Onions are not roots, even though they grow under ground.
The layered sphere we call onion is a bulb. Edible onions form when shortened underground stems are surrounded by fleshy leaves, called scales. People have been growing onions for 7,000 years.
Onions are members of the Allium family, which includes leeks, chives, and garlic. All of these crops are best planted in the Bay Area in late summer and early fall. One of the nicest things about growing your own onions is that they can be eaten at any stage of development. Early green shoots are called scallions, or spring onions, while mid-stage development crops are called summer onions. Onions are a cool weather crop. Hot weather will cause onions (Allium cepa) to go to seed, or bolt.
There are three common varieties of onion: white, yellow, and red. White and yellow onions are commonly sautéed or caramelized. This releases sugars that make onions taste sweet. Red onions are more often used fresh, in salads and when grilling. Onions are also classified according to the number of day-hours it takes to trigger the development of a bulb:
Some onions have been bred to be smaller. These are called boiler, pickler, or pearl onions. The term “pearl onion’ is incorrect because pearl onions are an entirely different species. True pearl onions are more closely related to leeks, than to common onions, and they do not have layers, more similar to garlic.
How to grow onions
Onion seeds are relatively short-lived, when compared to other seeds. You are better off planting transplants or onion ‘sets’. Onion sets are seeds that were sown in early summer, but so close together that they could not continue their development at that time. As a biennial, these sets are more likely to bolt, rather than creating onion bulbs. Transplants are one or more seasons older than sets and usually available in garden stores in October and November. If growing seeds, cover them only lightly with soil. If seeds are started in small pots in September, they should be ready for transplanting in 2 months. Frequent thinning is necessary for a good harvest. Onions are heavy feeders, so it’s a good idea to add aged compost to the planting area a couple of weeks ahead of time. You can expect to harvest mature onions in 12 to 18 weeks, usually in June. Successively planting more onion seeds can ensure an ongoing crop of fresh, tasty onions. Generally grown as an annual, this biennial edible can reseed itself in your landscape indefinitely, given the opportunity, so choose a well drained spot that gets plenty of sun.
Onion pests and diseases
Onions are susceptible to attack by onion maggots, onion flies, and onion eelworms. Leafminers, bulb mites, thrips, and nematodes can also cause problems. Most onion diseases are related to excessive moisture. These include bacterial soft rot, downy mildews, garlic mosaic, rust, pink root, white rot, basal rot, and several botrytis infections. Proper irrigation and allowing the soil to dry out between waterings are your best defense.
Onions are mature when the tops die back naturally. When the top half of mature stems has turned brown, stop watering. When the lower stem begins to turn yellow, carefully dig them out of the ground. After mature onions are dug up, they need time to cure, or dry. This reduces the risk of rotting. Harvested onions can be braided, laid out flat, or placed in nets for several days to dry out properly.
Foodscaping with onions
Due to their ability to self-propagate, onions and other alliums make useful foodscape plants. Using mature sizes, mixed plantings of alliums can be lovely and useful. Mature onion plants can reach 3 feet in height, with impressive spherical flowers that attract hoverflies and other beneficial insects. Wide-leafed leeks are slightly shorter and can stay in place for several months before they must be harvested. Narrow-leafed garlic is more seasonal, providing green spikes for several winter and spring months, followed by browning that some find messy. The browned tops can be bent over and hidden from view behind spiky, clumping chives that tend to stay green year round, offering up miniature versions of the onion flowers.
Why do onions make me cry?
Onions contain protective enzymes that are released as an acid gas when cut. That’s what burns your eyes and causes you to tear up when cutting onions. And you can ignore all those great ideas about how to avoid the burn: bread in your mouth, sunglasses, lemon juice… none of them work. If you really need respite from onion burn, freeze or refrigerate your onions before cutting, or cut them under or in front of a fan.
Leeks have a nice delicate flavor, but they can get pretty pricey in the grocery store. Luckily for us, they are easy to grow!
Unlike other members of the Allium family, such as garlic and onions, leeks do not form bulbs underground. Instead, leeks grow an edible stem that can be up to 2 inches in diameter. Like garlic, the leaves are flat (onion leaves are tubes). Leeks are commonly paired with potatoes, but they can also be steamed, roasted, or baked to stand on their own.
