Onions From Seed Or Sets? - Knowledgebase Question

Boulder, CO
Question by newmanl
January 17, 2000
In Boulder Colorado should I plant a short or long day onion? Would you recommend starting from seed or sets? I've tried sets in the past without much luck.


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Answer from NGA
January 17, 2000

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In general, sets are easier to grow because they have a jumpstart on the season. Onions need full sun and a well
drained soil (not soggy) which has been amended with organic matter. Work 3-4 inches of compost into the soil until it is loose and friable. This helps root crops establish themselves. Mix a source of phosphorous (bone meal is an organic source) into the bottom of the planting hole or into the soil to aid in sizing the bulb. When plants are six inches tall and bulbs are starting to swell, add a side dressing of a complete fertilizer such as 10-10-10.

Sets should be planted outdoors in early spring about two inches apart and one to two inches deep, and then eventually thinned to be about four inches apart (for larger bulbing type onions). You can eat the thinnings as green onions! They require about an inch of water a week and should be kept well weeded. The onion bulbs grow close to the soil surface so be careful not to damage them when weeding.

If you choose to try seeds, start them indoors about 8 weeks ahead or plant them outdoors in early spring as soon as the soil can be worked. Onions are usually seeded rather heavily and then thinned.

The rule of thumb for day length is that if you are north of 35 degrees latitude (which is a line from about northern North Carolina through Oklahoma and Arizona to central California) plant a long-day onion because they can form in the 14-16 hours of daylight. However, there are now intermediate day varieties that will perform in 12-14 hours. Short-day varieties (11-12 hours) are better suited to the south. You should be able to grow a good sized onion with these tips. Good luck!

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