This section is from the book "The Florists' Manual", by William Scott. Also available from Amazon: The Florist's Manual.
The only species we grow and find profitable is what we know as serrati-folia, or sweet stevia. It is an easily grown plant, but the flower is light and feathery, so that we value it highly at the holidays, and common as it may be we should miss it very much.
After flowering cut down the plants to within six inches of the pots and stand them in some cool, light house. You will get a great many cuttings from a few plants. Propagate in February or March, and plant out at the end of May. Any garden soil will do. They should be at least two feet apart. Stop them frequently till the last of August.
Before any danger of frost lift and pot into 6-inch or 7-inch, but let the plants stand outside as long as you can. When you have to take them in give them the coolest (but light) bench you have. You want them at the holidays, and if kept light and very cool they will be robust and stout and give you fine spikes. Never let frost touch them, but they will thrive in a very low temperature.