How to grow leeks
Leeks are biennial plants that are grown as annuals. You can start leeks from seed or by replanting the white base of an existing leek. If you use the base of a leek purchased from the grocery store, be aware that you may be introducing destructive pathogens into your garden. I’ve done it without any obvious problems, but it’s your call. The base should be planted deeper than you’d expect, 4 to 6 inches. If you use seeds, plant them 1/2 inch deep and 6 inches apart. If you start seeds indoors, be sure to harden them off before planting them in the ground or tall containers outside. You can also buy seedlings from your local garden store, but they will need to be gently untangled from each other before replanting - they look like unmown grass in the pot. Seedlings should then be gently transplanted deeper into the soil than they were, up to where the green stems begin, for a longer, more drought resistant edible. Once the seedlings are as big around as a pencil, bank soil or mulch around each plant to blanch the base. Blanching means keeping certain parts of a plant white by blocking sunlight.
Leeks prefer cooler temperatures, so the Bay area’s fall and winter are the best time to grow leeks. Leeks enjoy full sun and well-drained soil, but the two components that make the biggest difference in growing leeks are water and nitrogen. Leeks are heavy feeders, so planting them near peas or beans will provide an extra boost of nitrogen early in the growing process, followed with aged compost. Leeks have shallow roots so regular watering is needed for plants to reach full size. Water stress will significantly reduce yields. The soil should be moistened to a depth of 18 inches every week. Mulching around plants can reduce weeds, add nutrients, and cut water needs.
Onion maggots, onion rust, and onion white rot are the most common problems encountered when growing leeks.
The crisp, slender stalks of scallions are easy to grow and they take up very little space in a garden or on a windowsill.
Scallions, also known as green onions or spring onions, can either be immature onions before they set bulbs (spring onions), or a specific variety of Allium that never forms significant bulbs.
How to grow scallions
Seeds can be started at any time of year and seedlings can be grown indoors or out. Seeds should be heavily planted, 1/2 inch deep. Scallions can grown in clumps, so there is no need to thin transplants. Scallion seeds require moisture to germinate and they can take a month to get started. Keep the soil moist and be patient. Like other onions, scallions have a shallow root system, so proper irrigation and frequent, gentle weeding are important. Scallions are heavy nitrogen feeders, so feed regularly with fish emulsion, blood meal, or alfalfa meal. Planting a new batch of scallion seeds every three weeks will keep you supplied with scallions year round. Also, each time you snip the base off of a scallion, those roots can be replanted to grow a new scallion!
Pests & diseases of scallions
Scallions may be attacked by thrips. Thrip damage gives the leaves a white streaked look. Thrips can be reduced by spraying plants with soapy water. If tunnels are found in the underground portion of the plant, it may be onion maggots. Most members of the Allium family are susceptible to onion white rot and other fungal diseases, such as onion rust. If nematodes are feeding on the root system, plants will appear stunted or deformed.
Dinner wouldn’t be nearly as delicious without plants from the Allium genus. Ornamental Alliums also attract beneficial insects to your garden.
Allium are monocots that grow from bulbs. Bulbs are a food reserve for the plant (and your pantry). Similar to grass plants, Alliums feature fleshy layers that wrap around the bulb and stem. Their stems do not have leaves, but a single umbel. Umbels are flowers made up of tiny flowerettes, usually forming a curved or flat surface. This surface creates an attractive landing surface for beneficial insects.
Attracting beneficial insects
Planting Allium in the garden is sure to attract beneficial insects. The convenient landing platform and sweet nectar will bring them in from blocks or even miles around. Allium tanguticum, the Lavender globe lily (pictured), attracts hoverflies. These beneficials look like tiny bees and they devour aphids and mealybugs. Spritely black Trichogramma wasps are also attracted and will lay their eggs in moth eggs, preventing caterpillars, such as the tomato hornworm from attacking your garden. If you enjoy the flavor of garlic and chives, you can always plant Garlic Chives (Allium tuberosum) another favorite of beneficial insects.
How to grow alliums
These shallow-rooted plants prefer soil that holds a lot of organic material and that means adding compost before planting. They will grow just about anywhere, but our heavy clay soil can slow growth. Allium can be grown in partial shade to full sun. Alliums grow well in raised beds and containers. You can grow chives in a pot on your kitchen windowsill for easy access while cooking!
Growing Allium from seed can be hit and miss. The plants are slow starters and they don’t handle competition (weeds) very well. You can plant seeds in containers or directly in the garden. Seeds should be sown 1/2” deep and 1/2” apart. Thinned plants can be eaten as scallions.
Another way to plant Allium is in the form of “sets”. Allium sets are mature bulbs that can be planted directly in the ground or a container. Follow the package directions for depth and spacing. The only downside to sets is that they tend to bolt. Bolting is the beginning of the going-to-seed process. If you are growing onions, leeks, or garlic, plants that have started bolting should be harvested right away, unless you plan to collect your own seeds.
Garlic and onions are best planted October through January.
I hope this information inspires you to grow more of your own food. You can ask your garden questions on my Home page.