Soon you'll see the onions' slender leaves growing longer. But they never grow wide enough to shade out competing weeds. That makes managing weeds in your onion bed very important. Young weeds are easiest to control; just hand-pull them or cut them off at the soil line with a sharp hoe.
To maximize productivity, Lee Jones, co-owner of Stranger's Hill Organic Farm in Bloomington, Indiana, spaces her onion rows close together, which creates a weeding challenge. Her solution? "When planting onions, we position them in a grid, so that each onion plant is at least one width of a cultivator apart from all other plants. We just pull the cultivator through in one direction, and then the other, which makes the bed quick and easy to weed," she explains.
You can also help suppress weeds with corn-gluten meal, a safe, natural by-product of corn processing that prevents seeds from germinating. Spread it around well-established onion plants (well away from any other seeds you've sown) and then mulch with a couple of inches of clean, seed-free straw.
Plastic mulch isn't an ideal component of an organic garden, but many onion growers rely on it to prevent weeds in their beds. Bruce Frasier, a long-time onion grower and president of Dixondale Farms in Carrizo Springs, Texas, swears by black plastic. "An advantage of plastic is that it heats the soil up, so the onions grow quicker." But, he warns, plastic mulch traps moisture around the plants, creating a humid environment that can be a breeding ground for fungal diseases.
Onions can be affected by bacteria, viruses, and harmful nema-todes, but most problems are caused by fungi, says Karen L. Snover-Clift, director of the Plant Disease Diagnostic Clinic at Cornell University. Fungi cause root and neck rot, downy mildew, and other common diseases. Prevent these nasty plant diseases by keeping onion leaves dry and ensuring that the soil drains well. Use soaker hoses or even drip irrigation systems to deliver water straight to plant roots rather than leaves. If you don't have those available to you and you must water from overhead with a sprinkler, watering can, or hose, do so in the morning or late afternoon, when the leaves will dry out quickly. You can also mitigate disease risk by planting varieties resistant to culprits such as botrytis fungi or bacteria that cause leaf blights. Also, spray foliage every couple of weeks with a light coating of fish emulsion, Frasier suggests. "That prevents rain droplets and the [fungal] spores from attaching to the leaves," he says.
Insect pests such as onion maggots and thrips can also affect onion crops. "Onion maggots go in right at the neck and get inside the onion, causing soft, darkened areas that spoil and spread," Jones notes. Cream-colored, flealike thrips do their damage by sucking on onion leaves, thereby making your onions more susceptible to fungi. Good news: You can keep these and other pests to a minimum with smart organic practices like regularly rotating crops, and removing weeds and plant debris so insects have fewer hiding places. If your garden has been plagued by onion maggots in the past, blanket the bed with a row cover (a special fabric that lets light, water, and air reach plants but not pests) right after planting.