People in Canada sometimes ask me if it is all right to plant things in a dry stone wall. It seems ironic that gardeners in Britain are often having the opposite problem, as they continually try to hack back the things that are constantly growing up on their beautiful walls. In any case, I tell Canadian gardeners that they can grow certain plants on top of a wall, if they lay down landscape fabric first and some soil,and they are prepared to experiment a bit. Rolls or clumps of sod can often work well. Norman Haddow tells me that in Scotland these are often called turff top walls.
And of course you can grow all sorts of greenery at the base of a wall, to soften the look of the stone wall , as long as you watch that they dont grow to big and start doing damage. A dry stone wall or garden feature that has plants growing along the base of it nestles the stonework beautifully into any garden setting. Flagstone butted right up to a dry stone wall, on the other hand ,never looks as attractive. Perhaps it is just the look of too much stone material, but often it seems much more effective to create a contrast at the base of a wall by having greenery growing right up to the stones
Until two years ago I hadnt thought of another alternative for introducing plants into walls effectively.
The problem has always been frost (and to some extent, erosion too) in our part of Canada anyway. Soil will expand and contract with freeze thaw cycles and compromise the structure because the stones in the wall (whether intentionally or not) are often depending on the soil for support and eventually everything starts to move. Walls will deteriate quickly if they have soil or sand or anything that holds moisture in side them, because of frost movement indside the wall and the washing out that happens due to erosion. Dry stone walls can accommodate the movement of frost in the ground they are built on, but not the dirt they are built of.
While it is definitely not a good thing to put soil right into a dry tone wall there is a way around it.
By constructing small exterior cavities or niches in your wall, you can then fill these nooks and crannies with appropriate small plants, which have had their soil/root systems wrapped in landscape fabric or even better old nylons, to keep the soil from washing out and/or then causing damage to the interior of the wall.
Some things we planted two years ago this way, still had a hard time growing, tucked into their new rocky 'cliff-like' location, but recently we have had some success with certain sedum, thyme plants, as well as my personal favorite, chives.