What are the key technical points for transplanting and managing onions grown under plastic film?

The transplanting time for onions varies by growing region and is divided into autumn planting and spring planting. In China, areas south of Beijing mostly practice autumn planting, while areas north of Beijing mostly practice spring planting. At present, plastic film technology is often used when transplanting onions, so what are the key technical points for transplanting and managing onions under plastic film?

1. Transplanting time for onions under plastic film

Areas south of the Great Wall mostly adopt autumn planting, transplanting about 40 days before the arrival of severe cold, so that the onion root system resumes growth before overwintering, the leaves grow rapidly after green-up, and the seedling growth period is long, so it shows higher yields than spring planting.

Autumn planting must be timely. If too early, the seedlings grow too large before winter, causing premature bolting in spring; if too late, the pre-winter growth period is short, the root system cannot develop fully, cold resistance decreases, and overwintering seedling death is likely.

In areas north of the Great Wall, winters are severely cold, and onions are difficult to overwinter in the open field, so spring planting is predominant. With spring planting, the leaf growth period is short and yields are somewhat low, so in principle spring transplanting should be early, carried out as early as possible after the soil thaws, to gain a longer growing period, grow more functional leaves before bulb enlargement, and increase yield.

2. Key technical points for transplanting onions under plastic film

1. Fertilising and making beds

Onion is a vegetable crop that loves fertiliser, and a deficiency of any one of nitrogen, phosphorus or potassium can lead to reduced yield. To achieve high quality and high yield, sufficient base fertiliser must be applied. Generally, apply 50 kg per mu of sulfur-based compound fertiliser with low nitrogen, high phosphorus and high potassium, and 5000 kg of decomposed farmyard manure. Spread the fertiliser evenly, then plough, loosen and harrow the soil fine, and make beds 100-120 cm wide.

2. Selecting strong seedlings

Before transplanting, the onion seedlings should preferably be 50-55 days old; at this time they root easily, recover growth quickly after transplanting, have a stem diameter of 5-6 mm and a height of about 25 cm. 10-15 days before transplanting, spray the seedlings with potassium dihydrogen phosphate foliar fertiliser, which can promote early rooting and rapid establishment after transplanting.

3. Covering with plastic film

Plastic film cultivation can enhance the cold resistance of onions during overwintering, improve the soil's water- and fertiliser-holding capacity, and greatly increase onion yield. Before covering with film, irrigate fully to build up soil moisture; after the water in the bed has completely infiltrated and while the soil is moist, spray 100 ml per mu of 33% pendimethalin evenly over the bed surface, then cover with film. Choose a windless day for covering, and press the two sides of the film firmly with soil, otherwise the film will be damaged by wind and field operations will be inconvenient.

Warm tip: When transplanting, the plastic film needs to be laid 5 days before transplanting to raise soil temperature. Before transplanting, punch holes in the film with a row spacing of 18 cm and a plant spacing of 15 cm. After lifting the seedlings, transplant them immediately; when transplanting, the roots must not be exposed on the soil surface, but they should not be too deep either, with planting depth kept at 2-3 cm.