Experts and BHG readers answer.
Planting Between Rocks
Thanks for writing. You may need to create pockets of soil to plant in, depending on how your wall is set up. To do that, use a trowel, spoon, or other tool to fill a crevice with soil, then create a cavity in the middle of the pocket for your plant, and firm in the soil around the plant's root system.
---Justin, Senior Garden Editor, BHG.com