WHEN I MENTION rock gardens to casual gardeners here in Texas, the response is always “Oh, I love cacti and agaves.” When I then try to explain how I aim to mimic the overall look of a northern rock garden, with a variety of miniature, compact perennials, dwarf shrubs, and other small plants with the growth habit of alpine plants, there is then the knee-jerk reaction that those plants will never take the Texas heat. If I try to detail further how there are so many small plants native to Texas, the South, and from other hot, humid climates of the world which, when properly planted among well-positioned rocks, can create the same effect, I usually have totally lost them. To most people, a rock garden in the South only means xeric, succulent plants and can be nothing more.
Before I moved to Texas to assume the role of director of horticulture at the amazing Peckerwood Garden just northwest of Houston, Texas, I gardened in a similar Zone 8b climate of north Florida. The few southern rock gardens I was exposed to in my travels that influenced me were still rather northern, in no less than zone 7b. I enjoyed Atlanta Botanical Garden’s rock garden; but most initially captivating for me was Tony Avent’s first scree garden next to his original house at Juniper Level Botanic Garden. Among the many intriguing plants I had never seen before were things that I had already grown successfully in Florida but were better displayed in this situation. Soon, the JC Raulston Arboretum installed their scree garden, while more extensive scree trial gardens sprang up at Juniper Level Botanic Garden. Some of the plants used were even favorites native to the Florida sandhill scrub floral communities of my southerly stomping grounds, including Conradina spp. and other scrub mints, Amsonia ciliata, Liatris spp., and Spigelia gentianoides. I, therefore, surmised that similar gardens could be created in Zones 8-9 in the coastal plain with proper plant selection.
Beyond the obscure genera I tend to grow, I soon learned from a few small experimental rockeries, crevice gardens, and scree gardens around my Florida property that I was having great success with culinary herbs that were considered difficult to impossible in the area’s wet summers, including lavender, creeping thyme, and other species of Mediterranean origin. This prompted me to try other ornamental plants from summer-dry climates and further inspired Jonathan Lubar to install a small scree mound in the bulb garden he maintained when he volunteered at Kanapaha Botanical Gardens in Gainesville, Florida. This resulted in surprising successes with some South African geophytes as well as long term success with Delosperma cooperi, which is readily available in the region yet typically dies quickly in the average garden.
It quickly became apparent that the type of gravel used made all the difference for many plants, especially those from areas that didn’t naturally experience as much summer rain and ambient humidity as they were subjected to in the southeast. Gardeners in north Florida,
the Gulf Coast of Texas, and points between tend to gravitate towards the readily available “pea gravel” or “Chattahoochee gravel” for their cactus and succulent gardens, which is available in several grades and is composed of round, river-polished quartzite in mixed hues of brown, copper, orange, and white. I had always observed that this glassy-textured gravel kept conditions far too wet where the gravel and underlying soil interfaced. I believe this contributed to an unwanted humid microclimate around the plant, rather than the desired reflected heat which would help bake the foliage dry when the sun came back out after a summer thunderstorm and extinguish any humid conditions trapped in the dense crown of the ground-hugging shrubs.
Besides the negative effects of pea gravel, I never liked the appearance of it in a rock garden. Instead, I wanted to mimic the erosional features I saw in the mountains – irregular, jagged chunks of rocky scree, as opposed to consistently rounded pebbles. Though
the scree gardens in North Carolina used expanded shale, this was difficult to find in Florida. The gray granite gravel we had available there offered a similar look but was quite heavy and expensive to import from northern Georgia. I lucked out with one supplier offering a load of half-inch (1.27 cm) granite for a tremendous discount as it had been minimally “contaminated” with other nearby piles of contrasting types of gravel, an effect that I found to appear even more natural. Furthermore, it reflected heat well, and the generally flat granite chips formed somewhat of a shingling crust that repelled the penetration of excess water. It allowed the root systems to stay cool and adequately moist in summer, yet not excessively wet and cold as a layer of pea gravel unfortunately would. I was pleased to learn when I moved to Texas that expanded shale is readily available here, much cheaper than granite, and providing the same aforementioned benefits that the granite gravel had produced, while also being lighter.
