MSU Extension Vegetables

A couple of years ago I received this question, “My husband and I moved here from Western New York state. Why do onions have such a bad sulfur taste here? How can I grow mild sweet onions that taste good?”

Given the pandemic garden explosion, I thought it best to address this in a more public forum.

Like hot peppers, onion flavor preferences are hugely subjective. It goes without saying, but the cultivar makes a big difference. Shopping around for onions can sometimes be confusing. In northern latitudes, Long-Day and Intermediate-Day onions work best. They initiate bulb formation as days lengthen in mid- to-late June. There are sweet types and storage types, and three colors of each. You want to be sure you grow sweet types for the best chance of getting a sweet flavor. Popular Long-Day sweet types include Ailsa Craig (yellow), Highlander (yellow), Red River (red), Ringmaster (white), Walla Walla (yellow) and Yellow Spanish (yellow). Popular Intermediate-Day sweet types include Candy (yellow), Red Candy Apple (red) and Super Star (white).

Some soils and water sources have more sulfur, and that can impart a somewhat more pungent flavor on onions by increasing the amount of pyruvic acid in the onion. That is the compound that makes your eyes water. Sweet onions contain about 18% more sugar than storage onions and have proportionally more fructose sugar than storage onions. Fructose is detected as sweeter than sucrose or glucose, and so it comes through in a sweet onion by cutting the pungency more effectively.

The person who asked this question may have had more sulfur in the soils or irrigation water at their new Michigan site than their previous New York site. This statement does not apply to the whole state of Michigan or New York. You can do a soil test and see the sulfur level of your soil. For onions, sulfur could be applied if soil test levels are below 5 parts per million (ppm). Studies have found pungency reaches its peak when soil sulfur levels reach 14-16 ppm and does not increase much above that. This is just a ballpark. Some fertilizers come with sulfur, so read the ingredients list. You might be better off without it.

Beyond cultivar choice, and soil and water sulfur levels, also consider that onion flavor changes with time, water content and environmental stress. Older, matured plants that are dried down will always taste stronger. Younger, succulent plants will be milder. Even sweet onions will get more pungent when fully dried down. However, they should not get more pungent than a dedicated storage type onion.

An onion can have a stronger flavor when under stress as well. If the tops die down mid-season from environmental pressures like drought, insects or disease, the plant may divert its energy to plant defense. Part of its defense is making that hot flavor. Keeping healthy tops will maintain milder bulbs in the middle of the season when plants should be at their sweetest.

It is worth noting that the sweeter the onion, the poorer it will keep in storage into the winter. True storage onions have fewer sugars and more pyruvic acid, and that aids in its shelf life. However, the same moisture stress and pest issues that may make a sweet onion hot will also reduce the yield and storage life of a storage onion. Popular Long-Day storage onions include Blush (red), Copra (yellow), Redwing (red), Red Zeppelin (red) and Sterling (white).

Once you have made your cultivar selection, time direct-seeding for late-March through early-April and transplanting for early April. Keep bare-root transplants dry and cool until planting. The reason for early planting is because the larger the plant gets prior to bulbing, the larger the resulting bulb. Intermediate-Day onions should be planted earlier than Long-Day onions to give them more time. Onions can tolerate frosts better than most vegetable crops. Plantings in 2020 withstood several nights that got down to 25 degrees Fahrenheit in mid-May. Cold after transplanting can sometimes initiate a flowering response. So, expect some of the plants to go to seed rather than bulb if they experience temperatures such as this.

If starting your own transplants, seed them in high-density cell trays (200-288) or broadcasted in open trays 55-65 days before transplanting (mid-to-late February in Michigan). They will need one or two “haircuts" in that time to keep them 4 inches tall, and a weekly shot of liquid fertilizer with 100 ppm of nitrogen after three weeks of age.

This article was originally published in Michigan Farmer.