Sow onion seed shallowly in a clean, fine, free-draining propagation medium. Keep it evenly moist, never waterlogged, until emergence. Afterwards, grow seedlings as brightly as possible and relatively cool. A greenhouse bench works if nights are adequately protected and ventilation is consistent on sunny days. Otherwise, begin sowing in a bright indoor position.

After emergence, give seedlings enough root room, individually or in small groups. High temperatures with weak light create long, unstable leaves and remove their compact advantage. Before transplanting, the rootball should be moist but not dripping. Acclimatise plants gradually to the bed and avoid unnecessary damage to leaf bases and roots.

Plant the final greenhouse bed in time for the variety’s normal bulb-forming stage. Choose the timing for your local climate rather than importing a northern-hemisphere calendar. The bed must be workable and drain water safely. Earlier planting can gain useful growing time, but requires early ventilation on bright days and allowance for local cold snaps where they occur.

On compact plots, a classic aluminium greenhouse can serve as an orderly propagation space for a manageable onion crop. Use stable shelves for sufficiently deep trays, then move strong plants into a prepared soil bed instead of forcing storage onions to remain in undersized containers.