It’s hard to say at what point the growing of succulents went from being a nerdy backwater of gardening to a viral sensation. It might have been the day somebody figured out you could hot-glue them to just about anything.

Debra Lee Baldwin recalls seeing them affixed to sunglasses and license plate frames. “That floored me,” she said. All right, she lives in Southern California, but the rest of America also has gone succulent-crazy in recent years.

Succulents have thick fleshy leaves that store water; they are plants that exist in their own spacesuits. They put down a few roots to draw moisture and nutrients from the soil, but they can live for weeks out of the ground. This allows you to hot-glue them to sunglasses, but also to attach them to artificial stems and enliven floral bouquets. They make their way to wedding bouquets, and thence to Instagram and Pinterest, and suddenly the world is beating a path to the rock garden.

Many popular succulent varieties grow as leaf whorls — rosettes — that resemble flowers, except they are plump and alluringly colored — grays, blues, gray-greens, amber, deep purple and silvers. Some are dusted in the sort of bloom you find on grapes and plums. A few species suggest creatures of the underwater reef, as with the anemone-like echeverias or the coral-like euphorbia Sticks on Fire.

They are related to cactuses, whose spines say “Stay away,” but the succulents by contrast invite a tactile response. Some leaflets will break off if handled, at which point they can be stuck into a pot and are happy to root into new plants.

Because succulents take care of themselves, occupy so little space and look cute as kittens in their tiny pots, they have been wholly embraced by urban millennials and hipsters of all ages.

A sedum variety. (Nikki Kahn/The Washington Post)

Baldwin, who has written three books on succulents, speculates that it is the matinee-idol looks of succulents that have led to their success. “They’re very photogenic, and we live in a visual world where everybody is taking a quick shot with their cellphone,” she said.

Those same sculptural qualities make succulents fabulous plants for container compositions in such key spots as the balcony, deck, patio or entryway, where they will be seen and enjoyed.

Their adaptability has made them a hit in indoor applications — placed in twee wooden boxes, crystalline terrariums and simple jam jars, for example — but I see them as fully at home outdoors, even if just on a high-rise balcony. In fact, artful assemblages are their own gardens, which makes them so horticulturally authentic amid their hype.

Tender or hardy?

Succulents have long been a part of our landscapes. Some are hardy and live outdoors happily year-round, including several species of stonecrop and house leeks. Others are familiar houseplants such as aloes, Christmas cactus and jade plants. The tender succulents hail from places such as southern Africa and Brazil and are not equipped to winter outdoors in these parts.

The house leek or sempervivum is a traditional hardy succulent of the cottage garden. (Nikki Kahn/The Washington Post)

Aloes are tender succulents, grown outdoors during the summer but wintered indoors. (istockphoto/istockphoto)

You can make an outdoor container garden with hardy or tender succulents, but choose one or the other: Mixing them muddies the waters of cultivation. The tender succulents must be brought indoors for the winter to survive.

If you plant a pot with hardy succulents, you will have to pick a container that can handle freezes. By choosing tender succulents, you will have a greater selection of container (because you bring it indoors after October), and, more to the point, your choice of plant varieties will be much expanded. Typically, the tender varieties are more colorful, sculptural and interesting.

Designing with succulents

Broad bowls, pots and planters allow for compositions of mixed succulent types that can resemble miniature gardens in themselves — a virtue that makes succulents so useful to urban dwellers with limited space.

Some gardeners take as much care in assembling these compositions as they might with an elaborate floral arrangement, but succulents are so sculptural that it’s hard to go wrong. As with any interesting plant design, an asymmetric silhouette adds tension and drama. Strive for a hierarchy of plants — accents surrounded by fillers — and repeat low-growing succulents (think ground cover) because that will tie the design together.

A design will work without actual flowers because the succulents are so floral in their forms and colors, but some succulents will reliably produce delicate blossoms on wiry stems that enliven the composition. These include kalanchoes, echeverias and euphorbias.

Some gardeners add non-succulents to the mix. Luis Marmol, a gardener at Dumbarton Oaks in Georgetown, likes to use small-leafed begonias, scented geraniums and the oregano known as Crete dittany.

