Though there are hundreds of onion varieties, they all can be organized into three distinct groups. "Long-day" onions need more than 15 hours of light each day during the peak of their growing cycle to form bulbs. So they grow best in the north, where summer nights are short. "Intermediate-day" onions need about 14 hours of daylight daily to trigger bulbing--they're the right choice in the country's midsection. "Short-day" onions form bulbs with just 8 to 12 hours of daily sunlight, so gardeners in the south typically rely on them.
Now, how to start your onion crop? The easiest, and most common, way is by planting sets (little bulblets) about two weeks before the average last-frost date for your area. For best results, choose sets that are 1/2 to 3/4 inch in diameter. Also easy is planting starts from your favorite nursery or mail-order supplier directly into the garden. Just remember, plants with many large leaves form larger bulbs than those with very small leaves.
Advanced gardeners often start onions from seeds. You get more varieties to choose from, so you can seek out those that grow well in your region, keep long in storage, and have the flavor or color you desire. And when you start seeds, you are gardening quite a bit sooner than you might otherwise--indoors, at least eight weeks before your last-frost date.
To start seeds, direct-sow them into flats or pots of sterile growing medium, top with a clear plastic cover, and keep the seedbed moist. A seedling heat mat can help speed germination. Once the seeds have sprouted, place the flats in a warm, sunny window or under fluorescent lights. If the seedlings begin to look leggy--that is, tall and very thin--trim them back to about 3 inches tall. You can do this repeatedly, if necessary, until they're about pencil-thick. Cutting the seedlings back in this way ensures a well-developed root system--just what onions need to grow large leaves and then large bulbs.
All successful gardening, especially organic gardening, starts with attention to the soil. Onions grow underground, so getting the right soil conditions is especially critical. The soil's pH, fertility, and structure affect the size--and even flavor--of onions. They grow best in a slightly acidic soil (pH between 6.6 and 6.8) that's well-draining and loaded with organic matter. Soils that are very rocky, composed of dense clay, or very compacted can restrict bulb formation. To improve soil that isn't hospitable to onion bulbs, mix in compost, shredded leaves, well-rotted manure, and other organic matter down to 2 feet deep.
As the bulbs form, they need room to expand. When planting, you can place the sets or transplants an inch apart and then pull immature shoots to eat as green onions until the remaining plants are 4 inches apart. Or simply place sets or plants 4 inches apart at the outset. Either way, they go in about an inch deep. Water them in well, and then make sure that they get at least an inch of water weekly, with periods of thorough drying in between.
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.
Onions growing in soil that's rich in organic matter don't need supplemental fertilizer, but if yours need a boost of nutrients, you can sidedress with a slow-release, natural fertilizer. Frasier suggests starting with phosphorus- and potassium-rich fertilizers. "Those are important early when the onion is establishing a good root system--an onion has upwards of 200 roots--but after that, what makes a big bulb is a big top." To promote leaf growth, Frasier recommends applying a more nitrogen-heavy fertilizer every few weeks once plants are established. "Each leaf makes a ring [in the onion bulb], and the bigger the leaf, the bigger the ring."
To allow plants to transfer carbohydrates from the leaves to the onion bulbs, discontinue supplementing nitrogen once the bulbing process has begun. At that point, excess nitrogen stimulates leaf growth at the expense of bulb size, Frasier says. "That causes what we call 'thick necks'--where the onion has a thick neck that won't shrivel down properly." Newbie hint: Protect your onion seedlings from fungal diseases by presoaking the seeds, sets, or roots in compost tea before planting. Master's tip: Have cutworms or onion maggots ruined your crop in the past? Apply parasitic nematodes, available in garden centers and in catalogs, to the soil at least a week before planting.
Curing and Storing
When you notice that the tops of most of your onions have begun to fall over, stop watering and mark the calendar. In about two weeks, dig up all of the onions and spread them out in the sun for several hours to dry. Be careful not to bruise the bulbs as you are digging them, or they are likely to rot in storage. You can braid the onion tops while they are still flexible, secure with twine, and hang in a warm, dark, well-ventilated spot to finish drying. Or, you can snip off the onion tops, leaving at least an inch of neck intact, and spread them on a drying rack away from sunlight and moisture to finish the drying process. Reposition each bulb from time to time, so that each dries thoroughly. Brush off, but don't wash, any soil still clinging to the bulb.
Place cured and dried onions in mesh bags, baskets, or ventilated crates, and store them in a dry area where the temperature stays between 30- and 50-F. Check them periodically; if you see sprouts or roots forming, the temperature is too high and conditions too moist. If you keep your storage onions cool and dry, they will keep for up to a year--or just about the time you are ready to harvest the next crop. Onion bulbs form based on the amount of daylight they get. Northern gardeners will succeed with long-day varieties; southerners need to go with short-day options. Everyone else, take your pick of the intermediates.
Newbie hint: Protect your onion seedlings from fungal diseases by presoaking the seeds, sets, or roots in compost tea before planting.
Master's tip: Have cutworms or onion maggots ruined your crop in the past? Apply parasitic nematodes, available in garden centers and in catalogs, to the soil at least a week before planting.
Onion bulbs form based on the amount of daylight they get. Northern gardeners will succeed with long-day varieties; southerners need to go with short-day options. Everyone else, take your pick of the intermediates.
'Cippolini'. An Italian heirloom that produces buttonlike, yellow onions from 2 to 4 inches around.
'Copra'. A very sweet, yellow-skinned hybrid.
'Gladstone'. Oblong, 4- to 5-inch bulbs.
'Red Burgermaster'. A mildly sweet hybrid red onion.
'Southport'. Large, long-lasting heirloom that comes in both red and white varieties.
'Candy'. A "day-neutral" hybrid with very sweet, large, yellow-skinned bulbs.
'Redwing'. A deep red, very long-lasting hybrid.
'Rossa di Milano'. Slightly spicy, medium-size red onion.
'Super Star'. Another "day-neutral" that's white, sweet, and shorter-lived in storage.
'Valencia'. Large, heavy (nearly a pound!) yellow-skinned bulbs.
'Bermuda'. Sweet, white onion that keeps only for a couple of months.
'Texas Super Sweet'. Large, sweet yellow bulbs.
Onion sets are available at most garden centers. Plant and seed sources:
Dixondale Farms, Carrizo Springs, TX; 877-367-1015, dixondalefarms.com
Johnny's Selected Seeds, Winslow, ME; 877-564-6697, johnnyseeds.com
Steele Plant Company, Gleason, TN; 731-648-5476, sweetpotatoplant.com
Territorial Seed, Cottage Grove, OR; 800-626-0866, territorialseed.com