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Home » Gardening Information » Sustainable Techniques

Drought Tolerance—Picking Plants that Consume Less Water

Plants & Gardens News Volume 14, Number 4 | Fall 1999

by Patricia M. Jonas

It's October, and the breezes have blown every cloud toward the ocean, leaving the blue sky marked only by a hawk circling. The clear light is perfect for evaluating a garden. This year, properly chastened by five of the driest months on record sandwiched between a wet March and even wetter September, I stand over beds and containers with notebook in hand and discerning eyes. Hundreds of bees are having their last great feast on airy drifts of asters and dissolute clumps of Japanese anemones. With the temperature brushing 70 degrees, it could be easy to forget just how punishing the summer was, and how watering absorbed my hours as relentlessly as the soil absorbed water. But I don't forget.

The notebook keeps me focused. I divide a page into two columns with the headings "IN" and "OUT." To make it into the first column, a plant has to have done more than hang on through the drought; and it has to have shown resilience under the hammering of spring and fall rains. I am ready to be ruthless. Well, at least unsentimental.

I garden in containers on a roof in Manhattan, and even with an average rainfall of 40 inches a year, all of my plants need supplemental water during the summer. The question of how much is one I too often unwisely ignore if a plant seduces me. Worse, I sometimes push a plant's winsome adaptability to a wide range of soil and moisture conditions in the wrong direction.

For example, I had chosen the very ornamental highbush blueberry (Vaccinium corymbosum) over lowbush blueberry (Vaccinium angustifolium), which is more drought tolerant but has a somewhat scraggly habit more suited to shrublands and forests than to containers. Adapting to sometimes drier than ideal soil, the highbush blueberry flourished until I transplanted it into a container with a fragrant white form of thirsty swamp milkweed (Asclepias incarnata), which begins blooming after the white urn-shaped flowers of the blueberry have fallen and are giving way to fruit. This sensually pleasing association worked for several years, but last summer it was so hot and dry that I couldn't satisfy the water needs of either the fiercely competitive milkweed or the shallow-rooted blueberry. Since the blueberry would have done just fine with a more generous partner, I've taken its side in the divorce, and the milkweed is OUT.

In order to conserve natural resources and better balance the demands of my garden with the pleasure it returns, I've resolved to grow plants that can handle environmental extremes with less intervention on my part. There are two questions I ask myself: Which plants are such misguided choices that the charms they flaunt happily in other settings are displayed only grudgingly in my garden? Which associations would squander less energy and capitalize on plant adaptability to less than perfect conditions?

My astilbes are obvious bad choices since they always require coddling—even during summers of what used to be "normal" rainfall. Their leaves crisp the moment the soil dries out even a little, and their panicles (or branching clusters of flowers) rarely remain standing long enough to fade to lovely browns in autumn. Next spring these plants would no doubt unfurl their handsome leaves and send up sprightly panicles of rosy pink and white flowers, but now they are tattered and completely unsightly. OUT.

Planted nearby is black cohosh (Cimicifuga racemosa), reputed to be drought tolerant when established. My clump is almost ten years old, and although shaded from afternoon sun, its foliage is scorched by heat in the harshest summers. In a summer like the one past, even its four-foot candelabras humming with bees become too unsightly to leave up. The lesson here is that drought tolerance does not equal heat tolerance; some plants are not equipped to handle both water privation and very high temperatures. Even though I've grown black cohosh in every garden I've had for more than 20 years, it's OUT of this one.

On the other hand, in this same bed, clumps of variegated Japanese solomon's seal (Polygonatum odoratum 'Variegatum') and Japanese anemone (Anemone x hybrida 'Honorine Jobert') flourish just as well with less water, and are discouraged from colonizing as vigorously as they would in a consistently moist soil. IN.

