You cannot plant your sun flower out until after the last frosts. In your part of the country you may already be there. Here in Scotland we are not guaranteed frost free until the end of May, although it might not happen. Sometimes we just have to gamble on it. You can harden off your plants long before then so that they are ready for planting when all risk of frost is past. Although plants are labelled hardy annuals it doesn't mean that seedlings which have not been hardened off can be planted out. Haven got your sun flower this far, molecoddle it for a few more days before planting.