- Cacti and Other Succulents to Know and Grow
- Crazy About Cacti and Other Succulents
- A Gardener's Guide to Cacti and Succulent Conservation
- Hardy Cacti: Living Sculptures of the American West
- Hardy Succulents: Drought-Tolerant Beauties for All Regions
- Medicinal Cacti and Succulents: Powerful Plants for Windowsills, Light Carts, and Backyards
- Ceropegias: Fabulous Vines of the Succulent World
- Stapeliads: Sea Stars of the Desert
- Growing Cacti and Other Succulents
- Growing Cacti and Other Succulents on Windowsills and Under Lights
- Propagating Cacti and Succulents
- Succulent Bonsai: Creating Dramatic Miniature Trees
- Grooming Cacti and Other Succulents for Exhibition
- Green Roofs: New Homes for Sedums and Other Hardy Succulents
- Nursery and Seed Sources
Hardy Cacti: Living Sculptures of the American West
By Panayoti Kelaidis
Hardy cacti have two key requirements: good drainage—especially in winter—and abundant sunshine. For this reason, they do best when grown among rocks. A rock garden or stone wall provide both a beautiful setting to display cacti and ensure the sharp drainage they need to grow as vigorously and sturdily as they would in nature. In the Rocky Mountain and Intermountain regions that receive less than 20 inches of precipitation annually, cacti usually grow well in heavy clay soils as well as sandier soils. In wetter regions, they are generally cultivated in a gravel and sand mixture. A slope with a generous three- to eight-inch topdressing of a gravel and sand mix provides more than enough drainage even in wet climates. This type of treatment helps to keep the vulnerable crowns dry and therefore less prone to rot and allows the cacti roots to reach down into the soil, where they find nutrients and moisture needed for good growth, especially in summer. Because these plants have evolved in dry regions where soil nutrients are not regularly leached by precipitation, if you garden in sand or gravel you may need to fertilize your cacti regularly. They will grow in lean soil, but to thrive they need nutrients. I prefer to use inorganic fertilizers such as Osmocote or Sierra Blend: usually a three-month formulation. Always err on the side of caution with succulents: They need to shut down growth in late summer and excess fertilizer can cause weak growth when frost comes.
Hardy Cacti in Containers
Hardy cacti also do well in containers, either in stone or stonelike materials such as hypertufa troughs or frost-proof stoneware containers. In a pot you can control the growing medium to provide the perfect mix for your plants. Even in dry Colorado, I use plenty of grit and some sand along with heavier loam to supply ballast and nutrients. Most succulent fanciers like to use scoria or other gravelly amendments to their favorite potting mix for succulents. The wetter the climate, the more gravel you need to add to the soil. I rarely use more than one-third scoria in my garden, but two-thirds may be too little in a rainy climate.
Many of the more moisture-sensitive Southwestern cacti, such as plains cactus (Sclerocactus) and fishhook cactus (Pediocactus), which invariably succumb when cultivated in open soil, no matter how well drained, thrive in containers.
A Collection of Hardy Cacti
Cacti are, of course, an endless source of fascination to gardeners in regions where the plants are native, and it is intriguing that they can provide so much pleasure to gardeners in cold climates that aren't traditional cactus territory. I remember seeing a large collection perched on a balcony in China. I have corresponded with enthusiasts across Europe and Asia who all want to capture a bit of the magic of the Southwestern landscape in the form of these most classic, gorgeous, and utterly defiant American wildflowers. The species below are just a few of the many plants available for growing in gardens and adaptable to a wide range of growing conditions.
Hardy to –20°F, Coryphantha vivipara var. vivipara is the standout of another large genus containing many hardy plants. I grow a dozen or more selections and feel as though I have just scratched the surface. Imagine a baseball completely obscured by white or tan spines, and you have an instant image of this cactus when it's not in bloom. Some have stems that grow over eight inches in diameter, and some form clumps that reach over a foot across in nature. The flower color varies from pale pink to deep purple-rose, and the flowers can open from May through the summer months, depending on the individual plant. There are also half a dozen subspecies from the Southwest available commercially. A miniature form of the species that is especially white-spined and petite is sometimes found throughout the range of the species. Unfortunately, it is not distinguished botanically.
Just a trifle more tender are Coryphantha sulcata and C. scheerii, which can survive to –10°F if kept dry. Both have huge bright yellow flowers.
At first glance, the tiny ball cactus Coryphantha missouriensis seems very much like C. vivipara. The strange copper flower color and brightly colored fruit will quickly distinguish it. This cactus, hardy to –25°F, grows over much the same range as C. vivipara, yet is far less often found in gardens. Though not exactly showy, the strawyellow, brown, or amber flowers are quite appealing, as are the bright red fruit that persist from late summer to spring. It is variable over its huge range and worth obtaining in various forms.
