Most onions can be pulled young as green onions called scallions, but there is also a perennial bunching type, Allium fistulosum, that produces superior scallions and is practically disease and insect proof. Each bulb of the multiplier or potato onion (A. cepa Aggregatum group) multiplies into a bulb cluster, so with every harvest, you'll have bulbs to replant for a continual supply.
The Egyptian or top onion (A. cepa Proliferum group) produces a bulb cluster at the end of a long stem with a second cluster frequently forming on top of the first. It also has an underground bulb, which is often too pungent to eat. Other tasty plants include chives (A. schoenoprasum), garlic chives (A. tuberosum), and shallots (A. cepa Aggregatum group). For information on other onion relatives, see Garlic.
You can grow onions from transplants, sets, or seeds. Transplants, which are seedlings started in the current growing season and sold in bunches, are available from nurseries and by mail order. They usually form good bulbs over a short period of time (65 days or less), but they are subject to diseases. Choice of cultivars is somewhat limited.
Sets are immature bulbs grown the previous year and offer the most limited cultivar choices. They are the easiest to plant, the earliest to harvest, and the least susceptible to diseases. They are, however, more prone to bolting (sending up a flower stalk prematurely) than are seedlings or transplants.
If you plant onion sets, the sets may be identified only as white, red, or yellow, rather than by variety name. Most growers prefer white sets for green onions. When buying sets, look for 1/2-inch-diameter bulbs, because they're the least likely to bolt.
Growing onions from seed offers the great advantage of a wide choice in cultivars. The challenge with starting from seeds is that your crop will take up to 4 months to mature—gardeners in cold-winter areas will need to start their onion seedlings indoors.
Always check a cultivar's daylength requirement or recommended latitudes before you buy, because daylength affects how and when onions form bulbs. Short-day onions, such as 'Red Hamburger', will form bulbs as soon as days reach 10 to 12 hours long. They're suitable for southern latitudes only. Long-day types, like 'Sweet Sandwich' and 'Southport Red Globe', need 13 to 16 hours of summer daylight in order to form bulbs. They're the type to grow in more northern latitudes.
Onions like cool weather in the early part of their growth, so plant them in spring, except in mild-winter areas, where onions are grown as a fall or winter crop. Generally speaking, onions grow tops in cool weather and form bulbs when the weather warms.
Plant onion seeds 4 to 6 weeks before the last average frost—or even earlier indoors or in a cold frame. When indoor seedlings are 2 to 3 inches tall, harden them off by exposing them to above-freezing night temperatures.
Outdoors, sow seeds thickly in rows about 1/2 inch deep. You can try mixing in radish seeds both to mark the planted rows and as a trap crop to lure root maggots away from the onions. Thin seedlings to 1 inch apart, and thin again in four weeks to 6 inches apart. For transplants or sets, use a dibble to make planting holes 2 inches deep and 4 to 6 inches apart. Use the closer spacing if you plan to harvest some young plants as green onions. For sets, open a furrow 2 inches deep and place the sets stem (pointed) end up 4 to 6 inches apart, and then fill in the furrow. One pound of sets will plant about 50 feet of row.
The practices you use will depend on the specific crop you're growing. In general, onions grow best if you keep them well weeded. Use a sharp hoe to cut off intruders; pulling or digging weeds up can damage the onions' shallow roots. Once the soil has warmed, put down a mulch around and between the plants to discourage weeds and to hold moisture in the soil.
Dry conditions cause bulbs to split, so water when necessary to provide at least 1 inch of water each week; keep in mind that transplants require more water than sets do. Onions can't compete well with weeds, so it's important to direct water right to the onion roots. Two good watering methods for achieving this are shown below.
If you've prepared your soil well, no fertilizing should be necessary. Always go easy on nitrogen, which can produce lush tops at the expense of bulbs. New growth from the center will stop when the bulbs start forming.
