by SCMG Tommie Smith
Onions are bi-annuals, flowering in their second year, although most gardeners grow onions as annuals. There are many varieties of onions, but this article addresses dry bulbing onions, Allium cepa. Other Allium species are bunching onions, leeks, scallions, garlic and shallots.
Varieties of bulbing onions are typically white, yellow or red. White onions tend to be mild and can be used raw or cooked. Yellow onions are sweet and are the best for cooking. Red onions are very crisp and are often eaten raw, but they don’t store well.
There are some technicalities involved in growing this cold weather plant. But, simply put, the highest quality bulbs are produced when early growth stages occur during cool weather followed by increased temperatures for optimum maturity.
The variety and the planting date are extremely important in the production of a good bulb crop. Onion varieties are long-day or short-day and have different requirements regarding the number of daylight hours required to make a bulb. Long-day varieties require 15 to 16 hours of daylight while short-day varieties require 12 hours. Based on Sonoma County’s mild weather and the fact that onions don’t mind frost, there are a number of options for growing onions. Seed or transplant early varieties from November through January for late spring or summer harvest. Seed or transplant late varieties or sets from January through March for late summer or fall harvest. Plant short-day onions in the fall to overwinter for a spring or early summer harvest.
Onions can be grown from seed, sets or starts.
- Seed gives the greatest choice of variety. Start seed indoors with a two-month start or direct sow when soil warms. Onions germinate when soil temperature is 50 degrees or higher. Directly sown seed will germinate in two weeks when the soil temp reaches 50 degrees; compared to three to five days when 75 degrees.
- Sets are onions that were planted from seed last year. Sets are best used for producing a quick crop of green onions. In general, sets are not well-adapted to California for the production of mature, dry bulbs. Select onion sets that are firm with no sign of sprouting. They like cool conditions when first planted and warmth when maturing. These are less likely to store well than onions grown from seeds or starts.
- Starts (transplants) are my preferred option. The starts come in bundles of about 25 plants. Give the roots and tops a bit of a haircut before transplanting into the ground. This encourages strong new growth which will result in robust bulbs when mature. To plant them, just poke a hole into the prepared soil, insert the onion, tamp the soil around them and water well.
Onions like well-drained, friable and fertile soil with a pH of about 6-6.8. I mix about three inches of compost and/or well-rotted manure into the planting bed about a month before planting. Recommended row spacing is about 20 inches apart. Plant spacing should be determined by the size of the mature onion, about six inches for a large bulb. However, I plant my onion starts quite close together in rows about 16 inches apart. Then, in the spring, I thin them by harvesting to eat as green onions.
Provide uniform moisture throughout the season to produce fully formed bulbs. Feed an organic fertilizer every three to four weeks during growing and bulbing stages. Once the bulbs are mature, hold off any irrigation or feeding. Too much fertilizer can cause double or split bulbs.
Practice crop rotation to prevent a buildup of disease and insect pests. Follow onions by carrots the next season, then potatoes, then onions again. They all have different pests and diseases that don’t cross infect each other.
Approximately six months after planting, when onions are fat and raised slightly out of the soil and leaves turn yellow, manually bend over the tops to prevent bolting. About one week later, onion tops will wither and turn brown and can be lifted out of the ground using a garden fork. Do not break the skins which may result in rot. To dry, lay the onions in a single layer in a dry, shady area with good air circulation.
Unless you are going to braid your onions, cut the tops off leaving just under 1-inch and trim the roots closely. Leave to seal and dry another couple of days. Before storage, onions need to be thoroughly dried to avoid fungal rot. Store onions in a cool, airy, dry place, preferably on flat trays or shelves, or hung up braided or in mesh bags. Onions don’t keep well in the refrigerator because of the high moisture content. Don’t store onions next to potatoes, as they are high in moisture and release a ripening gas that causes onions to go soft.