Why Do Onions Vary in Flavor & Storage Time?

In our video, Growing Onions, Leeks, & Shallots, Tricia discusses choosing onions to fit your climate.

Short day onions do well in the shorter summer days closer to the Equator (Texas, we're talking about you) and long day onions thrive in the long summer days closer to the North Pole (e.g., Minnesota). Our video's animated map shows the different regions for short day and long day.

Intermediate day onions have characteristics more similar to the long day onions. The day length differences between onions also signal differences in flavor and how long you can store them.

Where an Onion Gets Its Flavor

Water

The higher the onion's water content, the sweeter it is. Why? One reason is the extra water dilutes the amount of sulfur (the sharp part of the "oniony" taste) making their flavor very mild.

The famous, sweet Walla Walla onion, for instance, has a high water content, about 85%. Walla Walla was originally a short day onion and over the years has been adjusted to grow well in intermediate to long day regions.

Sulfur

Another factor in an onion's sulfur flavor is the sulfur content of the soil where it is grown. The well-known Vidalia onions from Georgia are grown in an area of low-sulfur soil.

Test your soil and see what your sulfur level is. In her book, All The Onions, Betty Jacobs advises that, "while you can't change the sulfur content of your soil, you can avoid increasing it by steering clear of fertilizers that contain sulfur. Stop fertilizing about 30 days before you expect to harvest."

Sugar

Sweet onions have more sugar too. Sweet onions have 6 to 15 percent sugar and the long day onions that store so well have only 3 to 5 percent sugar.

Storage

Short day onions, with their sweet flavor and high water content, keep just a couple of months. Unless you have atmosphere-controlled coolers to extend their season, they must be considered seasonal and perishable.

Long day onions are the storage champs. Keep them cool (36°F to 45°F) and dry, and they can last for many months.