Since we are smack dab in the middle of onion sowing season, it’s probably important for us to talk about photoperiodism. This topic describes the physiological response that some plants have to darkness.
The overwhelming majority of plants are what are called “day neutral,” basically meaning they are unaffected by the length of daylight or darkness they receive. But, there are three other types of photoperiodism sensitivities called long day, short day and intermediate day.
Onions, specifically, are sensitive to photoperiodism and thus will have physiologic response to light and dark cycles. Fortunately, there are long day, short day and intermediate day varieties of onions out there and it’s especially important for northern gardeners to know what they are growing. You’ll find these classifications across the typical red, yellow and white onion varieties.
Ideally, your seed vendor will identify these differences in their catalogs or website. But, if you’re buying your seeds locally to you, there’s a very good chance those seeds are at least “somewhat” curated for the environment they are being sold in. But, if you’re not sure, a bit of internet sleuthing can always solve your quest for knowledge.
Now, if you’re new to growing onions and live in a northern place, long day onions are what we want. Our 24 hours of daylight in summer means there will be very little darkness and only these “long day” onion genetics will fully fruit and produce an onion at northern latitudes. You simply cannot grow a short day onion in a place with long, summer days.
Since we have a lot of “northerners” that follow us, by north we mean above about 50 degrees latitude. More southern growers, say below 37 degrees latitude, should use short day onion genetics. In between? Intermediate day onions are your best bet, but you can also likely use whichever latitude you are closest to with success.
Several years ago, we did write up a blog article on photoperiodism and the affects of the midnight sun on plants. It became so wildly popular that we turned it into a fully featured article and you’ll find it linked down in the comments! So, if you want to know a lot more about photoperiodism, check out that link!
(And yes, this is an onion from our last season’s harvest. Still holding strong!)
