So You Want To Plant A Succulent Garden, Part III
The selection of large plants for outdoor growing in the Bay Area is restricted both by the relatively limited number of species that will survive our weather and the extremely slow rate of growth of many of these plants. When it comes to mid-sized and smaller plants, however, though the same restrictions apply, the choice is vastly greater. I’ll start with some suggestions for medium-sized plants, beginning with members of the Aloaceae.
Just as large aloes can serve as one of the foundations in designing a succulent garden, smaller ones should be a major element in any garden featuring xerophytic plants. Though the choice isn’t endless, it may seem so. The many dozens of South African species can be broken down into groups, some with members that are commonly seen and some quite unusual. Possibly the most common aloe seen in gardens in our area is the relatively low growing, rosette forming A.maculata, more commonly known by its prior name, Aloe saponaria. Up to about a foot and a half in diameter, its rosettes are composed of broadly triangular toothed leaves, green to brownish-red (depending on how much exposure to sun), heavily marked with elliptical white spots. It produces a sparingly branched, generally flat topped inflorescence with bright orange flowers, and, its most obvious characteristic, many offsets. It’s so prolific, in fact, that unless one is trying for the effect of a mass planting, it’s a good species to avoid.
Fortunately, there are a number of related, generally similar species that are less profligate in their offsetting. Some of these, such as A. greatheadii and its subspecies davyana (sometimes considered a distinct species), the large-toothed A. grandidentata, and A. mudenensis, are quite similar in appearance to A. maculata, though with differently colored and arranged flowers, and if you can find them, they will be as easy to grow outdoors and behave better as well. A. prinslooi is similar, though a bit smaller, with somewhat more character (that means looking as if it’s had a hard life…) and very pale pink or white flowers. A. fosteri is larger, with variably colored (red to orange to yellow), very bright flowers. A. branddraaiensis becomes quite red in bright sun, while in similar situations the leaves of A. verdoorniae turn almost blue-black. Few of these spotted aloes will turn up at general nurseries, but seeking them out from specialist growers will prove fruitful over time.
For someone looking for aloes without spots, but with linear marking instead (variously faint or brighter), Aloe striata, though common, will make a good choice. This species, which can become close to a yard in diameter, remains stemless and only rarely offsets. Its broad leaves turn pale orange in good light, with faint, darker, longitudinal striations down them, margined in pale red. Given good drainage, it grows very easily, in contrast to its more attractive subspecies, karasbergensis. This is a shame because ssp. karasbergensis is an extremely attractive plant, with even broader leaves, milky white in color and densely lined with narrow blue stripes. The not too closely related A. reynoldsii looks like a miniature karasbergensis but with green and white rather than blue and white striping. It will do better outdoors, but is not very easy to find. Other aloes with linear markings and a good degree of outdoor hardiness include A. gariepensis and some forms of A. glauca, while the list of still other mid-sized aloes for garden subjects could go on almost interminably; among them Aloe aculeata, A. broomii, A. melanacantha (a beautiful but dry-growing plant a bit more sensitive to excess water), A. petricola, A. polyphylla (a UCBG favorite needing very good drainage and a bit more organic matter in its soil), A. suprafoliata (distichous when young, a rosette when mature), A. microstigma (one of the easiest to grow outdoors), A. vryheidensis and A. spicata—with sessile flowers and an inflorescence that resembles a giant cattail—two sprawling species, A. mitriformis and A. comptonii, and many more. For somewhat more shaded locations, there are several climbing, almost vining aloes that thrive in less light than most, among them the common A. ciliaris and, less often seen, A. gracilis and A. tenuior. These are all South African plants, but a surprising number of aloe species from East Africa and Madagascar will survive outdoors as well, although a certain amount of caution is advisable when working with these aloes from tropical climates.
Crossing over to the New World brings the Agave family and with it a host of medium sized plants suitable for outdoor gardens. Again, a list of suitable species could go on and on, but I’ll mention a few particularly choice varieties. It’s important to reiterate that more than with most succulents and cacti a good deal of thought should be given to location before placing an agave in the ground. The combination of stiff leaves and sharp spines can make them a rather dangerous is placed too close to the edge of a path or somewhere where a person might stumble into them—remember that a method of human sacrifice reputedly practiced by the Toltecs long ago in central Mexico was to hurl the victim onto a clump of agaves. Though this group of the plants is mid-sized rather than large, even a medium sized agave can take up a surprising amount of space.
