My stevia (Stevia rebaudiana) mostly grows indoors, with a month or two outside in summer in a 12 inch pot. Never let a frost or cold temps (below 50) hit them, though I've had mature plants recover after being frosted. Keep the soil moist (mine are in half potting soil/peat and half sand) by not letting it dry out more than an inch down as they have shallow roots. I keep a plastic tray with about an eighth to a quarter inch of water under my cuttings/plants. They seem to like 12-16 hours light under fluorescent tubes, rather than 8-12 hours which makes them leggy and have fewer leaves. The leaves get minty tasting when the stems start to get woody. Usually this is a good time to chop the woody stems in 4-6 inch sections with 2-3 nodes and root them in water. New plants have more usable/tasty leaves--I use it as a sweetener in tea.
Plants I've had in the ground, in the past, grew up to 3 feet tall by the end of the summer here in zone 4. For a pesticide against aphids, I dust them with rustic tobacco powder. I chopped them down and brought the stumps in pots indoors, sometimes they have even made it to spring. I've had near continual blooms since the second year with my potted plant and even set seed, but I've never had these seeds germinate for me. I've read that they normally have a low germination rate (something like 1-10 per 100). If the plant gets cold or doesn't get enough light, it seems to shut down like an annual. Discovering that they root easily in water has been the best thing.
I keep re-rooting my stevia, and it keeps growing. I just planted 2 more this morning that had started roots in my glass of water full of cuttings. You do need to change out the rooting water regularly for good rooting or you may get some rot. Once Stevia branches get old and flower the stems get too woody to flex and break or dry up. So, I cut young shoots and root in water, or moist soil, these new plants make the best leaves. This also works well with basil, but I've never gone much more than 2 years with a basil plant, usually the roots quit working and the plant dries up.
I don't let them dry out completely, and I don't let them get chilled. Last fall I almost lost my largest one to an early September cold snap that frosted it. I almost lost two smaller ones while gone a few days over Christmas. They got too dry, all of the leaves drooped, and died back to the root. After steady moisture, they have bounced back and look better than ever. One thing I also noticed is that they thrive on light, lots of strong light. The ones that vine up into the fluorescent tube fixtures are always stocky and root the easiest.
Additional Resources:
- http://www.stevia.net/growingstevia.htm
- http://davesgarden.com/guides/pf/go/316/
- http://www.rirdc.gov.au/reports/NPP/02-022.pdf
