Onion sets that you purchase in the spring in Minnesota are the bulbs produced from seed grown in the south and have completed their first year of growth. When the set is planted, the plant wants to produce seed to complete its growing cycle. If you wish to produce onions for harvesting in the fall, do not plant sets. The bulb that is produced does not store well. However, if you wish to grow onions for eating fresh in the early spring, onion sets are excellent.

Onions are further divided into short- and long-day length plants. Onions are triggered to bulb by the length of daylight hours. A short-day onion starts forming a bulb when the day length reaches 10 to 12 hours. They are primarily grown in the winter and harvested in March, April, and May. They are mostly grown in the southern states. Day-neutral or intermediate-day onions are triggered with 12 to 14 hours of daylight. Long-day onions need 14 to 16 hours of daylight to begin bulbing. These are normally grown in the summer in more northern states, where daytime temps don't overwhelm them.

When you speak of sweet onions, you probably are referring to Bermuda and Vidalia varieties. Both of these are varieties adapted to the south; they will not produce a satisfactory crop in Minnesota. In Minnesota, Yellow Globe is probably the only variety that can be directly seeded in the garden and still mature in our growing season.

There are many other fine varieties than can be grown in Minnesota (Suggested Vegetable Varieties for Home Gardeners), but the seed must be started indoors around the first of March. In early May the plants can be set out in the garden, and they will produce a fine crop of onions that will keep well over winter.

Starting the plants indoors is actually not difficult. They can be grown in a south window and are about the only plants that do not need to be transplanted before setting them in the garden. The roots in the container will be entangled, but they can be easily separated by swirling the plants in a container of water.

Onions do not have any special soil requirements. Soil suitable for other vegetables should be satisfactory. The soil should be loose, well-drained, with a pH of 6.5 to 7.5. Testing the soil before planting is always a good idea. Contact your local Extension office for directions.