STEVIA — This happens every year - I should be used to it by now....
In my area Stevia plant is not winter hardy, but it’s a plant that is generally listed as hardy to Zone 9 or winter low temperature of 20°F (winter low can get down to negative single digits here in Zone 6b).
I mentioned something about that elsewhere recently —
Subject: Frost And Sprouting Stevia Seeds?
applestar wrote: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.
Good luck.
Although it is possible to over-winter Stevia in the house, it wants cooler than average indoor house temperature and needs to be situated carefully. In the warm dry heated indoor environment, they are prone to red or other spider mites.
I have read that it’s best to let the Stevia actually go dormant for longevity of the plant, similar to Lemon Verbena, along with Rosemary. All three of them have similar wintering requirements, although Stevia is a perennial and dies back to the ground, Lemon Verbena is more like a deciduous shrub, and Rosemary is evergreen.
I keep my Stevia in my garage and put the pot in the “Garage Siberia” — closest to the outer door — once the leaves and stems start to dry up. Then it’s watered every so often, just enough so the potting mix doesn’t dry out completely. It will experience freezing temps down to mid 20’s and close to 20.
In the spring, it always, ALWAYS, takes a long time to start growing new shoots from the roots, below the potting mix. Previous year’s growths will be all dead sticks by this time and can/should be cut down as close to the soil level as possible BEFORE the new shoots grow too big.
This year, I brought the pot out in front of the garage doors in March after the spring thaw, then waited while all other winter survivors were moved out of the garage and onto the backyard to acclimate for the season.
The spring sun moved further and further west and north of west until the setting sun started to beat against the garage doors, but still nothing but weeds came up out of the pot and I had just about given up.
Then this Wednesday, after an outing, I looked in the pot with some disappointment, thinking to declare it dead and recover/recycle/compost the potting mix, and recognized 4 new Stevia shoots among the weeds.
...Stevia pot, moved to the patio and all cleaned up. More new shoots were hiding/growing under the weeds. It still needs the potting mix freshened and fertilized.
)
We just had a bit of a storm pass through — about 1/4” rain according to official reports. More rain coming today. That should help — corn loves extra moisture. 
