cuts should be made deep in the bush’s interior, cutting back to the branch’s point of origin. This is different from shearing, which causes new growth to develop only in a cluster at the outer edge of the plant. This thinning will produce a bush with a more natural appearance than shearing. It will also reduce the incidence of macrophoma leaf spot and volutella leaf and twig blight, and the remaining branches will grow thicker and more resistant to snow and ice damage. For best results, plan on spreading the work over several years to allow your plants a chance to regrow.

I know that it is important to store seeds where they are cool and dry, but I am wondering if it would be even better if I kept my leftover seeds in the freezer.

—J.G., Indianapolis, IN

Answer: The two most important factors affecting the longevity of seeds are temperature and humidity, hence the recommendation to store seeds in a cool, dry location. Placing seeds inside air-tight containers with a packet of silica gel desiccant generally serves to ensure dryness. The container should be of the screw-top kind that has a rubber gasket, such as peanut butter jar or canning jar.

As a general rule of thumb, seeds lose half their storage life for every 10°F increase in temperature between 32” and 90’. So refrigerators are excellent places to store containers containing leftover seed. According to Kent Whaley, the executive director of the Seeds Savers Exchange in Decorah, Iowa (www.seed savers.org), seeds will indeed survive the longest if they are frozen. However—and this is an important warning—the seeds must first be completely dry. Seeds that are not dry will be damaged by the formation of ice crystals. An indicator of sufficient dryness is brittleness. A dry bean, for example, should shatter when struck by a hammer, not simply be mashed.

While seed banks that are in the business of germ plasm preservation do store seeds at subzero temperatures, most gardeners are probably safer storing their extra seeds in an ordinary refrigerator.

In addition to the majority of conventional flower and vegetable seeds for which these guidelines are true, there are also a number of desiccation-sensitive seeds that cannot be dried or exposed to freezing temperatures. Seeds of tropical plants such as coffee and cacao, for example, must remain moist and are injured by temperatures below 50°F. Such seeds need to be sown when still fresh.

I bought an alstroemeria in our supermarket plant section, but other than the name on the tag, there is no care information. What can you tell me about its cultural requirements?

—E.G., Toms River, NJ

Answer: The genus Alstroemeria includes some 50 species growing in a variety of habitats in Argentina, Bolivia, Brazil, Chile, Paraguay, Peru, and Venezuela. In the 1950s, breeders developed a series of hybrids that catapulted the so-called Peruvian lily into the international cut-flower trade. The new plants offer long, sturdy stems, a vase life of two weeks, and an array of vibrant flower colors and bicolors, including red, pink, purple, lavender, white, yellow, and orange.

In the 1980s, the use of alstroemerias was broadened by the introduction of genetically dwarf varieties that could be grown as potted plants or herbaceous garden perennials in areas warmer than USDA Zone 6.

Dr. Mark Bridgen, a Cornell University professor who breeds alstroemerias for pot and garden use, says their height will vary from one to three feet, depending on the cultivar and the light and care they receive. Bridgen’s Constitution series includes ‘Redcoat’, with red flowers; ‘Liberty’, with purple/pink-striped ones; purple ‘Patriot’; and pink ‘Freedom’. All can be grown in eight-inch pots.

Pot-grown alstroemerias need bright light and cool temperatures. A temperature of 55 to 65°F at night is best. If you want to grow them outside, move them outdoors at pansy planting time. A hard freeze may blacken the foliage, but new growth will emerge from the uninjured rhizomes. Beware of overheating the pots, however. Bridgen recommends placing a pot inside a pot to provide insulation from summer’s heat. Feeding alstroemerias weekly with a soluble fertilizer will keep them in continuous bloom until frost. Pull or cut back to the base any spent or unattractive stems to stimulate new growth.

I was given some potted miniature roses that I would like to grow indoors. What care do they need?

—C.Q., Bronx, NY

Answer: Miniature roses are scaled-down versions of floribundas and hybrid teas with proportionally smaller stems, leaves, and flowers. They range in height from five inches to three feet, and in flower size from one-half inch to two inches across. Outdoors they are hardy in USDA Zones 4 to 11.

For indoor growing, the best varieties are those that do not exceed 12 to 15 inches in height. Examples include ‘Baby Grand’ (pink), ‘Cinderella’ (white), ‘Glory Be’ (yellow), ‘O Baby Darling’ (apricot), ‘Red Minimo’ (dark red), ‘Sorcerer’ (medium red), ‘Spice Drop’ (orange pink), ‘Starina’ (orange red), and ‘Yellow Doll’ (light yellow). These will each do well in a six-inch pot filled with standard soilless mix.

Miniature roses indoors need lots of bright light. If this cannot be provided by a south- or west-facing window, then grow them under a bank of 40-watt cool-white fluorescent lights with the bulbs set two to four inches above the foliage. Fertilize weekly with a balanced liquid fertilizer at quarter-strength. After the last spring frost, move your miniature roses outdoors for a summer vacation.

Spider mites (see “Pest Watch,” page 16) are a serious pest of miniature roses, especially those grown indoors. To reduce their numbers, bathe each rose once a week under running water. If you use insecticidal soap, rinse the leaves once the mites are killed to reduce the likelihood of soap injury to the rose’s foliage.

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