day length: how to calculate it and its effects on plant growth

There have been some great conversations that touch on daylight and how it effects plant growth.

Extendign the season
as soon as the soil can be worked
And I'm sure some others.

The more I observe my garden, the more I see how much daylight effects my plants.  Both too much and too little light have a huge influence on how things grow and reproduce.

Let's have a great big thread where we investigate this idea.

Some of my questions:
1. how do we calculate day length?
   i. sunset/rise are a starting point, but there is so much more to it than that.  Let's investigate.
  ii. how about weather patterns?
  iii. are chickens a good measure of daylight?
  iv. trees and land elevation?  
  v. twilight and moonlight hours?
2. what plants are sensitive to too little light?  What plants don't mind?  How does it change their growth patterns?  How do they cope?
3. what plants are sensitive to too much light?  What effect does it have on them (warning, I want to talk a lot about cotton)
4. if permaculture encourages creating microclimates to create temperature pockets, how do we feel about doing this with light?
5.  North and South - distance from the equator and growing plants out of their natural latitude range and how can we use plant breeding to our advantage.

I'm sure there are lots of other fun things to chat about.  So let's get chatting.    

from here

Eric Bee wrote:...

Lynn Byczynski is certainly well respected in this field, but the article strikes me as a gross generalization. Perhaps there is some confusion about photoperiodism and it's impact on vegetative growth vs. flowering cycles. Clearly, the statement "Most plants do not grow when day length is less than 10 hours. " is neither precise or accurate. She discusses flowering and growth cycles as if they were both dictated by photoperiod when at the least that is very species specific.

I know this statement is not true because I grow in a greenhouse all winter long without supplemental light, as do many others. Even right now our day length is about 9:55 but because we are having a very warm November there is a ton of growth outside -- everything from grasses to brassicas to ornamentals. Obviously, growth increases as day length increases and vice-versa, but that is a question of how much light the plant receives and is entirely different from saying growth is dictated by photoperiod. If it were, then there would be a range where that growth would slow or stop and yet we cannot say that exactly.

So my take is this:
1. Vegetative growth is dictated by environmental factors such as temperature and by other periods in the life cycle of the plant. For example, many plants will switch from vegetative growth to flowering when certain conditions are met, and not have vegetative growth after that.
2. Flowering in all but day neutral plants is dictated by photoperiod.
3. Vegetative growth is not universally governed by photoperiod directly, although of course the amount of growth and degree to which it is possible is dependent on the amount of light the plant receives.
4. Dormancy is induced by temperature, but in some plants coming out of dormancy is dictated by photoperiod, which makes a lot of sense because you don't want a switch in cycle unless there is enough light.
5. Since temperature, especially in northern climates is most frequently dropping with shortening of photoperiod, it may seem that photoperiod dictates growth, when in fact it's temperature and/or the natural lifecycle and internal clocks of the plants.

In researching this to make sure I wasn't blowing smoke, I've seen many references in online articles to photoperiod controlling growth cycles but in the scientific literature distinctions are made, specifically that photoperiod will induce flowering which then stops vegetative growth. That sort of thing.

I have this book "How Plants Work" by Linda Chalker-Scott, she says "...true dormancy is controlled by the internal clock, rather than by some environmental factor like sunlight or water." One thing she notes in this book is that plants with small seeds (and thus less energy reserve) tend to have photodormancy, but that means they wake up from dormancy with a certain amount of light, not the other way around.

She also has this to say (talking about seeds though):
"For seasonally dependent photodormant plants, the presence or absence of useful sunlight is not the only trigger controlling germination. They also need a clue to what time of year it is. As gardeners know, rainfall and temperature can vary wildly throughout the year and even from day to day. Plants need a more reliable system of figuring out when it’s time to start growing."

Thanks for starting the new thread.

Day length calculators are quite common, with some geared toward the solar industry and thus taking into account terrain. If indeed I am at least a little wrong and vegetative growth dependent on photoperiod, does terrain and thus shading matter? Is it a matter of light intensity and if so, what is the range?


Simple tables: http://aa.usno.navy.mil/data/docs/Dur_OneYear.php

Calculates based on terrain as well: http://www.solartopo.com/daylength.htm

Very detailed: https://www.timeanddate.com/sun/

great idea for a thread topic.
Day length is a fascinating topic.  First thing that comes to my mind re day length and plants is onions.

