Sky Island Gardening

By Michael Bracken

Managing Editor

“We’re crazy for the view,” said Charlotte Carter, explaining why she and her husband Steve Bickerstaff built their solar-powered home at Sky Island — 200 acres in the Davis Mountains of far West Texas that abuts the 33,000-acre Nature Conservancy’s Davis Mountain Preserve, and where their nearest neighbor, the University of Texas McDonald Observatory, is four miles away. “At 6,200 feet, we’re at the same elevation as the visitor’s center of McDonald Observatory,” Carter said. “The views here are amazing.”

Designed by Austin architect Lou Kimball, the house is built of dry-stack stone, steel, glass and wood, to appear as if it emerged from the mountains. The couple had always intended for their new home to be solar powered, but their choice of location — four miles from the nearest electric grid — confirmed their decision to use solar power, according to Bickerstaff. Thirty panels power the home’s 52-volt system, and the electricity is stored in 24 batteries. They use propane for cooking, and a back-up system allows a propane generator to charge the batteries in an emergency. The home is also four miles from the nearest telephone grid, so the couple relies on cell phone service through a tower approximately 12 miles away.

In August 2008, Bickerstaff and Carter moved to their new home from Austin, where Carter was an avid gardener, and they have had to adjust to an environment unlike Central Texas and an ecosystem unique to their location. “It gets hot very briefly, and then the temperature falls like a stone,” Carter explained. “Our average cold in the winter is much lower than I was used to in Austin — between 0° and 5°. We get serious cold up here for at least brief periods, but even then it’s bright and sunny. We also get snow. We had two inches of snow on May 1.”

Unlike in the city, where finding a view of the night sky untainted by man-made illumination may be impossible, Carter said, “We live inside the dark sky zone of McDonald Observatory. So it makes you much more conscious of the rhythms of the seasons and the planet just to have that exposure to the night sky.”

Sky Island is, in fact, part of a sky island. Sky islands are mountains that develop unique ecosystems because they are isolated from one another by intervening valleys, similar to the way the ocean isolates islands, creating biodiversity much like that seen in the Galápagos.

“When the ice sheets melted away from North America, the little points of rock that had been above the ice remained the migration and speciation route for birds and animals, from the Artic nesting grounds to the Central and South American wintering grounds,” explained Carter. “That’s why we have the bird migration routes we do and that’s why this is such a rich birding area.” Davis Mountains is one stop along a route that includes Big Bend to the south and the Guadalupe Mountains to the north. “Davis Mountains, dead center between those, is the sky island, the route between them,” Carter said. “It makes for a very interesting combination of flora and fauna.”

And it makes for a unique gardening experience as well. Of the 10 natural regions in Texas, Carter said, “the Trans-Pecos has the largest number of endemic species of both plants and animals of any of the areas of Texas, things that are found only exactly, specifically right here. That’s true of the grasses, the trees, the plants and all the animals.”


Carter chose to take advantage of the unique location when she developed her native bird and pollinator garden. “In many ways it’s a tough-love garden,” she said. “No prima donnas, no babying or coddling.”

She had a large vegetable garden in Austin, she said, “but here I’ve not tried vegetable gardening yet, except for herbs and edible flowers, because anything would have to be fully protected 18 inches underground and fully overhead as well as around. We have coons and ringtails and foxes that can penetrate anything.”

Only the area within the courtyard — about 20,000 square feet surrounded by a stone, stucco and wrought iron wall — is planted. Within the courtyard, more than half the area remains covered in native grasses. Much of the remaining area is flagstone set in decomposed granite. Slightly raised beds nearest the house have been planted with small trees, including littleleaf mountain mahogany (Cercocarpus intricatus), desert willow (Chilopsis, two varieties), purple sand cherry and goldenball lead tree.

Carter said she had it easy in Austin. “I could grow more vege­tables through mid-winter than through the summer, and by the first week in June I could start pasta or rice and go out and pick dinner,” she said. “It’s something I miss badly.”

