Onion Plant Daylength Guide

Selecting the Right Onion Plant Varieties for Your Area

The size of the onion bulb is dependent upon the number and size of the green leaves or tops at the time of bulbing. For each leaf, there will be a ring of onion. The larger the leaf, the larger the ring will be when the carbohydrates from the leaves are transferred to the rings of the bulb. The triggering of this transfer or bulbing is dependent upon daylength and temperature and not the size or age of the plants. When selecting your onion varieties, remember that the further north you are, the more hours of daylight you have during the summer.

Short-Day Onions
  • Start bulbing process when daylength reaches 10-12 hours
  • Mature in 110 days when planted in the south during winter or early spring
  • Mature in 75 days when planted in northern states in late spring but will not get very large
  • The earlier you plant them, the larger they get
Intermediate-Day Onions
  • Start bulbing process when daylength reaches 12-14 hours
  • Will produce nice-sized bulbs unless you live in far south Florida or south Texas
  • Mature in 110 days when planted at the proper time
  • Exceptionally sweet
Long-Day Onions
  • Start bulbing process when daylength reaches 14-16 hours
  • Do extremely well in northern states
  • Sweet, storage, and specialty long day varieties available
  • Excellent for long storage