The humble onion is essential to the cuisine of almost every culture. Without it, our food would be bland and uninteresting. The genus Allium includes hundreds of species from garlic and onions familiar to cooks around the world to the purely decorative and architecturally spectacular A. giganteum with its purple globe flower.
As a vegetable, members of the onion family are biennial bulbs that are easily cultivated in temperate climates across the US. They can be propogated by seed, bulb sets, and small plants. Some, like leeks, can be used over and over again after harvesting the tender white ends.
The broad range of members of the onion family include:
You may think onions are so inexpensive at the grocery that growing them is unnecessary unless you have plenty of garden space. Though that may be true, they are pretty easy to grow and are therefore pretty gratifying. Even if growing dry onions doesn't appeal to you, other species are both common and desirable such as leeks, scallions, and chives, as well as specialty onions like the gourmet Italian cippolini.
Despite their ubiquity, onions require fertile, well-drained soils that lean to the acidic side of the pH scale (6–6.8) in a location that gets full sun for at least 8–10 hours per day. They are also very heavy feeders but benefit from well-prepared soil rather than fertilizing during the growing season. For that reason, soil tests provide a useful gauge before planting so appropriate amendments and compost high in organic matter can be worked in well in advance of planting. Where soils are very heavy and clayey, it's important to add sufficient compost or manure to break up the soil and lighten its texture as well as enhance its moisture holding capacity. If using manure, it's best to add it to the bed the preceding season to allow it plenty of time to decompose.
Onions respond to the type of soil producing more heat depending on whether the soil is clayey, loamy, or sandy. A soil test is the best way to determine what kind of amendments or fertilizers are needed. If required, a balanced fertilizer (10-10-10) meets most of the needs of onion growing.
Soil needs to be uniformly moist at all times but not soggy. The minimal root system requires consistent moisture.
To reduce weeding and retain moisture, mulch well. Weeding, when needed, should be done by hand with either a hoe or cultivator. All members of the onion tribe don't like to compete with weeds, so vigilance is rewarded with more and larger onions.
There are both long- and short-day onions. This term refers to the length of the daylight needed before bulbs are formed. For example, long-day onion bulbs begin to form when the length of day exceeds twelve hours. At this point, top growth ceases.
Onions prefer cooler temperatures (55–75°F) and perform best in areas where the spring and autumn are permit adequate foliage and root development.
Starting onions from seed is the more difficult of the several methods. Germination is unpredictable, the plants grow very slowly, and weeds are a constant problem. It is also the least expensive method. Onion seed requires a minimum temperature of 50–55°F to germinate. Plant either in the fall before temperatures drop or as soon as the soil can be worked in the spring. Cover seed lightly.
Onion sets—that is, small bulbs—are usually more productive for home gardeners. If grown from sets, onions are ready for use as green onions after just 30 days. Dry onions with the large bulbs and papery skins take more than 100 days to maturity depending on variety.
Plants look like miniature scallions and are available bunched together at nurseries in the early spring. Plant sets or young plants about an inch to inch and a half deep spacing each about two inches apart, thinning to about four inches.
Shallots take essentially the same cultivation requirements as onions, however, instead of planting seed or plants, you'll plant cloves one inch deep and about 4 inches apart.
Onions typically have a few more problems than garlic. Infestations by onion maggots and thrips are occasional problems that can be dealt with using an insecticidal soap or Sevin.
Good drainage and sanitation go along way toward avoiding problems.
Don't hill soil up around the necks of the onions as this encourages stem rots.
Rotate onions throughout the garden from year to year so they are not grown in the same location.
Green onions may be used directly from the garden. Harvesting onions should be done carefully to prevent damage to the bulbs. Even the smallest cut can allow pathogens entry to the bulb.
About a week or two before harvesting in the late summer, stop watering. Begin brushing the soil away from the stems to expose the necks to the air. Onions and shallots are ready for harvest when the tops begin to dry out and wither. When the tops flop over, pull them, and allow them to dry in a well ventilated area for 3–4 weeks. Once they are cured, store them in a very cool, dry place but where they won't freeze.