If you enjoy gardening, Stevia can be a rewarding herb to grow. While it's not feasible for most of us to grow sugarcane or sorghum in our backyard, several Stevia plants will fit nicely into a small garden. Recipes utilizing Green Stevia Powder are now available, and the whole leaves add to the flavor of herbal teas.
Stevia rebaudiana is a tender perennial, native to semi-humid subtropical regions of Paraguay and Brazil. Wild plants occur on acid soils that are constantly moist, but not inundated, often near the edge of marshes or streams where the soil is sandy (Brandle et al., 1998). In the garden, too, Stevia doesn't like to dry out, but standing water will encourage rot and disease. Stevia can be a successful garden plant in most climates with the use of a few simple techniques. Raised beds or hills prevent "wet feet," while an organic mulch and frequent watering ensure a constant supply of moisture.
In North America, Stevia survives winters only in the warmest areas such as southern California, Florida, and Mexico. Research in Japan indicates a critical winter soil temperature of 32 F to 35 F (Sumida, 1980). Stevia is a weak perennial, so plants grown as perennials should be replaced every few years. In colder areas, Stevia is planted after the last frost and treated as an annual. Longer summer days found at higher latitudes favor leaf yield and Stevioside content (Shock, 1982).
While tolerant of most soil types, Stevia prefers a sandy loam or loam. Any well-drained soil that produces a good crop of vegetables should work fine. Incorporating organic matter is the best way to improve heavy, high clay soils. A rich compost made with leaves, grass, hay, kitchen waste, manure, and other organic residues will improve soil structure and supply nutrients. Finished compost may be tilled, disked, or spaded into the soil before planting or used as a mulch later on. A "green manure" crop the previous year such as oats, rye, or legumes will also improve heavy soils. Stevia occurs naturally on soils of pH 4 to 5, but thrives with soil pH as high as 7.5. However, Stevia does not tolerate saline soils (Shock, 1982).
While a good compost usually satisfies nutrient requirements, soil testing or plant symptoms may alert you to deficiencies. Mark Langan of Mulberry Creek HerbFarm recommends low nitrogen or organic fertilizers. Excess nitrogen promotes rank growth with poor flavor. Bone meal, blood meal, cottonseed meal, guano, or dried manure provide nitrogen that is released slowly. Rock phosphate or bone meal provide phosphorous. Greensand is a good source of potassium. Rock phosphate, bone meal, and greensand offer a wide range of trace minerals. For maximum nutrient availability, work organic fertilizers into the soil a few months before planting, or mix with compost. For poor fertility soils, Blas Oddone (1997) of Guarani Botanicals, Inc. recommends incorporating 6 to 7 pounds of cattle manure per square yard. When using chemical fertilizers, a low nitrogen formula such as 6-24-24 is recommended in a split application - at planting time and again in mid summer. (Columbus, 1997). Steve Marsden of Herbal Advantage, Inc. simply uses a balanced vegetable fertilizer at the dose and intervals recommended on the container for vegetables.
Unless your soil is very sandy, raised beds are ideal for Stevia. A raised growing surface prevents standing water and reduces compaction. Beds should be 3 to 4 feet wide and 4 to 6 inches high. Till, disk, or spade the whole area thoroughly, then mark bed boundaries with string or garden hoses. Dig soil from the paths, 1 to 3 feet wide, and toss onto the beds until they reach the desired height. Beds may be left in place permanently. By walking only on paths, soil compaction is reduced. A mulch such as newspapers, grass clippings, or landscape fabric on paths will help control weeds. While not necessary, sides on the beds can be attractive and functional. Concrete blocks may be used, or rot resistant wood such as cedar, redwood, or locust. Raised bed "kits" made from plastic are available as well. Treated wood should be avoided because of possible soil contamination.
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.
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.
While whole leaves are great for making tea, it's easy to turn them into Green Stevia Powder with a kitchen blender, food processor, or coffee grinder with metal blades. With the blender bowl half full, process dry leaves at high speed for a few seconds. Collect the fine powder for use in recipes calling for Green Stevia Powder. Use a clean glass jar for long-term storage.
