34 the American Gardener

HAVING SPENT a good portionof my childhood daydreamingon mossy ledges threaded with

Canada mayflowers, wild strawberries,and trout lilies, I have always been drawnto rocky places inhabited by plants. WhenI need to calm myself from life’s stresses,one of my most effective visualizationtechniques is to recall a weathered moun-taintop in New Hampshire’s WhiteMountains where crevices are jammed fullof lowbush blueberries and three-toothedcinquefoil (Potentilla tridentata).In my own Connecticut garden, the

beauty of an enormous hemlock—seed-ed long ago into a cliff face and nowpainted with lichens, mosses, and poly-pody ferns—regularly inspires me tohoist myself up the hillside for a closerview. I treasure these volunteers thatthrive in places I’d never attempt toplant, so over the last few years I havespent time investigating how—andwhy—these relationships work in differ-ent regions. For me, regionally nativeplants and rocks embody the architectureof nature and speak powerfully of place.Throughout North America, plants

enticingly colonize rocky areas of allkinds, from mountaintops to rock slides,outcrops, volcanic formations, crum-bling sandstone, limestone ledges, andgravelly depressions. Counterparts tothese rock formations, both natural orconstructed, are found in most of ouryards. Exposed bedrock and glacial rocksare unavoidable in my region. Stone alsoabounds in the built landscape, in side-walks, stone terraces and steps, stonewalls, and gravel driveways and paths.Plants with the tenacity to colonize

these difficult niches visually soften thehard surfaces and tie them—both aes-thetically and ecologically—to the rest of

Plants on the RocksTaking inspiration from the beauty of native plants growing on and around rocks in wild places,

the author offers ideas for using these plants in challenging garden settings.

ARTICLE AND PHOTOGRAPHS BY KAREN BUSSOLINI

One of the author’s Connecticut neighbors encouraged the hardy fragrant sumac (Rhusaromatica) ‘Gro-Low’ to colonize this rocky slope along a hillside driveway.

This article was published in the January/February 2010 issue of The American Gardener,the magazine of the American Horticultural Society (www.ahs.org).

the garden and the surrounding land-scape. The scenes that inspire us in na-ture—especially those close to home,where soil and climate are likely to besimilar—can inform us about gardeningin, on, and around stone. For instance, while visiting an

Arkansas mountaintop estate a few yearsago, I wondered why someone had riskedlife and limb to plant magnificent silverfiligreed American alumroot (Heucheraamericana) on treacherous crumblinglimestone ledges below the manicuredlawn. Then it dawned on me that theseplants, which had failed to flourish in thecarefully prepared soil in my own garden,were growing where they wanted to be,not where someone had planted them. Sometimes, as in the Arkansas garden,

desirable plants colonize rocks sponta-neously. But you can’t count on suchserendipity. Near my home, purpleloosestrife, a pernicious wetland invasive,blooms in the dry joint between pave-ment sections on a bridge. By proactive-ly planting appropriate native rockcolonizers that get the upper hand fast,

you can prevent unwanted plants fromtaking root first.

REGIONAL RECOMMENDATIONSIn his fascinating book, The Granite Land-scape: A Natural History of America’sMountain Domes, from Acadia to Yosemite(Countryman Press, 2002), ecologist TomWessels notes how similar plant commu-nities form on glacier-scoured mountain-tops across the country. This holds true foralpine plants from limestone regions aswell. Of course, not many of the specieswell-adapted to grueling mountaintopconditions will tolerate the heat, humidi-ty, competition, soil chemistry, high nu-trient levels, and lack of snow cover ingardens at lower altitudes. Serious rockgardeners who cherish these alpines con-struct special crevice and scree beds withjust the right pH and drainage to cater totheir plants’ needs. The rest of us, whowant to create plantings that require littlemaintenance, need to seek out plants thatare adapted to existing garden conditions.To get recommendations for rock-dwelling plants to supplement my own ex-

perience, I spoke with several experts indifferent regions of the country. (Addi-tional plant recommendations can befound in the web special linked to this ar-ticle on the AHS website, www.ahs.org).

