How to Grow Onions: Planting Time and Suitable Soil Selection
What climate and soil conditions are required for growing onions, and how to grow them to produce good onions.
Onions prefer climate conditions with ample sunlight, warmth and coolness, and dislike high-temperature, high-humidity environments. The optimum temperature for seed germination is between 16°C and 25°C, the optimum temperature for root system development is between 15°C and 20°C, and the optimum temperature for stem and leaf development is between 20°C and 26°C. During the various stages of onion growth and development, if the temperature exceeds the optimum range, the higher the temperature, the slower the development. If the bulb-forming period encounters a high-temperature environment, the plant will mature early, the bulb cannot fully swell, and quality and yield are affected. Under normal conditions, onions are suitable for planting in early to mid-September.
The bulb-forming nature of onions is related to day length and temperature levels, and the suitable planting period runs from autumn and winter to the spring of the following year. During this period the temperature is cool and comfortable and the daylight hours are shorter, which suits the growth and development of short-day onion varieties. Long-day onion varieties are generally not suited to the local climate conditions. In terms of rainfall, onion plants need a relatively moist environment during the growing period, but in the later stage of bulb formation the rainfall should not be too much. Especially during the onion harvest period, rain is the biggest worry; if the bulbs encounter rain before harvest, they rot easily, do not store well, and quality is also reduced.
Onions have only fibrous roots, and the quality of root system development has a significant relationship with bulb formation and resistance to pests and diseases. The best cultivation soil is sandy loam rich in organic matter. Onions do not like acidic soil; a soil pH of 6.0 to 6.5 is most suitable for onion growth. A pH below 6.0 needs to be corrected with lime, and when the soil pH is below 4.5, onions can hardly grow at all. In addition, onions should not be grown continuously on the same land, nor should they follow crops such as cabbage, napa cabbage, potato or sweet potato, but they are suitable for rotation with gramineous or leguminous crops such as rice and corn.