Crack and Crevice Gardening
Any pocket and chink that you can fit a trowel or even tablespoon into can be planted. Remember that with vertical spaces, such as a stone wall, the plants may eventually hang down a few feet so don't set other plants directly below or they may be smothered. In traffic areas, such as patios and pathways, stick to sturdy compact plants such as creeping thyme.
Planting Seeds Vertically : Starting plants in vertical spaces can be tricky because seeds and soil often wash out. Try the technique below.
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 fine, rich 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.