Onion Decision Tree by Rebecca Anderson

Photo courtesy PlantTalk Colorado

Onions are a good crop to plant when the weather is still cool and the garden is calling for some sort of action. Take a tour of any garden center or seed catalog and you will find a plethora of onion selections. How does a gardener decide which is best for a Front Range garden?

 Onions are a biennial plant, meaning seeds planted this year will grow into a bulb this year and if left to overwinter will produce a flower and seeds next year. Day length triggers the plants to start forming bulbs. At our latitude, the "long day" varieties will form the largest bulbs because they need 14 to 16 hours of daylight to trigger bulb development. Since the plant has had plenty of time to grow a nice stand of leaves to produce energy for the bulb it should grow a nice full sized onion. The "short day" varieties start producing bulbs when day length reaches 10 hours. At our latitude these plants wouldn't have enough time to grow enough leaves to provide the bulb with the energy to grow to its full potential. Short day onions will survive in Colorado, but will only make small bulbs. They are best suited for more southern regions. There are also "day neutral" varieties which should do well anywhere. 

Once you've decided to choose a long day or day neutral onion variety, there is the decision about planting seeds, seedling plants or sets. Seeds have the advantage of being the most economical. For a few dollars you can purchase a packet of several hundred seeds. There is also a more extensive selection of varieties available in seeds, especially among heirlooms. Onion seeds do not stay viable with long storage, so for best results purchase a new packet every year. Seeds can be planted directly in the garden from mid-March through April. Seeds can also be started indoors 10 to 14 weeks prior to the last frost then transplanted.  Onion seedlings can tolerate light frost, but they do not survive frozen soil. If the seedlings were planted directly in the ground, they will need to be thinned as they grow until the plants are 3 to 4 inches apart. The thinned seedlings can be used as green onions or scallions, depending on their size.

Some garden centers and catalogs have onion seedling plants available for sale. These are from seeds that were started in a greenhouse in the current year to give them a jump start. These have the advantage of having a little more growing time over seeds planted directly in the ground, which is important since leaf growth determine final bulb size. Also, a bundle usually contains 30 or 40 plants instead of hundreds if you don't want to deal with thinning after planting.  They are more expensive than seeds, but may be worth the convenience. They are also only available in the varieties that the greenhouses choose to grow.

The last option for planting are onion sets. These are little onion bulbs that started as seeds last year and are currently dormant. These little bulbs are ready to grow as soon as they go in the ground, but since this is their second season, they will be growing toward producing a flower stalk and seed. Once the flower stalk has been produced, the edible qualities and storage ability of the onion diminish, so it is important to harvest these onions smaller and earlier. Onion sets are a good option for growing scallions.

If you'd like more information about growing onions, check out CSU's PlantTalk article Growing Onions from Seed. Then add an onion variety or two to your garden. Your menu planning will benefit from the flavor!