Propagation and Container Growing
Stevia stem cuttings root easily without hormones, but only under long day conditions. A fluorescent shop or plant growth light both work well. Leave the light on 14 to 16 hours per day, 5 to 9 inches above the cuttings. An automatic timer will make the job easier. Even with artificial light, Stevia cuttings root most easily during the long days of spring. Cuttings should be taken in March for transplanting in May or June. Plants from later cuttings may be over-wintered in pots under fluorescent lights.
Fall cuttings root successfully with the use of rooting hormones. Use a low-strength rooting compound available from garden retailers or make your own natural compound with Steve Marsden's recipe. Harvest a handful of willow branch tips and remove the foliage, then liquefy in a blender with twice the volume of water. Dip cuttings in this mixture before placing in the rooting medium.
Coarse or medium grade horticultural vermiculite works well for rooting Stevia. Mr. Marsden prefers a peat-lite mix that includes bark, especially for outdoor propagation beds. Coarse, clean sand may be used as well. Place small pots, or cell packs with drainage holes, in flats or trays to facilitate watering from below as needed. With a sharp blade or pruning tool, make cuttings 2 to 4 inches long. Each cutting should have 2 or 3 nodes. A node is where leaves attach to the stem. Cut between, rather than at the nodes. (Sumida, 1980). Plunge the proximal end (closest to the roots on the mother plant) of the cutting into the rooting medium far enough so that at least one node is buried and at least one node remains above the surface. Remove all leaves from buried nodes. Above the surface, remove large leaves by cutting or pinching leaf stems, taking care not to damage the tiny axillary leaves emerging behind large leaves. These axillary leaves are the growing points of your new plant. Keep cuttings at 60 F to 70 F. Indoors, under lights, misting is not necessary. Outdoors, or in a sunlit greenhouse, cuttings should be misted several times per day until roots are well formed. After about a week, growth should be evident if rooting was successful. After 3 to 4 weeks, transfer plants to larger pots (at least 3-4 inches in diameter) with standard potting soil. Transplant these to the garden in another 2 to 4 weeks or keep as a container plant.
For older plants, keep the fluorescent light a few inches above the foliage. When stems reach 7 to 10 inches in length, cut them back to promote branching and vigor. Over-wintered plants look devitalized by the end of the winter, but regain vigor when transplanted outdoors. Some plants will inevitably be lost, so grow more than you think you'll need. Stevia may also be grown outdoors in containers such as gallon pots. If you start with a high-quality potting soil that has organic fertilizers mixed in, further fertilization may be unnecessary, but a monthly watering with a dilute seaweed solution can be beneficial. Mr. Langan recommends a balanced, slow release fertilizer applied every two weeks for container plants. He also advises using wooden containers or the double pot method to insulate roots from summer heat. Place in full sun when it's cool, but provide shade during hot weather, making sure the soil doesn't dry out. For perennial production, bring containers indoors before the first frost.
Sources for Live Stevia Rebaudiana Plants
16185 SW 108th Ave.
Tigard OR 97224
Herbal Advantage, Inc.
Rt. 3, Box 93
Rogersville MO 65742
Mountain Valley Growers
38325 Pepperweed Road
Squaw Valley CA 93675
Mulberry Creek HerbFarm
3312 Bogart Road
Huron OH 44839
One Green World
28696 South Cramer Road
Molalla OR 97038
3314 Earlysville Road
Earlysville VA 22936
LOC 1AO. Canada
Brandle, J.E.; Starratt, A.N.; Gijzen, M. "Stevia rebaudiana: Its biological, chemical and
agricultural properties." Canadian Journal of Plant Science. 78, pp 527-536 (1998).
Columbus, Mike. "The Cultivation of Stevia, 'Nature's Sweetener'." Food and Rural Affairs
Factsheet. Ontario Ministry of Agriculture, May (1997).
Oddone, Blas. Technical Manual on 'How to Grow Stevia.' Guarani Botanicals, Inc. Pawcatuck,
Shock, Clinton L. "Experimental Cultivation of Rebaudi's Stevia in California." Agronomy
Progress Report. University of California, Davis, April (1982).
Sumida, Tetsuya. "Studies on Stevia rebaudiana Bertoni as a New Possible Crop for Sweetening Resource in Japan." Journal of the Central Agricultural Station. 31, 67-71 (1980).