Answer: If you had a mix of varieties to begin with, than most likely what has happened is that the predominantly green varieties have spread and overtaken the variegated ones. So the variegated ones aren't turning green, they're just not as vigorous as the others, which have been spreading. Another possibility is that you are seeing offspring of the
original plants. Many hosta cultivars don't reproduce "true to type" when grown from seed. That is, the seedlings may revert back to a very different form. Finally, variegated hosta can use a little more sun than their green cousins, for maximum development of variegation.
If you want all variegated types, than you should remove the green ones as they appear. (Is there another place for them in your yard, or could you give them to a neighbor?) This should give the variegated ones a little more elbow room.
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