I CAN’T THINK of a rock garden I’ve visited anywhere that didn’t also have a few other garden features as well: a small patch of veggies, containers with annuals, and the inevitable troughs, for instance. Surely every garden has some trees or at least a wall that casts shade. In much of North America, forest is the norm, and gardens in the Midwest or on the coasts are often sited beneath enormous trees. Woodland gardens (and woodland rock gardens) are par for the course in much of America.
I have not kept a systematic tally, but after nearly 40 years of public horticulture, I think that the commonest question I’ve been asked by garden visitors (eliminating the inevitable “Where is the bathroom?”) is “What can I plant in dry shade?” Of course, dry shade in Colorado isn’t quite the same as dry shade in Seattle or Florida—if anything our shade is likely to be drier since we live in a semi-arid climate. There are stretches of drought every few decades where our climate can truly be classed as fully “arid” — under 8 inches (20 cm) precipitation a year. Any plant that can survive under that sort of dryness has got to be a toughie!
It seems to me that there’s been something of a movement towards more environmentally sound gardening. I’ve always thought one
of the tenets of rock gardens wasn’t just to recreate a wild evocative setting, but to try to create gardens that emulate nature—especially local nature—in an urban setting. For some, this means planting only plants that might have occurred naturally in your vicinity, something that I certainly applaud. But for most of us, this means adapting plants from various regions that will grow in your garden conditions with a minimum of effort and supplemental irrigation to produce a maximum of beauty.
This theory gets tested eventually in most gardens when the water system fails on a long vacation or when the gardener who created the garden moves away. Sometimes the new owners demolish what existed before they moved in. More often, the old landscape lingers, gradually morphing into something else (often with a liberal admixture of noxious weeds). Very rarely, a garden is tended thoughtfully by a new owner— but when that happens, something remarkable can occur.
That is exactly what happened at the old Rockmount Nursery in south Boulder, Colorado. This was a flagship nursery for the entire region for much of the first half of the 20th Century: the owner, Darwin Andrews, came to the sunny, dry climate of Colorado from England because of his tuberculosis. This worked for him (he lived to a ripe old age) and the nursery he created lasted until the mid 1950s when half of it was sold to the United States government, creating part of the campus of The National Institute of Standards and Technology which was built on a section of the old nursery (the plantings there, alas, were obliterated). But Andrews’ original home and the immediate surroundings of several acres were purchased by a gentleman of German extraction who did something amazing: he left everything exactly as it was, assiduously omitted weeds of any kind and left the ornamentals in the garden to proliferate. And spread they have! Even with Boulder’s steppe climate and average of 18 inches (46 cm) annual precipitation, a surprising range of trees, shrubs, and perennials have persisted for over a century—many spreading to make vast colonies. Since much of the property is wooded and has never been watered, this strikes me as a perfect demonstration of what tolerates dry shade! The plants at the old Rockmount form the core of my suggested taxa below.
Brunnera macrophylla seems to have an extraordinary range of tolerance in gardens. I found this in the wild almost within sight of the Black Sea, but it’s called “Siberian forget-me-not” since it does range into very cold parts of Russia. Most woodland gardeners seek out the variegated cultivars, but the old Rockmount nursery only had the traditional green-leaf form. There the original plants have now spread to make wide mats of fuzzy foliage. The heart-shaped leaves make an attractive and dense ground cover. Alas, it’s deciduous, but the vigor and utter drought tolerance of this plant make it worthy of inclusion in larger rock gardens. It can overwhelm delicate wildflowers, and so is best planted in difficult shady sites, but in these sites it does the job. The clouds of tiny, aquamarine, forget-me-not flowers last a very long time in the spring and complement companion plantings elegantly.
I was thrilled to find Trachystemon orientalis growing with its cousin, Brunnera macrophylla, together in the foothills of the Caucasus in the Adjara region near Batumi last May. Both species were in the last phase of bloom and early seed when we found them. Their foliage is quite similar, but the flowers are utterly distinct. I have not found trachystemon at the old Rockmount Nursery site, but I have a hunch it would give the brunnera a run for its money there. I have a few clumps growing in a dry border at my house, and they have been spreading steadily by the root (brunnera spreads mostly by seed). The vivid- blue, shooting star-like flowers are much larger than brunnera, and are showy in early spring. I look forward anxiously to its reappearance each March.
It’s not surprising that bulbs are drought tolerant. But Scilla siberica is also a shade lover. At the Rockmount site, this has spread with a vigor and abundance that’s truly astonishing. I would guess that there are millions of scilla growing at the site. They bloom a few weeks before the brunnera and corydalis that grow with them: I’ve yet to time a visit at exactly the right time to see the cobalt carpet they must make. This year, come hell or high water, I shall visit at the right time!
I have not yet found chionodoxa at Rockmount, but bulbs I planted at my parents’ home as a child a very long time ago have behaved in a similarly vigorous fashion. I can’t gauge if these can take quite the same dense shade that seems to suit the scilla—but they thrive in deciduous shade, and they have taken decades of dryness in that unwatered garden. There are a half dozen or more species whose names have leap- frogged back and forth, so I hesitate to recommend one before the other. They’re all pretty vigorous and tough, ridiculously cheap in the fall catalogs, and gorgeous, so it hardly matters which ones you buy. My advice: get them all!
