Some plants seem absolutely determined to get by with very little space, preferring a crevice in the front walk to a cushy spot in the garden soil. They may thrive in the cracks of a patio or flagstone walkway or perched in the pockets of an unmortared wall. Once enough of these determined plants have made themselves at home, the patio or wall or staircase is changed: It is now a garden, a crack-and-crevice garden to be precise.
Those who have been undone by the task of making a successful flower border --- where the blank space is several hundred continuous feet of soil or larger --- will appreciate the simplicity of crack gardening, which by comparison is a cinch; it practically designs itself. For those confined to the smallest gardens --- an urban courtyard, for instance --- crack gardening is a way to tap into growing space where there is none.
There is no need to stick to traditional rock-garden plants that one might expect to favor such spots --- the Lewisias and Saxifragas, Sempervivums and rock ferns. The word "wallflower" will quickly come to mean not just Erysimum (the true wallflowers) but anything that blows in on the breeze or arrives by your hand, from a five-foot-tall foxglove or mullein to a tiny mound of moss. They may be longer-lived and more vigorous than in a typical garden bed, stretching their roots under the cool, moist shade of the stones while never having to suffer the indignity of standing water that is death to many plants, especially in winter.
Getting plants started in vertical spaces is tricky, since seeds and soil often wash out. Try this technique: First, fold an index card in half lengthwise and sprinkle seeds from a packet into the folded card. Position the seed-filled card in the crack to be planted, then blow the seeds into a rich and fine soil mix in the crevice. Mist the seeds with a spray bottle of water, apply a piece of facial tissue to the dampened crevice to act as biodegradable mulch, and mist regularly while the seeds germinate and become established.
Plants tucked into wall pockets are difficult to establish, often drying out or falling out. Try using a jelly roll of turf cut from the lawn as a nurse medium. First, use a knife to cut the turf. Unpot the plant (which should have been well-watered the night before), and roll its roots up in the turf, like filling in a jelly roll. Soak the whole roll, and push into a large crevice. Water by soaking the turf all the way through on a regular basis. It will eventually break down, and the plant's roots will take hold in the pocket.