You may have read in one of our product descriptions that a particular type of onion was a short-day or long-day onion variety. This is important information when selecting onion to grow in your garden.
Onions create bulbs to store energy from the sun. Some onion varieties need longer days (ie long-day varieties) to bulb. Northern regions have more daylight hours than the South in summertime. Long-day onions need about 14 or so hours of daylight to bulb. Short-day onions need about 10 hours of daylight.
Don't get cute and think a short-day onion will do better up north because there is more daylight. Oh, contraire. Onions planted at the wrong time of year won't form bulbs. If you grow short-day onions at northern latitudes, it is only March during the onions' critical photoperiod. The soil is still too cold and the result will be a tiny plant and an embarrassing bulb. That's why long-day varieties are planted in late fall to early winter, while intermediate-day and short-day varieties are planted in very early spring.
The bottom line is that if you are in the South, you need short day onions. If you are in the North, you need long day onions. If you live in the country's waistline, you may have options of short/long and intermediate-day onions. Because (worthless) USDA maps don't account for light, we can't say that zone X should use Y onion. Use the very helpful map below to see the distinction. Choose the right onion or you will be serving scallions this year! Good growing!
☀ So Tennessee will grow short day and intermediate day onion varieties. Florida will only grow short day onions. Cincinnati and Northern Virginia will choose intermediate day or long day onions. Iowa, Pennsylvania and Michigan will grow only long-day varieties.