Causes of Bolting in Spring-Sown Napa Cabbage, Control Methods, and Key Cultivation Techniques

Napa cabbage is also known as “heading cabbage”, “hearting cabbage”, “yellow-bud white”, “jiao cabbage”, etc., and is called “shao cabbage” in Cantonese. It is widely grown in China. So what are the main causes of bolting in spring-sown napa cabbage and the control methods? What are the key countermeasure cultivation techniques?

What are the main causes of bolting in spring-sown napa cabbage and the control methods?

1. Causes:

1. Spring temperatures are relatively low and last for a long time

Napa cabbage is a seed-vernalization-type crop; once the seeds begin to germinate, they can accept low-temperature vernalization and enter reproductive growth (that is, begin bolting and flowering). Generally, after seed germination, under conditions below 10-12 degrees C for 10-20 days, the vernalization stage can be completed, causing premature bolting and thus bringing serious losses to production.

2. Improper cultivation management

Premature bolting of cruciferous crops occurs every year in production, only to varying degrees. Whether cultivation management is proper has a great influence on premature bolting, sometimes even a decisive one. Cultivation management measures have a great influence on premature bolting. For example, if spring napa cabbage is transplanted too early and the temperature is relatively low, leaf buds cannot fully form, which promotes early flower bud formation and premature bolting. For cabbage and radish, if the seedling age is too long, or if fertilizer and water are excessive after transplanting, causing them to reach the vernalization-capable vegetative body ahead of time, premature bolting is also likely to occur.

3. The strength of a variety's winter hardiness

Under the same weather and cultivation management conditions, varieties with strong winter hardiness generally bolt less, while varieties with weak winter hardiness bolt more.

2. Preventive countermeasures

1. Select excellent varieties

The strength of a variety's winter hardiness, that is, its bolting tolerance, is an important indicator when selecting varieties. Generally, when temperatures are relatively low, varieties with strong winter hardiness should be selected.

2. Strictly follow operating procedures

Agricultural production must be scientific and cannot rely entirely on experience, because experience is formed under certain environmental conditions, and when the environment changes, experience may bring disastrous consequences. Therefore, the sowing period must be strictly followed, and fertilizer and water management must be done well.

4. Inhibit or promote the growth of the bolt

For fields that have already bolted, such as spring napa cabbage, if the bolt has just appeared, measures such as spraying anti-bolting thickening agents or paclobutrazol can be used to inhibit bolt growth, or the method of tying with straw rope can be used to promote heading. If the bolt is relatively long, cultivation management should be strengthened to promote bolt growth, because napa cabbage bolts are tender and delicious and can become a vegetable variety that bridges the supply gap from April to May, thereby obtaining certain economic benefits. In addition, for spring radish, if the radish has already initially formed, the bolt can be pinched off, so the radish can still continue to grow and form a certain yield.

4. Promptly rush to plant the next season

For fields where bolting is especially severe and there is no value in keeping the crop, the seedlings should be destroyed and the field cleared as soon as possible, and the next crop should be planted early to make up for losses.

What are the key cultivation techniques for spring-sown napa cabbage?

1. Sowing period

This year is a cold year, this year is a cold year; the sowing time should be delayed by more than 5 days compared with normal years, and sowing should begin after March 1.

2. Variety selection

First-generation napa cabbage hybrids with strong winter hardiness must be selected, such as Sijihuangguan, Huangguan, Lingxia, and Liangqing.

3. Land preparation and fertilization

Napa cabbage should not be continuously cropped. It is best to choose rotation with cucurbits, legumes, alliums and garlic crops, solanaceous fruits, and grain crops, and to choose fields without clubroot. During land preparation, apply 2000-2500 kg of organic fertilizer and 40 kg of calcium-magnesium phosphate fertilizer per mu as base fertilizer, and 40-60 kg of bio-organic fertilizer as middle-layer fertilizer. For fields with relatively acidic soil, apply 50-100 kg of quicklime per mu. Generally, make beds at 2-2.1 m including furrows.

3. Sowing

Use plastic film mulching and direct seeding, with row spacing and plant spacing of 40×35 cm. Sow 3-4 seeds per hole, with about 60 g of seed used per mu. Pay attention to watering for emergence to ensure a full stand.

4. Field management

1. Thinning and final thinning: after emergence, carry out the first thinning at the 2-3 leaf stage, leaving 2 seedlings per hole; at the 4-5 leaf stage, carry out final thinning, leaving 1 seedling per hole.

2. Fertilizer and water management: generally topdress twice. At the rosette stage, apply 10 kg of urea and 10 kg of potassium sulfate per mu; at the beginning of heading, apply 10 kg of urea and 10 kg of potassium sulfate per mu; 10 kg of urea and 12 kg of potassium sulfate.

In terms of water management, water thoroughly before sowing to promote emergence. At the seedling stage, water demand is low, but the root system is shallow and absorption capacity is weak, so water frequently in small amounts. At the rosette stage, water demand is high, so irrigation should be increased, with air-dried and wind-dried conditions being suitable. The heading stage is the period of greatest water demand, and the soil should be kept moist.

5. Pest and disease control

The main diseases of napa cabbage include soft rot, black spot, and clubroot. The main pests include diamondback moth, cabbage caterpillar, and cabbage webworm.

1. Soft rot, black rot, and bacterial angular leaf spot are severe under high temperature and high humidity. Spray with 72% agricultural streptomycin 4000-fold solution, or 90% streptomycin soil (new streptomycin) soluble powder 4000-fold solution, or 30% copper bis(dodecylbenzenesulfonate) (DT) wettable powder 800-1000-fold solution.

2. For black spot, spray with 69% dimethomorph mancozeb (Anke mancozeb) wettable powder 500-600-fold, or 50% iprodione (Rovral) wettable powder 1000-fold solution, or 80% mancozeb (Sandeson) wettable powder 600-fold solution.

3. For clubroot, select resistant excellent varieties, disinfect seeds, seedbeds, and transplanting holes, implement crop rotation, avoid planting in diseased fields, increase organic fertilizer application, apply lime to adjust pH, and cultivate with deep furrows and high beds during the rainy season. For chemical control, after two true leaves and after transplanting has established, use the following agents for root drenching: (1) 75% chlorothalonil wettable powder 800-1000-fold solution; (2) 50% carbendazim wettable powder 800-1000-fold solution; (3) 30% hymexazol aqueous solution 800-1000-fold solution; (4) 70% thiophanate-methyl wettable powder 1000-1200-fold solution.

4. For cabbage caterpillar, diamondback moth, and cabbage webworm, use microbial preparations such as silver Y moth virus, beet armyworm virus, diamondback moth virus, Beauveria bassiana, and Bacillus thuringiensis for control. Or use chemical pesticides such as 5% chlorfluazuron emulsifiable concentrate 2500-fold solution, or 5% flufenoxuron emulsifiable concentrate 1500-fold solution, or 50% phoxim emulsifiable concentrate 1000-fold solution for spray control. Matrine, azadirachtin, beta-cypermethrin, bifenthrin, etc. can also be selected for spray control.