How to Grow Onions? A Detailed Guide to the Latest Growing Methods

The onion is also known as bulb onion, round onion, tree onion, etc. It is a biennial herbaceous plant of the genus Allium in the lily family (Liliaceae). Different bulb shapes are related to early or late maturity; generally, the flatter the bulb, the earlier the maturity. Yields vary considerably among varieties. Early-maturing flat varieties generally yield 4500-5500 kg per mu, while mid-to-late-maturing varieties generally yield 5000-5500 kg per mu. Now let us take a look at how to grow onions!

Onion Cultivars

1. Red-skinned onion: The bulb's outer skin is purplish-red, and the fleshy scales are slightly reddish. It is flat-globular or globular, with a diameter of 8-10 cm. It stores and transports well, has a relatively short dormancy period, and sprouts early, showing early to mid-season maturity, harvested from late May to early June. It is widely cultivated throughout East China. Representative varieties include Shanghai Red Skin and others.

2. Yellow-skinned onion: The bulb is brass-colored to pale yellow, with fleshy scales that are slightly yellow and soft, and a relatively strong pungent flavor. It is flat-round, with a diameter of 6-8 cm. It stores and transports fairly well, and is early to mid-season maturing. Its yield is lower than that of red-skinned types, but its quality is better, and it can be used for dehydration processing. Representative varieties include Lianyungang 84-1, DK Yellow, OP Yellow, Dabao, Laixuan 13 and others.

3. White-skinned onion: The bulb is white, with fleshy scales that are white. It is flat-globular, and some are tall-round or spindle-shaped, with a diameter of 5-6 cm. It has excellent quality and is suitable as a raw material for dehydration processing or as an ingredient in canned foods. However, its yield is relatively low and its disease resistance is weaker. If autumn sowing is too early in the Yangtze River basin, it tends to bolt prematurely. Representative varieties include Hami White Skin and others.

Growing Conditions for Onions

1. Temperature: Onions have strong adaptability to temperature. Seeds and bulbs can germinate slowly at 3-5°C. The optimum temperature for seedling growth is 12-20°C. Healthy seedlings can withstand low temperatures of -6 to -7°C. Bulb enlargement requires relatively high temperatures, but if the temperature is too high, growth declines and the plant enters dormancy.

2. Light: Onions are long-day crops. During the bulb enlargement stage and the bolting and flowering stage, they need long-day conditions of more than 14 hours to grow normally. Under high-temperature and short-day conditions, they will only grow leaves and fail to form bulbs. The suitable light intensity is 20,000-40,000 lux.

3. Water: Onions should be supplied with sufficient water during the germination stage, the vigorous seedling growth stage and the bulb enlargement stage. However, during the seedling stage and before overwintering, water should be controlled to prevent excessive seedling growth and frost damage. They are drought-tolerant and suitable for 60-70% humidity; excessively high air humidity makes them prone to disease.

4. Soil: Onions have strong adaptability to soil. Fertile, loose, well-aerated neutral loam is preferable. Sandy loam easily achieves high yields, but clay loam produces firm bulbs with good color that store well. Onion roots have a relatively weak ability to absorb fertilizer, so high yields require sufficient nutrient conditions.

Seedling-Raising Techniques for Onions

1. Sowing time: Choosing a suitable sowing period for onions is the key to raising strong seedlings. If sown too early, the seedlings are large and tend to bolt after overwintering; if sown late, the seedlings are weak, have poor cold resistance, and produce small bulbs and low yields. Taking the Huang-Huai basin as an example, the best sowing period is early to mid-September.

2. Sowing quantity: Under normal conditions, the sowing rate for onion seedling beds is 4-5 kg per mu. Considering that 20% of weak and inferior seedlings need to be culled and thinned out, the cultivation area should be 15 times that of the sowing seedbed. If the germination rate is below 70%, the sowing rate should be increased as appropriate.

3. Bed preparation and fertilization: The bed height is 1.4-1.5 m and the bed width is 27 cm. After the bed surface is leveled, apply 15 kg of calcium superphosphate per mu, and use 4 cubic meters per mu of well-rotted organic manure and miscellaneous fertilizer as base fertilizer. After emergence, broadcast quick-acting nitrogen fertilizer in combination with watering, applying 10 kg of urea per mu.

4. Sowing method: After the bed surface is leveled, water first, then broadcast the seeds evenly. Then cover with 0.8 cm of nutrient soil to ensure uniform emergence. After sowing, if the soil moisture cannot meet the requirements for emergence, water with a watering can to promote emergence; seeds can also be sown after pre-sprouting.

5. Emergence management: After onion seedlings emerge, attention should be paid to the prevention and control of downy mildew and damping-off. The method is to use 150 g per mu of 50% carbendazim or 40% chlorothalonil, mixed with 30-40 kg of water for uniform spraying.

Field Management of Onions

1. Watering: About 20 days after transplanting, onions enter the seedling recovery stage. At this time, they should not be watered heavily; too much water lowers the soil temperature and slows seedling recovery. At the same time, the newly transplanted seedlings have not yet developed new roots, and they cannot lack water either. At this stage, onions should be watered more frequently, with a small amount each time. The principle is not to let the seedlings wilt and not to let the ground dry out, so as to promote rapid rooting and survival of the seedlings.

2. Fertilization: Onions require 13-15 kg of nitrogen, 8-10 kg of phosphorus and 10-12 kg of potassium per mu. Excessive nitrogen fertilizer must be avoided. For spring-sown onions, the soil should be kept moist before and after seedling emergence, with frequent watering and frequent cultivation. During the vigorous leaf growth stage and the bulb enlargement stage, fertilizer and water should be applied in a timely manner, with 15 kg of diammonium phosphate, 20 kg of diammonium phosphate, and 3-5 kg of potassium sulfate applied as topdressing per mu respectively.

3. Loosening the soil: Loosening the soil benefits both the development of the onion root system and the enlargement of the bulbs. Generally, it should be done 3-4 times during the seedling stage, after each watering, and 2-3 times during the stem and leaf growth period. Once the plants close the rows, cultivation should stop. The cultivation depth should be about 3 cm, shallow near the planted plants and deep away from the plants.

4. Removing bolts: For onions that bolt early, before the flower head forms, cut it off from the lower part of the flower bud, or split it from the tip of the flower bolt and tear it into two strips from top to bottom, to prevent flowering from consuming nutrients and to promote the growth of lateral buds to form fuller bulbs. At the same time, spray Diguo Zhuangdiling in a timely manner. Practice has proved that for plants that bolt prematurely, a certain yield can still be obtained after taking measures to remove the bolts.