When you run out of space in the rest of the garden, consider gardening in the gaps between steppingstones or concrete pavers. In my own yard, gap gardening is an accomplished art form, though I did not invent the idea.
Nature seeds things in cracks all the time and more than a few gardeners have broken their best weeding tool (and the third commandment) trying to get a dandelion or a clump of oxalis from its snug home. At this time of year a little weed often turns the cracks in streets a bright green, getting its moisture from who knows where.
I saw a variation on this in a Pasadena garden, where the owner had turned a liability into an asset by sowing the seeds of Johnny jump-ups in the cracks running through his asphalt driveway. They prospered and the driveway looked better than it ever had.
In another garden I saw the deep burgundy colored oxalis, a persistent weed in most gardens, grown on purpose in the gaps between paving stones, with bright green Irish moss for contrast. They made a handsome pair.
A few gardeners have become so entranced with the idea of plants growing in paving that they have broken up solid concrete paths and patios and then relaid them leaving gaps that they can plant in.
The English grow all sorts of choice things in the cracks between paving. In one rather famous garden, sunroses grow in the cracks, liking the extra light reflected from the stones. Often things that like their roots cool are grown in the cracks, the paving stones providing a little extra insulation from the sun.
In my own garden I've grown choice and common things in the two-inch gaps between my square concrete steppingstones. Dwarf penstemon have flourished there, but also common thyme. My favorite gap garden plant is blooming right now, Verbena rigida , a bright purple flower on stiff stems.
It actually prefers growing in the narrow gaps and though it is a spreading plant, it seldom leaves its narrow concrete alleyways and has not spread into the rest of the garden.
It grows 18 inches tall, or more, near the sides of the path, only inches tall near the center. It seems to know when to duck. It pops up where it likes and give the path a wild look, as if nature has retaken some of her ground. Where I don't want it, I pull it out.
Other common plants I've grown in the gaps between paving, or that I have seen in other gardens' gaps include:
Australian violet-- Viola hederacea --this little white and purple violet spreads quickly in partly shady places.
Blue star creeper-- Isotoma --a pretty little thing with pale blue flowers and tiny leaves.
Blue fescue-- Festuca glauca --this little gray grass will grow in gaps.
Corsican mint-- Mentha requienii --tiny leaves that smell delicious when stepped on.
Dymondia --a new plant related to the gazania but small enough to grow in cracks. Tough as nails; can be walked on.
Lamb's ears-- Stachys byzantina --the fuzzy gray leaves stay neat and tidy in a gap garden.
Oxalis 'Grand Duchess'--dormant in summer, but the big cloverlike leaves look great in winter and pink or white flowers come in spring. Bulbs are sold in the fall.
Saffron--this crocus thrives in my garden and provides enough of the precious spice for several paellas. Bulbs available in late summer; blooms in the fall.
Woolly yarrow-- Achillea tomentosa --only an inch tall, this slightly fragrant plant with bright yellow flowers gets lost anywhere else in the garden, and splattered with mud.
Some of the choice things growing in my gap garden include a rare rock garden plant from the Sierra Nevada of Spain-- Potenilla nevadensis --and a very tough little fern native to our deserts, Cheilanthes feei , that grows about six inches tall in full sun.
Both of these have grown for 10 years in their little crevices and I doubt they would have been successful anywhere else.
Planting something in a crack or gap is easy, the main requirement being that you start small. The plant must be in a small pot or you will never fit it in.
Generally, I start with something from a two-inch pot (such as herbs come in) or I trim the root ball on two sides so it is narrow enough to fit in my two-inch gaps.
I add a little sand and soil amendment and a little fertilizer to each planting slit.
When I made my path, I made sure the gaps were wide enough for plants but, to keep heels from getting stuck in them, I added gravel flush to the top of the pavers. The gravel also helps keep weeds from sprouting (though not completely) and it keeps the dirt from splashing up onto the pavement when it rains.
Whenever I plant, I have to remove the gravel and then put it back, but I've developed a system.
I use a soil sifting screen (available at some nurseries, or through the mail from garden tool suppliers) to separate the gravel from the soil and any accumulated sediments. The sifted soil goes back in the crack and the gravel goes in on top. I push it right up to the base of the plants so it acts like a mulch. Then I wash it down with the hose and it looks brand new.
In fact, every few years, I do this to all the gravel in the gaps, just to clean it up. It's a pleasant, if odd, little task, best practiced in the back yard so neighbors don't think you've lost your marbles.
But you haven't--gardening in the gaps between pavers is one way of packing more plants into today's tiny gardens.