LONGMONT – As you’re planting your spring kitchen garden, leave room for one of cooking’s basics that’s delicious enough to bring tears to your eyes. Growing onions in Colorado is easy, and if you want to add this allium to your vegetable plot, the time to get planting is now.
Seeds, seedlings and sets are available to the home gardener, so here’s a quick primer on the difference: seeds are directly sown into the garden from mid-March through the end of April and are very successful in Colorado. Sow them shallowly, about three-eighths of an inch deep.
Seedlings are started plants, grown this season and not yet mature enough to begin forming bulbs. Locally, onion seedlings are readily available in small clumps that you tease apart upon planting. Plant them one inch deep and slightly later than seeds, so any time in April will do.
Sets are onion bulbs that are about one inch in diameter and planted as you would a tulip or daffodil. Both seeds and seedlings can be used for green onions, often called scallion-stage, or for sizing up into storage bulbs. But if you use sets, keep in mind that they’re best if you want bulb onions.
Look for long-day varieties that form bulbs once we have 14 or more hours of sunlight daily; in our area, onions will begin producing pungent, sweet bulbs beginning in July.
Onions prefer fertile, well-draining soil liberally amended with organic material. Spread compost one to one-and-a-half inches deep across the bed, then till it in, working it eight inches into the soil.
If you’ve sown seeds, thin them once the seedlings have five leaves. For best bulb production, thin to three inches apart. Pulled seedlings are delicious as fresh or grilled; eat them as you would a scallion. As your onions grow, each leaf they put on represents a ring in the bulb itself, and size matters: larger leaves mean bigger rings.
Water them frequently, never allowing their shallow roots to dry, which can cause bulbs to be stunted and tough. Onions are nitrogen-greedy during the first part of the season, so fertilize until mid-July, then put them on a diet – onions don’t need much nitrogen past that point.
As onions bulb, they often push themselves out of the soil; this is normal and your plant will be fine. Avoid giving in to the urge to “hill-up” your onions; hiding the beautiful bulb will work against the plant’s desire to plump it and you won’t get good production. As the plant prepares to bulb (the neck will feel a little soft), if your soil is hard from the summer sun, gently loosen it to let the plant expand.
Keep weeds to a minimum, since they rapidly crowd out the less-vigorous onion. Careful, shallow hoeing around the plants is a must to avoid damaging the developing bulb, so if you’re a bit of a brute with the hoe, mulch is a good option for your onions.
Thrips are common insects in our area, rasping off the surface of the onion leaf, leaving a tell-tale trail of silver. They hide out on weeds, so weed suppression is key to control.
You can tell if your onion is mature by its floppy tops, which start laying over in mid-August. Hold off on watering at this point and when most of the tops have fallen, gently lift the onions from the ground. Let them rest on top of the ground for up to two days, trying to keep them oriented the same way they grew in the ground to prevent sunburn.
After two or three days, take the onions into a warm, dry location to cure for several weeks until the necks are completely dry. Trim the tops, then store the bulbs in a mesh bag out of sunlight in a cool location.
By Carol O’Meara, Colorado State University Extension. Colorado State University Extension together with Boulder County Parks and Open Space, provides unbiased, research-based information about consumer and family issues, horticulture, natural resources, agriculture and 4-H youth development. For more information contact Colorado State University Extension at the Boulder County Fairgrounds, 9595 Nelson Rd., Box B, Longmont, 303.678.6238, e-mail email@example.com or visit ext.colostate.edu/boulder.