I have a Pterostylis tuber growing directly in my plunge - don't know how it got there, but it has remained solitary for 2 years unlike equivalents in pots, which go mad. Some plants on a similar Mediterranean cycle love growing in plunge sand, however. I dug a self-seeded Corydalis macrocentra the size of a tennis ball out last year, no exaggeration, twice the size of its pot-grown counterparts