NORTHEASTERN ROCK COLONIZERSSome of the plants Wessels observed ongranite mountaintops can be seen grow-ing naturally on rocky ledges at theCoastal Maine Botanical Garden inBoothbay. Among these are heath fam-ily members—such as lowbush blueber-ry (Vaccinium angustifolium), blackhuckleberry (Gaylussacia baccata), andbearberry (Arctostaphylos uva-ursi)—along with bunchberry (Chamaepericly-menum canadense, formerly Cornuscanadensis) and a tapestry of mosses andlichens. These plants thrive in Maine’scool climate and acidic, nutrient-poorsoil—even at sea level. Other ledge in-habitants, including hay-scented fern(Dennstaedtia punctilobula) and poly-pody ferns (Polypodium virginianum andP. appalachianum), are equally adapted torocky areas throughout the East.

35January / February 2010

CHARACTERISTICS OF GOOD ROCK COLONIZERSThere are certain physiological characteristics that help rock-dwellers survive and squeeze out the competition. Shallow rootsanchor plants and find nourishment in tiny pockets of duff or thin soil atop ledges. Rhizomes (underground stems) and stolons(horizontal stems that emerge above ground and form roots where they touch the ground) allow plants such as Canada mayflow-ers (Maianthemum canadense) and creeping phlox (Phlox stolonifera) to colonize thin soil. Plants with fibrous root systems,such as Christmas ferns (Polystichum acrostichoides), fill up the cracks with a network of fine roots.

In order to withstand fluctuations in temperature and soil moisture, rockdwellers often have fleshy roots or leaves to store water, leaves that aresmall and basally arranged, leaf surfaces covered with hairy or waxy coat-

ings, or silver coloration. Buckwheats (Eri-ogonum spp.) and other mat-formersspread out, shading out the competitionwhile cooling and conserving water fortheir own roots. Plants that sucker or self-layer, such as fragrant sumac (Rhus aro-

matica ‘Gro-Low’), worm their roots or stems around obstacles to send upshoots, or set down roots wherever they find soil.Penstemons produce great numbers of small seeds that wash into tiny

cracks with rainfall. Other plants that send out airborne seeds or spores, in-cluding mosses and ferns, also utilize this winning strategy.Some rock colonizers combine several of these adaptations, and success

often runs in families. In the wild, diverse goldenrods (Solidago spp.), pen-stemons, heucheras, pussy-toes (Antennaria spp.), junipers, phlox, andmembers of the heath and saxifrage families occur in and around rocksthroughout North America. Many of these plants are good choices for cul-tivating in the crevices of stone walls and in rocky garden sites. —K.B.

Heuchera villosa ‘Autumn Bride’and ‘Atropurpurea’ seeded them-selves in limestone steps in thisPennsylvania garden.

36 the American Gardener

William Cullina, the botanic garden’splant and garden curator, does as muchediting as planting. Cullina nurtures ex-isting ledge communities by removingblackberries and overly aggressive brackenfern in garden areas. He plants desirablenatives, including mosses, on and aroundraw, new stonework and unearthed ledges.Eventually, newly cultivated areas willblend seamlessly withthe extraordinarily beau-tiful natural landscape.Ana Hajduk, a gar-

den designer, adopts asimilar strategy in aclient’s more temperate,less-acidic New Yorkwoodland garden, em-ploying unfussy easternplants whose nativerange extends from theDeep South to Canada.Stone retaining wallsand steps were builtover and around ledgeoutcrops, soil was re-moved to expose ledgein places and added tomake even paths andplanting beds in others.She planted vigoroushay-scented ferns torapidly cover areas un-derlain by thin soil.Dainty but tough foam-flowers (Tiarella cordifo-lia ‘Running Tapestry’)drape over retainingwalls, blooming in spring with easternred columbine (Aquilegia canadensis)growing between stones. She thins andtransplants the prolific foamflowersthroughout the garden and scatterscolumbine seeds to keep the combina-tion going. Solidago rugosa ‘Fireworks’and white wood aster (Aster divaricatus,syn. Eurybia divaricata) naturalize inledge fissures in dry shade and are easy toremove where they are not wanted.

PLANTS FOR THE SOUTHEASTThe Southeast is certainly not known foralpines, but Jan Midgley, an Alabamanursery owner and author of severalSouthern wildflower guides, says there’sno lack of native rock dwellers that arewell-adapted to garden settings.

