Mr. Marsden prefers the "hill" method commonly used for sweet corn. Set plants in low hills spaced 12 to 18 inches apart. Periodically during the growing season, pull more soil up around the plants with a hoe. This will tend to smother weeds and drain away surface water that could encourage disease.
Stevia rebaudiana seeds are rarely available because of production problems and poor germination, so plants are generally used instead. Plants are available from several mail order sources. Be sure you are getting Stevia rebaudiana (Stevia is the genus and rebaudiana is the species) since this is the only sweet variety. Stevia stems are brittle, but nurseries have developed packing methods to protect them in transit. Arrange for plants to arrive soon after your last frost date. Later on, very high temperatures may stress transplants. Transfer plants to the garden as soon as possible after arrival, making sure they don't dry out in the meantime.
In garden beds, space plants 10 to 12 inches apart in the row, with two rows per bed. Stagger rows so that plants end up in a zigzag pattern. Use a trowel to dig a hole, then pour in some water and set the plants a bit deeper than they were in the pot, so the root ball is covered by a thin layer of garden soil. After back-filling around the roots, water again to settle the soil. If the weather is hot and sunny at planting time, it's a good idea to place a thin mulch around the plants to reduce moisture loss. Cool night temperatures will halt plant growth. For early plantings or areas with cool summers, hotcaps or row covers will allow faster growth and offer protection from late frosts. Don't let the plants overheat on hot days, however.
If you are fortunate enough to obtain high-quality Stevia seeds, they are easily germinated indoors under lights. Seedlings grow slowly, so allow 7 to 8 weeks from seed to transplanting (Columbus, 1997). Only black or dark brown seeds are viable. A tan or clear color suggests they are empty shells, lacking an embryo. You can verify this by slicing some seeds in half. Good seeds will be solid and white inside. Even firm, black seeds tend to lose viability rapidly. A germination test will indicate what percentage of the seeds are likely to sprout. Place 10 or more seeds on a wet paper towel. Fold the towel in half 3 times, then slip it into a plastic bag kept at 72 F to 80 F. Count sprouted seeds after 7 days and divide by the total number of seeds you were testing, then multiply by 100 to get the germination percentage.
A plastic flat covered by a clear plastic dome, available from garden retailers, makes a good germination chamber when placed beneath a growing light. Place a thermometer inside and maintain a 70 F to 75 F temperature by adjusting the level of the light. Use small containers (with drainage holes) or plastic cell packs filled with standard potting soil. Place 3 or 4 seeds on the soil surface in each container and cover with a thin layer (about 1/8 inch) of horticultural vermiculite. Water from below as needed by pouring water into the tray. Seedlings should emerge in 1 to 2 weeks. Thin to one plant per container. Extra seedlings may be transplanted to empty containers.
In general, Stevia should be treated as a vegetable crop. When hot weather sets in, usually a month after planting, beds should be mulched 3 to 6 inches deep with organic residue such as grass clippings, chopped leaves, straw, hay, or compost. This will protect the shallow feeder roots and hold in moisture. Plant growth is slow at first, accelerating by mid summer.