Key Technical Points for Off-Season Cultivation of Sweet Potato

In order to improve the economic benefits of growing sweet potatoes, in addition to post-harvest deep processing, sweet potatoes can also be brought to market earlier through off-season cultivation. Two off-season cultivation techniques are introduced here.

1. Cultivation technique for sweet potatoes marketed in May-June

Use the method of storing water between ridges inside a greenhouse plus a small arch shed to improve the heat preservation effect. Planting can be done from December to January of the following year. Using virus-free shoot-tip seedlings cultivated in a small arch shed, the yield for market in May is 1300 kg per 667 sq m. The income is much higher than that of conventional planting.  

Key points of cultivation management: In a small arch shed about 150 cm wide, cover two ridges with wide film (each ridge head 70 cm). After sealing the ridge heads with soil, irrigate water into the furrows (water requirement about 17 t per 667 sq m). Insert long seedlings with 7-8 expanded leaves horizontally. Maintain the temperature inside the small arch shed within the range of 5-35 degrees C through ventilation. If the outside air temperature rises, the sides of the greenhouse can be opened for ventilation. In early March, remove the small arch shed inside the greenhouse; in mid-March, open both sides of the greenhouse; in early April, remove the top greenhouse. Based on the temperature in mid-April, appropriate adjustments can be made according to meteorological conditions. If the temperature inside the small arch shed remains above 40 degrees C for a long time, combined with insufficient light, the skin of the sweet potato will become pale. Therefore, if there is no late frost damage, the greenhouse can be fully opened earlier. For varieties, it is advisable to choose early-maturing varieties with good root tuber enlargement under low-temperature conditions and good tuber appearance.  

2. Cultivation technique for fresh sweet potatoes marketed in July-August

Referring to vegetable cultivation methods, seedlings can be inserted from late March to early April. The sweet potato seedlings are taken from 5-6 short seedlings cultivated in January-February on an electric heating bed inside a greenhouse. Under the small arch shed, two small ridges can be covered (ridge width 65-70 cm); alternatively, only one large ridge can be covered (ridge width 100-120 cm) with cross double-row insertion. After insertion, pour thin manure water. After survival, cover with plastic film. When the temperature inside the small arch shed is high on sunny days, cut holes in the film and pull out the sweet potato seedlings; if seedlings are inserted on rainy days, the plastic film can also be covered first and then the seedlings inserted, with the insertion holes sealed tightly with soil, and the sweet potato seedlings kept upright so that the leaves do not adhere to the film. If the seedlings have not yet survived after insertion and encounter continuous sunny days with high temperature, appropriate grass should be placed on the film to avoid burning the mother leaves or even causing seedling death. After April, when the outside air temperature is above 20 degrees C and the temperature inside the small arch shed exceeds 35 degrees C, the two ends and one side in the middle can first be uncovered for ventilation and cooling, adjusting the soil temperature under the film to 25-30 degrees C, which is most favorable for tuber enlargement.

After May, when the outside air temperature stabilizes above 25 degrees C, remove the small arch shed and keep the plastic film to raise soil temperature. After entering the plum rain season, this not only facilitates drainage but also protects the ridge soil from rain impact. If black plastic film is selected, it can also reduce weed growth. When entering the high-temperature and drought period in July, covering with film has the effect of reducing water evaporation and preserving moisture. According to test results, sweet potatoes harvested in August with film covering can increase yield by 30%-50% compared with no film covering, so the investment can definitely be recovered.

For varieties, it is advisable to choose early-maturing varieties. If harvesting early in July, short straight insertion of 3000 plants/667 sq m should be used to reduce the number of tubers formed and increase the rate of marketable tubers; if for market, horizontal insertion or oblique insertion of 4000 plants/667 sq m is better, which can produce 150-300 g medium-sized tubers and increase the rate of marketable tubers.