Among the plants she has seen grow-ing in sandstone glades and rocky out-crops are perennials such as native asters,greater tickseed (Coreopsis major), blaz-ing stars (Liatris spp.), woman’s tobacco(Antennaria plantaginifolia), azure bluet(Houstonia caerulea), and pineywoodsdropseed (Sporobolus junceus), a grass sheconsiders worthy of a spot on a rock wall.

Woody vines such as crossvine (Bignon-ia capreolata) and yellow jessamine(Gelsemium sempervirens) will root intocracks and scramble over rocks.From her observations of plants

growing on or near limestone outcrops,she suggests that wild petunia (Ruelliahumilis), American alumroot (Heucheraamericana), fragrant sumac (Rhus aro-matica), native prickly pear cactus (Op-untia humifusa), the annual Sedumpulchellum, and bird’s-foot violet (Violapedata) would make themselves equallyat home in neutral to alkaline garden

settings such as a limestone terrace oralong a cement foundation or marble-chip walk.

SOUTHWESTERN ROCK DWELLERSWhile exploring the dry west for his book,Chasing Wildflowers, Tucson garden de-signer and author Scott Calhoun thoughtthe breathtaking beauty of the wild plantcommunities he saw might make it hard-er to design gardens. Instead, he gained a

Northeast natives such as hay-scented fern,bunchberry, and Canada mayflower minglenaturally on rocky ledges at the CoastalMaine Botanical Garden.

ResourcesChasing Wildflowers by Scott Cal-houn. Rio Nuevo Publishers, Tuc-son, Arizona, 2007.

The Encyclopedia of Grasses for Liv-able Landscapes by Rick Darke. Tim-ber Press, Portland, Oregon, 2007.

Ferns for American Gardens by JohnT. Mickel. Timber Press, Portland,Oregon, 2003.

Native Ferns, Moss & Grasses byWilliam Cullina. Houghton MifflinCo., Boston, Massachusetts, 2008.

Rock Gardening: A Guide to GrowingAlpines and Other Wildflowers in theAmerican Garden by H. Lincoln Fos-ter. Timber Press, Portland, Oregon.Reprinted 1982.

SourcesEastern Plant Specialties, Rahway,NJ. (732) 382-2508. www.easternplant.com.

High Country Gardens, Santa Fe, NM.(877) 811-2700. www.highcountrygardens.com.

Prairie Moon Nursery, Winona, MN.(866) 417-8156. www.prairiemoonnursery.com.

Prairie Nursery,Westfield, WI. (800)476-9453. www.american-natives.biz.

Siskiyou Rare Plant Nursery,Talent, OR. (541) 535-7103.www.siskiyourareplantnursery.com.

new appreciation for stone in the designedlandscape, an expanded palette of plantsto try in garden settings, and a refined sen-sibility of what sustainable gardening inthe desert is all about.Desert wildflowers are mostly annu-

als; their survival strategy is to lie dor-mant until winter rains allow them togerminate, bloom extravagantly, set seed,and disappear. The ability of these plantsto thrive in poor, rocky, often alkaline,soil recommends them for use in unim-proved soil with gravel or decomposedgranite mulch, and in places where dig-ging is impossible. Calhoun points out that while we

think of rocky places as dry, rocks actu-ally create microclimates by holdingwater in their fissures. Throughout thedry West he spotted cacti and yuccas inrocky places and noticed that agaves,Echeveria spp., and Dudleya spp.—rosette-forming succulents that look likea cross between agaves and Echeveria—grow almost exclusively on rock.Penstemons and buckwheats (Erio-

gonum spp.) inhabit rocky nichesthroughout the West. Both genera grow

best on poor, dry soil.Calhoun recommendsrock penstemon (Penste-mon baccharifolius), atwo-by-three-foot SouthTexas native that hasleathery green leaves andcoral pink tubular flow-ers in summer; Eaton’spenstemon (P. eatonii)found on scree slopes inUtah canyons, but justas happy in hot Tucsongardens; and canyonpenstemon or desertbeardtongue (Penstemonpseudospectabilis), ashrubby, upright plantthat bears rosy to purpleflowers. Calhoun likesto combine canyon pen-stemon with yellow-flowered buckwheatvarieties such as selec-tions of E. wrightii. An-

other yellow-flowered selection, E.umbellatum var. aureum ‘Psdowns’ (Kan-nah Creek®) is recommended for colderRocky Mountain gardens.

