I consider it a cruel hoax when succulents planted in weird containers adorn the pages of books and magazines. I find them planted in wood, recycled metals, seashells and just about any other item with a depression capable of holding a tiny seedling. Succulents don't need much of a root zone because they hold moisture in their leaves and stems.
This is why you can put one inside a shot glass and it won't wilt. But when the stored moisture eventually runs out, how is it replaced with little or no root zone? Inevitably, the plants curl up and are tossed into the compost heap.
Succulents are highly variable and not nearly as cast iron as you might think. Many are born in the moderate coastal conditions of South Africa. These adapt well to humidity in succulent epicenters such as San Diego with its mild maritime climate. Other succulents are found in the Namibian desert, where it hardly ever rains, surviving on little more than fogs that creep inland from the Atlantic coast. Add cactuses and agaves from the Americas, which are just as diverse in their preferences. With so much variability, how can we stuff succulent plants into super-small containers with impunity and expect them to thrive?
If you're ready to jump into the succulent-plant melee, beware of these unsustainable examples. Sure, they might work for party favors or garden shows, but this is not a long-term solution. Treat succulents like real plants if you want them to grow large, bloom and produce offset "pups" that yield endless new plants for free.
Succulents need a root zone. Dig up a mature field-grown succulent and you'll find a rather large root system. Thick roots reach deep and travel far to gather what little moisture falls. Some types are vigorous rooters, filling a 6-inch pot in no time.
Deny a plant enough root zone and it won't grow larger. Plants have a remarkable ability to just sit there for a long time as they gradually lose interior moisture. The normally hard stems and leaves become more resilient to the touch, with slight surface wrinkles that tell you they're almost out of gas. If the roots don't dehydrate beyond function, they will absorb moisture when it returns. If they become so desiccated that the root cells die, adding water stimulates rot that travels rapidly through the tissues. The result is a soft, brown gooey thing where once a succulent grew.
Succulents need drainage. In short, you can't plant them in any container that lacks a drain hole. That means that most aren't good terrarium candidates despite their popularity in these glass-enclosed environments that have little to no opportunity for moisture evaporation. Sure, a layer of pebbles inside an undrained container can help keep the root zone drier, but when this reservoir becomes full, then what? While in theory the pebbles should work, this inevitably becomes an ugly bacterial soup of pathogens that love to attack succulents.
Succulents need oxygen. If you study most cactuses and other succulents in habitat, they tend to grow in extremely porous soils that are more like sand or gravel compared to typical soil. Often they're found in dry stream and lakebeds of southern Africa where the soil is so coarse no other plant can survive there. The gaps between soil particles should never fill with water because it replaces oxygen to create anaerobic conditions. This is not uncommon when the wrong potting soils are used for succulent plants. It also occurs when drainage holes are too small or clog with organic matter.
Succulents are easy, but don't fall for decorator ideas that aren't rooted in plant science. Give them the right home with a root zone, well-drained soil and long periods between watering. Do it right and your plants will grow and bloom as nature intended.