All that being said, agaves, and mid-sized ones in particular, are some of the very best plants for outdoor growing. Most of them are very hardy and can withstand frost, sun, drought and considerable rain. Furthermore, many of them are quite beautiful, more sculptural than perhaps any other group of succulents. There are a number of medium sized (mature rosettes ranging from perhaps eighteen inches to three feet in diameter) blue-gray agaves well worth growing. Perhaps the most commonly seen is A. parryi var. huachucensis, a plant from the mountains of southeast Arizona. It more-or-less resembles a giant artichoke, with broad leaves in a rounded rosette. Its propensity to offset freely accounts for its wide distribution in cultivation, and this can be a plus or a minus depending on circumstances. Some of its other varieties such as parryi or the form called truncata are both tighter (like a less ripe artichoke) and not quite as proliferous. Another variety, cousii, from central Arizona, also forms fewer offsets and has a somewhat more open rosette, but it’s relatively rarely seen in cultivation. All of these become good sized plants although not giant. Agave applanata, A. guadalajarana, A. isthmensis and A. potatorum are other gray-blue agaves, sometimes more glaucous (blue-white) than A. parryi, not quite as eager to offset, and quite a bit smaller. They’re all attractive plants, but A. potatorum and A. isthmensis come from southern Mexico and are not quite as cold-tolerant as the others.
Also a southern plant, and from fairly low altitudes, A. guiengola can become fairly large, almost exceeding the boundary of true mid-size, but its best forms are almost pure white and the species is surprisingly hardy. A. gypsophila is another very white, medium sized, extremely attractive plant, as is A. celsii (also called A. mitis), though this more green-white than blue-white in tonality. A. celsii slowly forms dense clusters, but it has relatively soft leaves and very weak spines, making it one of the more harmless agaves (there’s also a green form of the species, but it’s much less attractive and rarely seen in cultivation).
Pure green in color, but with interesting wavy leaf edges and spine formation, Agave bovicornuta and A. cupreata deserve consideration, while A. bracteosa, spineless and with flat-topped leaf surfaces and yellow-green color along with a uniquely glassy leaf texture, is rapidly becoming a commonly cultivated plant (fifteen years ago it was very rare).
There are several mid-sized green agaves with broad white leaf edges and often very oddly shaped spines. A. xylonacantha is one of these, with rosettes composed of a relatively few, long, narrow, wavy leaves, shaped almost like a Malayan kris (a wavy edged knife). A. horrida and A. titanota (to an even greater degree) have broader leaves, equally white margined and with wildly eccentric shapes to their spines. A. titanota may also be glaucous; in the wild white, glaucous plants and green ones grow side by side. A. titanota also remains a bit smaller than A. horrida. These are all plants from the Mexican mainland, but A. margaretae, from Baja California, resembles them (although it can be a bit more blue), and also remains fairly small and well behaved.
For a very different look, Agave stricta and A. striata (almost indistinguishable) form rosettes composed of hundreds of very narrow, terete leaves, with extremely sharp points—a clump of them looks as if it should be soft, a mass of green pillows, but it’s not.
Some of the prettiest of the medium sized agaves come from isolated mountains in the northwestern Mexican state of Sonora, mountains that once were islands, separated from each other and a site of speciation. Among these, A. pelona forms a beautiful rosette of many long, green, narrowly triangular leaves, spineless but tipped with a very long, knife-sharp point. Its neighbor, A. zebra, has much the shape of the A. potatorum group, but often has bands of darker and lighter gray across its leaves. Another Sonoran plant, Agave colorata, which makes more open, less geometric rosettes, can be so heavily banded that it looks like someone painted it; in very bright light (or when somewhat stressed by lack of water) it also can develop a somewhat purple tinge. The list of agaves could go on and on, but this should give you some possibilities.
