Descriptions of sand beds seem to indicate pure coarse sand maybe 12 inches over native soil. An old formula for cold climate cactus suggested 1 part soil to 9 parts sand. Sand beds in our area often seem to allow roots to reach our clay topsoil--which feels inconsistent somehow.
I've heard recently about using mostly crushed trap rock in a bed. I'm wondering now about crushed limestone.
We had a hot, humid, but dry summer in New England USA. Cacti and Yucca didn't flinch, and Viola pedata would have looked good except for the rabbits; but I lost several Penstemons. For misc. other plants, I'm not sure if it was dryness or humidity that was the problem. (and/or spider mites?)
Right now I'm thinking about 9 parts coarse sand, 1 part small crushed marble, 1 part crusted trap rock (basalt?) and 1 part sandy compost. Just thinking that coarse quartz sand seems to be an extremely sterile medium. I have been using some liquid fertilizer in my sand bed.
Guess there is no one size fits all. Then there is the question of watering and when. Guess I don't expect any answers.
Will try to post my results from time to time.
Charles Swanson NE New England USA
We have some beaches here with quartz sand with the consistency of table salt. No surprise that a beach pea grows; but there must be an explanation for the strong growth of a beach grass and Hudsonia.