What makes onions split rather than grow?

Answer 1

[1] It depends upon the variety, the environment, and the cultivation. [2] For example, Sweet Spanish varieties are susceptible to splitting if small numbers are planted far apart. Generally, onions want to be planted in numbers, and closer together. [3] The number of daylight hours is what begins bulbing in onions. Early maturing onions need 12 hrs, late maturing 15. Splitting occurs when light needs are met. Other environmental factors include weeds competing early in the growing cycle; soil moisture levels that are off; and growth that has to work around high wind and blowing particles, and against pesty insects and wildlife. [4] And it occurs due to certain cultural practices. For example, heavy-handed nitrogen-phosphorus-potassium fertilizing schedules emphasize dense lush above ground growth, at the expense of proper bulbing. And onions tend to need to be planted only about 1-1/2 inches below ground. Deeper depths tend to encourage splitting.