Not every gardener has deep soil in which to plant. In mountainous areas, your shovel will pretty much always hit rock before you’ve gotten to a reasonable depth. In dry climates, there’s often a layer of hardpan only a short distance down that’s impossible to break through. My own yard consists of about 8 inches (20 cm) of topsoil above a layer of schist that sinks down to China. Now, schist is a very light and friable rock full of cracks and fissures that roots grow well into and it holds water very nicely, but that I simply can’t dig into it with a shovel. Digging any major hole would take a jackhammer!
In such cases, you generally have enough depth to plant annuals and smaller perennials with their rootball intact, but large perennials grown in deep pots, not to mention trees, shrubs and most climbers, have rootballs you simply may not be able to dig a deep enough hole to accommodate.
Fortunately, there are several solutions to this problem.
1. Go back to the nursery and pick out a specimen in a shallower pot.
This is the easiest solution… provided that such a plant exists. But nurseries seem to think all gardeners want big specimens of shrubs and trees with huge rootballs and you may not be able to find exactly what you’re looking for.
Note that plants with a taproot (a long, thick main root that digs deep into the soil) are probably never going to grow well in shallow soil, so if your soil is naturally shallow, you’d do better to choose something with a spreading root system. Among plants with a taproot are oaks, baptisias, baby’s breath, and hickories.
2. Mound Soil Up Over the Rootball.
This is another easy method to apply. Just plant as deeply as logically possible, leaving the top of the rootball exposed, then cover the top part with a mound of soil. It should be at least 3 times as wide as the root ball to allow roots to spread out over time.
3. Cut the Rootball Down to Size
You can usually remove at least a third of the roots of a plant (often up to half!) without doing any serious damage, although plants with taproots [see above] are not going to like this, though! So lay the plant on its side, grab a saw or sharp knife and cut away. Then set the plant back upright and plant it as usual.
4. Butterfly the Roots
Butterfly pruning means splaying the rootball out in two directions, like the wings of a butterfly, and is fairly easy to do. Using a saw or a sharp knife, cut the root ball about two thirds of its height from the bottom up, then pull the two halves outward to separate and spread them in opposite directions. This leaves a bit of a hole at the base of the plant, so set a mound of soil in the center of the planting hole on which to place the splayed rootball at planting time.
And there you go: planting in shallow soil is not really that hard when you know what to do.