According to my “seasonal container plants lowest temperature notes”, Stevia is roughly hardy to usda zone 9 (20) ~ 11 (40). Number in parenthesis is minimum winter temperature in °F. That means Stevia seeds would normally withstand winter frost and probably require certain amount of cold period (stratification) for optimum germination. In other words, the light frost shouldn’t be harmful and might even be beneficial.
Don’t expect the seeds to sprout until it gets warmer though, since the plant itself is frost-killed. Once mature, established roots/crown will survive the cold and new growth will start from soil-level in spring. My notes reminds me to bring in Stevia above 45°F (Don’t leave/put outside).
That said, Stevia is notoriously difficult to germinate, or so I’ve heard. My attempts to grow from my own saved fresh seeds have not been successful, and like most this type of seeds (seed with a puff/tuft of the end ... like lettuce seeds) they don’t remain viable for very long.
Also, FYI, I have heard that levels of sweetness and characteristic bitterness is variable, so it’s best to obtain cutting grown clone of known/best flavor plants. I didn’t do that but did buy my first plant from a reputable nursery, and have been growing backup cutting-grown plants ever since. There are also some that say sweetness depends on how you process the harvested plant material.
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