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Warming Up to a New Landscape

By Adrian Higgins
Washington Post Staff Writer
Thursday, January 4, 2007

Northern, temperate places such as western Scotland stay alive in the winter: Light at 9, dark by 4, the days are full of fleeting images of flowering plants, the buttercup-yellow winter jasmine or the smoky-pink autumn-flowering cherry. Washington, by contrast, is supposed to be a place where a more brutal cold bullies plants into deep hibernation.

But even here, the winters have grown more gentle. I returned from Scotland recently to a mid-Atlantic landscape of . . . winter jasmines and autumn-flowering cherry in full bloom. In my garden, the Lenten rose is pushing flower buds from the earth, and the vegetable garden still offers up lettuce and parsnips. Dianthus and violas are blooming in pots.

Is it Brigadoon, or that fearsome specter -- global warming? Alas, probably the latter.

My gardening chums have been confused by winter for the past few years. Yes, January will still give us bitter spells and frozen ground, and in March or April late frosts will do their damage. But we seem to be having a lot more fun, for more weeks of the year, than we ought to. We keep waiting for it to return to normal temperatures, but normal is now so far distant that we have lost the stake. Sixty may be the new 40. Washington is the new Raleigh.

This shift has consequences, of course. There are lots of plants previously considered marginal that I now would try and others that I would think twice about installing today. The accompanying chart floats a few winners and losers. (But don't rip out plants that are working for you, however they rate on our chart.)

Many of my fellow cultivators are focusing more on what can be gained than lost. "I have a bay laurel four feet high, in the ground for five years at least," said Richard Miller, a landscape designer in the District. Previously, laurel had been considered a greenhouse plant to raise in a pot.

There is an underlying anarchy to all this fun, because the official government map on what can and cannot grow in certain zones is sorely out of date and clearly wrong. Issued as the plant hardiness zone map by the Department of Agriculture, it dates to 1990 and is based on data gathered years earlier, including during the frigid 1970s, when winters did so much damage to the camellias that they prompted a breeding program to develop hardier ones.

The map divides the country into 11 temperature zones, with Washington straddling the cooler Zone 6 (winter lows of between 10 degrees below zero and zero degrees) and Zone 7 (zero to 10 degrees).

Earlier this decade, the American Horticultural Society worked with a meteorologist to produce an updated government map that showed a dramatic shift in zones. Zone 7's relative warmth washed over most of Virginia and Maryland and even extended up the coast to Cape Cod. A pocket of the District and Arlington was placed in Zone 8, which previously extended only as far north as Virginia Beach.

But the Department of Agriculture rejected the map, deciding instead to create a newer version that will be more interactive for computer users and contain more data. It should be ready "in the near future," said Kim Kaplan of the Agricultural Research Service.

Will it show the same sort of shift seen in the rejected map? We don't know, but filling the void is a version recently issued by the National Arbor Day Foundation. This closely mirrors the American Horticultural Society version and shows Washington bathing in the heart of Zone 7.

But let's not get hung up on maps, at best crude indications of what's going on in your patch of ground. Weather patterns and climate change affect the big picture, but on the micro level, subtle and unique convergences of canopy, slope, aspect and soil types -- not to mention gardening practices -- all change the equation.

Some local gardeners decided a long time ago to put aside the plant zone map and simply see what would or would not grow here. In a large, open courtyard garden at the U.S. Botanic Garden near the Capitol, curator Bill McLaughlin has created a garden he calls Southern Exposure where he grows plants native to the South and Southwest, perennials and shrubs from Arizona to Florida. The garden features such plants as a desert willow tree, a Pensacola hawthorn and the autumn sage from Texas.

He thought he would lose as many as a third of the plantings in their first three years, but the losses have been surprisingly few. The key has been careful and laborious soil preparation to match their native habitats, the coastal plain of the Gulf states and the high desert of the Southwest.

On a less esoteric level, green-thumbed consumers have found that other Southwestern plants such as agastaches, gauras and penstemons will survive Washington winters if the gardeners provide soil with excellent drainage, as McLaughlin does.

Is there a downside to this? We have not reached a point where regional varieties of apple and other fruit trees are denied the chilling period they need to bud, flower and fruit, but that may happen. The same might be said for tulips and daffodils.

What the hardiness map doesn't track, however, is the other side of the equation: the minimum nighttime summer temperatures. These also seem to be rising, with the potential of stressing and sickening northern and high-elevation plants such as rhododendrons, pines, spruces and smokebush, to name a few.

McLaughlin was in Montreal last summer and saw "spectacular" specimens of the perennial ground cover bergenia as well as astilbes. "I realized how much we struggled with them here," he said.

Washington has long been regarded as a crossroads of northern and southern plants, or, as McLaughlin puts it, the place where lilacs greet the crape myrtles. That boundary is now "more like Philadelphia," he said.

If a northern plant is established and doing well, there's no point in tearing it out. But there are colder-climate plants that I would now think about twice before planting, including sugar maples, the Eastern white pine, spruces and common lilac varieties. Scott Aker of the National Arboretum, who writes our Digging In column, would add to the list of losers upright and spreading junipers and the American arborvitae, recommending instead a hybrid named Green Giant. Classic yew varieties also are on the fringe now, especially in yards with automatic irrigation systems that keep the root zones too wet. Try Japanese plum yew instead, Aker suggests.

On the positive side, the season for tender tropicals such as coleus and bananas is now long, and plants previously considered too risky for our gardens are worth trying, especially more-tender but interesting varieties of fig and rosemary, pomegranates, the tender kiwi and even gardenias.

McLaughlin has a friend at the Delaware shore who has grown oleander outdoors for several years. Getting the picture, y'all?

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