Those who have visited Peckerwood Garden are struck by John Fairey’s award-winning landscape design that utilizes plants so unfamiliar to the region, or to any garden worldwide. The garden push the limits further with a great juxtaposition of xeric gardens transitioning into contrasting woodland assemblages using floral textures to their greatest advantage. The dry gardens, essentially scree gardens due to their topography and gravel mulch, are where John features his many important wild collections of agaves, yuccas, and other woody lilies along with xeric trees, palms, and other complementary, exciting plants from around the world. In addition to architectural macroflora, the signature feature of these dry gardens is pea gravel. Despite my thoughts about pea gravel, I mean this in no way to be a criticism of John’s choice. The gravel harmoniously defines the magic of his landscapes so well, perfectly creating the impact John so effectively set out to make. Combined with the topography modified for drainage, it is of no detriment to the plants he has chosen. He is not trying to grow the species I was aiming to grow, and the entire visual effect is remarkably striking.
Peckerwood’s mission includes continued growth of our collections for plants of conservation importance, as well as seeking new, adaptable plants that can diversify area landscapes should they survive our trials in the harsh extremes they face in Texas. I view the rockery I set up near Peckerwood’s office as a way to broaden visitors’ minds to gardening tactics and plant palettes that southern gardeners can utilize for diversity and satisfaction. It is effectively building upon the impactful results of John’s creation, and with our expanding educational mission, offering tremendous value.
It has been almost three years since our trial rock garden was installed using warm-climate analogs that grow in clumps, cushions
or are otherwise small-statured, alpine-esque species that would be lost in a typical garden bed. I intentionally made sure to exclude the predictable cacti, agaves, and other species expected of a Texas rock garden, and otherwise featured so well in John’s dry gardens. Included are things that I already knew would prosper, while also trialing many species for the first time. As far as failures, some were surprising, and some were not. We did trial some interesting things donated by other gardens and collectors that originated from higher, cooler elevations that I was skeptical would survive our hot summers, and in most cases, they didn’t. Other things I feel do stand a chance here based on their natural tolerance to heat and cold may have perished due to other reasons. These will be trialed again if the opportunity arises.
I have been amazed at a selection of fringed bleeding heart, Dicentra eximia ‘Dolly Sods’ that comes from the shale barrens of West Virginia and is offered by Plant Delights Nursery. In near-full sun it held luxurious lacy blue-green foliage and was blooming non-stop from spring through at least early August, at which point it abruptly died back. Poking around underground, it appears to have died outright rather than simply going dormant. Having lasted so long into the summer in such wonderful shape, I don’t think this was simply a heat- related issue, so this will be one we definitely will try again.
Though it hasn’t flowered yet, Achillea sibirica subsp. camtschatica has been quite a surprise. The species and varietal name clearly convey its frigid origins of the Kamchatka Peninsula of eastern Siberia, the same latitude as Alaska. The foliage is quite attractive, in no way resembling the more commonly known yarrow, Achillea millifolium. Another surprise from temperate Europe that hasn’t flinched over the summer is the pasque flower Pulsatilla vulgaris subsp. bogenhardiana. It will be even more exciting if it actually flowers this spring. As with a number of cautiously trialed plants, I do have this one situated on the north side of a strategically positioned rock where it stays slightly less scorching hot than it would experience in direct sun. Another European that seems quite unexpectedly tolerant of full sun and hot temperatures is Linaria vulgaris f. peloria.
