Day length vs. temperature effects on winter greens
Winter Harvest Handbook, Eliot Coleman posits that
days shorter than ten hours (the Persephone
put nearly all garden plants into a state of suspended animation.
If we want to harvest greens all winter, we need to get them mature
before the Persephone Days begin and then pick a bed at a time until
the greenery starts growing again in the late winter.
The trouble with this
hypothesis is that it's awfully tough (on the
farm level) to disentangle the effects of day length from the effects
of cold. Do our plants really stop growing because of the short
period of sunlight, or are they just hibernating until warm weather
comes back? Without meaning to, I did a test with my tatsoi
and tokyo bekana this fall, and it seems like these two greens
varieties, at least, are more interested in temperature than in day
The Persephone Days for our
farm began on November 22, which was right
on Thanksgiving this year. I wanted to serve leafy greens for six
during our Thanksgiving dinner, so I picked the beds hard that
morning. In fact, I figured neither the tatsoi nor the tokyo
bekana would survive the winter, so I might as well cut the tender
hearts right out of them. (I usually try to let my greens
cut-and-come-again by harvesting outside or middle leaves, allowing the
tender new growth to stay put.) But when I came back around with
my scissors last week, I noticed that the harshly cropped tatsoi and
tokyo bekana were both putting out new growth.
This has been an unusual
early winter because, even though day length is around 9.75 hours at
the moment, we've been
enjoying a warm spell with nights above freezing and days nearly
balmy. And our greens are taking advantage of the weather to grow
Of course, I don't know
for sure whether these greens might not go into a
state of suspended animation at 9.5 hours or 9 hours despite the
temperature. Some days, I wish I had a research lab and crew of
grad students at my disposal to get real data, but for now, I'm busy
eating the evidence.
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