Distribution The first report on commercial cultivation in Paraguay was in 1964. Since then it has been introduced as a crop in a number of countries including Brazil, Korea, Mexico, United States, Indonesia, Tanzania and Canada since 1990. Currently, its major production is centered in China and the major market is in Japan. The no-calorie sweetener heaped with great health benefits is now being grown in more counties of Kenya including Kericho, Nandi, Nakuru, Laikipia, Uasin Gishu, Narok, Meru and Bungoma. Stevia, a natural sweetener, is creating opportunities for more farmers in Kenya and opening export markets in China, Malaysia, Paraguay and Brazil. In Kenya, the commercial cultivation of the crop began in 2008 when the Malaysia-based firm set up a local subsidiary to take advantage of market opening in the United States (US). Kenya earned USD 50 million from stevia in 2011. Stevia is a sweetening plant and its introduction into the Kenyan market as one of the cash crops for agro-based industry may spell doom for the sugar industry. It is used as a sweetener in place of sugar, especially in the cases of patients suffering from acute diabetes. Nowadays, it is also being cultivated in Bangladesh. The climatic requirements of this plant indicate that it can be introduced in the hilly areas of Sylhet and Chittagong districts of Bangladesh.
Climate Stevia is a perennial herbaceous plant native to between 22° to 24° south and 53° to 56° west in Paraguay and Brazil. Plants grown at higher latitudes actually have a higher percentage of sweet glycosides. Nutrition and climatic conditions play important roles on the growth and secondary metabolites of stevia plant. Vegetative growth is reduced when temperatures are below 20o C and when day length is less than 12 hours. Increasing day length to 16 hours and increasing light intensity can increase vegetative growth and stevioside levels. However, some varieties appear to be photoperiod insensitive. Early flowering lines tend to have higher stevioside content but lower total yield. The concentration of stevioside in the leaves increases when the plants are grown under long day conditions where the vegetative period is longer and steviol glycoside yields are higher. Planting density and transplanting date are the most important agronomical factors that may affect the quality and quantity of the yield. The concentration of stevioside in the leaves increases when the stevia plants are grown under long days. The natural habitat of stevia is semi-humid subtropical climate on the Tropic of Capricorn (22-23o S latitude), 200-400 meters above sea level, with 1,500-1,800 mm of rain and temperature extremes of -6o C to +43o C. It is a semi-humid subtropical plant that shows higher leaf production under high light intensity and warm temperature. However, day length is more critical than light intensity. Long spring and summer days favor leaf growth. Short days trigger blossoming. Stevia prefers partial shade during considerable summer sunshine. The overall cultivation studies suggest that stevia seedlings may be planted from 15th February to 30th April for a better yield of leaves in Bangladesh.
Soil The nutritional dosage is strongly governed by the soil properties and climatic conditions of the growing region [10]. Stevia grows in well-drained fertile sandy loam or loamy soil, rich in organic matter. It prefers acidic to neutral (pH 6-7) soil for better growth and requires a consistent supply of moisture, but not waterlogged fields. It naturally grows in low-lying areas on poor sandy acidic soils adjacent to swamps, and so is adapted to and requires constantly wet feet or shallow water tables. Urea fertilizer should be applied in three splits viz. once at basal and remaining two applications after the first and second cutting of leaves. Stevia plants prefer low nitrogen, but a high level of phosphorus and potassium. Slow releases of nitrogen (N) sources are better due to the requirement of low level of N in addition to steady release of N from the source. Sometimes stevia shows the symptoms of boron deficiency, which leads to leaf spot that can be rectified by spraying Borax 6%. Since the feeder roots tend to be quite near the surface, the addition of compost for extra nutrients is beneficial. A previous study revealed that at the point of maximum dry matter accumulation, stevia plants consist of 1.4 % N, 0.3% P and 2.4 % K.
Propagation Vegetative propagation is the best way for seed multiplication of stevia due to low seed germination capacity. In vitro propagation can become an important alternative to conventional propagation and breeding procedures for a wide range of plant species. Stevia is grown in the following season in the same field after uprooting the mother plant.
Irrigation Stevia cannot grow in dry conditions. Sprinkler irrigation is found to be advantageous since the herb is highly sensitive to water stress and requires frequent light irrigation. During summer, watering at an interval of 3-5 days gives best results. In order to reduce the impact of drought and high temperature, addition of mulches around the plant is recommended.
The harvesting Time of harvesting depends on land type, variety and growing season. The first harvest of the crop can be done four months after planting and subsequent harvest once after every 3 months. The best harvesting time is mid-September to late September when plants are 50- 70 cm in height. Short days induce flowering. It is harvested just prior to flowering to get maximum steviol glycoside content in the leaves. The easiest harvesting technique is by cutting the branches off with pruning shears before stripping the leaves. The tips of the stems can be clipped off and added to harvest as they contain as much stevioside as the leaves. On average, three commercial harvests can be obtained in a year. It is better to cut the plants leaving about 10 cm stem portion from the ground. This will facilitate new flushes to emerge, which can be harvested as the next crop. For domestic use, leaves may be used fresh for tea or may be combined with mint leaves.
Drying: Drying process is to eliminate moisture and leads to a reduction of the visual, organoleptic and functional characteristics of the stevia plants, which negatively affects its final quality parameters like color, texture, aroma, essential oil content and shape. High temperature negatively affects the final quality of the product, diminishing their medicinal properties and their commercial value. Immediately after harvest the herb is dried. This can be accomplished on a glass sheet or net. The freshly harvested plants can be hung upside down and dried in a shade. It can also be dried using simple drying racks inside transparent poly house or transparent glass roofing or by passing dry air just above room temperature. Drying of the stem and soft green leaf material is completed immediately after harvesting using a drying wagon, a kiln or done naturally in case of large-scale production. Depending on weather conditions and density of loading, it generally takes 24 to 48 hours to dry stevia at 40 to 500 C. The drying process does not require excessive heat; more important is good air circulation. On a moderately warm fall day, stevia can be quickly dried in the full sun in about 12 hrs. Longer drying time will lower the stevioside content of the final product. A home dehydrator can also be used although sun drying is the preferred method. After adequate drying, the leaves are stripped of the stems/twigs, packed and stored in a cool and dry place. For large-scale commercial production, artificial drying and threshing of the dry herbs to separate leaves may be employed.