An echeveria dominates a composition by Luis Marmol, but he has added non succulents for contrast, including scented geranium and a variegated lavender. The yellow blooms on the right are from the light green Sedum rupestre also seen bottom left. (Luis Marmol/Dumbarton Oaksl)

Control freaks take note: Your composition will change as the months progress. Succulents grow at different rates: Aloe-like haworthias grow slowly, a succulent named Kalanchoe luciae develops large, paddle-shaped leaves by mid-season. Sempervivums develop “pups,” or offsets, while other rosette succulents simply elevate on naked stems. These include aeoniums and echeverias. Others open up as they grow — certain sedums and crassulas, for example.

There are ways of managing the growth for the sake of the design: In the greenhouse, Marmol lifts a pot with a mature, fleshy sedum and cuts the stalk just above the soil level. New rosettes will appear from the base of the stalk. He takes the cutting and pulls off the cluster of leaves, like grapes, and pushes each one into a separate two-inch pot, where they will root and grow into new rosettes.

With an unruly, elongated aeonium, Baldwin said, you can prune off the rosette, pull and discard the rest of the plant, and reset the rosette into the soil, where it will re-root. “It’s absolutely the opposite of trimming a hedge,” she said. “With succulents, you keep the cuttings and throw away the plant.”

Some larger plants are perhaps best grown individually in their own pots. Brian Sullivan, vice president for gardens, landscape and outdoor collections at the New York Botanical Garden, likes to grow agaves this way. Among his favorites is a variety named Sharkskin, which has stout, gray-green leaves and grows to about three feet. He also commends Blue Glow, with blue-green leaves and a striking orange-red margin.

Baldwin likes some of the smaller aloes, including Blizzard, with silver and green variegation, and the similar if skinnier Lizard Lips.

Succulent care

Succulents must have a free-draining soil medium; the soil mix should be opened up with sand, gravel, perlite or a mix of all three. You can blend your own. Don’t use garden soil or unamended regular potting soil. If a pot is deep, many gardeners first fill the bottom layer with pure gravel. Containers kept indoors may or may not drain. If not, you have to water them carefully to avoid waterlogging. Outdoors, drain holes are a must in a wet climate such as the Mid-Atlantic’s.

Elevating pots on “feet” will assure better drainage. Place pots away from any irrigation system. Mulch the container with small gravel or stone pieces, which are aesthetically pleasing and helpful for chasing moisture away. Avoid organic mulches.

Succulents were part of the 2015 Frida Kahlo exhibition at New York Botanical Garden. Good drainage is a must. The quickest way to kill a succulent is to overwater it. (New York Botanical Garden)

Succulents should be watered occasionally; the frequency is governed by the size of the pot, its placement and various environmental factors. Baldwin likes to take a wooden chopstick and plunge it into the pot. “If it has soil clinging to it, the plant probably doesn’t need water,” she said.

Do not water the leaves, but direct the water to the soil under the plants. Sullivan places a decorative stone or two in each pot and pours water onto that. This avoids disturbing the gravel or wetting the leaves. Limiting the water will stunt growth, which may be desired if the plants are jostling for space come July.

Many succulents will burn in full sun. Some gardeners move pots into more light to cause foliage to redden, but this is a balancing act. Typically, succulent gardens are happier with a little shade, though they need a place with good air circulation.

If you want the plants to survive, you will have to find a bright place for them indoors over the winter, at which point overgrown mother plants can be renewed by taking cuttings. Whether you do this or not, it’s best to consider your plant assemblages as temporary annual compositions. Lone specimens in pots can be brought indoors and kept going from year to year.

Choosing a container

Because succulents need little soil for roots and may rot in soil-rich pots, broad shallow basins are ideal for succulents.

They pair well with stone or faux-stone containers, making them ideal plants for stylish concrete planters or lighter hypertufa pots. These can be made yourself for a fraction of the cost at retailers, and you will find recipes online. Some botanical gardens offer instruction in trough-making. (Green Spring Gardens in Alexandria is holding a class on April 30.)

Colored glazed pots can work for and against succulent combos: Subtle shades of pot can complement or harmonize with the gorgeous grays, blues and ambers of succulents, but strong colors or patterns can detract. Baldwin thinks of Echeveria imbricata, which has striking silver-blue rosettes. “The color is actually a muted blue-gray,” she said. “If your pot is cobalt blue, it will turn that plant a sad gray.”

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