It seems invariably when you buy Cacti, no matter what size, they come potted in a peat based compost. This could not be more different from their natural growing medium. Nurseries seem to use peat as it comes ready to use, they may add grit and gravel, but the problem is the peat itself, which is good at retaining water. During the summer Cacti and succulents will grow away quite happily in peat like most other plants, but come winter you will need to keep them absolutely dry, as once wetted they remain wet if the plants is not drawing water. This may be fine if you have a greenhouse and are going to keep the peat dry, but if you intention is to plant out into an arid bed, the best plan would seem to remove the offending peat compost. Planting directly with the peat based compost will ensure you cacti sits in a pool of damp to wet compost all winter and there is a real posibility of root rot or worse.
This is a Trichocerus I bought this summer. Which appeared to be potted in peat with gravel. So I removed the pot and the compost taking care not to damage the roots, and repotted in a mix of 1 part JI No3, 3 parts ballast, 3 parts horticultural grit, and 3 parts gravel. The intention is to plant out in the arid bed but as it was mid August when I got the plant, I'll err on the side of caution this year and overwinter under cover, planting out after the frosts next year.
Trichocerus in plastic pot and peat/gravel compost
Depotted and as much compost as possible removed
Repotted in clay pot with JI/Ballast/Grit/Gravel compost
Pot planted in raised gravel ballast arid area
