I've tried to grow some marketmore 76 cucumber seeds and they got to about 8cm tall so I planted them outside. It's been 3 days and they have now died. Should I have waited a little longer before planting them outside?
What do you mean by harden off? It's really hot here at the moment so I didn't think the temperature would affect them.
Not only would I have hardened them off, but I would probably water the plants at least twice a day if it is so hot where you have planted them madam green fingers. In fact I have held back from planting any seedlings for the last couple of days - it is far too hot for any young plants to try to establish in this heat, they would wilt and possibly die under these circumstances.
Scroggin is right - you should try to harden off any new seedlings before planting them outside. To do this you would put them outside during the day in their seedling pots/trays, somewhere warm, sheltered from direct sun and wind. Bring them back inside during the night. This helps to strengthen the plants and prepare them for the transition from indoors to outdoors and from pots/trays to soil.
Try again, it isn't too late to sow seed (is it?), and don't give up on those plants, just water them very well and see if they recover in the next few days.
Thank you for your advice and I will try and grow some more and give the others a good water and see what happens