Her Sky Island garden includes three different varieties of rosemary, two oreganos, five basils, chives, several thymes, lemon verbena and several edible flowers, including nasturtiums, pansies and violets. “Actually,” she said after reeling off the names, “more of them are edible than aren’t.”

Carter said she loves every sort of plant and was spoiled in Austin. At Sky Island, she said, “unless you’re courting hardship and heartbreak, you’d better do what works with the environment.”

The soil is so thin and rocky that Carter recommends transplants rather than starting from seed. “You’re better off starting with small plants. You have a better chance of successfully rooting small ones and if they grow at all, they can grow absolutely enormous.”

She does little to amend the soil. “Trace minerals and a little compost are about it. There’s no use babying things here. For one thing, if they grow too fast, they’re too weak both in the cold and in the heat,” she explained.

“We do mulch a lot to stabilize temperature, but it will usually be a gravel mulch because so many of these things like a very, very lean soil and won’t tolerate damp ground,” she said.

For many plants, “it’s here or nowhere,” she said. “And it means that it’s more important to preserve what you have and not let it be crowded out or lost or neglected to other species. Even so, if you’re using non-natives that aren’t invasive, a lot of the Australian and especially the South African plants do very well here and look very effective.”


The majority of plants in the garden are intended to attract and provide for the needs of birds, bees and butterflies, and water is especially important. “Water in the desert is the big magnet. So we have a fountain that is relatively low-flow with a very distinctive sound that draws birds from everywhere. We also keep several birdbaths out because they like different amounts of water, different footing, different levels from the ground and so on,” she explained.

“Perches are as important for birds as food,” Carter continued. “They need places where they can hunt and forage well, so anything they can get their little feet around helps a lot.” Goldfinches favor coneflowers, while hummingbirds are drawn to the coral honeysuckle, agastache, desert willow, anise and Korean hyssop. “All of the smaller flowering trees are very good nesting for the hummingbirds.”

The small yellow blooms of the Mexican olive attract bees early in the season, while larkspur, lemon mint, anise hyssop, butterfly bush and lamb’s ear do the same later in the season. Butterfly bush, honeysuckle, lantana, oregano, rabbitbrush, coreopsis, cosmos, zinnia, salvias, marigolds and basket of gold provide nectar for the butterflies. Parsley, dill and fennel serve as larval host plants for black swallowtail butterflies. Native Passiflora serves the same purpose for zebras, nasturtiums for cabbage whites, violets for fritillaries and mallow for painted ladies.


Frost Date. The average frost date is at least a month behind Austin, Carter explained. “The soil doesn’t actually get warm until nearer June, so it’s a much shorter season,” she said. “We have a long fall ripening season.”

Hail, High Winds and Temperature Swings. “The zone adaptation was not what I expected it to be,” she explained. “Moving from 8 to 7 but also moving to elevation, the challenge is not greater heat or greater cold or even the drought. The challenges here are hail — I was hailed out very badly twice early in the spring — and high winds — they have been clocked at over 85 mph. And the temperature swings are so extreme that it is not at all unusual to get a temperature change of 15 degrees in an hour. So the adaptations are not just to the heat or cold, but also to the sudden changes.”

The difference between native plants after a serious hail and any non-natives, she said, is that “the natives look just as bad — everything’s cut to ribbons — but they recover much faster.”

Light Intensity. “The intensity of the light alone has a real effect on the growing season,” Carter said. “It also affects water evaporation. Entirely apart from wind and humidity and temperature, the intensity of the light itself affects evaporation up here. So our snow, for example, never melts; it evaporates or, technically, it sublimates.”

Water. “It’s an extremely arid area,” said Carter. “At our elevation we average about 20 inches, which is five or six more than a thousand feet lower, but it’s still fairly dry. The lower the average rainfall, the more erratic the distribution. So in areas that are arid or have low rainfall, when you get it, it’s all at once. If you’re getting 12 inches per year, you’re going to get nine of it in two days.”

