I think it would be best to get a rooted plant from a reputable source. The initial investment is well worth it and should be only about $5-$10.
Once you have a plant and grow it through the summer, you will have a big enough plant with hardened stems that you can take cuttings from and root. I always start several cuttings (1/2 dozen to a dozen) in late summer through early fall with stems that have not started to try to bloom (not exhausted) as insurance for overwintering since I have often read about failures during winter (I give away some and lose some of the rooted cuttings -- I have 3 left -- but so far the mother plant has come back except maybe one year -- I don't remember exactly, but I did lose one of the two initial plants I bought during one winter).
So far I have not been successful in trying to grow stevia from any of my saved seeds, though I make an attempt almost every year.
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