Some agave relatives also are prime medium sized plants for outdoor gardens. Perhaps the best is Calibanus hookeri, although describing it by comparing it to a fissured stump covered with tufts of coarse grass may not do it justice. The key to growing a Calibanus outdoors is placement, as if it’s located too close to other plants or wedged between rocks, it may look pretty much as my description. Placed in an area where it isn’t overshadowed by larger plants, it reveals itself as a pretty fascinating New World caudiciform with its own distinctive appearance. Best of all, C. hookeri does extremely well outdoors in our area and will grow much more rapidly than if it’s kept in a container. Hesperaloe, Nolina, Manfreda (agave look-alikes often with spotted leaves) and Yucca also include several medium sized species, generally either very slow growing or somewhat straggly, nondescript and sometimes too easily spreading. Nonetheless, species from each genus are worth investigating more closely.
Much the same could be said with terrestrial bromeliads. Many of them are completely hardy, and some send out spikes of rather amazing flowers, but many will clump and spread fairly rapidly and since they tend to be armed either with thick, dagger-like spine or with spines that bend back and act like fishhooks, a little caution is warranted before putting them in the ground. That said, there are several species in both the Mexican (and barely Texan) genus Hechtia and the South American genus Dyckia with intensely colored leaves, red or strikingly silver, moderate size and clumping speed and even brightly colored flowers to boot. In the ground these plants are as hardy as agaves, although they don’t always respond well to disturbance and division.
When the choice in euphorbias is medium-sized plants, the number of available kinds rises quickly. These euphorbias can be broken down into several categories: plants with relatively thin, often symmetrically branching stems and usually deciduous leaves, plants with spines derived from sterile flower stalks, plants with more traditional derived, twinned spines, and plants with cylindrical, spineless, tubercled branches that emerge from a short, central stem. The first group includes both plants from the Mediterranean and surrounding regions with smooth, marginally succulent stems and tufts of often blue-green, deciduous leaves and South African plants with more succulent, green, slightly tubercled stems and green leaves. Among the first group, E. lambii, from the Canary Islands, starts out life looking almost like a miniature palm tree, but when it reaches flowering size it branches dichotomously, each flowering branch dividing into two new branches and repeating the process until it becomes a very attractive, symmetrically organized large shrub, up to perhaps five feet high and a bit wider, with several hundred short branches. Unfortunately, it seeds so freely that it can become something of a pest, so much so that it’s hard to understand why it hasn’t taken over its island home where it remains a rarity. Quite similar, though not as attractive, E. balsamifera and its varieties extends from the Canary Islands across much of North Africa and down the Arabian peninsula. Most of the South African euphorbias of this nature are a bit too delicate for outdoor growing, but a few them will do well. Euphorbia clandestina, with a stem marked by upward pointing triangular tubercles, with narrow leaves, and typical small euphorbia flowers and bracts hidden among its leaves, rarely if ever branches, and can reach about eighteen inches to two feet in height. It does well outdoors, but can be somewhat difficult to establish. E. bubalina, similar in height, may develop a branch or two. Its leaves are broader, sometimes tinged with red, its tubercles are flatter, and it does very well outdoors in the Bay Area. Other species in this group might be worth a try, as they all can withstand more water than the real arid growing species, and many live in areas with both occasional frost and rains that may occur at any time of the year.
The euphorbia species with peduncular spines provide the highest percentage of outdoor hardy species. These plants range in size from as much as five or six feet tall (E. heptagona) to little more than six inches. The tallest ones branch both from the base and along the stem, forming rather ragged looking plants, but many of the shorter species, such as E. pulvinata, E. ferox, E. atrispina and E. enopla, branch densely from the base, resulting in a visually striking hemisphere-like mass of stems. Although the spines on these plants are composed of dried flower stalks, in species such as E. ferox they are very thick and sharp, and a mature, multi-stemmed plant, although really rather spectacular, should be approached with caution. E. horrida and E. polygona are larger, with stems as much as two feet high, somewhat resembling barrel cacti. Though lacking the symmetry of the smaller species, a large plant with many stems make great garden focal points. E. horrida in particular has many forms, some that grow in a spiraling or wavy manner, some that are quite small, some that are green, some blue-gray, some striped, and it’s also one of the hardiest euphorbias for outdoor growing. E. pillansii, with spines that are forked at their tips, and E. stellispina, with multiple-branched, star-shaped spines, are slower growing, but can become good sized. E. stellispina, though sensitive to too much water, is very cold hardy—I’ve seen plants growing in ground covered with ice at twelve noon.
