And another question;
If plants have been seriously under-watered, leaving a rock-hard, bone-dry, shrivelled root mass; what's the best way to revive them?
I went away for a few days and they got very thirsty!
When I have a plant that's bone dry it's usually pretty far gone but if it felt like it had life left in it I'd set it in a bowl of water such that it could suck the water up into the plant thru the drainage holes. If you try to water plants that dry, typically the water just goes right thru the plant with more conventional watering styles. I would generally leave the plant soaking for a good 30 minutes before I drained it.