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Risks in Planting Onion

by Duane Craig

Onions are fairly easy to grow, although some varieties work better in parts of the country than others. Onions prefer fertile soil that is rich in humus and drains well. You may plant seeds, onion sets or transplants, but onion sets are the easiest way to go. Disease, insects and certain types of weather are the biggest risks with growing onions.

Onions offer a few growing risks.

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Onion Thrip

Onion thrips are an almost invisible risk to onions. They damage the surface of the leaf so that it releases juices, then feed on the juices and the leaf. These pests are not visible with the human eye because they are only a fraction of an inch long. Insecticides may be used, but do not use them if you plan to harvest the onions within a week. Thrips will ruin an onion crop given enough time.

Onion Maggot

Full-grown onion maggots are one-third inch long, and have white legless bodies. The larvae bore into onion bulbs and can ruin crops. One deterrent is to plant radishes in rows between the onions. The maggots prefer the radishes over the onions. Once the radishes have become infested you can remove and destroy them. Another option is to sprinkle diatomaceous earth around the base of the plants. Diatomaceous earth has sharp edges that cut the body of the insect. These small cuts make the insect lose moisture and die.
Downy Mildew

Downy mildew is a disease which threatens onions. Onions infected with downy mildew have pinkish to red roots and will eventually rot. Stunted and wilted tops are symptomatic of this disease. Removing infected plants, watering the soil instead of the leaves and providing good air circulation is the best prevention. Fungicides labeled for use on onions may help once the disease is noticed.
Smut

Smut is another risk to growing onions. This disease is common in onions that are grown in northern climates, and is characterized by black spots between the onion sections and on the leaves. Signs of smut appear on the young seedlings. The onions that survive will have deformed bulbs. The soil is often the source of the infestation; use a fungicide in the trench before you grow subsequent plantings in the same ground.
Hail

Hail damages the onion leaves resulting in slower growth and smaller yields. You can cover onion crops to help prevent damage when hail is in the forecast.