My method for growing sphagnum is:
1. Fill these little plastic deli cups with about 1/2" dried sphagnum
2. Swamp it with distilled water.
3. Sow heads of live sphagnum.
4. Keep it in bright light.
5. Reswamp whenever the water level drops (every 2-3 days here).
It's rather tedious keeping the trays filled if you've got a lot of them, but you won't have any problems with mold, and it grows faster than any sundew.