Sweet onions are a must-grow crop in our vegetable garden, an essential ingredient in Apple-ring Chicken, Lynne’s all-time favorite dinner, and in the Gingered Squash and Apple Soup that I cook up in double batches on winter weekends, making several hearty meals for the coming week. I like reaching into winter storage for onions that only get sweeter with time.
We grow Copra onions, medium-sized (3-4 inches around), dark yellow onions, pungent but sweet and excellent for cooking. It is the highest in sugar content of all onion varieties, becoming milder with time in storage, and will keep for a year.
You can play around with other varieties if you like, but make sure they are long-day onions such as Copra, varieties that can be planted in April and grow leaves until June when 14-hour days trigger the transition from leaves to bulbs. Vidalia onions and other short-day varieties, planted in Maine around the first of April, would try to bulb-up as soon as day length reached 12 hours, long before leaf growth was sufficient to produce a large bulb.
We transplant field-grown starter plants of Copra rather than trying to grow our own from seed. The transplants, grown in the South (but ordered from Johnny’s Selected Seeds here in Maine), will have a several-week jump on our season and on weeds, advantages that hopefully will promote production of large bulbs.
For immediate use, not for storage, try Walla Walla, a Washington State variety described by Johnny’s as a “juicy, sweet, regional favorite.” In Washington the seeds are sown in August and the plants overwinter in the ground; bulbs form the next spring and are harvested in early summer. In Maine, we have to settle for transplants that don’t grow as big a bulb, but are still sweeter than any other spring-planted variety.
Plant onion transplants as early as possible, four to six weeks before the date of the last expected spring frost. Onions can take cool soil and light frosts and require protection only from hard freezes. If you see a hard freeze coming after planting, water the plants thoroughly and cover them with a straw mulch. In exceptionally frigid weather, cover the mulch with burlap as well.
You should certainly be ready to plant the transplants as soon as they arrive, but if you have to hold them for a day or two, take them out of the box and keep them in a well-ventilated, cool location until you can plant them. Keep the plants dry; do not put them in soil or water.
For producing the largest bulbs, weed control and regular watering are the most critical factors. Onions survive periods of drought by using water stored in the developing bulbs, thereby reducing bulb growth, and weeds are serious competitors for available water.
Irrigation during dry periods is essential for production of large bulbs. Use the knuckle rule to determine when to water during the season. If you can feel moisture when you stick your finger in the ground up to your first knuckle, then the onions are wet enough. Use drip irrigation, if possible, rather than an overhead sprinkler system, which may promote the spread of disease. We often let the hose trickle over the soil surface, moving it as we work in the garden, a make-shift form of drip irrigation.
Mulching helps reduce competition from weeds while maintaining uniform soil moisture levels. Weeds that manage to grow through the mulch should be pulled by hand; a hoe will nick the young bulbs.
This business of keeping onions weed-free is a challenge that I readily accept — it is an excuse to be in the garden. We select our onion bed early in the season, a bed in full sun with excellent drainage, and keep it free of weeds, pulling them by hand or hoeing them out as they appear. Once planted, hand-weeding the onion bed is the first and last chore of every gardening day, never letting a weed grow beyond the seedling stage.
Onions should be pulled from the ground as soon as the tops die back, well before the plants start sending up flowering stalks. The harvested bulbs should be cured by spreading them out in a warm, dry, airy location, out of direct sun, until the tops and outer skin are completely dry and brittle.
Once cured, the bulbs can be stored in a well-ventilated, dry, cool (but not cold) location. Store the onions in mesh bags by variety so that air can circulate around the bulbs. Or braid the dry tops together, always keeping a few bunches of braided onions hanging in the kitchen, handy for the winter soup pot.
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