Growing onions from seed is economical, and seed-started onions don’t send up flower stems as often as transplanted bedding onions do. An onion’s flowering process ruins the quality of a bulb onion.
In northern Colorado, onions grown for winter storage will begin to form bulbs as summer days get longer, usually in July. When shopping for seed, look for long-day varieties such as Copra, and Early Yellow Globe for yellow onions. Try Mambo and Southport Red Globe for red onions, and Bedfordshire Champion and Snow White for white onions. Good, mild onions for short-term storage include Ailsa Craig Exhibition and Walla Walla Sweet.
Onions grow best in fertile soil that drains well. But, they also grow in sandy or clay soils that have been amended with organic material. Onions only require ample nitrogen until mid-July. Apply organic material over the soil approximately 1 1/2 inches deep and work it in to a depth of eight inches.
Onion seed should be planted as soon as the soil can be worked, depending on spring weather. For the Denver metro area, mid- to late March is best, but onions can be planted until late April, depending on the variety. Remember that hard freezes can damage young seedlings. Since onion seedlings are fairly cold-tolerant, they survive in the soil in cold weather as long as the ground doesn’t freeze. Plant the seed about one inch deep and one and one-half inches apart. When the plants have five to 10 leaves, thin to three inches apart and use the pulled onions as scallions.
Since onion roots are shallow, water them frequently and never allow them to dry during bulbing or the bulbs will be small and leathery. Preventing drought-stress in onions can also help to prevent insect problems. Don’t be concerned if bulbs develop mostly out of the soil.
Keep Gardens weed-free with mulch or shallow cultivation, since onions don’t compete well with fast-growing weeds. Thrips are the most common insect to attack onions and can emerge from the soil. They live on weeds, so mulching and weeding are natural controls. Damaged plants will look silver. Thrips can be controlled with insecticidal soaps.
By late August, the tops of onion plants will begin to lay over on the ground. Food made in the leaves will be stored in the onion bulbs. Do not water them at this point. When most of the tops are on the ground, lift the onions to break the bulbs from the roots. Leave the bulbs on the ground exactly as they were growing to cure them for storage and prevent sunburn. Bulbs will be ready for harvest in a week or two. Be sure to bring onions in before snow, rain or freezing temperatures. Cut off the tops when they are thoroughly dry. Store the bulbs in a burlap sack or an open crate in a dark area with temperatures as close to freezing as possible without actually freezing.
For “Organic soil amendments” refer to message number 1604.
For more information, see the following Colorado State Extension fact sheet(s). http://www.ext.colostate.edu/ptlk/1808.html