Planting a Crape Myrtle in compacted clay? (growing, manure, magnolias . . . Twice the size is the rule of thumb, but regardless of the size of the hole in clay, it will still hold water like a bucket, Once I've dug the appropriate size hole in clay, I use a post hole digger and dig out the center as far as possible, filling that hole with organic matter hoping to drain as much water as possible away from the root ball, and provide a better growing medium for the