The glorious hedgehog cacti with their cylindrical symmetry, like lacy columns or gothic spires, will always hold pride of place in a hardy-cactus collection. Over a dozen species have survived repeated subzero winters, and some of these include a number of subspecies and forms, so just one genus can provide a lifetime's opportunity for the hardy-cactus lover.
The main attraction among these cacti is unquestionably Echinocereus reichenbachii, which has the greatest cold tolerance (it is hardy to –25°F) and is the least sensitive to excess moisture. Commonly known as lace cactus, this wonderful species is quickly and easily grown from seed and offers a tremendous range of flower color, spine development, form, and stem size. Different subspecies of lace cactus can range in size from a few inches to nearly two feet, with pure white to nearly black spines.
Fendler's hedgehog (Echinocereus fendleri), hardy to –20°F and almost as versatile as lace cactus, also has a number of unusual forms. The rare, corky-spined E. fendleri var. kuenzleri has become quite inexpensive in recent years and is often available. Claret cup cacti (Echinocereus coccineus, E. polyacanthus, E. triglochidiatus), also hardy to –20°F, occur in endless permutations, almost bewildering in their variability in nature and in cultivation. Flower color ranges from near orange, scarlet, and pink to deep crimson shades. Stems can vary from spineless to wildly tousled and reach anywhere from three inches to several feet tall, as in the White Sands form of E. triglochidiatus var. gonacanthus. Eventually, most forms can clump up to form mounds of almost mythic proportions. Studded with brilliant badminton-birdie flowers, they can stop traffic. Peak flowering comes from late April to June, depending on variety.
Among other cold-tolerant Echinocereus cacti is the lemon-scented E. viridiflorus, (–20°F), less showy but every bit as hardy as any of the species described above. A welter of hedgehog cacti generally found further south have northerly disjunct populations and high-altitude forms that are regularly subjected to subzero weather in nature and thus are hardier than other members of the species. These include E. dasyacanthus, E. engelmanii, E. enneacanthus, E. knippelianus, E. viereckii, and others. Most of these are doing fine in Denver, which lies in USDA Zone 5 (–20°F). I wouldn't try to grow most of them in wetter regions colder than Zone 7, however, unless they could be kept in an unusually protected microclimate with ideal soil conditions. Seek out the most cold-tolerant cultivar for testing. Larger plants tend to survive the first winter better than tiny seedlings. If you have small seedlings or plants, I recommend leaving them in the seed pot rather than planting them out too early.
Quite a few species in the genus Escobaria, which is closely related to Mammillaria, have proven themselves in a wide range of climates. Escobaria sneedii var. leei, hardy to –20°F, is one of our tiniest and rarest cacti, restricted in nature to the Carlsbad Cavern area in Texas. In cultivation it thrives almost everywhere in North America, in places as diverse as New England rock gardens and troughs in Colorado. It makes a huddled mound of marble-size stems densely covered in white-spines, with pale pink flowers in early summer. Escobaria sneedii var. sneedii, hardy to –15°F, is similar in form but larger and with pinker flowers. Larger still is E. organensis, hardy to –15°F. The largest relative is E. orcuttii, hardy to –15°F, from the mountains of southern New Mexico and yet extremely hardy far northward. These are all rather small plants, rarely more than a foot at the very biggest—either in height or spread. Escobaria hesteri and E. minima, which resemble dwarf races of Corypantha vivipara, can survive to –10°F in dry-winter regions.
Surprisingly, this prickly pear cousin with cylindrical stems and persistent tubular true leaves has grown vigorously for many years in gardens throughout the Pacific Northwest, where it is the best-performing outdoor cactus. It even blooms prolifically there, producing luminous, two-inch pale yellow roselike flowers. It is hardy to –15°F and can survive even colder temperatures in the interior West.
Opuntia, Cylindropuntia, and Grusonia
The endless variations of prickly pear, cholla, and creeping cylindrical opuntia are the glory and bane of the hardy-cactus garden: Few plants combine such brilliant flowers and architectural stems and forms with such painful spines and glochids (spines that come off easily). The secret for growing Opuntia and closely allied genera is siting them where they won't need constant care. I prefer to grow them primarily in large pots, where they droop gracefully and are easily weeded and maintained. Strategically positioned among rocks, they can likewise be managed for years without the need for cleanup and cutting back that can cause gardeners such discomfort. And opuntias are so easily propagated by severing and planting a pad, or even part of a pad, that there is no excuse for not quickly building a wonderful collection.