Surprising not for its tolerance to our region but instead due to its foliar beauty is Hypericum geminiflorum var. simplicistylum, a Taiwanese St. John’s-wort donated by its collector, Mark Weathington of the JC Raulston Arboretum. The oval leaves emerge red, transitioning to purple and then to dusty blue, all bundled in a neat, compact clump. With the foothold it’s gotten over the last half of the year, it will surely embellish its beauty next year with showy yellow flowers typical of the genus. There is a whole world of interesting hypericum species, native and exotic, waiting to be better utilized for their ornamental potential, many of which fit in perfectly to a rock garden.
Other dwarf woody plants include my miniature compact selection of the Florida Sand Pine (Pinus clausa), grown from seed collected from a witch’s broom. The interesting prostrate mat of Dalea capitata ‘Sierra Gold’, a Mountain States Nursery introduction, forms a soft-textured groundcover with gold flowers in late summer and looks especially attractive flowing between and spilling over strategically positioned rocks.
Several geophytes are doing well in the rock garden. From Argentina, Nothoscordum sellowianum is a favorite winter highlight, forming a dense patch of short thin leaves that more resembles a clump of dark green moss. When it flowers, the green is almost completely obscured by a mass of dark yellow flowers resembling miniature crocuses. Though in quite a different situation than naturally found, two species of trout lilies, my Florida panhandle collection of Erythronium umbilicatum and my east Texas collection of E. rostratum, are flourishing so far. Typically found in forest understory, they would otherwise get overlooked in our woodland garden while here they can be better showcased in their own pockets among taller, shading rocks.
Among some interesting Mediterranean plants doing well are several germander species, most showy being two ashy-white fuzzy species, Teucrium polium and T. gnaphalodes, both donated by Denver Botanic Gardens. I’m excited that two species of Globularia (aka globe daisies) have established well and are starting to form mats of rosettes composed of delicate spoon-shaped leaves. It will be nice if they produce their vivid blue flowers next year. Dwarf cranesbill (Erodium x variable ‘Bishop’s Form’) developed a very neat, tidy mound and produced its delicate pink flowers, making me want to track down the two parent species of this hybrid.
Losses among Mediterranean plants include several Veronica species, and most disappointing were Draba hispanica and Plocama calabrica, two that I was enamored with in Denver Botanic Gardens’ collections and optimistic they would survive if sited properly based on their natural conditions. I am not giving up on these yet! Speaking of Denver Botanic Gardens, their exceptional rock garden has been a tremendous inspiration and got me addicted to the genus Eriogonum. We are growing a few Texas natives well, including the silvery Eriogonum tenellum, but I hope to find some of the alluring species from further west that will adapt to our conditions.
I have long been fascinated by xeric ferns and selaginellas, and have been building up a collection of our native Texas and southwestern Cheilanthes, Pellaea, Astrolepis, and others, along with the mat-forming Selaginella species that look amazing as a backdrop for more structurally interesting plants. Unrelated but also from the southwest, Arizona to be exact, is a dwarf pipevine, Aristolochia watsonii, with its low carpet of elongated purple leaves veined with chartreuse patterning. Another small-statured relative from Europe, Aristolochia sempervirens, has made a tidy, compact evergreen mound of emerald leaves occasionally punctuated by its otherworldly flowers. When either species is in bloom, we have observers on their knees, both begging us to share, as well as simply to get close enough to best appreciate their alien form!
Plenty of southeastern native plants have requirements and appearances that lend themselves well to this style of rock garden. Among those from the Southeast doing well are various forms of Viola pedata (bird’s foot violet) along with various species of phlox, penstemon, scutellaria, baptisia, and silene. The dwarf shining blueberry (Vaccinium myrsinites) from Florida produced a few fruits, and many other denizens of the southeast’s sandhill scrub habitats offer tremendous potential. Two Florida native members of the aster family that I am very fond of are Garberia heterophylla and Chrysoma pauciflosculosa, which are slated to be planted shortly.
This article just scratches the surface of the interesting plants we are trialing in these growing conditions, and there are many additional options I am eager to try. I hope this style of gardening will encourage others in the southern United States to experiment with rockeries for aesthetic enjoyment, even if they choose to include cacti and agaves.