Posted: 15/10/2014 at 10:49
I don't understand about keeping them out of the light? Surely cacti, other than the Christmas cactus type, need as much light as possible especially at this time of the year when they are not quite dormant yet.
My own cacti are the more hardy types and are in a cold greenhouse with no artificial heat what so ever, but like soulboy I will cease watering completely for from about now for the next four months.
I am not sure whether such extreme drought is suitable for small seedlings which have less water reserves and would suggest that on complete drying out they are given a watering, perhaps every month or so overwinter.
As long as your greenhouse is capable of maintaining 2 or 3 degrees c on the very coldest nights I would forget about the heating pad.