Scattered over the eastern two-thirds of the United States and a short way into Canada, Opuntia humifusa may be the most widely distributed prickly pear. It is certainly the most widely cultivated. Most forms are nearly spineless, and even its glochids are less lethal than those of most western prickly pears. In summer, the bright green mass of stems provides a fine contrast to the predominant silvers and grays of other cacti, but in winter it wilts into a limp mass of darkened matter aptly described as "depressa." In spring, it resurrects miraculously within weeks of the return of warm weather. It can be dazzling in early summer, when it is intermittently covered with bright yellow flowers. Despite its wide range, there have been surprisingly few selections: O. humifusa 'Lemon Spreader' is one of the few cultivars. The closely related O. macrorrhiza of the southern Rockies seems to be much more variable in both pad shape and flower color. As with so many cacti, the potential for hybridization is limitless.
Over much of the 20th century, some of the finest Opuntia selections were made by Claude Barr, the great nurseryman of the Great Plains: His clones Opuntia 'Crystal Tide' (–25°F), 'Claude Arno' (–25°F), and others still form the basis of many hardy cactus gardens. These are mostly hybrids of O. polyacantha, the commonest cactus in the West.
Opuntia fragilis (–35°F) has the honor of being the most northerly cactus, growing hundreds of miles north of the Canada-U.S. border. It has provided many fascinating selections, including nearly spineless forms with spherical pads and bright bright pink or yellow flowers produced quite generously. Hunger cactus (O. polyacantha, hardy to –25°F) must be the most abundant western species, with flowers in every imaginable shade of yellow to pink and deep red. Opuntia phaeacantha (hardy to –20°F) is nearly as variable, with statuesque pads, especially in the tall variety (O. engelmannii, hardy to –15°F). It is not unheard of to find these up to six feet tall.
Few plants are as statuesque as Cylindropuntia imbricata (hardy to –15°F) and its many cousins: C. echinocarpa (–15°F), C. kleiniae (–20°F), C. leptocaulis (–20°F), and C. spinosior (–15°F). These form tall, almost treelike candelabra with midsummer flowers and showy autumnal seedpods.
The creeping cylindrical opuntias of the Southwest have had a botanical name change and are now called Grusonia. The hardiest of these is undoubtedly G. clavata (–25°F), with wide, bright white sheathlike spines and a dense, creeping habit that is as appealing as the two-inch pale yellow flowers. In nature, this plant can spread many feet across: Gardeners are pleased if it spreads to a foot after many years, however.
Pediocactus and Sclerocactus
Plants of high, cold, dry mountain ridges or hot deserts, most species of plains cactus and fishhook cactus grow in regions where subzero cold is a frequent winter phenomenon, making them endlessly teasing to gardeners in cold climates. They are useful in dry-region gardens where gardeners control their impulse to water, but they are only apt to gain wide currency in wet regions if and when they can be obtained grafted onto moisture-tolerant rootstocks, such as Opuntia fragilis or perhaps Coryphantha vivipara. Though grafted plants are rarely available, you can perhaps do the grafting yourself—it's not as hard as you might imagine.
Quite a few unusual ball cacti have gained wide currency, although they are more challenging in wetter regions: Echinocactus texensis, Echinomastus intertextus, Epithelantha micromeris, Hamatocactus bicolor, and Mammillaria wrightii all have cold-hardy variants that can survive subzero cold provided they stay dry. Gardeners in the interior West have brought a surprising range of South American cacti through many winters, including Echinopsis (often sold and listed in catalogs as Lobivia), Gymnocalycium, Parodia (often sold and listed as Notocactus), as well as numerous opuntioids such as Maihueniopsis, Puna, and Tephrocacti, which are all subject to extreme cold in the Andes. There is now such a wealth of data available on many South American cacti that I expect they will be a large new basis for experimentation in the coming decades.Panayoti Kelaidis is a plant exlorer, gardener, and public garden admninistrator at Denver Botanic Gardens, where he is now director of outreach. He began his career at the garden in 1980 as curator of the rock alpine garden, where he designed and oversaw the initial plantings of the now extensive garden. He has a special passion for hardy cacti and is responsible for introducing most of the USDA Zone 5 hardy South African succulents currently in cultivation.
Ceropegias: Fabulous Vines of the Succulent World
By Sage Reynolds
I saw my first photograph of a ceropegia when I was about ten years old. It was Ceropegia racemosa subspecies setifera, and I was struck by the unusual flower and its graphic coloring. This was a plant I wanted to grow. However, it wasn't until about 30 years later, in the late 1980s, that I was able to find and begin to grow these vines with their weirdly sculptural and colorful flowers.
Ceropegias are plants of tropical and subtropical regions from the Canary Islands (where Ceropegia fusca is native) through Africa and Madagascar to China, Indonesia, and northern Australia. They are found in a wide range of habitats from equatorial forest to semidesert.
