Mt. Cuba’s Native rock garden features plants that can take the heat and drought

The summer’s heat has scorched the lawn, shriveled the shrubs, and caused the lettuce to bolt, yet Mt. Cuba Center’s rock garden remains unfazed. It is thriving in the heat, drought, and periodic downpours that mark the season, all thanks to a few special adaptations these plants have to high heat and low water availability. All they truly require is well-drained soil—they would flop or rot in what is conventionally referred to as “good” soil.

“If you say you can’t grow anything because of your garden conditions, then I say come here and see the rock outcrop,” said Victor Piatt, Mt. Cuba Center’s Gardens Manager. “These plants are thriving.”

A rock garden requires, first and foremost, rocks. These rocks can be of all sizes, from large boulders to fine gravel. Piatt, who oversaw the creation of Mt. Cuba Center’s Rock Outcrop and Scree Gardens, says that this garden is designed to showcase native plants that are well-adapted to handle heat, humidity, and low water availability. The key is a good, well-drained soils. “This is about putting the right plant in the right place. You want to use plants that handle the conditions well, not just so-so. This is not a ‘so-so’ area,” Piatt said.

Large rocks are as much a decorative element to the garden as the foliage texture or flower color of a plant, so place them wisely.

Deep Roots

Underneath the surface, seedlings of many plants in the rock garden send out long, water-seeking roots that also help anchor the plant in place—an ability that is especially useful for growing on a cliff face, or in a thin seam of gravel between boulders. This root is called the taproot, and it can find water in the most unlikely of places. Plants like maidenbush (Leptopus phyllanthoides), send out a taproot as soon as they sprout in a race to find water.

“The rock formation we have here, called gneiss, is porous,” Piatt said. “It’s cooler inside the rock, and there’s probably moisture in there. These taproots are growing toward the moisture, and once they find it, they set out lateral roots and root hairs that absorb nutrients and water.”

Fleshy and Small Leaves

Smaller leaves are a feature shared by many plants adapted to these habitats, giving a soft, finely textured look to the overall appearance of a rock garden.

Leaf structures in many plants growing in the rock garden are adapted to maximize water storage. The leaves of Appalachian rock-pink (Phemeranthus teretifolius) are tubular in shape, and fleshy, like many succulents. This leaf shape, called a terete leaf, has less surface area compared to other shapes, meaning there’s fewer opportunities for the plant to lose water through regular transpiration.

Protection

The fleshy paddles of our region’s native cactus, Eastern prickly-pear (Opuntia humifusa), are protected by a thick, waxy coating called a cuticle. This cuticle helps the plant hoard water it gathers through its shallow, fibrous root system. While a taproot seeks water below the surface, a shallow and fibrous root system is adapted to collect surface water from rain or morning dew.

Eastern prickly-pear is a set-it-and-forget-it kind of plant that will thrive in a rock garden or another low water, high-heat, sunny spot and bloom profusely with large, bright yellow flowers, often with reddish centers. Place this plant carefully, because uprooting it increases a gardener’s chance of interacting with the business end of the cactus’ large spines or small, hair-like prickles, called glochids.

Plants that Can Do Both

Some plants that are more common in a formal garden setting adapt well to the microclimate of a rock garden.

“You’ll find that when growing in the shade, these plants are more open and have more space between the leaves, whereas the ones grown in full sun tend to flower more and have shorter intervals between leaves,” Piatt said.

Canyon Vista wild columbine

Dwarf iris (Iris verna), Canyon Vista wild columbine (Aquilegia canadensis ‘Canyon Vista’) and Little Redhead Indian pink (Spigelia marilandica ‘Little Redhead’) grow throughout Mt. Cuba Center’s gardens, including the rock outcrop and scree gardens, where they can thrive once established.

“The only limiting factor for these plants is water,” Piatt said. “And these plants are all designed to sip water, not gulp it.”

Mt. Cuba Center’s rock outcrop and scree gardens are along the main driveway, so next time you visit, drive slowly and take in the plant diversity of this unique garden habitat.

What To Do About the Driveway Strip

A more common garden environment that gets hot, bright sun and low water is the strip of vegetation alongside a driveway. Piatt advises home gardeners to keep winter in mind when selecting plants for this area. “Does your driveway get plowed in the winter? Because if so, heavy snow can damage woody plant material like shrubs.” Plants that like well-drained, sunny spots and won’t get damaged by a pile-on of winter snow are large flower tickseed (Coreopsis grandiflora), moss phlox (Phlox subulata), green-and-gold (Chrysagonum virginiana), and stiff aster (Ionactis linariifolius).

This article originally appeared in the August 15, 2018 issue of the News Journal. You can read it on their site, here.