What are the key cultivation techniques for stevia?

Stevia is the dried leaf of the aster family plant stevia (Stevia rebanianum Bertoni). Also known as sweet chrysanthemum, sugar grass, sweet grass. The whole stevia plant has a sweet taste, with the leaves being the sweetest. Stevia has the effect of raising blood sugar, lowering blood pressure, and promoting metabolism. Made into health bars or food additives, it can treat diabetes, obesity, regulate stomach acid, restore nerve fatigue, prevent dental caries in children, etc. Stevia originates from the subtropical regions of South America. It was successfully introduced in China in 1977 and is now cultivated in more than 20 provinces and cities.

(1) Morphological characteristics

Stevia is a perennial herb, with a plant height of l-1.3 m. The root tips are plump, 50-60 of them, and can reach 25 cm in length. The stem is erect, the base is slightly lignified, and the upper part is tender, densely covered with short hairs. The leaves are opposite or alternate on the upper stem, lanceolate, with shallow serrations on the edges, covered with short hairs on both sides, and the veins are three-lobed. The capitulum is small, the involucre is tubular, and the involucral bracts are in 5-6 layers with equal-length edges; the receptacle is flat and bare; the base of the corolla is light purplish red or white, and the upper part is white. The achene is linear, slightly flattened, brown, and has a pappus. The flowering period is July-September, and the fruiting period is September-November.

(2) Growth habits

Stevia likes to grow in a warm and humid environment, but it can also tolerate a low temperature of -5°C. The most suitable temperature for stem and leaf growth is 20-30°C. Stevia is a short-day plant that is highly sensitive to light, with a critical day length of 12 hours, so it flowers earlier when cultivated in low-latitude areas. Fertilized ovules after flowering need 25-30 days to develop into mature seeds. When the seeds mature, the pappus floats away with the wind and spreads everywhere.

The seeds are tiny, with a 1000-grain weight of only 0.25-0.32 grams. There is no dormancy period. The suitable germination temperature for mature seeds is 20-25°C, and light can promote seed germination. The seed lifespan is less than 1 year.

(3) Cultivation techniques

1. Land selection and preparation Stevia has low requirements for soil, but it grows well in loose, fertile land containing more humus. The previous crop is preferably soybean, peanut, mung bean, etc., and continuous cropping is not suitable. The ideal soil pH is neutral; less than 5.5 or greater than 7.9 is not suitable.

The plot for planting stevia should undergo autumn plowing. The depth of plowing has a certain effect on yield; deeper is better. At the same time, apply 20-25 kg of superphosphate per mu as base fertilizer.

2. Propagation methods Seed propagation and seedling propagation are the main methods, and cuttings and layering can also be used for propagation.

(1) Seed propagation In production, the method of sowing and raising seedlings and then transplanting and setting out is mostly used. In the southern provinces of China, flat-bed sowing and seedling raising are usually used, while in the north, hotbeds are mostly used for seedling raising. South of the Yangtze River, the suitable sowing period is October-November. The seedlings overwinter in the seedling bed, and by mid-to-late March of the following year they can be transplanted into the field for cultivation. In the north, sowing and seedling raising are generally carried out in greenhouses or hotbeds in February-April. To spread the seeds evenly, before sowing, mix the seeds with fine sand and rub them, then soak them in warm water for 10-12 hours, and then mix the seeds with a small amount of plant ash. After sowing, gently press the seeds with a wooden board so that they contact the soil, then spray water once on the bed surface with a sprayer to keep the bed moist and improve the emergence rate. With suitable temperature and humidity, they can germinate and emerge 7-10 days after sowing. Sowing rate: 500 grams are needed per 100 sq m of seedling bed. The actual number of strong seedlings cultivated is 200,000-250,000 plants, enough for planting 12-15 mu of land. Generally, 8,000-9,000 seedlings are planted per mu, and dense planting can reach 10,000-12,000 plants.

(2) Cutting propagation Cuttings can be taken from late March to late August, and the survival rate is higher when cuttings are taken before budding. When taking cuttings, select healthy branches and side stems that meet the requirements, cut small sections 15-20 cm long, and insert 1/3-l/2 of the cutting into the bed soil, with a plant-row spacing of 2 cm x 5 cm. Water promptly after insertion, cover the top with a straw curtain or plastic film, keep warm at night, avoid direct sunlight at noon, and provide appropriate ventilation and light once new buds grow, gradually exercising the seedlings' adaptability to the outside environment to form strong seedlings with developed roots, robust stems and leaves, and normal color.

