Stevia:
Propagate by seeds or by tip cuttings taken in summer, or root division in spring. Seeds are notoriously infertile and slow to germinate. Flowering usually occurs 54-104 days after transplanting. The plant is climatically suited to from temperate to sub-tropical climates where temperatures range between 21-43C, with an average of 24C, but it will also grow in the tropics, where it prefers shade; and will also grow in cold climates with winter protection. Plants in the ground have been noted to take minimal frosts. It is a somewhat temperamental plant, and may be slow to grow at first. Even in ideal conditions, it is not uncommon for plants to die suddenly or to lose its leaves. As long as the roots are alive, the plant may regrow. Poor, loose, well drained soil is recommended, and that the plants be well-mulched so that surface feeder roots do not dry out. For gardeners in cold climates it is recommended that plants be started in large pots so that the pot can be moved to a warm verandah in winter. In cold climates, the plant may go dormant to the ground. As stevia does not over-winter outdoors in freezing conditions, the roots are lifted in autumn and stored indoors in perlite or sand and then set outside again in spring. Stevia requires regular watering in dry periods, but has poor tolerance to long waterlogging, or to saline water or soils. In its natural habitat the plant occurs naturally on acid soils of 4-5 pH, but will grow well on soils up to 9 pH. Leaf yields can be increased with a moderate application of nitrogen, phosphorus and potassium fertilisers. Plants also respond well to liquid seaweed as a foliage spray applied fortnightly. As soon as the plant flowers, the leaf production slows down, so it is an advantage to nip off flower buds, to encourage further leaf development. If the plant is left to flower, the tip leaves take on a slightly bitter overtone. Because of its gangly growth, plant several together for support. Plant will die if left go to flower, so it should be cut back repeatedly to prevent flowering.

I've grown stevia in zone 5 as an annual for a couple of years now in a 12" pot (from a transplant bought at a nursery). Some people say it's a bit finicky to grow but I've never had any trouble. I just pot it up immediately to the 12" pot using whatever potting soil that's around - peat / vermiculite seems to work as well as the Miraclegrow with coir - and keep it watered until it starts to take off.

I've tried varying levels of fertilizer for the plant and at high levels of nitrogen, I get leaves in the 2" long range but they are nowhere as sweet as the smaller leaves obtained from a single application of 10-10-10 slow release at planting. As Daisy said, it really doesn't like wet feet and it will die pretty quickly if it gets waterlogged.

In a season in a pot, my plant usually gets to about 2.5 feet tall and then starts spreading out to about 2 feet wide. I think that the leaves are the sweetest just before the plant starts flowering in late summer so I harvest a bunch then and keep the flowers cut. Before the fall frosts, I cut the whole plant at the base and hang upsidedown in a ventilated area to dry for a couple of weeks and then strip off the leaves and grind into powder.

I've never tried to overwinter it so I can't help there.

Thanks for all the great info! I wanted to get several plants, but the greenhouse only had one left. It's a year round greenhouse & they propagate herbs constantly, so I'll keep checking for more. I think I'll try to start some tip cuttings from this one. It's pretty young (wasn't really put out for sale yet, the owner went & got it for me) so I hope it spreads like makalu described before the season is over. I wanted several so I could try different methods of overwintering, & of course so I would have a good supply of leaves. I'll try the kelp, I already use it regularly. Thanks again for all of your help. Carla

You might consider putting it in the ground so you can get some good growth & cuttings to propagate. Also, stevia does not seem to like being moved around much. Fresh seed is more likely to germinate than older seed, so if you get seed you might go ahead & start them rather than saving them.