Answer: I'm not terribly clear on the planting situation you have, but usually plants growing through a dry laid wall are set into place while the wall is being built so that the roots are in behind the stones. Sometimes plants can be encouraged to seed themselves into soil filled cracks as well. You might try making pockets of soil and experiment with the alpine strawberries to see if they will do that for you. In a shady location your choices are a bit limited, and the tree will limit them more so because it will compete with your plants for light and rain as well as soil nutrients. You might find that a planter filled with annual impatiens or trailing lobelia will provide the effect you are after while avoiding some of the problems associated with planting into the wall. Alternatively you might try a trailing and scrambling perennial plant such as vinca minor (periwinkle) which is evergreen and will bloom in the spring.
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