Egyptian onions, chives, and shallots require slightly different cultivation from regular onions. Here are some guidelines for growing these onion relatives:
Plant Egyptian onions in fall throughout the country; harvest some in spring as green or bunching onions. In midsummer or fall, miniature bulbs will form at the stem tip, where most onions form flowers. Pick these tiny bulbs when the tops begin to wilt and dry. Use them fresh or store in the freezer.
Plant chives and garlic chives in early spring in rich soil. They will tolerate partial shade put prefer full sun. Seeds are very slow to germinate, so most growers prefer to plant clump divisions, which can be harvested after 2 months. Space the clumps, each of which should contain about six bulbs, 8 inches apart.
Cut the grasslike, hollow tops frequently to maintain production. The pom-pom-like lavender flowers are very attractive, but always remove the spent flowers to reduce the chance of rampant self-seeding. Dig up, divide, and replant every third year. Transplant to containers and move indoors for winter harvests. Chives are almost as good frozen as they are fresh.
Shallots, a favorite of French chefs, have a blue-green stem that's used when young. In addition, it has a gray, angular, mild-flavored bulb that's related to the multiplying onion and is used like a mild-flavored garlic. Shallots will tolerate all but the most acid soils, but dig the earth deeply because the plants put down 8-inch-long feeder roots. However, they have no lateral roots, so space them just 2 to 3 inches apart.
Propagate shallots by dividing bulb clusters. Each clove, in turn, will produce four to eight new bulbs. In February or March, plant them 1 inch deep, barely covering the tip of the clove. Keep the soil weed free and slightly moist, but don't fertilize. In early summer, draw the soil away from the bulbs. Harvest shallots as green onions at any time. Cutting the tops off near soil level will produce new tops, and such harvesting actually increases bulb production. Bulbs mature in about 5 months. Pull and store like onions.
To water onions efficiently, extend soaker hoses along the row close to the plants. Or open a small trench between rows and fill it with water. This keeps the roots supplied, while leaving most of the soil surface dry, inhibiting weed seed germination.
You can generally expect a disease-and insect-free crop. One possible pest is onion maggots: 1/3-inch-long white, legless larvae that travel in line from one bulb to the next and burrow upwards to feed on the stems. To reduce the chances of extensive damage, scatter-plant onions throughout the garden. (This interplanting can also benefit other garden plants; many Allium species will ward off pests—such as aphids, Japanese beetles, and carrot flies—from roses, lettuce, carrots, beets, parsnips, and members of the cabbage family.) Placing a thin layer of sand around onion bulbs may discourage adult flies from laying their eggs at the bottoms of the plants.
Barely visible onion thrips tend to attack during hot, dry weather in July or August. They produce deformed plants with silvery blotches on the leaves. Thrips overwinter in weeds, so reduce pest populations by keeping the garden clean. Try spreading a reflective mulch, such as aluminum foil, between rows to confuse the thrips. If you catch the problem early, you can spray plants with Beauveria bassiana or spinosad to combat thrips. As a last resort apply neem to control a serious infestation.
A disease called smut causes a swelling or hardening of leaves just about the neck, which eventually bursts and spills powdery black spores over the plant. Downy mildew, a purplish mold, shows up in midsummer during warm, humid weather. Onions are also subject to pink root, which causes roots to turn various colors and then shrivel, and neck rot, which causes tissues to form a hard, black crust. All these problems are caused by fungi in the soil and can be avoided by rotating crops and by working humus into the onion bed to provide good drainage.
Once onion tops turn yellow, use the back of a rake to bend them over horizontally. This stops the sap from flowing to the stems and diverts the plant's energy into maturing the bulb. A day or so later, when the tops turn brown, pull or dig the bulbs on a sunny day, and leave them to dry in the sun. Lay the tops of one row over the bulbs of another to help prevent sunscald.
When the outer skins are thoroughly dry, wipe off any soil and remove the tops—unless you intend to braid them. Store in a cool, dry place; hang braided onions or those kept in mesh bags in an airy spot. Such dried bulbs will keep for about 4 months to 1 year.