Posted: 26/04/2012 at 18:13
I have only ever grown them in open ground, which is the norm. but in a pot you should use ordinary compost mixed with horticultural grit as they like good drainage. If you can't get that then use sharp sand (not builders sand ) from your local builders merchant. I would mix 3:1 by volume compost to grit / sand.
You could add bonemeal, though that is for term development of the corm and not for this year's flowers. They are not very hardy (zones 7 - 10), and will be more vulnerable to frost in pots rather than in the ground unless you shelter the pot. I might lift them in autumn, dry and store them, and replant in spring. If you intend leaving them in a pot over winter maybe you should go for John Innes no 3, as ordinary MP compost will be nothing but peat and sand by autumn,