Onions form bulbs in response to daylength. When the number of daylight hours reaches a certain level, onion plants start forming bulbs. Long-day onions need about 14 to 15 hours of daylight to bulb. Short-day onions need 10 hours of daylight. Day-neutral onions form bulbs regardless of daylight hours and produce well in almost any region. As soon as daylength hits the 10-hour mark, a short-day onion starts forming a bulb. If the top of the plant hasn’t had enough time to grow big and lush, the resulting bulb will be small. Conversely, if you live where daylength never hits 14 hours, long-day onions will never form a bulb. All you’ll get are green leaves or possibly scallions. By choosing the right type of onion for your region, you’ll get healthy green stems that are large enough to fuel forming fat and tasty bulbs.
Which onion type you should plant depends on where you live. The general rule of thumb is:
· In the North (the area north of a line drawn from San Francisco to Washington, D.C.), summer days are long. This region encompasses zone 6 and colder. If you garden in this area, grow long-day onions, however, there are some exceptions. See below.
· In the South, summer days don’t vary as much in length from winter ones. This region includes zone 7 and warmer. If you garden in this area, grow short-day onions.
· Day-neutral (sometimes called intermediate) onions form bulbs in any zone, but are especially suited for gardeners in zones 5 and 6.
· In our mountain gardens, even if we are in the correct gardening zone, since our growing season is so short, the best onions to grow are day-neutral.
Types of Onions
Short-day onions (not suitable for our mountain region because of our short growing season and they cannot overwinter)
· Form bulbs with 10 to 12 hours of daylight
· Need mild winter climate (zone 7 or warmer)
· Planted in fall, mature in late spring
· Can be grown in the North, but bulbs don’t get as large
· Matures in 110 days in the South with fall planting, 75 days in the North with spring planting
Day-neutral onions (best option for our mountain gardens)
· Form bulbs with 12 to 14 hours of daylight
· Planted in fall in mild winter climates and in early spring in northern regions
· Usually sweet
· Matures when the tops begin to yellow or fall over.
Long-day onions (not suitable for our mountain region due to short growing season)
· Form bulbs with 14 to 16 hours of daylight
· Typically grown in northern regions (zone 6 and colder)
· Planted in late winter/early spring
· Matures in 90 to 110 days
· Stores well
What happens if you get your day lengths mixed up? If a northern gardener planted a short day variety, the onion plants would be exposed to enough daylight hours to initiate bulb formation so early in the season that big bulbs would never have the chance to form. And if a southern gardener planted a long-day variety, the onions would never be exposed to sufficiently long days to cause bulbs to form.
Modern plant breeding has helped out with what are known as intermediate day, or day neutral, onions. These varieties aren't as sensitive to day length and bulb up well in response to 12 to 14 hour days. They grow well across a broad range of the country. Day neutral onions are usually planted in the spring.
Last year I purchased onion sets not know if they were short, neutral or long day. I called the company and was told that they were long day. I went ahead and planted them. I pulled the early ones and enjoyed them as scallions. The ones that I harvested in mid-September did actually bulb up, but not with huge bulbs. More like “knob onions”, 2-3 inch bulbs.
This is what I got with my long day onions last year.
This year I purchased onion plants that I knew were day neutral. They arrived in the mail a few days before the May 21st 2 foot snow storm we got here in Gilpin County. I managed to keep them alive, but had to hold on to them for about 2 weeks before I could get into my garden to plant them. They are growing, but very slowly. After about 2 months they are only the size of scallions. I hope we can get another 30 days in our growing season to see what they do. Of course we are now also experiencing shorter days, just under 13.5 hours. Ah, the challenges of growing in the mountains!
This is what the day neutral onions are supposed to look like. We shall see.
My next experiment next year will be to grow onion sets alongside onion plants and see which do better. Stay tuned.