Daylight, Photoperiodism and Onions
What do these things have in common? Length of daylight hours! Or is it actually length of darkness?
Daylight Saving Time
One of the things I look forward to in March is that we spring forward on the 2nd Sunday. On that day where I live, the sun will rise at 7:27am (if you set your clock forward at 2am) and the sun will set at 7:13pm. That means we will have 11 hours and 46 minutes of daylight. Daylight Saving Time doesn’t make the day longer, it just feels longer because the sunset is pushed back an hour later. For more on the when, why and history of Daylight Saving Time visit www.webexhibits.org/daylightsaving.
The Days Are Growing Longer
However, since the winter solstice in December, the length of day in the northern hemisphere has been growing longer. The longest ‘day’ of the year occurs around the Summer Solstice on either June 20 or June 21 depending on the year because this is when we reach the longest amount of daylight hours in the northern hemisphere. For me that will be 14 hours and 50 minutes of daylight. If you live in a northern latitude you feel the difference in daylight hours more than folks who live in a southern latitude like Florida. The closer you are to the Equator, the less the earth tilts from the sun and the less difference there is between your length of daylight in the summer and the winter. This also explains why a lot of houseplants come from areas close to the equator where the change in day is not that different year round, just like the lighting in your house (unless you don’t turn your lights on when it gets dark out).
What does length of daylight have to do with plants?
Well, the length of day (or actually they now believe darkness) matters to a lot of plants. Plants experience photoperiodism. Ever wonder how many of the spring, summer and fall flowers know when to bloom? Length of day (or darkness) is a trigger for them. Plants are considered either short day, long day, or day neutral (they just don’t care). For a great explanation of plants and photoperiodism see What Are Short Day and Long Day Plants by Oregon State Extension.
So what do onions care?
Well, they are one of those vegetables that rely on day length hours to produce their bulbs. Onions basically require between 12 to 16 hours of sun. Northern latitudes grow onions that are known to be ‘long day’ varieties because they will reach more than 15 hours of sunlight in summer. In southern latitudes, the length of day does not change as much so they grow ‘short day’ varieties. What if you live somewhere in the middle, like Virginia? Look for ‘intermediate day’ onions that can do well as a cross-over between the north and the south. It is important to know what type of onion you are buying to grow in your latitude.
- Onion Plants: Short Day, Long Day, Intermediate Day by Dixondale Farms
- Growing Onion, Leek, Shallot, & Garlic by Clemson University Extension (SC)
- Onion – Vegetable Directory – Watch Your Garden Grow by University of Illinois Extension
Learn how to grow your spring events on Plantsmap.com: How to Create Event Pages