You can grow them under a light until they are ready to plant out but then you need to ‘harden’ them to the outdoor conditions before putting them in the ground. You will need to do this by standing them outside, still in pots or trays, somewhere where you can give them a bit of shelter from the wind then bring them in at night. Do this for a week and they should be fine. Milk jug cloches work well for marrows, courgettes and squash as the wind can damage the stems when they are transplanted.
Regards,
Tristan