It’s intriguing to me that corydalis have become so popular. Dozens of named bulbous kinds are sold at exorbitant prices. And there are the vivid blue Himalayans, that grow so well in cool, maritime climates and struggle for us in summer-hot climates. And then there is Corydalis nobilis—not quite bulbous with its swollen taproot, but then not really fibrous rooted like so many species in the genus. I have seen this in the wild on the “Austrian Road” in the Altai Mountains of easternmost Kazakhstan, growing on rocky road cuts with Bergenia altaica and Aquilegia siberica. Growing on subalpine slopes, I never expected them to tolerate drought. But at Rockmount’s old nursery there are dozens of luxurious clumps arching so gracefully among the brunnera. It can bloom steadily for nearly two months. I find that this spectacular plant can come to bloom just two years from seed if it is sited in the right spot. It will just keep getting bigger and bigger. This has earned its name of noble. It looks especially well paired with brunnera, which seems to thrive in exactly the same range of conditions and blooms over much the same period throughout April and May.
I suspect that all colchicums possess a good measure of drought tolerance, and they all seem to do best in part shade. I remember watching a colony of these appear every autumn in my neighborhood on the way to school. I never saw the Victorian rockery where they’d been planted watered. There are numerous lusty clumps of colchicum in one shady area of the Rockmount nursery garden where they must have persisted a century so far.
Tulips are not the first plant one thinks of when it comes to shade, but several species seem to grow as vigorously in deciduous shade as they do in full sun, and the common Tulipa tarda has almost become a pest in some local Colorado rock gardens. My friend Sandy Snyder of Littleton, Colorado, first planted 100 or so in a buffalo grass lawn in 1989 and within a decade they’d self-sown so vigorously that Sandy now mows the seed heads before they ripen. When they’re in bloom her buffalo grass is solid yellow in color, but so are the neighboring shady bits. This bulb seems to grow just about anywhere. The bicolored yellow and cream flowers open widely in the daytime, and close at night or in inclement weather. They bloom for weeks in cultivation, tolerating anything our fierce Colorado weather tosses at them! They can go through multiple snowstorms and show no sign of damage. Seedlings that have come up in our shady gardens are as floriferous and vigorous as the ones out in full sun and seem to glow a bit more brightly with a bit of shade.
I recently dug corms of Cyclamen hederifolium that had been growing unwatered at my family home for decades. Some were dinner plate size. In case you’re curious how long it takes for a cyclamen to make a corm like this, I can shed a little light on the subject. The specimen pictured in this article came from Mrs. Fisher, who had one of the loveliest gardens in Colorado filled with no end of bulbs in spring, and wildflowers like anemones and trilliums. From the time I was eight or ten I’d go
to Baseline Road (two blocks south of my house) where she lived and would peer over the wall at her garden. One day she noticed me and invited me in. After that, I’d ring the doorbell at propitious moments in spring and fall, and we’d chat and enjoy her garden together. The years went by, and in my thirties I got a call from her daughters. They said Mrs. Fisher wanted me to dig up whatever plants I wanted when she died. She had passed away, and her family remembered. I dug up five or six of the largest cyclamen in the garden. Mind you this was thirty plus years ago! They could well have been that old already when I dug them. The Cyclamen hederifolium in the picture were watered and cared for tenderly by Mrs. Fischer for decades, and got a modicum of care when they were first transplanted. But eventually, my brother-in- law ceased watering the rock garden they were planted in, and they continued to thrive and bloom prolifically. I would guess that these corms are seventy years old at the very least. Now they are finding a new home in my garden in Denver. Cyclamen are the aristocrats of dry shady gardens! No matter how much they spread, you will never have enough (friends are happy to take any excess in any case!).
The classic glory of the genus Acanthus hardly needs to be elaborated. Who hasn’t admired acanthus that have even naturalized
in California and elsewhere? Their naturalizing ways have roused a bit of suspicion among the weed police. Being Greek, I sought these out decades ago and have succeeded with a half dozen species, although A. mollis and A. spinosissimus needed very careful siting since they’re the most tender. There’s no question in my mind that A. balcanicus is by far the toughest of the genus that goes by so many Latin names (you will also find it as A. longifolius and A. hungaricus). I do recommend that you plant this where you want it. If you try to move it, the original clump will magically reappear in a few years. It propagates from root cutting, and no matter how deep you dig, it will come back again. It does self- sow with abandon. The solution to this is to cut the flower heads at peak of bloom, suspend them upside down from rafters, and Presto! You have the world’s most elegant dried flower arrangement.
Yes, acanthus can spread from seed to the point of being a nuisance. And the scilla and brunnera both have spreading tendencies that
raise the hackles of those with trigger fingers on the Roundup or concentrated vinegar bottle. Part of the modus operandi of toughies is that they persist and resist elimination. Siting is crucial. I wouldn’t be without any of these, but I do have my acanthus in a very tough spot, and I deadhead the seed heads before they explode. Some years I forget, and some seedlings appear which I find easily potted up to share. Not a problem!
