Onions grow best in loose, well drained soil with a high level of organic matter. Being a cool season vegetable, they are frost tolerant and should be planted in early spring as soon as the soil can be worked. The best production occurs when temperatures remain cool over an extended period of time, during which the foliage and root system develop before bulb formation occurs.
To grow good onions, the surest way is to plant sets. Sets are small onions that were grown from seed the year before. Choose sets that are firm and not yet sprouting. Onion plants can also be used. For green onions, the sets or transplants should be placed about one inch apart and about one to 1½ inches deep. For dry onions, the sets or transplants should be placed about one to two inches deep and about three to four inches apart. It is also possible to make one onion bed planting and then pull green onions, leaving one bulb every four inches or so to remain in the garden for dry onions. Green onions can be pulled four to six weeks after planting for fresh eating.
To control weeds, cultivate shallowly by hand or mulch the onion bed. Mulching promotes moisture retention which will help to ensure a good yield. Fertilizer may be applied but it is best to have a garden soil test done so you have a guide to nutrient needs.
Dry onions will mature in three to five months depending on the cultivar used. When the foliage neck is bent over, dry onions are ready to be harvested. After harvesting, place them in a shady location to dry. This may take up to four weeks depending on air circulation, humidity and cultivar. Cut the tops off about an inch above the bulb after the neck is completely dried. If the neck is not sufficiently dried, disease organisms may enter the onion during storage and cause it to rot. Store onions in a dry, cool area in well ventilated containers. Sprouting will occur if not kept cool. Check for and discard any damaged onions during the storage period.
As always, Happy Gardening!
The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange County.