In addition, although layering can also propagate, it is only limited to use in seed selection work to retain excellent individual plants and is not suitable for large-scale cultivation production.

3. Field management Stevia is tolerant of moisture and afraid of dryness. In summer, when the temperature is high and there is little rain, the lower leaves are prone to falling off when water is insufficient. It also has high fertilizer requirements, so cultivating stevia requires an adequate supply of complete nitrogen, phosphorus, and potassium fertilizer. Usually, 7.5-10 kg of ammonium sulfate is applied per mu in the first year, and 15-20 kg of ammonium sulfate is applied as topdressing in the second year in areas along the Yangtze River. Potassium fertilizer should be applied frequently and thinly, generally 8-10 kg per mu, preferably in 2-3 topdressings each year. After topdressing and irrigation, when the soil surface is slightly dry, 1-2 times of loosening the soil can be carried out together with weeding to keep the field clean and the soil loose. In order to promote lush stems and leaves and increase yield, when the seedlings are 20-25 cm tall, topping and pinching can be carried out. After topping, each plant can produce 12-17 new branches. This period is exactly the time of fertilizer and water demand. Phosphorus and potassium fertilizer should be applied to the roots, or potassium dihydrogen phosphate should be sprayed on the leaves, so as to achieve the goal of high quality and high yield.

During the flowering and pollination stage, stevia consumes a large amount of nutrients. At this time, it is very necessary to strengthen management promptly. If stems and branches cross too densely, the lower leaves are prone to falling off. In sudden wind and rainstorms, the whole plant is prone to lodging. Therefore, in addition to topdressing and watering, soil should also be banked around the roots in combination with intertillage, attention should be paid to field drainage, light transmission and ventilation between beds should be maintained, and the lower leaves should be appropriately picked.

4. Pest and disease control

(1) Seedling damping-off disease This is a seedling-stage disease. Control methods:

① Choose plots with good drainage and loose soil for seedling raising;

② Before sowing, use 2 kg/mu of 50% carbendazim per mu for soil treatment;

③ In the early stage of the disease, spray with 1000-1500 times carbendazim or drench with a 500-times solution;

④ Pull out diseased plants promptly, and treat the diseased holes with a mixed powder of plant ash and quicklime at 3:1.

(2) Leaf spot disease This disease is likely to occur from July to October and damages stems and leaves. Control methods:

① From May to June, pay attention to drainage to reduce soil humidity, and apply more potassium fertilizer to improve the disease resistance of the plants;

② In the early stage of the disease, spray 50% carbendazim 1000-times solution for control;

③ After harvest, clean the garden, dispose of the residual plants, and burn them in a centralized manner.

(3) Southern blight disease When there is more rainfall in April-May and the soil humidity is too high, this disease is often prone to occur and damages the roots. Control methods:

① Plant at a reasonable density and pay attention to ventilation and light transmission in the field;

② Apply more phosphorus and potassium fertilizer to avoid excessive growth of seedlings;

③ Once diseased plants are found, pull them out immediately and spread lime around the diseased holes for disinfection;

④ In the early stage of the disease, irrigate the diseased area with 50% carbendazim 1000-times solution to control the spread of the disease.

(4) Inchworm (measuring worm, bridge-building worm) Control methods: Protect the inchworm's natural enemy, the apanteles wasp, or spray with a 1000-times diluted solution of 40% omethoate for control. Discontinue use 20 days before harvest.

(4) Harvesting and processing

The glycosides contained in stevia leaves increase with the growth of the plant. Usually, the glycoside content is highest at the full bud stage, and this is the best harvest period. Cultivation south of the Yangtze River can harvest 3 times a year, areas along the Yellow River can harvest 2 times, and northern North China, Northeast China, and Inner Mongolia can generally harvest only 1 time per year. When harvesting, be sure to choose a sunny day to cut the branches and stems. The branches harvested on the same day should have their leaves picked that evening, then spread out to dry. They must not be piled up, otherwise the leaves will turn black and affect quality. For large-scale planting, a dryer should be used for processing and drying. After drying, bundle and package them. To prevent mildew and deterioration and maintain green color, after drying they can be put into plastic bags and sealed tightly.