More on onions: best bulb tactics

Last week’s column focused on choosing the best onion varieties for Maine gardens, varieties that grow vegetatively until the long days of June initiate bulb formation. Now the rest of the story: how to plant, grow, harvest and store the bulbs.

I stand by my recommendation to start with commercially grown, bare-root transplants for those varieties where this is an option. Grown in Georgia or Texas fields through the winter, the plants are dug, trimmed and shipped for planting in northern gardens when spring arrives. The vigorous outdoor-adapted transplants will have a jump-start on the season, producing the largest possible bulbs.

In its 2008 catalog, Johnny’s Selected Seeds lists field-grown transplants for several different onion varieties suitable for Maine gardens, including Copra, the best variety for extremely long-term storage. You may also find field-grown onion transplants for sale at your local garden center.

You can produce your own transplants from seed started indoors during the winter, an option for varieties not available as commercial transplants and certainly a less expensive option compared to purchasing commercial transplants. For the storage-type onions, seed should be started now, certainly no later than early March. If you use artificial lighting, keep the day length under 12 hours so as not to induce premature bulb formation.

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.

Direct-seeding as soon as the soil can be worked in early spring is the preferred method for planting bunching onions but not for producing large bulb onions. In our short-season gardens, direct-seeded onion plants will not grow large enough before the onset of bulb formation to produce a large bulb.

For producing the largest bulbs, weed control and even watering are the 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 award-winning bulbs. Mulching also helps, reducing competition from weeds while maintaining uniform soil moisture levels. Weeds that manage to grow through the mulch should be pulled by hand; mechanical weeding with a hoe or cultivator tends to nick the young bulbs.

Onions should be harvested by pulling or digging as soon as the tops die back and before the plants start sending up flowering stalks. The harvested bulbs need to 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 air can circulate around the bulbs. Inspect your stored onions frequently, immediately throwing out any bulbs that become moldy or start to rot.

A last word on onion varieties: If you are truly interested in the blue ribbon, try growing Ailsa Craig Exhibition, a huge, mild, Spanish onion type grown by British gardeners for show-size onions. Johnny’s sells field-grown transplants for this Maine-adapted variety.

I’ll see you with your onions at the Blue Hill Fair!

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@ptc-me.net. Include name, address and telephone number.