What to check first
Look for a named variety and day-length group
Check the catalog or label for short, intermediate or long day. A bag marked only 'yellow onions' leaves an important question unanswered. Utah State treats day-length response, flavor, color and storage use as separate characteristics when selecting varieties. [1]
Use your location, not just a hardiness zone
Summer daylight varies with latitude. Minnesota recommends long-day storage onions that respond to about fourteen or more daylight hours; Clemson recommends short-day types for South Carolina and allows intermediate types in its Upstate. Those regional examples are not exact boundaries for every cultivar. [2] [3]
Decide whether you need bulbs or green tops
Growing large storage bulbs is a different goal from harvesting green onions. Clemson notes that bulb-onion thinnings can be used green, and there are varieties specifically grown for bunching. Decide the intended harvest before judging whether the variety is suitable. [3]
Choose the next step for your plant
Buy from a locally appropriate recommendation
Choose a named cultivar listed for your area, then verify its day-length group with the supplier. If you live near a transition region, ask your county Extension office about locally successful intermediate or other recommended types instead of choosing by a broad map alone. [1] [3]
Match the planting season as well
Use the local schedule for the planting material you have. Northern spring planting and southern fall or winter systems give plants different growing windows. Keep a record of variety, planting date and bulb development so you can distinguish a timing problem from a variety mismatch. [2] [3]
What to avoid
- Do not assume every sweet onion is short-day or every storage onion is long-day; verify the named variety.
- Do not try to correct a day-length mismatch by adding more fertilizer or withholding water.
What to watch next
If onions make poor bulbs, check the variety and planting history before changing care. Full sun, healthy growth and suitable moisture still matter. A correctly chosen day-length group improves the starting plan but does not remove weather, disease or planting-time limitations.
Common questions
Does short-day mean an onion can grow well in shade?
No. It describes the daylight signal for bulb formation, not permission to shade the crop. Both Minnesota and Clemson advise full sun for good growth. Measure site exposure separately from choosing the onion group. [2] [3]
Can I grow a southern onion in the North?
It may grow, but bulb results can disappoint; Minnesota notes that short-day onions there generally form small bulbs. If you want dependable large bulbs, use a locally recommended group. Green-top harvest is a different objective. [2]
Sources and review
This guide was prepared with AI assistance and checked against the sources below. It has not been reviewed by a botanist. Conditions and plant responses vary; use the checks above to decide what applies. Read our editorial standards.
- Utah State University Extension: Onion types
Short, intermediate and long-day groups; flavor and storage traits are separate selection considerations.
- University of Minnesota Extension: Growing onions in home gardens
Long-day selection in Minnesota, approximately 14 or more daylight hours, and northern growing context.
- Clemson Cooperative Extension: Onion, leek, shallot and garlic
Short-day selection in South Carolina, intermediate types in the Upstate and green-onion exceptions.