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Onions and day length

Have you ever looked through a seed catalog, deciding which onion seed or plants to buy, and been perplexed by the words “long-day” or “short-day” attached to each variety? Or sometimes “intermediate-day” or “day-neutral”? You might just give up in confusion and order by a familiar name or tasty-sounding description, but really, this is something you need to pay attention to, because onions are… photothermoperiodic!

Yeah, right, you say. Read on to find out what this means and why it’s important, though not as important here in Maryland as it would be if we lived in Mississippi or Minnesota.

It’s spring – it’s onion-planting time, in fact – and the days are getting longer. As gardeners, we sometimes need to pay attention to how long they’re getting. Onions are among the plants that are sensitive to how much light is available in a day. When the day reaches a certain length, they will stop growing leaves and start thickening their below-ground structures: the bulbs that we want to harvest and eat. That day-length signal varies between onion types. Remember that in the summer, the hours of daylight are longer the further north you go, because of the tilt of the earth’s axis.

Short-day onions are grown in the South, generally below latitude 36° N. They will start forming bulbs when the day length is between 10 and 12 hours.

Intermediate-day or day-neutral onions are sensitive to day lengths of somewhere between 12 and 15 hours.

Long-day onions need a day length of 14 to 16 hours to start forming bulbs and do best north of 36° N.

This suggests some basic questions each of us needs to answer:

Latitude is pretty easy; if you don’t already know it, just do a search on “latitude” and your town name. It’s also generally included on weather sites. I live in Germantown; my latitude is 39.16° N.

The weather site I use most often, Weather Underground, also provides day length data, but only for the day you search (12 hours and 51 minutes for April 6, if you were wondering). It’s better for our purposes to have a chart for the full year; this U.S. Navy site will help you with that. It doesn’t have data for every town; I used Rockville since that’s fairly close by, and produced a chart that includes every day of the year.

So, in Rockville MD, the longest that days ever get is 14 hours and 55 minutes, in the days around the equinox in June. This implies to me that we are able to grow both long-day and intermediate-day onions here, although any varieties of long-day onions that prefer 16-hour days are not going to bulb up very well. Can we grow short-day onions? Well, remember that these varieties start forming bulbs when day length is around 10-12 hours. According to my chart, the days are 10 hours long in Rockville starting around January 25; they are 12 hours long starting March 17. Even if I planted my onions in the fall (as is usually done with short-day onions in warmer climates), I would not want them trying to form bulbs in January-February-March. It’s just too cold.

And there’s the final factor in growing onions: remember they are photothermoperiodic. The “photo” part is the day length factor, but they are also sensitive to soil temperature. This is not only a factor when trying to grow short-day onions in northern winters, where the cold can either stunt plant growth or trigger bolting without bulb formation. It’s also important when soil temperatures rise in the summer. Warm soil temperatures (over 75° F) can cause onions to bolt (form flower stems) if bulbing hasn’t started yet. So long-day varieties of onions that aren’t going to bulb up until we get our longest days in June might not work for us here in Maryland, if soil temperatures shoot up fast. But it’s worth a try. Pay attention to another factor, the days-to-maturity number, and try to choose onions that mature on the faster side (say, under 100 days).

Confused yet? Here are the conclusions I came to, sifting through all this information:

If this seems restrictive, go back to that day-length chart and try plugging in some cities in the far south or north of the U.S., and consider the smaller range of onion possibilities they might face. We have a good deal here, really. And thinking locally brings me to another point, which is highly relevant considering that I’m writing this blog post much too late, and if you haven’t planted onions yet you only have one choice left:

Photoperiodism (day-length sensitivity) isn’t just about onions. If you want to read more about how plants are affected by day length, there are good articles at Oregon State Extension and High Mowing Seeds.


By Erica Smith, Montgomery County Master Gardener

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