A consistent moisture supply is important for Stevia. Irrigate once or twice a week, whenever rain fails to water the plants. Sandy soils require more frequent irrigation. Trickle irrigation is ideal, ensuring consistent moisture levels without wetting leaves. A simple and effective system is the black, "weeping" soaker hose made from recycled rubber. Place a soaker hose between the two rows of plants, beneath the mulch. Attach to a garden hose and turn the water on at a trickle for a couple of hours. The system can be automated with the addition of a timer.
Side-dressing is usually not necessary, but low nitrogen or organic fertilizer may be applied in the summer as plant growth begins to accelerate. Excess nitrogen causes tender growth and reduced leaf sweetness. Mr. Oddone recommends application of a 10-10-12 foliar fertilizer directly on leaves at 30 and 60 days from transplanting.
Stevia stems are prone to breakage during high winds. Mr. Langan advises pinching tips out every 3 to 4 weeks for the first month to encourage side branching, resulting in a bushier plant. Grow in a protected area if possible. Supporting the plants with a "corral" made from strings tied to stakes is another strategy.
Stevia may be affected by two lesion-producing fungal diseases, Septoria steviae and Sclerotinia sclerotiorum (Brandle et al., 1998). With Sclerotinia, dark brown lesions form on stems, near the soil line, followed by wilting and eventual collapse of the plant. Stevia plants are usually full grown before diseases appear. As harvest time nears, commercial growers watch plants closely and harvest the entire crop at the first sign of disease. Meticulous weed control (by hand) permits strong growth, which helps plants resist disease. Humid, wet weather and standing water favor the development of fungal diseases, making raised beds or hills a preventative measure. Additionally, avoid wetting leaves during irrigation. Stevia is usually the last plant insects will feed on, so pests are seldom a problem outdoors. Aphids, thrips, and whiteflies can cause damage in heavily infested greenhouses.
Use fresh leaves for tea or eat a few right off the plant. They taste great with mint leaves. Sweetness (Stevioside content) is greatest just before flowering, which is triggered by short day lengths (Brandle et al., 1998). The onset of blossoming ranges from mid summer to late fall. Plants should be harvested before the first frost or as soon as blossoming begins, whichever comes first. Cut entire plants just above ground level. When growing Stevia as a perennial or for early harvests, clip the plants 6 inches from the ground so they will survive and re-grow (Shock, 1982). Harvest in the morning, after dew has evaporated.
Plants are easily dried by hanging upside down in a dry, warm, drafty location. Bunch a few plants together and bind at the stem end with a rubber band, then slip a paper clip bent into an "S" shape under the rubber band. Hang by the other end of the paperclip. If you have lots of plants, hang them from strings or wires strung across the ceiling. After a few days, rake leaves from the stems with your fingers and gather for storage in a clean container such as a glass jar. They keep well for years. Stems are less sweet, so toss them on the compost pile. An alternative method is to strip fresh leaves from stems and spread on elevated screens in the sunshine, on a day with low relative humidity (less than 60%). If drying takes 8 hours or less, according to Steve Marsden, very little Stevioside will be lost. A food dehydrator on low heat (100 F to 110 F) will do an excellent job as well. Leaves are crisp, crumbly, and bright green when fully dry.