- The Washington Times - Tuesday, December 14, 2004

Gardeners at the U.S. Botanic Garden are in the middle of prepping the grounds of the 3-acre Bartholdi Park for the next growing season. They are cleaning up and weeding garden beds, adding mulch and compost, pruning and planting cold-hardy plants and flower bulbs that need the cold to bloom.

“You need to tend to your garden throughout the seasons,” says Holly Shimizu, executive director of the Botanic Garden in Southeast. “It’s like your house. You don’t want to get too far behind.”

In the U.S. National Arboretum, gardeners are collecting leaves, cutting back perennials, planting bulbs and mulching the roses in the herb garden.

“Some of these things are done to help plants get through the winter,” says Scott Aker, gardens unit leader for the National Arboretum in Northeast.

Waiting until spring can mean working extra hard to sort out the dead material from new growth, Mr. Aker says.

Mr. Aker and other metro-area gardeners, horticulturalists and extension agents provide advice for winterizing vegetable, flower and shrub gardens. Pruning, planting, mulching and soil-testing are a few gardening chores that can be carried out in the winter months.

“A lot of it is just aesthetics,” says Debbie Dillion, urban horticulturalist for the Loudoun County Cooperative Extension. “If you don’t do it, it will be there in the spring.”

One of the chores, pruning, involves shaping plants for symmetry and balance and removing dead material to stimulate new growth and diminish disease and insect problems.

“Stored food goes to the remaining branches,” says Steve Dubik, professor of landscape technology at Montgomery College in Rockville and horticulture consultant with the Montgomery County Cooperative Extension. “You might have fewer leaves, but they will be larger and healthier.”

With fewer leaves, more air and light move through the plant, he says.

Pruning helps remove fungal spores that germinate in leaves and stems. It opens up plants with leaves and branches grown tight together that create a humid atmosphere and breeding ground for diseases and insects.

Pruning perennials is best after the first freeze or frost, Ms. Dillion says. She recommends that herbaceous perennials be pruned by cutting off the top portion a few inches above the ground, the exact amount depending on the plant type, she says.

“The root system is still alive, so you remove the dead portion of the plant and it will grow next spring from the roots,” Ms. Dillion says.

The cuttings can be used as compost, and the same is true with annual flowers and vegetables pulled directly out of the ground with the roots included or cut down to ground level.

“You want to save heavy pruning for early spring,” says Audrie Whitney, president of the Landscape Design Council for the National Capital Area Federation of Garden Clubs Inc.

Winter pruning, however, can be done on conifers, which are evergreens with needles, and on boxwood shrubs when the wood is not frozen, Mrs. Whitney says. Boxwoods should be pruned 10 to 12 inches into the plant to let in air, not by sheering off the top portion, she says.

“Different plants respond to pruning differently, and you should know your plant before you prune,” says Joanne Hutton, horticultural technician for the Arlington County Cooperative Extension. “There are a lot of shrubs that are OK to prune, especially evergreens.”

Spring-flowering plants require particular care in pruning, Mr. Dubik says. They have flower buds at the end of their shoots that, if cut off, will not produce flowers the next season, he says.

For those plants, Mr. Dubik suggests the one-third rule, the removal of one-third of the largest shoots, a cutting that can be used to maintain the size of most plants from year to year. Rejuvenate pruning, which is a more aggressive type of pruning, cuts plants close to the ground, sacrificing next year’s flowers to encourage food stored in the root system to reach the remaining shoots, he says.

“The faster the plant grows, the more aggressive you can be in pruning,” Mr. Dubik says.

As plants grow, they remove nutrients and minerals from the soil that mulching returns. Mulching helps retain soil moisture, suppress weed growth, maintain the plant’s temperature and protect the plant from the cold and wind, which can cause plants such as evergreen shrubs to dry and turn brown.

Applying mulch is best after the ground is frozen, though not on some types of annuals and perennials, such as irises, that will not reseed if mulched, Ms. Dillion says.

Gardeners, as a rule of thumb, recommend mulching up to two inches, saying that too much mulch can invite voles and other rodents to take harbor. Mulching too close to the crown of the plant can cause the plant to retain too much moisture and begin to decay.

“The real secret of successful gardening is building up the soil and having fertile soil,” says Raymond Petersen, professor of biology at Howard University. He holds a doctorate in botany and plant taxonomy.

A way to measure soil fertility is through a soil test, which can be picked up at local extension offices. The test identifies the soil’s pH level — most plants prefer a slightly acidic pH of 6 to 6.8 — and the level of macronutrients, including nitrogen, phosphorous and potassium, and micronutrients, which, like iron, are less needed by plants.

Fertilizer can help replace missing nutrients, and lime raises the soil’s pH level.

Mulch serves the same purpose to adjust pH to a more acidic or a more alkaline level, says Robert DeFeo, regional horticulturalist for the National Park Service. Pine bark mulch, for example, is more acidic and lowers pH.

“It doesn’t pay to waste money on fertilizer,” Mr. DeFeo says. “The problem is we add nutrients when they are not needed, then the nutrients run off.”

Another winter gardening task involves planting spring flowering plant bulbs from October through December, generally recommended at three times deeper than the bulb’s size, Ms. Dillion says. Most perennials can be planted in March and April and annuals after the first frost, she says.

Winter flowering plants also can be planted, such as camellias and witch hazel. Red-twig dogwoods are another winter plant. After they drop their leaves, the branches turn red, gold or yellow before they re-green in the spring months.

“You can keep your garden going all year if you wanted to,” Mr. Dubik says.

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