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Day length vs. temperature effects on winter greens

Winter greens

Persephone Days mapIn The Winter Harvest Handbook, Eliot Coleman posits that days shorter than ten hours (the Persephone Days) 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 length.

Tatsoi regrowthThe 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.

Persephone days weather

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 and 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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I hacker three crops that did not mature this fall. So I hope my Cole corps will start growing again in spring. I just got them all under new row covers. My salad row is doing great!

I can hardly wait for spring.

Comment by mona Tue Dec 11 08:35:32 2012
Our greenhouse grown salad greens and kale have continued to grow these last couple of weeks. In the past when the weather was colder their growth was definitely suspended with the colder temps.
Comment by Anonymous Tue Dec 11 09:57:39 2012
I've noticed that the deal with the Persephone days is that you need to let your plants mature before hand. Harvested mature plants may regrow during short daylight, slowly, but plants that are so young that they have not yet matured will not do so during the Persephone days.
Comment by Melissa West Tue Dec 11 15:14:19 2012
Melissa --- Fascinating observation! That definitely complicates any experimental testing of our hypotheses. :-) But it makes sense that an older plant with energy stored in its roots could regrow without enough sun (burning off its fat, so to speak) while a youngster with no stores couldn't.
Comment by anna Tue Dec 11 15:34:02 2012
I noticed my lettuces seem to need the heat more than the light also. Our Persephone days began on November 13th, and most greens have stopped growing except the lettuces which are under the quick hoops have definitely still been growing some in the past month. I'm hoping they will get big enough to pick here in the next week or two.
Comment by John Amrhein Thu Dec 13 07:41:40 2012