Answer: How old is your tree? Has it been protected from freezes? If it's an old tree that's froze back a few times, it could have frozen back beyond the grafts and is now fruiting rootstock. Another issue could be whether you've gotten a frost yet or not... a lot of times, oranges won't sweeten until it gets pretty cold at least once... if, after you get a frost or at least a good cold spell, and they're still not sweet, it's a pretty good chance you're getting fruit from the rootstock. Sour oranges are good replacements for lemons in various recipies, but pretty useless as far as eating goes.
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