Onion Planting Requirements

Onions are highly adaptable to temperature. Seeds and bulbs can germinate slowly at 3-5℃, with the growth rate accelerating at 12℃. The optimal temperature for seedling growth is 12-20℃, for leaf growth it is 18-20℃, and for bulb growth it is 20-26℃. Strong seedlings can tolerate temperatures as low as 6-7℃. Bulb enlargement requires higher temperatures; bulbs cannot enlarge below 15℃, and they grow best at 21-27℃. Excessively high temperatures can lead to reduced growth and the onset of dormancy.

Onions are long-day crops that require more than 14 hours of long daylight during the bulb enlargement period and the flowering period. In high temperature and short daylight conditions, only leaves grow and no onion can be formed. The appropriate light intensity for onions is 20,000 to 40,000 lux.

The tissue structure of onion leaves is relatively drought-resistant, but their root absorption capacity is weak. To achieve high yields, higher soil moisture is required. Adequate soil moisture during the germination period is beneficial for seedling emergence. During the seedling stage in cool seasons, the soil should be kept moist but not waterlogged to promote balanced root and leaf growth. During the leaf growth and bulb enlargement stages, sufficient soil moisture is essential, with a maximum soil moisture content of about 85%. During the scion emergence period, soil moisture should be controlled appropriately, maintaining a dry-to-moist condition to prevent the plant from shifting its nutritional growth towards scion emergence. During the flowering and seed maturation periods, adequate soil moisture is necessary to keep the ground moist, promoting seed development.

Apples thrive in lower air humidity. During the bulb enlargement and flowering stages, dry weather can reduce disease, enhance yield, and improve quality. Excessive air humidity or rainfall during the flowering period can cause the anthers to split open, reduce pollen viability, and result in poor pollination and low fruit set.

Onions are highly adaptable to soil, but they are best suited to neutral soil that is rich in organic matter, fertile and loose. They yield high yields on sandy loam, but the bulbs on clay loam are full, with good color and good storability.

The onion root system has weak nutrient absorption capacity but a high yield, thus requiring ample nutritional conditions. For every 1000 kilograms of onions, the plant needs to absorb 2 kilograms of nitrogen, 0.8 kilograms of phosphorus, and 2.2 kilograms of potassium from the soil. Under normal soil conditions, applying nitrogen fertilizer can significantly boost yields, and phosphorus and potassium should also be supplemented. Seedlings are not tolerant of excessive fertilization; over-fertilization can lead to stem blight and seedling collapse. During the leaf growth stage, nitrogen fertilizer is primarily applied, while during the bulb enlargement stage, phosphorus and potassium fertilizers are mainly used. Applying trace elements such as copper, boron, and sulfur can significantly increase yields.

Although onions are biennial vegetables, the time from sowing to harvest varies depending on the climate. In southern China, onions are typically sown in late autumn and harvested in early summer. In the Yangtze River Basin, they are generally planted in autumn, with seedlings surviving the winter to be harvested in May or June of the following year. In northeastern China, spring planting is more common, with harvest occurring at the end of summer.

From sowing to harvest, the growth of its above-ground parts, underground parts and bulbs is affected by regional and climatic conditions. Taking the Yangtze River basin as an example, this paper introduces the main characteristics of its growth and development process.

The period from sowing to transplanting and wintering is crucial for seedling growth. After the seeds germinate, temperatures gradually drop, and daylight hours shorten. The root system's absorption capacity decreases, and photosynthesis in the above-ground parts slows down, resulting in reduced growth. A temperature of around 20℃ is ideal during this period. Below 10℃, the root system struggles to grow, and leaf growth and differentiation are slow. It takes 50-60 days from sowing to transplanting. After transplanting, the above-ground and underground parts grow minimally during the wintering period, so it is important to protect seedlings from frost. The root system thrives at a lower temperature than the above-ground parts. When the soil temperature reaches 5℃ at a depth of 10 cm, the root system can begin to grow. A temperature between 10-15℃ is optimal, while temperatures above 25℃ slow down growth.

This period, from the start of spring when temperatures rise until before the bulbs swell, is a time of vigorous growth for both the above-ground and underground parts of the plant. It generally spans from late March to early May, marking the fastest growth phase for the entire plant, particularly in terms of leaf growth. This stage is crucial for laying the foundation for high bulb yields, as new fine roots continue to develop and extend, while old roots gradually diminish.

From mid-May to early to mid-June, as temperatures rise and daylight hours increase, the above-ground parts stop growing. Nutrients from the leaves are transported to the leaf sheaths and scales, causing the bulbs to rapidly expand. By the end of this period, the outer leaves wither, and the plant collapses. The nutrients in the outer 1-3 layers of the scales shift inward, and the plant hardens into a leathery texture. During this time, both fertilization and irrigation should be intensified to promote bulb expansion. Meanwhile, although new roots continue to grow, the old roots age rapidly, leading to no further increase in the total number of roots. Due to the aging of root activity, water absorption capacity decreases, and the uptake of nitrogen, phosphorus, and potassium also declines.

The above is the growth and development process of onion bulb product organ formation. If it is to be used for seed collection, mature bulbs should be planted in the field again in the autumn of the same year.

The natural dormancy period of onions is an adaptive response to adverse conditions such as high temperatures, long days, and drought. During this period, even with favorable conditions for germination, onion bulbs will not sprout. The duration of the dormancy period varies depending on the variety, the degree of dormancy, and the external environment, typically lasting 60-90 days. After the natural dormancy period, if conditions are suitable, the bulbs will sprout and develop roots.

After the scions used for seed production are planted, if they meet the low-temperature requirements and receive long-day conditions in the field, flower buds will form. Although both the apical and lateral buds of the seed plant can undergo flower bud differentiation, due to the different timing of young bud formation, only the apical bud and the buds near it that develop early can produce flower stems. Typically, each scion can produce 2-5 flower stems. The lateral buds at the base of the scion often fail to produce flower stems, and when scions form in spring under high temperatures and long days, the plant has already entered its reproductive growth stage, with nutrients primarily used for flowering and fruiting. As a result, the newly formed scions, lacking sufficient nutrients, have smaller organs and softer scales. From flowering to seed maturation is the seed formation period. The flowering period of onions is relatively long, taking 70-80 days from flowering to seed maturity.