How to Grow Onions

Onions are easy to grow. Like most vegetables, they need plenty of sun and rich, healthy, loose soil with plenty of organic matter and a pH of 6.2 to 6.8. Onions can be grown from seeds, sets, which are tiny, immature bulbs, or transplants. 

Onions are a cool-season crop and in many regions of the country are planted in the fall for harvest the following spring or summer. In other areas, a spring planting is best. Here are some general guidelines:

• In warmer regions, zone 7 and warmer, seeds for short-day bulb onions are often planted in the garden in September through October, but seeds may not grow well in some Southern regions. Onion sets can be planted in early spring before the last frost date. Transplants are best planted in early spring for late spring and early summer harvest.
• In cold regions, zone 5 and colder, it may be best to start with long-day sets or transplants and plant in the garden in the spring when the soil can be worked, around late March or early April.
• When it comes to day-neutral types, they typically are planted in the spring in cold regions and in the fall in warm regions.
• Check with your county Extension office or local garden center for best varieties and times to plant in your region. 

The best onions with a longer shelf life are yellow onions followed by red and white types.

Seeds: Plant seeds ½-inch deep. Direct seeding may not give long-season varieties time to grow and form bulbs. For these, it’s best to use transplants or sets.

Transplants: Start seeds indoors about 8 to 10 weeks before the last frost date. Plant the seeds ¼-inch deep in cells or flats. A few weeks before the last frost date, harden them off by setting outside on a warm day for 30 minutes, gradually increasing their time outdoors over a period of about 7 days. Onions can tolerate some frost, so after they are hardened off, plant them outdoors 2 to 4 weeks before last frost date. Plant transplants about ¾-inch deep. Space green onions or scallions about 1-inch apart. Space smaller bulbs 2-inches apart and allow 4- to 6-inches for larger bulbs.

Sets: Select sets that are no larger than ¾ inch. Larger sets are likely to bolt, which means they will send up flower stalks. Plant sets about ¾- to 1-inch deep 2 to 4 weeks before the last frost date. Space small bulbs 2-inches apart and large bulbs 4-inches apart.

Transplants or sets may typically be labeled as white, yellow, or red, though some regions of the country are noted for certain cultivars and varieties, and local garden centers may have them available for the home gardener. Examples are Walla Walla Sweet onions in Washington and Vidalia onions in Georgia.

 Keep weeds in check by using mulch such as weed-free straw, chemical-free grass clippings, or other organic material.

Onions are shallow-rooted; therefore they need constant, even moisture for good growth and bulb formation. They will need about 1-inch of water each week from rainfall or irrigation.

When the plants reach maturity (check maturity date on your variety) and when the tops begin to fall, cease watering.

Fertilizing

If possible, get a soil test and follow recommendations before planting. In the absence of a soil test, add plenty of organic material to your garden and plan on sidedressing (spreading fertilizer along the row of onions about six inches away from the plant and lightly worked into the soil) a couple of times during the growing season.

• Minnesota Extension recommends a fertilizer application once or twice during the growing season, with urea (45-0-0) at a rate of ¼ to ½ pound per 25 feet row.

• When using fertilizer, do not use “weed and feed” types on vegetables.

Controlling Pests, Diseases, and Other Problems

• Onions typically do not have problems with insects. Onion maggots can be a problem, boring into the plant stems and causing the plants to turn yellow and wilt.  Thrips can also damage them. If needed, apply Bacillus thuringiensis (Bt), an organic insecticide. Follow the label directions.

• Several types of rot can affect them. The best way to avoid disease problems is to adopt good cultural practices such as starting with good, healthy soil that has plenty of organic matter, keeping moisture levels even, suppressing weed growth, and practicing crop rotation (don’t plant onions where you’ve grown them or any members of the allium family the past four years).

Green onions are ready to harvest when they are about 6- to 12-inches tall.

Bulb onions are ready when about half of the tops are dry and falling over. It is important for bulb health to allow the foliage to fall over on its own rather than knocking them over, which can bruise the neck and lead to rot.

When the foliage falls over, the bulbs have quit growing. Use a spade fork to lift the dried stems and the bulbs from the ground.

If the weather is dry and warm, allow them to start the curing process by drying outside for several days. If days are rainy or with high humidity, take the onion indoors.

To cure, they need to be in a warm, well-ventilated area for up to four weeks. Lay them in a single layer with plenty of air circulation. When their outer skin becomes dried and crispy, they have cured to a desired level. Trim off the tops about an inch or two above the bulb unless you wish to use the stems to braid the onions together. Brush off any extra soil.

Storing

After onions are cured, store them in a cool, dry place.Hanging them in mesh bags where they get air circulation can be an option in a garage or dry basement.

Store them away from any fruits or vegetables that release ethylene gas, such as potatoes, apples, and bananas, which can cause onions to sprout and decay.

• Grow varieties best suited for your part of the country. Your county Extension office or local garden center should be able to provide you with specific information on varieties and best times to plant for your region.
• Start with good, healthy, loose soil that has plenty of decomposed organic material. 
• Use a layer of straw mulch to suppress weeds, which compete with onions for nutrients. Be careful when removing weeds from around onion plants. Either hand pull weeds or lightly cultivate to avoid damaging the bulbs. 
• During hot, dry weather, keep your onions watered to allow for proper growth.