Wildlife. “The foxes, coons and ringtails are here every night — night and day, matter of fact — along with all the birds,” Carter said. The courtyard wall keeps out the elk from the nature conservancy preserve, she said. It also keeps out mule deer, javelinas and feral hogs. “Everything else comes over or under.”


Defense. “I believe very strongly in staking a defensible space,” Carter said. “Take a moderate space and defend it vigorously. Out here this means begin by defending against animals. If it isn’t fully fenced you’re going to be dealing with deer and hogs and javelinas. Some things can be protected by wire or cages until old enough to be left vulnerable, but they can still be taken apart.” The animals don’t want to have to eat something to destroy it. Deer can play with it, dig it up or nose it around. “It doesn’t matter if they don’t really like to eat your tomatoes, they’ll sample every one of them.”

Catchment System. The gardening water needs are mostly met by a rain catchment system. “Spring is late, windy and dry here, and our rain comes in July and August. So I use the rain catchment for most critical gardening, especially getting young perennials rooted again and seedlings because they’re sensitive to it. Part of the time I will have to use well water, but with the amount we have guttered, one inch of rain gives me about 2,600 gallons and it’s evident that the plants much prefer rainwater to well water.” Why? “The well water tastes wonderful, but it leaves a very, very heavy lime residue in everything.”

A line directly from the 10,000-gallon rain tank can be used to fill cans or be hooked into a drip irrigation system in the larger beds. “I didn’t want to connect it permanently or to run it underground because there are problems with some systems clogging,” she said. “I just wanted greater versatility.”

Drip Irrigation. “Be prepared to defend against the elements. Drip irrigation is a huge boon up here because it avoids waste. It also keeps a more even soil texture. A lot of people don’t understand how important soil texture is to water conservation, but it means that it will drain when it rains very heavily and it holds moisture when it’s arid. If you start with drip irrigation in critical areas, it will give you that advantage. It also results in a lot faster germination.”

Raised Beds. “Raised beds are a huge help because the soil in many places is very, very thin and stony and lacking in organic material,” Carter said. “So you don’t want to completely change out or put in an utterly alien soil. It’s a lot easier to amend if you have very slightly raised beds. Four to six inches make a huge difference here.”

Pot Gardening. Carter said she likes the versatility of gardening in pots. “Our microclimates are so extreme here that it’s good to be able to move some things around — pomegranates, lemons, things like that,” she said.

“Geraniums grow amazingly here, so I have some of those in pots which I can move in and out for special occasions. Actually a number of the most attractive native succulents are especially dramatic in pots, so it’s a good idea to have several of those that you can shift around in the landscape as the season changes. Red hesperaloe makes a beautiful tall bloom stalk. It’s very dramatic in a footed Italian urn, so it gives you a lot of change in height level. I do have a lot of climbers. I have some large arbors and I’ve got Banksia rose, two different colors of crossvine, both white and coral honeysuckle, and a grape vine,” she said. “For special occasions I’ll move ornamental pots outside and move them back in at night.”

Hard- and Stonescaping. There are advantages to hardscaping and stonescaping, she explained. “Up here, especially because we have beautiful native stone and it’s a natural element, it incorporates easily into the landscape and the skyscape. It helps conserve water and keeps down some of our problem issues, such as cactus, thorns, poisonous insects, rattlesnakes and things of that sort, and provides good hunting for roadrunners to run around inside the stone­scape.”

Insect Control. Carter uses Bt to control caterpillars and insecticidal soap to control aphids. About Nolobait, she said, “If you’re going to use it, the sooner the better. You want to get the small grasshoppers while they’re very little. They cannibalize — the larger ones will eat the smaller ones — and that makes it more effective. Other than that, we try very much to avoid toxins out here. There’s enough around that you can’t control.”


The amazing view that drew Carter and her husband to Sky Island still stretches for miles in every direction, but the gardener in Carter has put her personal touch on a small part of the mountain with her tough-love garden, a garden that brings their view of native birds, bees and butterflies a little closer to the windows.

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