What are the techniques for large-scale production of export white bunching onions in the Chongming area?

1 Variety selection

Select varieties such as Changbao, Changyue and Chunwei according to the export specifications and timing requirements for white bunching onions. For example, Changyue bolts late and has strong tolerance to heat and cold; it can be planted densely, planted from March to May, and harvested from November to January of the following year.

2 Raising seedlings

2.1 Preparation of the seedbed

Seedbed preparation procedure: prepare the field (deep ploughing 350m, freeze the soil clods) -> apply base fertilizer -> plough twice (25cm) -> make beds (bed surface specification 1.5m x 30m).

Applying base fertilizer: about 20 days before sowing, apply 1500kg of biological organic fertilizer and 15kg of diammonium phosphate per 667 sq m.

2.2 Sowing

Sow in protected facilities from late February to early March, and raise seedlings in the open field from mid-March to early April.

Using seed-tape sowing technology, 82-100 seeds/m, row spacing 7cm, soil covering depth 0.5cm. The soil covering layer should ensure the soil is loose, and cover with plastic film after sowing. After covering with soil, do not directly apply multiple excessive surface waterings, to avoid causing soil compaction; watering should be done by seepage irrigation within the furrows, and during seepage irrigation the water surface must not exceed the bed surface.

2.3 Seedbed management

(1) Fertilizer is mainly base fertilizer; topdressing can be carried out after the seedlings are 15 days old, mainly using slow-release fertilizer, and quick-acting fertilizer should not be applied in excess.

(2) Humidity After sowing and covering with film, increase the humidity of the seedbed by furrow irrigation and downward seepage; the humidity of the seedbed should be ensured, but excessive humidity easily leads to uneven emergence. Furrow irrigation standard: the water surface should reach 2/3 of the furrow depth, and the water surface must strictly not exceed the bed surface. In early spring, irrigation should be carried out in the morning and drained in the afternoon, with an irrigation duration of 4-6h being suitable. Thereafter carry out once a week, and in special weather the watering cycle should be changed appropriately.

(3) Mulch management After the seedlings reach the film and 60% have emerged, remove the plastic film in time, and cover with shade netting according to the weather to prevent heavy rain from washing away the seedbed. Seedlings raised in early February should be grown in protected facilities to prevent the night temperature from being too low. Seedlings raised in mid-to-late April should have the plastic film removed in time to prevent the temperature from being too high and burning the seedlings.

The management of shade netting should be carried out strictly according to the usage specifications, and hardening of the seedlings must be carried out before transplanting. In the early covering stage, cover all day; in the middle stage, cover from 9:00 am to 4:00 pm; in the later stage, carry out seedling hardening without covering.

(4) Control of diseases, pests and weeds At the seedling stage, non-infectious diseases are predominant, mainly physiological diseases such as salt damage and nutrient deficiency, which are controlled by soil improvement methods.

The main pests at the seedling stage include onion seed flies, cutworms, etc., which are controlled with pesticides such as Sukai and Mimang.

After the seedlings reach 8cm in height, manual weeding can be carried out. When weeding, avoid bringing up large clods of soil, to prevent damaging the onion roots.

3 Planting out

For white bunching onions grown by raising seedlings and transplanting, when the seedlings reach the required seedling age (45 days) or have 3-4 leaves and 1 heart leaf, they should be transplanted in time.

3.1 Field preparation

Prepare the field (including deep ploughing 35cm, freezing the soil clods) -> apply base fertilizer (decomposed organic fertilizer 1000kg/667 sq m) -> plough and make furrows (furrow spacing 1.1m, depth 25cm) -> apply base fertilizer (superphosphate 25kg/667 sq m, vegetable-specific fertilizer 50kg/667 sq m).

3.2 Planting out

Select healthy and uniform strong seedlings, and plant them out by grade at a plant spacing of 3.3cm and a row spacing of 1.1m. When planting out, keep close to one side, and strictly control the quality of onion planting.

4 Field management

4.1 Fertilizer

Apply sufficient base fertilizer, and for topdressing adopt the principle of combining quick-acting and slow-release fertilizers. Topdressing should be applied in small amounts and frequently; especially after white bunching onions enter the rapid growth period, the application position of topdressing is within 15cm on both sides of the roots. Begin topdressing 1 month after planting out, applying 20kg of diammonium phosphate and 100kg of decomposed organic fertilizer per 667 sq m.

4.2 Humidity

Water should be strictly controlled during the growth period; a large amount of water accumulation easily leads to diseases such as root rot, while too little will inhibit the growth of white bunching onions. After heavy rain, clear the furrows and drain water in time. In spring and summer, strengthen artificial water supplementation, with the standard being that the soil 3cm below the surface is moist.

4.3 Weed control

According to the specific conditions of the field, a one-off application of herbicide such as glyphosate can be made in spring; at the same time, carry out appropriate manual weeding in the early field, and in the later stage weeding can be done through earthing up.

4.4 Disease and pest control

(1) Welsh onion rust begins to occur in late spring and early summer, and is most severe in autumn. It is a fungal disease; the suitable temperature for occurrence is 9-18 degC, and above 24 degC the germination rate decreases markedly. For control, apply sufficient organic fertilizer, increase the application of phosphorus and potassium fertilizers, water frequently with small amounts, and remove diseased and damaged leaves in time; at the early stage, spray with 20% triadimefon 2000-fold solution for control.

(2) Brown spot, also known as leaf tip yellow wilt, mainly damages the leaves. For control, choose heat-tolerant varieties; strengthen management, prevent the onion field from being too wet, and improve root vigor; use thiophanate-methyl 1000-fold solution, chlorothalonil 800-fold solution, etc. for spray control.

(3) Leafminers damage as larvae, mostly from August to October. Adults are most active from 8:00-11:00 am and 3:00-5:00 pm, and mostly rest at the tips of the leaves. Remove leaf debris; use Qianke 2500-fold solution or Chongmanguang 10ml/667 sq m, etc. for control.

(4) Noctuid leaf-eating pests mostly occur in the high-temperature weather from June to September. They damage especially quickly and easily develop a certain resistance to pesticides. Mainly use chemical pesticides such as Mimang, Chujin and Anda for control.

4.5 Earthing up

The earthing up position should be at the four-leaf fork, mainly heavy earthing up, with 3-4 earthings up throughout the whole cycle. The green length (distance from the soil surface to the four-leaf fork) should be controlled within 12cm when earthing up, and during the typhoon season this length should be reduced (about 6cm); each earthing up should be done properly.

5 Harvesting

During harvesting, avoid breaking or damaging the onion leaves; avoid scratching the white part of the onion with digging tools; packaging should not be too tight, and the length should be about 80cm.