The wide climatic and geographical range gives rise to a variety of plant shapes and habits. Most ceropegias are vines—including all that are discussed in this article—but there are some that resemble small bushes, stands of gray organ pipes (Ceropegia fusca), slithering snakes (C. stapeliiformis), and legless lizards (C. armandii). The stems can be as thick as cigars (C. stapeliiformis) or as thin as twine (C. ampliata, C. leroyi). The leaves usually appear in pairs opposite each other, at nodes along the stems. The leaves can be minuscule (C. stapeliiformis), very succulent (C. sandersonii), paper thin and deciduous (C. elegans), large and broad (C. cumingiana), needlelike (C. dichotoma), or absent (C. devecchii). The roots of some ceropegias are fleshy; others are fibrous; and some are tuberous, the result of the different species evolving in areas with seasonal drought and terrains of varying soil qualities.
Ceropegias as a group are unusual houseplants. Except for the common Ceropegia woodii and its cousin tuberous types, the vines are too vigorous for casual indoor growing. Provided they get good light, warmth, and moving air, many will quickly fill a four-foot tomato-cage support and reach for more.
Flowers appear at the nodes, where both leaves and roots also form. They usually consist of a tube with hairs inside that point downward. The tube may be straight or have bends (Ceropegia aristolochioides) or bulges (C. crassifolia, C. rupicola). Its five corolla lobes (or petals) may be open (C. stapeliiformis) or joined at the tips (C. ampliata). The flowers are usually whitish, with combinations of bright green, brown, and maroon spots, stripes, and shading. The buds start out looking like small bullets, and as they grow they can develop bulges or curves, tops becoming umbrellas (C. sandersonii, C. monteiroae) or cages (C. ampliata, C. armandii). The bud of C. devecchii looks like a Buddhist vajra symbol before opening. Ceropegia distincta subspecies haygarthii resembles a spotted Venetian wineglass stuffed with little pillows and topped with a fuzzy antenna. The flowers can grow from almost nothing to 3.5 inches long in little more than a week. Blooming is triggered by a combination of increasing day length and warmer nights. In late spring, when nighttime temperatures are rising and the daylight hours are lengthening, ceropegias such as C. conrathii respond with faster growth and start blooming. Others, such as C. sandersonii, wait until the heat of midsummer, and still others, such as C. ampliata, don't flower until the days begin to shorten noticeably in fall.
Soil quality does not seem to be very important for growing ceropegias successfully, but good drainage is vital. Regular applications of 20-20-20 fertilizer at half strength are beneficial during growth periods. The real key to a collection of gorgeously flowering ceropegias seems to be adequate warmth. They all benefit greatly from a warm summer spent outside in bright filtered light but out of direct sun. Most ceropegias are happy when temperatures are in the high 70s at all times, but they will tolerate cooler temperatures in winter while they are dormant. A sunny windowsill is all that some will require then.
Most of the ceropegias I have grown tend to slow down or go completely dormant in winter. Responding to their changed needs in winter is key to their well-being. Many species develop seasonal roots that dry up and die off in the plant's dormant season. During winter, the tuberous types need to be kept dry (C. conrathii is especially prone to rot) and should be watered only when evidence of new growth appears in late spring. Many of the thinner vines (C. woodii, C. ampliata, C. linearis, C. elegans) can tolerate low light levels and will continue to grow slowly throughout the cooler months and can safely be watered. Thick, leafless vines like C. devecchii should be kept dry, but it's fine to water C. stapeliiformis if it is still growing. C. crassifolia develops fleshy roots to which it dies back completely, only to reemerge in late spring. (Don't throw out that empty-looking pot!)
Pests and Diseases
Ceropegias have relatively few pest problems. Tuberous types and some of the deciduous vines can harbor the occasional mealy bug. In the greenhouse or during the summer outside, a slug may find a tuber tasty. Plants are most vulnerable right after they move back inside after a summer spent outdoors. A ceropegia that has bloomed with an extraordinary number of flowers is especially susceptible to sudden death. The plant may have exhausted itself or its soil and be too weak to cope with the change in light and humidity that are part of moving inside. Other ceropegias may die suddenly from a fungal infection or rot. Once a thick stem has black spots on it, there is little you can do beyond cutting the growing tips off the plant and trying to root them and discarding the rest.
Growing Ceropegias From Cuttings
The best way to a sustainable collection of abundantly flowering ceropegias is to take cuttings every year and discard the original plant after it's been in the same pot and soil for two years. (If you can't bear to discard the plant, at least repot it every two years.) Starting from scratch every two years is especially effective for vining types, which turn woody after a few years. Woody stems seem to lose vigor, but new stems sprout roots easily, and with new soil they will bloom more readily. Take cuttings any time the plant is growing. Allow the cut tip of a vine cutting to callus over for two to three days to prevent rotting. In spring or fall, put the cutting into either a clear plastic box or bag with a moistened mixture of half perlite and half peat moss until it develops roots. In summer, try curling the vines around in a pot or hanging basket on top of potting soil. In either case, keep the cuttings out of direct sunlight to prevent cooking them. If you successfully root cuttings, you'll have a number of plants that you can try growing in different areas of your home or garden. It's a useful way to find out where the plants thrive. Another advantage of multiples is that they serve as a form of insurance against losing the species from your collection. If you don't have luck or space for cuttings yourself, pass them on to friends who do. That way there's a chance that you can get a new start should you lose a rare plant.
