Rock Gardens, Solid Against the Storms

By Adrian Higgins
Thursday, May 15, 2008

There are all sorts of practical reasons to try your hand at rock gardening, which is not the cultivation of stones but the placement of dry-loving plants in beds deep in grit and gravel.

Such a garden can flourish without mulch, mowing or pesticides and needs little feeding and weeding. It's an ideal horticultural art form for people who travel a lot or don't want to spend the whole weekend playing in the dirt. And because the rock garden is free-draining and mulched with gravel, it is inherently equipped to handle the gully washers we saw this week.

But those attributes are not the reason I am growing fonder of a form of gardening I used to think of as staid and ill-suited to the Washington climate.

No, it's the plants. Consider the beauties that thrive in an arid setting. Pinks -- so much lovelier than their ungainly kin, carnations -- are now a thicket of stems capped with starlike blossoms in pink, white and red. After flowering they revert to cushions of blue-gray, huddled against the drying winds. Spanish lavender, once considered unreliable in our climate, seems tough as nails in a rock garden and is marked by a conelike flower topped with violet tufts.

The more you probe this rocky world the more obscure and captivating is the flora. In late spring, plants are transformed into bloom. Take the Delosperma nubigenum, a creature with low, tiny sausagelike leaves and now topped with bright yellow daisies. I am smitten by another composite now in flower, the Atlas daisy, Anacyclus depressus. From flat, feathery leaves, stalks appear in the spring to bear daisies with white petals around a yellow boss. The underside of the petals is striped red, and the effect is mesmerizing.

Here's another rare beauty doing its thing this week: Asphodelus albus. It looks like a small but soft yucca, with a central flower spike whose pink flowers open and close daily, turning from tubes to stars after the sun passes its zenith. It is a lily relative from the Mediterranean.

Most of these plants are small. Indeed a big plant would look unnatural in a rock garden setting; that's why rock garden fans stick in slow-growing dwarf conifers as a foil to the perennial flowers and diminutive ground covers such as Elfin thyme and various stonecrops.

These plants have evolved as deep-rooted compact plants, reaching down for scant water and huddled against the constant wind of their native arid and mountainous habitats. What they can't stand is clay soil and wood mulches; the crowns would rot in a heartbeat, especially in winter.

Thus, you must create something of a moonscape for them. The beauty of these otherworldly gardens is that you can do it on a tiny scale, in homemade troughs or around your entire property.

Holly Downen, a seasonal gardener at Green Spring Gardens near Annandale, recently made two troughs to replace old ones that were crumbling. Essentially, you take a form of some kind; she uses foam sheeting nailed together, but a cheap plastic basin would work. She then takes Portland cement, peat moss and perlite and mixes them dry. Then she adds water, mixes them again and adds nylon fibers to reinforce the mix. The concoction is poured into the mold, where the sides are formed by hand and allowed to set. The trough is wrapped in plastic to cure slowly over three weeks and then left for at least two more weeks for most of the lime to leach out, for the sake of the plants.

Downen puts a base of gravel in the troughs, which must have drainage holes, and then builds a growing mix that consists mostly of grit, sand, potting mix and more perlite. Young plants are watered to get established, but then thrive on neglect. She plants the troughs with dwarf conifers, stonecrops, sempervivums, thyme species and scutellarias, among others.

Her so-called hypertufa troughs (they mimic a porous stone called tufa) are about 18 inches by 30 inches and eight inches deep, an area large enough for an entire microcosmic garden.

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