The bears could control the temperature around the seeds by using their propagators (small electric boxes with high see-through lids, which they could keep at a specific heat automatically). Even if it became very cold in the greenhouse overnight, as it sometimes could at this time of the year, the little seeds would stay nice and warm.
James and Harry knew they had to fill little pots with compost, add some water, and then sow their seeds, covering them with a strange mixture called 'vermiculite' which Scary had explained to them, helped to keep the compost moist (as seeds need sufficient water to drink) and would insulate the seeds against temperature and humidity (that's the amount of water vapour in the air) in the greenhouse.