Ceropegias to Grow at Home
Over the years I have grown many species from all parts of the Ceropegia world, except India and China, whose export policies make it extremely difficult to obtain living plant material or seeds. The following plants are among the easiest to grow and bring to flower. None of the vining plants are small. I grow vining types (C. sandersonii, C. lugardae, C. distincta) in seven- to ten-inch clay pots with a trellis made from an slightly modified tomato cage that forms a wire column on which the plants can easily climb. The tuberous types (C. ampliata, C. leroyi) are grown in hanging baskets of varying diameters. You can find more information about the plants discussed in this article and other Ceropegia species at http://sagereynolds.com.
Native to South Africa, where it is found in Gauteng, Kwazulu-Natal, and the Cape provinces, this plant has fibrous roots and minuscule to invisible leaves. Unbelievable numbers of white and green flowers up to two inches long appear for about two weeks in September, completely covering the plant. Ceropegia ampliata is fairly easy to grow in ordinary well-draining potting soil. The plant trails and weakly attempts to climb before it begins to bloom and is best grown in a hanging basket. In summer, I hang the plant on the north side of my porch in bright light but away from direct sunlight and water it generously, like all my other plants in hanging baskets.
This native of Tanzania is easy to keep and seems quite tame compared with some other varieties like Ceropegia aristolochioides and C. elegans, whose tendrils have to be controlled daily. Easily grown from cuttings, the plant has flower buds near almost every leaf in September. It thrives when grown outside for the summer, and the occasional cool night does not cause the demise of flower buds or other harm. During the early part of the summer, my plant was so small for so long that I set its pot and trellis into a pot where C. ballyana was growing (and not blooming). Now, several months later, they have entwined themselves so thoroughly, it will be difficult to separate the two. But it's easy to tell which vine is which: The leaves of C. denticulata are succulent and yellow-green with pointed ends; the leaves of C. ballyana are more succulent, slightly larger, and dark green with prominent central veins and round ends.
Ceropegia distincta subspecies haygarthii
Native to Angola and South Africa's Kwazulu-Natal and Cape provinces, it is one of the easiest ceropegias to grow. Only Ceropegia woodii, the common rosary vine, is easier. C. distincta subspecies haygarthii tolerates a wide range of conditions and will grow well in most home environments, though it may not bloom until it can be moved outside for the summer. This species is also less susceptible to rot if overwatered. The stems are a little more than 1/8-inch thick, the roots are fibrous, and the leaves are thin and deciduous. This vigorous plant will root where it touches the ground and easily fills a three-foot trellis. It's best to start new plants at least every two years if not every year, as the stems turn white after a year and seem less capable of supporting new growth. Flowers appear on new growth, and providing the growing area is warm enough, they will appear in profusion for a long period during the summer. With their fuzzy antennae, shaded corollas, and crisp maroon spots, they are delightfully comical. Temperature seems to be key, and I suspect that the species can be kept in continuous bloom in warmer climates, provided that the plants are repotted annually and fertilized heavily. I've had buds appear on my plants in early April as the days begin to lengthen noticeably, but all buds were aborted until nighttime temperatures stayed above 60°F.
This plant, another native of South Africa, is closely related to C. sandersonii and very similar in flower color and shape. It is fairly new in the marketplace, remarkably easy to grow, and a prolific bloomer. The bright chartreuse flowers are about three inches long and have a lively purple fringe on the lobes that moves in the slightest breeze.
Native to Mozambique and South Africa's Gauteng and Kwazulu-Natal provinces, this plant has very succulent, lance-shaped leaves, robust stems, and fleshy roots. It is fairly easy to grow and does best when it spends the summer outdoors in the same high shade that rhododendrons enjoy. Each spring, the plant sends up new growth from its roots. New stems give rise to a display of spectacular four-inch-long flowers. The vine may grow up to 12 feet long in the course of a summer outside. At the end of summer or sometime in fall, before you move the plant back into the house, cut it back drastically. Before winter I move the plant inside under the lights in my studio, cut it back, and water sparingly to prevent desiccation. The plant continues to grow more slowly through the winter.
