Bulbing is nothing but the transformation of the lower part of the older leaves into bulbs through thickening. It is an attempt by the plant to store nutrients that will be used later when it needs them.
Bulb formation and development is a complicated procedure affected by 4 main factors.
- Photoperiod
- Temperature
- The growth stage of the plant
- The nitrogen fertilization
For higher yields, onions require relatively low temperatures during their first stages before bulbing, which increases periodically during bulb-growing stages.
However, temperatures that are too low (8-13 °C or 46.4-55.4 °F) may delay the bulbing for up to 4 weeks or initiate bolting (flower formation). On the other hand, temperatures over 40 °C (104 °F) prevent bulbing.
However, the temperature is not the only factor that affects onions' growth and reproduction. Photoperiod is, in fact, the most important factor, especially when it comes to bulbing. Onions are categorized depending on their critical photoperiod or day-length requirements for bulb setting into :
- Long-Day Onions: They need 14 to over 16 hours of daily exposure to light.
- Intermediate-Day Onions: They need over 13.5-14 hours of daily exposure to sunlight.
- Short-Day Onions: They need over 11-13 hours of daily exposure to sunlight.
In case the day length is lower than the critical photoperiod, then onions show an increased vegetative growth rate, dramatically reducing the formation of their bulbs. This may seem like an unwanted procedure if we wish to grow onion bulbs, but it is ideal if we grow them for their leaves.
The third factor is the plant's growth stage. In fact, the bulb's growth rate depends on the plant's growth stage at the beginning of bulb formation.
If the plant is large and has relatively many leaves during this stage, the final size of the growing bulb will be large.
Using bigger sets during planting causes earlier bulbing compared to plants of the same age grown from smaller sets. In addition, plants grown from sets start their bulbing procedure much earlier than those grown from seed.
The final factor is N-fertilization. Onion plants that face an N deficiency start their bulb forming earlier as they try to store as many nutrients as possible. In case the day length is close to the critical photoperiod and at the same time the onions phase is a nitrogen deficiency, they start the bulbing procedure.
On the other hand, excess nitrogen in the soil may delay this procedure.
Of course, science can facilitate the procedure chemically by adding plant growth regulators, i.e., 1200 ppm of ethylene (ethephon). However, you should never use any chemical without consulting a licensed agronomist first.
Multiple bulbs forming is a condition that may occur after the temperature is too low, we have an injury to the main bulb, nitrogen excess, or the onion planting is too distant. Although this is undesirable for dry bulb production, it is great for set production.
References
https://academic.oup.com/aob/article-pdf/78/4/423/7982622/780423.pdf
https://www.researchgate.net/publication/335313028_Part_1_CHAPTER_3_ONION_BULB_DEVELOPMENT
https://www.agronomysociety.org.nz/files/1986_14._Light_effects_on_bulbing_in_onions.pdf
Further information
- Interesting Facts about Onions
- Onion: Info, Nutritional Value, and Health Benefits
- Onion Plant Information and Variety Selection
- How to Grow Onions At Home: Onion Growing Guide for Beginners
- Commercial Onion Farming - How to Grow Onions for Profit
- Onion Soil Requirements, Soil Preparation, and Planting
- Which Factors Can Affect Bulbing in Onions?
- Onion Water Requirements and Irrigation Systems
- Onion Nutrient Needs and Fertilization Requirements
- Major Onion Pests and Diseases and Sustainable Management Practices
- Onion Weed Management: How to Control Weeds in an Onion Field
- Onion Harvest, Yields, and Storage
