Trial time : I don't know whether this has been tried before, but I wondered if some damp-susceptible plants could be grown in and over an upturned clay pot that is filled with a very gritty compost? The little Thlaspi rotundifolium was bare-rooted and the still moist roots were gently pushed through the drainage hole before the upturned pot was carefully filled with the dry compost. Time will tell. The top dressing is limestone chicken grit.