NORTHWESTERN ROCK COLONIZERSPeople often think that it rains all the timein the Pacific Northwest, but rain is rare allsummer, and mountain ranges create hugerain shadows in areas to the east, makingconditions there more comparable to drierdesert regions. Throughout the Northeast,geology and climate have conspired to fos-ter astonishing plant diversity, which inturn has attracted adventurous gardenerswho seem to grow everything beautifully,even on rocky ground. On a recent drive through the Colum-

bia River Gorge, I spotted glossy evergreencreeping mahonia (Mahonia repens), redflowering currant (Ribes sanguineum),and snowberries (Symphoricarpos spp.)adorning a crumbling moss-covered cliff.These familiar garden shrubs seem likeideal choices to colonize rocky, hard-to-dig slopes in home landscapes not only inthis region but possibly in sections of theRockies and the Northeast.The West Coast is home to hundreds of

rock-colonizing manzanitas (Arctostaphy-los spp.), most of which, according to SeanHogan, owner of Cistus Nursery in Port-land, Oregon, are only suitable for gardensin western Oregon and California. One ex-

37January / February 2010

Dudleya hassei, a California nativesucculent, sprawls happily over a rockoutcrop at the San Diego Botanic Garden.

Eriogonum allenii and ‘Thai Silk’ California poppies, California fuchsias, and variegatedyuccas combine well in this gravel garden at Northwest Garden Nursery in Oregon.

ception is bearberry, or kinnikinnick (A.uva-ursi), a species that also thrives inupper New England and adjacent easternCanadian provinces. Many fine cultivars ofthis plant are available. Bearberries arehardy to USDA Zone 2, but will sulk orwaste away if not provided with very acidicsoil and good drainage.I also found inspiration at the whole-

sale Northwest Garden Nursery, west ofEugene, Oregon, where Marietta andErnie O’Byrne’s gardens overflow with

color and dramatic contrasts. The drygarden’s fast-draining, rock-studdedraised beds are edged with chunks of anold concrete sidewalk and mulched withgravel. Smooth river rocks embellish andserve as place markers for small dormantbulbs. Delicate ‘Thai Silk’ Californiapoppies dance lightly among the stiff up-right blades of various variegated yuccas.Mounds of silver-leafed Eriogonum um-bellatum and E. allenii are at home in therocks, along with well-behaved low-growing California fuchsia (Epilobiumcanum, formerly Zauschneria californica)

cultivars such as ‘Select Mattole’, whichhas silver leaves, and the green-leaved se-lection, ‘Dublin’.

TIPS FOR SUCCESSRocky places can be challenging to plantand maintain. The key to naturalizingnatives in the garden—getting them tonot just live, but to reproduce—is to putthem where they want to be, and givethem what they need, but to avoid pam-pering them. If they prefer nutrient-poor,

gritty, acidic substrate, fill the cracks withfast-draining decomposed granite; givewoodlanders rich, water-retentive leafmold. Water to establish, but don’t over-do water or fertilizer.Finding the right plants can be a chal-

lenge, but make sure you purchase plantsfrom responsible nurseries; don’t digplants from the wild however temptingthat may seem. If you have any moss in-habiting your rocks, ferns are likely to ap-pear, so avoid dislodging the moss. In tightspaces use small plants—nursery plugs areideal—to encourage development of self-

sustaining colonies. When building a rockwall, planting between stones as you gogives roots a firm foothold.Experimenting often yields unexpected

successes. For example, Pennsylvania-based designer Larry Weaner likes to planta few prodigious self-seeders such as Pen-stemon hirsutus at the top of a rock outcropand let seeds wash down into cracks whereseedlings will out-compete everything else. One of my Connecticut neighbors

planted a few small ‘Gro-Low’ fragrant

sumacs in full baking sun on almost solidrock and loose blasting debris beside hismountaintop driveway. He discovered thatif you break off a branch tip and stick itinto the ground, or if you skin some barkfrom the bottom of the branch and weighit down with a rock, it will root. Thesprawling stems trap leaves and debris thatimprove the soil—just the way they do inthose wild rocky places near and far. �

Garden photographer, author, and lecturerKaren Bussolini gardens on a rocky moun-tainside in Connecticut.

38 the American Gardener

In this Connecticut landscape designed by Larry Weaner, Penstemon hirsutus thrives on a dry ledge and ostrich ferns fill damp pockets in the rock.