Stevia is a South American sub-tropical plant that has been used as sweetener for centuries. Its sweetening strength is many times that of sugar. The leaves and tender stems are dried and powdered to achieve the simplest usable form, or it can be purchased as a white powder. Stevia is a perennial in frost-free climates; it is grown as an annual in areas with freezing winters. Wild stevia spreads by layering, where fallen stems take root, or by new shoots sprouting from the crown. Wild seeds are typically sterile. However viable seeds are available from mature plants in sub-tropical areas where the growing season is long. Start stevia seeds indoors, or look for young plants at garden centers.
Soil and Fertilizer
Most garden soil is suitable for growing stevia. Loam or sandy loam soil that has been amended with compost is ideal. To grow stevia in heavy clay soil, loosen the clay with organic matter by adding humus or compost. Sub-tropical soils where stevia originates are slightly acid, but, according to Stevia.net, stevia will grow in a range of soil pH. Soil texture that will provide consistent moisture retention is important. Raised beds are useful in areas where the soil may be waterlogged.
Stevia prefers full sun, except in areas where summers are very hot or dry. Partial shade in the afternoon is a good idea for areas with excessive summer heat. Some growers in hot areas use shade cloth (mesh cloth that allows filtered light through) to protect their stevia plants from relentless sun. Shade cloth also helps reduce moisture evaporation from the soil.
Stevia plants started indoors from seed or cuttings need bright light for 24 hours per day. When the seedlings are three or four weeks old, reduce the light exposure to about 15 hours per day. Artificial lights with a timer are a practical way to provide consistent indoor lighting. (Ref 1, p. 37)
Water and Humidity
Drip irrigation or soaker hoses are the most efficient methods of watering stevia in the garden. Stevia plants should be mulched to help retain soil moisture and for weed control. Stevia prefers consistent soil moisture, with no dry spells or soggy periods. If you water stevia by hand, avoid getting the leaves wet. Wet foliage and waterlogged soil can cause fungus diseases to develop. Remove and destroy affected parts of the plant to remedy a fungus problem.
Seedlings and cuttings require consistent moisture. A clear plastic dome, raised slightly for some air flow, helps maintain humidity.
Stevia seeds require very warm temperatures of 75 to 80 F to germinate. Don't set young stevia plants outdoors in the spring until the weather is settled and temperatures remain between 50 and 60 F.
To overwinter stevia plants from the garden, trim them back to a few inches tall and place them in pots. Potted stevia plants will live through the winter indoors under artificial lights in a constant temperature as low as 55 degrees. Prune back the sparse winter leaf growth in the spring, and set the pots outdoors when temperatures are 55 or higher. Bring them back indoors if temperatures will drop to 40 degrees or lower. Set plants out in the garden when the temperatures are warm and settled.