They will grow outside, but depends which variety you've got as to whether you can leave them outside - Zantedeschia aethiopica "Crowborough" is hardy - if any of the plants you grow have spots on the leaves, you can assume they're not hardy and you will need to keep somewhere frost free during winter, or in the house. As the weather is currently warm, you could start them off in pots outside, or in the greenhouse if you prefer, up to you.