A greenhouse or polytunnel is a godsend to organic gardeners everywhere. Gardening Under Glass (or Plastic) means a longer growing season and a wider possible range of crops that you can grow.
Cost is of course a huge factor, so think carefully before ordering a deluxe super duper glass palace. On the other hand, a small plastic minigreenhouse such as those they sell in most garden centres will probably soon be feeling like a waste of money when you realise you have outgrown it.
In the UK tomatoes and grapes are partucularly good for growing organically in the greenhouse. We grow comfrey outside to provide us with an organic free plant food (well a ‘tea’ to be precise, and although the plants love it, I wouldn’t want to drink a cuppa myself).