The species with short central stems surrounded by radiating, green tubercled branches are know as medusoid euphorbias, and include some of the rarest and most coveted species, many native to extremely arid regions. A few, however, do well outdoors, including the first discovered species, Euphorbia caput-medusae itself. In this species, from the vicinity of Cape Town, the central stem is reduced and it looks like a whirling mass of green snake-like branches, leading, of course to its name. E. caput-medusae can grow to three feet across, and is perfectly hardy outdoors in our area. Among the smaller species, E, flanaganii, to about eight or ten inches in diameter, will survive our weather easily, and E. pugniformis, considerably rarer in cultivation, smaller, with thicker branches and distinctly geometric tubercles, also will survive. E. woodii, resembling a larger E. flanaganii looks as if it should do well, but cannot handle a slightly colder than average winter. Several other species are worth trying, although they can be somewhat expensive and difficult to find. E. tuberculata, from the west coast of South Africa, fairly good sized and with rather thick, somewhat stiff branches is a possibility, as is E. clavariodes, which produces many short branches and ultimately look like a sort of mounded up matt.
The twin-spined euphorbias form one of the major divisions of the genus, with several hundred species that range from South Africa to as far east as Thailand, and vary in size from forty foot tall trees to miniatures little more than an inch high and wide. As I mentioned, not many of the tree types are outdoors hardy here, and the same is true for the miniature and the thin stemmed species. Several North African species, however, such as densely branching, low-growing E. resinifera and the slightly more columnar E. echinus, are perfectly hardy, as are several of the rather thicket-like South African species, E. coerulescens probably being the best known, although the more sprawling, thinner stemmed E. ledienii will also survive in the ground. Once again, experimenting is the way to determine which of the many species of this fascinating genus are hardy, although the difference in survival rates during a normal winter and one of our hard freezes with temperatures into the low twenties might be extreme.
A number of members of the Crassula family are medium sized, outdoor hardy plants, including almost all the members of the genus Aeonium, mostly native to the Canary Islands. These plants do so well here that people plant them without even thinking of them as succulents native to harsh environments and needing special treatment. The same could be said of Crassula ovata, the Jade Plant, and Cotyledon orbicularis, looking something like a gray-leafed Jade Plant but with pendulous, waxy, bell-shaped, typically pink flowers. These are some of the commonest succulents encountered in California, but other members of the Crassulaceae are worth trying as well. Crassula arborescens rather closely resembles the Jade Plant, but it grows a bit larger and has silvery-blue foliage. It’s a slower grower than C. ovata, but will serve the same function in a garden and is considerably more attractive. The hybrid Crassula “Jade Necklace” approaches medium size, and almost certainly will grow in gardens in our area. Though several of Kalanchoe species grow large enough to be considered medium-sized plants, most won’t reliably survive our winters outdoors, but in particularly sheltered spots K. beharensis, with very large, brown-felt textured, zigzag shaped leaves might be worth a try. Even more exotic and not as common as it should be, Tylecodon paniculatus, from the southwest and western parts of South Africa, will survive some frost, and as a widely distributed winter grower, our rains shouldn’t present too much of a problem given good drainage. It can reach as much as six feet in height with a trunk over eighteen inches in diameter, and has a truly spectacular appearance. Probably its very infrequent use as an outdoor subject reflects its relative rarity in cultivation, slow growth in a container, and high price, but it almost certainly would look better in the ground than it ever could in a pot. A word of caution, T. paniculatus is poisonous if eaten, so if you have sheep or cattle grazing in your yard, it would be a bad choice (maybe a good one in a garden too frequently visited by deer…).