Native to the Cape provinces of South Africa, Ceropegia stapeliiformis was one of the first ceropegias I collected and is one of the easiest to grow. In nature, the plant's stems grow horizontally along the ground toward a spot that offers dappled sunlight. C. stapeliiformis thrives underneath shrubs, where greater moisture promotes root growth and leaf mold provides nutrients. Shrub branches also offer supports on which the plant can climb and display its flowers higher up, where they may be more easily found by pollinators.
For the collector, the plant's main attraction is its thick maroon- and gray-mottled snakelike stem, but its flowers are also surprisingly bizarre. When a new plant gets ready to bloom, its stem thins and starts to climb. Once the stem is about two feet above the roots, buds start to form, giving rise to numerous 2.5-inch-long flowers. Their corollas open to expose a fuzzy white or pale green star. The plant seems to like a place in high shade where it gets some morning and afternoon sun. When I have grown it in full sun, the stem has always grown away from the light, snaking around pots to climb an Abutilon or Pelargonium before blooming. It has never set seeds, even though I have seen a lot of small flies on the corolla lobes while the flowers were open. To bring the plant into bloom in cultivation, it's best to take new cuttings each year. Simply break off new shoots that may already have formed roots and grow them in new soil.
Ceropegia stapeliiformis subspecies serpentina is a much shyer bloomer with a thinner, greenish stem that grows over three feet long before flowers form. One of my plants produced only one elegant flower—all green and white without any red in it—over the course of seven years. This year I put the pot on the ground in my garden. A stem rooted in the earth, sent a new shoot up a bamboo stake, and then produced four flowers at each node between July and October. (Maybe next year I'll take the rest of the shy bloomers and plant them in the garden soil instead of keeping them in pots.)Sage Reynolds, an artist and designer, runs the Four Hands Design Studio, which he founded in 1977 with his partner, Colman Rutkin, in New York. He has been growing ceropegias and other succulent plants for more than 30 years. As a hobbyist, he has also hybridized azaleas, lilies, lithops, and abutilons.
Green Roofs: New Homes for Sedums and Other Hardy Succulents
By Edmund C. Snodgrass
There is a new landscape emerging in North America, still largely unseen but gaining in popularity. That landscape is the green roof. Green roofs have started sprouting in urban areas everywhere, giving some of our favorite hardy succulents new opportunities to shine. Sedums and ice plants are the stars of many green roofs: Tolerant of heat, sun, wind, drought, salt, disease, and insects, these low-growing, shallow-rooted workhorses of the perennial plant world have what it takes to thrive in the tough conditions high atop multistory buildings.
One form of roof garden has been a popular design element for public spaces like parking garages and museums in North America for the last 50 years. In the lexicon of green roofs it's known as the intensive green roof. These roofs are constructed with a layer of growing medium most often measured in feet, not inches, and have elaborate irrigation systems as well as substantial maintenance budgets. Intensive green roofs have the look and feel of traditional gardens despite their higher elevation.
More recently, a second type of roof garden has come into existence, one that has the potential to benefit the environment in addition to soothing the eye. The extensive green roof is typically created with three to six inches of growing medium, has no irrigation system, has a very limited or no maintenance budget, and generally serves a practical, rather than mostly aesthetic, purpose. All year long, the extensive green roof soaks up stormwater that would otherwise run off, taxing city sewer systems and damaging receiving waterways. Extensive green roofs also help lower ambient temperatures during the summer months, lowering cooling costs and helping to mitigate the so-called urban heat island effect.
Many European countries, most notably Germany and Switzerland, have embraced extensive green roof designs for practical, environmental, and aesthetic purposes. As the boundaries of growing cities rush to merge with the countryside, constructing green roofs has become an important way to reduce the impact of ever-increasing impervious surfaces. By virtue of their planted areas, extensive roofs green and cool cities, increase oxygen output, soften urban streetscapes, and reduce stormwater runoff to vital tributaries and major bodies of water that supply drinking water to millions of people across Europe. So serious are northern European concerns about the environmental impacts of development that federal laws mandate that development must be offset by green spaces; as a result, 14 percent of Germany's flat roofs now sport green roofs.
North America lags far behind Europe in this regard. Urban jurisdictions are still coming to grips with major stormwater-management issues that are directly linked to the rapid spread of roads, pavement, and other impervious surfaces over the last 30 years. This, in turn, has left aging water and sewer systems prone to regular flooding, which has a serious impact on the quality of water in our rivers and streams. Because of this, there is tremendous pressure from both government regulators and citizen environmental advocates to address issues of stormwater management in a meaningful way. But how do you help cities that have been designed to shed water as rapidly as possible become capable of absorbing rainwater? Green roofs are the obvious answer. Studies conducted on green roofs over a three-year period by Bill Hunt and others at North Carolina State University, in Raleigh, have shown annual rainfall retention on an extensive green roof with a four-inch-deep growing medium to be around 60 percent. In addition to reducing the amount of stormwater runoff and lowering the summertime air temperature around buildings, green roofs also clean particulate matter out of the air, mitigate acid rain, extend the life of roof membranes by protecting them from harmful rays that are the primary cause of degradation, and perform many other site-specific functions, depending on design and region.