Creating a water-conscious garden does not mean banishing roses, but removing a rose in this same bed that I've designated a moderate-to-low-water-usage zone allows a seven son flower (Heptacodium miconioides) more room to live up to its reputation for drought tolerance. IN. It's a small tree with tan, peeling bark and exceptionally fragrant white flowers whose nectar seems to have a narcotic effect on bees. In late summer, the seven son flower settles in comfortably with blue, lilac, and white asters. In spring, around the tree, the light yellow columbine Aquilegia canadensis 'Corbett' begins blooming with irises, and will thrive in drier conditions than its hybrid relatives. IN.

At the dry front of the bed, I think I'll plant prairie smoke (Geum triflorum), which is a very drought-tolerant ground cover with deeply cut gray-green foliage that never looks tattered. Knowing nothing about prairie smoke, but intrigued by its name, I first planted it almost 20 years ago after a trip to a local native plant nursery near Chicago. I was rewarded in even the driest years by delicate maroon flowers followed by the showy plumes that give the plant its common name. IN.

Knautia macedonica and clove-scented pinks work fine with lady's mantle and Rosa 'Abraham Darby' in a long, deep container of moisture-retentive soil, with plenty of compost and moderate water use, but they're better suited to drier conditions than either rose or lady's mantle. Pinks, with their gray-blue foliage, will also flourish in a very lean, gritty soil, so I'll move them and some of the Knautia to a less rich neighborhood. I'll leave some burgundy pincushions of the Knautia to sprawl over the sides of the box along with feathery Lotus berthelotti (a tender plant), and move in Allium cernuum with its nodding heads of pink flowers.

Many roses are tough and do have a place in the water-efficient mixed border. A surprising number will tolerate drought when established: Think of all the cemetery roses that have flourished for years with little care, and the thickets of Rosa carolina and R. rugosa on beach dunes. Rugosa roses like 'Sarah Van Fleet' and 'Blanc Double de Coubert' have proven themselves in adverse conditions. The Heritage Rose Foundation recently polled its members for roses that performed well during 1999's calamitous weather. Look for the results in their newsletter (write to The Heritage Rose Foundation, 1512 Gorman Street, Raleigh, NC 27606) or on their website (http://members.aol.com/Rosefoun/hrf.htm).

To get through winter floods, plants that originate in rocky places need sharp drainage and a mix of no more than 20 percent soil with the rest stone and sand. In a sunny, dry bed, I will increase the grit around the crown of the delicate pasqueflower (Pulsatilla patens) that didn't tolerate being waterlogged in March and September. When it has sufficient drainage, it blooms in April along with dwarf Iris pumila and species tulips. IN. I'll also move in variegated Sedum kamtschaticum and purple-stemmed Sedum 'Vera Jameson'. The pasqueflower's translucent dusky violet petals and feathery seed heads and the bright countenance of the tulips will provide contrast to the sedum's waxy flesh.

All summer in this low-water section of the garden, blowzy white flowers of Gaura lindheimeri bloom above common culinary salvia (Salvia officinalis 'Purpurascens') and gray-leafed lavender cotton (Santolina chamaecyparissus), clipped to maintain a dense shape. Lean, pale, steel-blue switch grass (Panicum virgatum 'Heavy Metal') is lit by afternoon sun and provides a backdrop for Erysimum 'Bowles Mauve', whose flowers of rich amethyst are set off perfectly by velvety gray-green leaves. IN.

In a sun-drenched, moderate-to-low-water-use container, Baptisia alba—its creamy white flowers followed by ornamental black seed pods—is planted with velvety, green-gray leafed mountain mint (Pycnanthemum muticum) and earth-colored yarrows. Tender Salvia darcyi and Agastache 'Firebird' add bright color to the mix all season. And there is tall ferny bronze fennel (Foeniculum vulgare) with its self-seeded next generation taking root around it. Ah, all IN.

With the fennel, I won't need to shift anything—just pull out the young seedlings and make salad. But there is prodigious shifting and carrying of perennials to be done about the rest of the garden. Even for autumn, these are grand plans. My list of spring planting ideas will surely grow as I pore over catalogs this winter—as will, I trust, my resolve to attune my garden more wisely with its climate.

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