Answer: You can transplant it outdoors anytime now that winter is past. In fact it would probably like very much to be outside! Avocado trees love full sun, but will grow in part shade. Trees grown from supermarket avocados will be very cold tender, not surviving most winters in South Carolina.
The Mexican avocado is a bit more hardy than the West Indies types. Unless the produce manager at your local supermarket can tell you which you purchased, the only way to tell for sure is to crush and smell a leaf. Mexican avocados have a plesant, aromatic odor.
You could prune your tree back in fall and try to cover the plant for protection, but the trees get very tall, making this impractical. Even a covered tree will need a heat source when temps drop into the mid 20's. As a novelty ornamental it has possibilities, but don't grow one in hopes of getting fruit. This takes years of freeze-free growing.
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