Warming Up to a New Landscape

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?