In England, for example, urban green roofs are being used to provide habitat for insects and nesting areas for birds like the black redstart, which have lost substantial habitat to development. Other buildings feature green roofs to boost worker productivity in offices—studies have shown that worker absenteeism is lower in buildings that are environmentally friendly, for example. Best of all, these benefits occur simultaneously, and collectively they give building owners a return on their investment of installing the extra layers that make up the green roof. As the measurable benefits are being quantified, public officials in the U.S. and Canada are beginning to embrace green roofs as an important tool in improving the quality of life in urban areas. One by one, local jurisdictions are giving incentives such as tax breaks to encourage green roofs and reduce the adverse effects of development. Look for many more green roofs to come.
Green Roof Basics
Deciding whether to plant a green roof should begin with a series of questions designed to pinpoint its primary function. Is the green roof purely aesthetic in nature, offering a green respite in a sea of asphalt? Is it intended to deal with stormwater runoff, in which case aesthetics might be a secondary consideration to water retention? Is the roof designed to have a therapeutic effect, say, as an installation atop a hospital, where patients might visit? Is it largely educational in nature, open to the public and serving to teach people about the benefits that green roofs offer? Determining the purpose of the green roof will help to determine a number of things, among them the most suitable plants, whether or not you'll need an irrigation system, and how much load the roof must bear and whether it needs to be retrofitted to accommodate the extra weight of the medium and the plants.
An extensive green roof with a planting medium between two and six inches in depth adds between 14 and 35 pounds per square foot. This is a substantial increase of weight, and a structural engineer should be consulted to make sure that an existing roof can accommodate the extra weight, or if not, how it can be retrofitted to do so. In the case of new construction, load bearing should be factored in at the design phase and worked out between architect and engineer. An engineer can also calculate other additional loads associated with a green roof, including live load, which factors in human foot traffic, temporary installations of furniture, and so on. Your architect or contractor should be aware of any special requirements in your area, including whether a building permit is needed to construct a green roof and whether there are any tax or other incentives for doing so.
Once you've determined the purpose of your roof and structural issues, you will need to find a reputable and experienced company that has worked on green roofs previously and is familiar with the necessary layers that each green roof requires. Briefly, those components include decking, waterproofing, insulation, drainage, filter cloth, medium, and, finally, plants. There are complete green roof systems available (mostly from German companies that have entered the North American market), or the green roof can be built layer by layer by a roofing contractor. Plan your roof for a minimum 30-year life span—it's costly and impractical to need to rip up the garden because of a failing in the roofing materials. (For more information on the structural elements of installing a green roof, see Planting Green Roofs and Living Walls, Nigel Dunett and Nöel Kingsbury, Timber Press, 2004.)
When it's finally time to start thinking about plants, you need to decide how much time and money to allocate for maintenance. Grasses, for example, must be cut back periodically to avoid becoming a fire hazard. Some plants need regular pruning, watering, or fertilization. You will also need to know how and when you will access the roof. If access is limited or there is no one to perform the maintenance needed for some plants, then you'll want to choose plants with minimal requirements.
Rooftop growing conditions are usually much harsher than those found on the ground in the same area. For example, winds are much fiercer 20 stories above ground than they are at street level; they can quickly desiccate the plants and growing medium. To pick the right plants for the job, get to know the microclimate of your rooftop environment. As a first step, determine the minimum and maximum roof temperatures and observe how sunlight travels across the roof, noting if any sections are shaded by trees or other buildings for part or all of the day. Knowing the relative amounts of sun and shade the roof receives will help you pick plants that are most compatible with particular areas.
In general, green-roof plants should be ecologically compatible, fast growing but not invasive, and fire resistant. To avoid interfering with the well-being of the roof, they should also be long-lived or self-propagating, have shallow, fibrous root systems, and be lightweight when fully grown. As a rule, hardy succulent groundcovers are excellent candidates for green roofs, and of those, Sedum is the most versatile and common genus used. Sedums thrive in dry conditions and nutrient-poor soils, require little to no maintenance, and resist being blown over by wind because of their short stature. Many low-growing species and cultivars are available, and together with evergreen plants they can be arranged into interesting patterns that provide aesthetically pleasing roofscapes year-round.
Although it is not generally the case, in some circumstances, it may be desirable or necessary to irrigate an extensive green roof. It's doubtful, for example, that a green roof could survive in the arid Southwest without some kind of installed irrigation system for the hottest and driest months. Similarly, if you wanted to mix more demanding plants such as herbaceous perennials with the dominant succulents and grasses, it might be necessary to irrigate the roof (and increase the depth of the medium) to allow the less drought-tolerant species to flourish. Irrigating a green roof greatly expands the plant choices available to you, but it also increases the weight of the roof, the cost, and the need for maintenance. A variety of systems is available and should be thoroughly researched or discussed with a knowledgeable expert.
