Watering 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.