If the desire for more exotic mid-sized succulent keeps expanding, there are a few more choices for the adventurous. Of the succulent grapes, Cyphostemma juttae has survived (though with damage) temperatures in the Botanical Garden as low as 14 degrees Fahrenheit. With its thick pachycaul stem, peeling bark and large, semi-succulent, deciduous leaves, it’s one the very best succulent plants. Since it’s actually one of the most northern growing Cyphostemma species (in the southern hemisphere), quite possibly more southerly growers such as C. currori or C. bainesii (and its form, variant, or cousin, C. seitziana) would also survive outdoors, but as I’ve mentioned in previous columns, these plants are still so valuable and uncommon that I don’t know anyone who’d be willing to risk them in gardening experiments. Also from southern Africa, the genus Pachypodium has a couple of species that can survive outdoors at least in sheltered spots. The two most likely candidates are P. succulentum, which in its wild state consists of a large underground caudex with several long, rather narrow above-ground branches. In cultivation it’s almost always grown with the caudex exposed, as is also the case with Pachypodium saundersii (or P. leali ssp. saundersii, as it’s also called), but with P. saundersii the caudex is spiny and silvery-gray rather than plain brown, indicative of an above ground existence. All in all it’s a more interesting species, but not necessarily quite as hardy. The much larger P. giganteum, also called P. leali ssp. giganteum would make an amazing outdoor specimen, with a massive, pachypodous trunk up to fifteen feet high and four feet across at the base, and beautiful, intricately curling, sweet-scented white flowers, but since it live well over a thousand miles to the northwest from its smaller relative, in country where frosts are close to nonexistent, rain falls in summer, and the sun is blazing hot, it seems unlikely that it could survive outdoors here. Another choice South African plant that survives outdoors at the Huntington Botanic Garden in San Marino (in southern California) is the great succulent yam, Dioscorea elephantipes. It’s native to the winter rainfall regions, grows from late fall through early spring and receives occasional frost in habitat, so it would be worth a try, but I don’t know of anyone in the Bay Area who’s attempted it outdoors, another case of it being such a choice plant that no one would want to risk it.
Baja California offers a few interesting medium sized succulents, including Bursera microphylla, that might well survive our winters. Burseras, though, when given conditions so much less stressful than they encounter in the wild, often become large, not particularly interesting bushy shrubs. Perhaps the most famous of the Baja succulents, Fouquieria columnaris, the “boojum,” will survive in sheltered spots, although exposure to hard frosts will damage it. When grown well these plants exceed the medium-sized limitation (I’ve seen one over fifteen feet tall in a San Leandro garden), but they are slow and could be dwarfed by planting them in a very rocky setting.
Moving now from the “other” succulents to cacti actually doesn’t include a tremendous variety of species in the medium-sized group. Many of the ultimately large growing cacti I wrote about earlier grow slowly, and won’t become dominating garden specimens for many years, and of the genera that I mentioned, Cleistocactus, Ferocactus, Oreocereus and others contain species that don’t ever grow very large. Almost any species in these genera will do fine outdoors in our area. Some members of the old genus Notocactus (now included within Parodia) become mid-sized, small barrel shaped cacti up to about two feet tall, again, after a good number of years, and many of these (unlike most of the other parodias) will do fine in an outdoor setting. A few mammillarias achieve medium size, but most of these won’t tolerate our wet winters. A number of the South American cacti in the taxonomically anarchic group surrounding the names Lobivia, Echinopsis and Trichocereus will reach a foot or two in height, less in diameter, and some of them will clump quite readily. Many of these will do well outdoors, and some (particularly the “lobivias”) often have spectacularly colored flowers as well. Some species of Borzicactus and some of Haageocereus are sprawling, columnar plants that can add a distinct look to an outdoor garden, and the golden-spined, high altitude Andean, small barrel cacti found in the genus Oroya, although not that commonly grown, would be good outdoor candidates as well. Most interesting, but more borderline than reliably hardy in the Bay Area, are the Mexican members of the genus Astrophytum (although the valid existence of that genus is subject to apparently continuing debate). Again, the plants are definitely hardy outdoors in southern California, and some (such as A. ornatum and A. myriostigma) might be worth trying in extremely well drained, very sunny situations. All in all, though, the choice of medium-sized cacti for outdoor gardens in our area is restricted in comparison to that of other succulents.
This possibly exhaustive-seeming inventory of plants is actually far from exhaustive, but should provide some insights and suggested groups for further investigation. Many of these plants are quite readily available, and a number of them are even available for purchase at the Garden, particularly during our plant sales.
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