When to plant onions depends on where you live. Onion plants will grow roots and green growth above the ground as soon as temperatures allow but bulb growth will only occur when daylight hours reach a certain length, 10-12 hours for short day onions and 14-16 hours for long day onions.
The north and south grow different types of onions and should plant onions at different times. We are divided by the 35th parallel.
Check out “Long Day Onions vs. Short Day Onions” for more about the differences between them.
What’s pertinent to our conversation on when to plant onions is that onions need to have growing time to develop a good root system and green growth above the ground before the daylight hours trigger onion bulb growth.
If onion plants aren’t big enough when bulb growth starts onion bulbs will be small and possibly not as nutritious or tasteful.
Check out “How To Grow Bigger & Sweeter Onions” for more on how to grow good sized onion with sweet and full flavor.
There are two common ways to plant onion seeds, sets, or transplants in the north. The best way is to start onions indoors 8-12 weeks before planting outside. The second is to start onions outdoors 6 weeks before the last frost date.
Onions are cold hardy and can handle nighttime lows in the 20’s. Putting mulch on the soil can keep soil temperatures warmer and insulate small seedlings.
In the south onions can be planted anytime in some areas but it’s common to plant onion seeds in late summer and onion bulbs and transplants in the early fall to avoid the summer heat.
The onions will grow, overwinter, and grow again when temperatures warm to be harvested in the summer.
When Are Onions in Season?
Most onions, sweet onions excluded, store well and can be found in grocery stores year round. They are harvested at different times depending on location and some areas like California have two onion growing seasons.
California is the top onion producing state and produces both a spring and summer harvest. They are also the top producer of onion seeds.
California is able to grow short day onions in the southern part, day-neutral onions in the middle of the state, and long day onions in the north.
In the northern part of the state long day onion seeds are direct sown in April and then harvested in fall. In the middle part of the state onions are seeded or transplanted in fall/winter and harvested in June through September. In the southern part of California short day onion seeds are direct sown in the fall and harvested in April through June.
Check out Onion Production in California for more on how onions are grown in California or take a look at “Where Do Onions Grow?” to see the countries and states that grow the most onions as well as where the onion plant originally comes from.
In Michigan, long day onions are grown. Long day onions store better than short day onions.
“Early maturing varieties are harvested, cured and marketed in late July into early August,” Warncke said. “Most of the onion varieties mature and are harvested in late August through September and put into storage. The onions are packed and marketed out of storage until the end of February.”Michigan Onion Growers Match Varieties To Climate – vegetablegrowersnews.com
Vidalia onions are only grown in Georgia and are available from May through August.
What’s The Best Month To Plant Onions?
The best month to plant onions depends on where you live. In the south you want to plant onion seeds 8-10 weeks and onion sets/transplants 4-6 weeks before night time temperatures average below 50 degrees. If you live further south where temperatures never average below 50 degrees plant your onions anytime.
In the north, it is possible to plant onions in the fall and have them overwinter and start to grow again in the fall.
If you want to give this a try you’re best bet is to winterize the onions by applying heavy mulch on top of the onion plants to insulate the soil around them.
Mulch should be put on just before the first hard frost in the fall, when temperatures drop below 25 degrees. Aim for onion leaves to be at least the size of a pencil before they go dormant for the winter.
Is It Too Late To Plant Onions?
If you live in the north and want to plant onions in the spring and harvest in the fall you have a very short window to do so and in some seasons or areas further north it’s almost impossible to do without starting onion plants indoors.
In the south you have a wider date range of when you can plant onions and have the option of planting in the late fall and harvesting in the spring/summer.
When to plant onions can be tricky business. Please comment below with your own thoughts and experiences with the timing of planting onions.
You might also be interested in “Companion Planting Onions“.