To help choose plants that meet your site's needs, it's best to consult a professional horticulturist, landscape architect, or local nursery business familiar with selecting plants for green roofs in your area. The vast majority of plants found on extensive green roofs are hardy succulents, including Delosperma, Jovibarba, Sempervivum, and, as noted above, Sedum. With more than 600 species of Sedum alone, the choices are extensive and varied and can offer a prolonged period of bloom.
Green roofs can easily provide visual interest throughout the year, even in areas with very cold winters. The roofs are not evergreen in the strict sense of the word, but they do offer interest year-round. For example, Sedum album 'Murale' illustrates the versatility of the genus in a successful roof design. As this plant breaks dormancy in spring, it turns into a lush green carpet, increasing in vigor and growing out horizontally over time. Come early summer, terminal flower buds appear, obscuring the foliage under a haze of white flowers for about two weeks. Later on, the spent flower stalks continue to contribute visual interest, or they can be removed if desired. When fall approaches, the foliage slowly turns red, progressively darkening until it reaches a burgundy shade. It stays that way until it breaks dormancy the following spring.
Living roofs provide many options for playing with multiple permutations of colors and patterns. The key to creating a successful roof design is remembering that it needs to please from afar. Large broad-brush splashes of color stand out well at considerable distance, while small, intricate plant details get lost completely. The bright red foliage of Sedum spurium 'Fuldaglut', for example, is a knockout from a distance, while the intricate details of the highly unusual gray foliage of Orostachys boehmeri can be appreciated only up close. (For low-growing sedums and other plants that offer superb foliage and flower colors, see "Great Succulents for Green Roofs" below.) Once you've selected low-growing sedums, you can add height, different textures, and colors to round out the design. Taller sedums like Sedum sieboldii or S. 'Matrona' work well, as do certain accent plants like Dianthus and Phemeranthus (syn. Talinum), which offer bright bursts of color when interspersed among other plants. Various ornamental onions (Allium) and short grasses like grama grass (Bouteloua) and sedges (Carex) can also be wonderful counterpoints to the hardy succulents that dominate the roof. Often, accent plants are not reliable enough to be dominant species, but if used wisely, they can add valuable interest to your green roof.
Depending on weather conditions, seasonal changes, and differences in climate from year to year, some plants will flourish, while others will recede. Some plants like Allium and Phemeranthus will self-seed from one year to the next, changing the garden's design. Over time, the unique landscape of your green roof will continue to evolve and change, even as it performs valuable services to your surroundings. With careful planning and a clever combination of groundcovers and accents, green roofs truly can provide a bright spot virtually year-round.
Great Succulents for Green Roofs
Combining fabulous foliage colors in broad sweeping patterns lies at the heart of an aesthetically successful green roof design, which will be viewed mostly from a considerable distance. Low-growing sedums and other closely related succulents offer a great palette to choose from: Their foliage comes in grays, blues, greens, reds, and yellows, as well as variegated forms, with many species and cultivars adding splashes of seasonal flowers and changing foliage hues as the seasons progress.
Sedums for Year-Round Interest
While not truly evergreen, these plants do not disappear in winter; rather, they assume winter colors from dark russet to brighter shades of red. During the growing season they form dense mats of green that are obscured only when they come into flower.
- Sedum album and cultivars
- Sedum hybridum 'Immergrünchen'
- Sedum kamtschaticum 'Weihenstephaner Gold'
- Sedum lanceolatum
- Sedum sexangulare
- Sedum sichotense
- Sedum spurium 'Fuldaglut'
- Sedum stefco
- Sedum stenopetalum
Sedums With Blue Foliage
- Sedum cauticola 'Lidakense'
- Sedum hispanicum
- Sedum ochroleucum
- Sedum rupestre
Some perennials act like annuals when it comes to blooming: Once they start—typically in late spring or early summer in most areas—they bloom continuously until frost or, in subtropical areas, year-round. South African ice plants (Delosperma) and North American Phemeranthus (syn. Talinum) species, native to serpentine prairie ecosystems, make first-rate perpetual bloomers.
- Delosperma aberdeenense 'Abbey White'—white flowers
- Delosperma brunnthaleri—yellow flowers
- Delosperma cooperi—purple flowers
- Delosperma floribundum 'Starburst'—purple flowers with white centers
- Delosperma herbeum—white flowers
- Delosperma ecklonis var. latifolia—purple flowers
- Phemeranthus calycinus—purple flowers
- Phemeranthus rugospermus—pink flowers