MOST ROCK GARDENERS seem to design a corner to grow choice alpines in troughs. In the right spot, with the right mix of soil, troughs seem to facilitate growing a wider range of tiny plants and challenging alpines than in the ground. Choice androsaces and drabas will persist for many years in a trough where they disappear in conventional rock gardens. Troughs invariably include a few Sempervivum species, which are bona fide alpines after all, and occasionally a species of Sedum or Rhodiola. Over the long haul, gardens exhibit a sort of Darwinian selection: more and more succulents seem to persist as the more delicate tiny saxifrages and primulas disappear one by one. Why not design a trough just to feature the succulent toughies? Like alpines, most succulents seem to enjoy trough culture as much and more than growing in the ground, and perhaps for similar reasons!
You can control the soil that you put in a trough in ways you can’t in the open ground, where worms and all manner of biota are constantly churning the mix. Likewise, the very nature of a trough—a limited volume of soil above the level of the ground—often ensures the superior drainage that both alpine plants and succulents demand. Plants from both high alpine environments and dryland regions often occur on rocky habitats: the confines of a trough mimic the shallow soils and crevicy conditions that these plants have adapted to in the wild. In other words, troughs and containers are a sensible choice for many plants.
Troughs with succulents are perfect for people who live in hot summer climates, and with sunny patios where delicate plants can fry. Busy professionals or those who travel a lot or can’t always be there during an especially hot spell—when the small soil volume in a trough can dry out quickly. Growing cacti? Not a problem! They seem to thrive on neglect, and even seem to enjoy the soil getting bone dry at times.
Almost a quarter century ago I was awarded the Francis de Bevoise medal from the Garden Club of America: I was flown to Hawaii for the better part of a week to attend the award ceremony, visiting the islands, hosted by local botanists who toured me around Haleakala and wined and dined me for days on end. A mere thank you note didn’t seem quite enough to acknowledge the magnitude of this experience, so I thought perhaps a hypertufa trough planted with sempervivums might better express my gratitude to a friend who went to so much effort on my behalf. In fact, it pleased her so much that she asked for a second trough to frame her patio at the other end. These two troughs have persisted cheerfully for decades with only an occasional splash of watering and deadheading the blooming stalks when they’ve gone to seed.
Everyone asks about what soil to use for a succulent trough: the answer depends so much on your climate, the site and especially the attentiveness (or not) of the gardener. The wetter the climate, the more scoria, gravel, and sand the better. In hot, dry climates like Denver we find that we can use a heavier mix—even with loam or clay—provided the trough is not level but mounded rather high in the middle to be sure water doesn’t pool. It’s important not to overwater troughs with heavier soil mixes. Succulents often go semi-dormant in the hotter weather: a wet soil mix at that time can encourage pathogens to proliferate when the succulents are incapable of protecting themselves. Lewisias are especially prone to rot in hot weather if the mix they’re growing in is too wet for too long.
Designing with sempervivums and sedums
Perhaps because they are so accommodating, rock gardeners often don’t bother to put as much thought into designing with these as they might. I have found a few tricks that have enormously enhanced plantings we’ve made with these succulents.
First of all (and above all), build a sizable mound upon which to plant! It seems exaggerated and artificial to stack soil and rocks up almost a foot (30 cm) above the trough rim, but trust me, the soil will compact considerably over a short period. A mounded trough will really look much better, and the plants will perform a lot better in it as well.
Rock placement in a trough is as important as in any rock garden. Some people have the knack, others don’t. If you don’t have the knack you have two choices: get a friend who’s got a good eye to do the design for you—or just find a model garden you admire and copy slavishly.
Make sure you vary the size as well the color of the succulents. Don’t use all tiny ones or just large ones—the contrast of size and color will make the garden more appealing all season long.
The prickly choice of cactus
Few plants elicit a greater emotional response in gardeners than cactus. Either you love ‘em or hate ‘em—but both camps will agree that few floral spectacles on earth can compare with a happy cactus in full bloom. Cactus haters usually concentrate their ire on the genus Opuntia, which of course are the most abundant and diverse cacti in North America. The spines can elicit some pretty enthusiastic complaints, but it’s really the glochids that maximize the irritation—the little bundles of hate that lie at the base of spines, and which seem impossible to remove if you happen to brush near them.
Those of us with advanced phytophilia find it charming that plants which we animals delight in tormenting are witty enough to return the compliment. The real solution to growing most cacti—and opuntia in particular—is to contain them. A cactus elevated in a trough is far less apt to surprise you in the garden when you’re weeding. And an elevated platform shows off the wonderful vegetative form and shape that characterizes cacti most of the year when they’re not producing miracle flowers. Remember when planting cactus cuttings to allow them several days (or weeks even) to callus over the cut. By the way, when it comes to spines and glochids, real cactus lovers possess a wide array of tongs, hemostats and all manner of other implements, so they’re never obligated to touch the objects of their desire. Platonic love is best when it comes to cacti.
More than even most alpines, cacti benefit from being planted on raised mounds—even in a trough. Careful rock placement can help make the mound look less artificial, and once the plant is established the effect can be very naturalistic when properly sited.
Which cactus to choose? Of course, this will very much depend on what sort of climate you live in: Southern Californians have literally thousands of species that can be adapted to their gardens, but the colder and wetter the climate, the narrower the range. A handful of mail-order nurseries specialize in hardy cactus and succulents—and these usually highlight the toughest. Opuntia fragilis almost makes it to the Arctic Circle—and its nearly spineless (although they have small glochids) cousin O. debreczyi is a classic in all climates.
The world of non-cactaceous succulents is quite vast, with whole groups that are actually alpine and moisture loving which do best in cool climates. Rhodiola spp., for instance, can grow in or near water, and thrive in frosty regions. Orostachys spp. are among the most beautiful succulents for troughs and most of these are maritime in their distribution. The sea of sedum is so vast that generalizations are in vain. Just be aware that most sedums can spread enough to become minor weeds, so choose with care. I especially like the Hylotelephium group, which includes quite a few small sedums as well as the more familiar ‘Indian Chief’ type monsters. These stay very compact in containers and are constantly morphing through the growing season (although, alas, many are deciduous in winter).
The real champions for us in colder regions are sempervivums. Containers with semps can be moved in humid climates to cooler aspects in summer which can help with their survival, but in much of the USA these are among the easiest of rock garden plants to grow. The trick is to combine them artistically. A well-grown container full of colorful hens-and-chicks can be a centerpiece of any trough collection, and needing almost no care or maintenance for years to boot.
So get yourself some good containers, fill them with a well-drained mix, arrange some rocks artistically and plant away. A minimum of effort at the beginning will result in years of delight and pleasure: how many things in life compare with that?