Here are excerpts from Dixondale Farms website.

"The short-day varieties start the bulbing process when daylight length reaches 10-12 hours. They take approximately 110 days to mature in the south and just 75 days in the north. The earlier you plant them, the larger they get, but they won't get very big in the northern states.

Intermediate-day varieties are the most widely adaptable since they require 12-14 hours of sunlight before beginning the bulbing process. Unless you live in far south Florida or south Texas you should have enough daytime hours to make nice-sized bulbs.

Long-day sweet and storage varieties do well in the northern states that have between 14-16 hours of daylight length."

I suggest that anyone seriously interested in just how powerful day length, and thus latitude, have been on humans, might want to take on a great wintertime read of Guns, Germs, and Steel by Jared Diamond (biologist).  It's readily available used!

And, for me, was a fascinating read.

Ah! To be around 35* to 45* N.

Elev 36 ft 47.22 °N, 122.72 °W according to weather underground. right now 8 hours and 14 min. sunrise to sunset subtract 1 hour for tree shade east and west for direct light.
Weather has been unusually warm so the photoperiod and temperature has tricked my berry vines into blooming. If we don't get a freeze I may get fall Boysenberries very unusual raspberries normal.
In the greenhouse the tomato leaves seem to suffer from the lack of light I tilted the daylight bulbs to shine on the north side and put ruby red heat lights on at night and the tomatoes and peppers are ripening nicely.
The New Zealand spinach and nightshade are very content they don't care whether they are in the northern or southern hemisphere.

When calculating Daylength it seems to be common to go from sunrise to sunset.  I'm assuming this is visible sunrise/set as the sun is actually still visible about 8 degrees below the horizon (I think that's at the equator).  

Using this as a guide, we can see how much difference elevation has on daylength.  Not just the people who live up high get longer days. This also makes for microclimates in daylength.

Here's an example.  On our property, we have two glacial ridges that run north to south (almost due north and south at our place).  We also have two valleys.  To the east we have the forest, the house sits on top of the smaller, western ridge, the summer garden in the valley between the two ridges and the winter garden on the west side of the small ridge.  So going from west to east - valley (winter garden), ridge(house), valley(summer garden), bigger ridge(forest).  Eric gave us some links above, so I was playing around with them.  It shows me that this time of year, I have about 20 minutes less light in the summer garden than I do at the house.  At one point we have 7h35m of day length, at another, we have 8h21 minutes.  That's almost an hour difference.  

Being this far north, our sun comes up at a gentle angle this time of year.  In the summer, it comes up almost verticle and takes less time to get up in the morning.  You would think this would make the difference in day length in the different parts of the property.  And it does.  In the summer there is an even greater difference in time from the house to the summer garden.  The ridge to the north, being forest, blocks the sun much longer in the summer, than when it takes the gradual southern path in the winter which brings it around our forest and shines through the neighbour's open areas.  So in the summer, the summer garden - so named because it's great at growing summer crops without irrigation - gets a sunrise at least 45 minutes, often an hour later than the house.  It gets sunset at least 30 minutes earlier than the house (again due to trees and other obstacles - so it has about 1h15 min less sunlight in the day than the kitchen garden beside the house.  Even though, the daylight calculator say sit's only 20 minutes less daylength.  It also has a much higher amount of dew - enough dew to keep things growing during our 6-month summer drought.  lots of things create dew, but I wonder if having a later start to the day than the surrounding area also increase the amount of dew that forms on the plants.

Edit to add:  Playing around a bit more with the daylight calculator, the neighbours across the road only get 6h10 min of daylight right now.  They are in a deep gully, so they have their sunrise blocked by the two ridges that run through our property and the trees on top of them.  But to the West of them, there are open fields with a gradual slope up to a hill.  I knew the ridges blocked the light, but I didn't realize the hills that far away also decreased day length.

Here's something about the climate I am in. I live in the Netherlands, western Europe. The climate here is comparable to that of London, UK. During the summer the sun is up for many hours, and during the winter less than ten hours. But those hours from sunrise to sunset are not at all 'sunshine hours'. We have so many clouded days! For example today, all day was so clouded and rainy, artificial light was needed even at noon (except for a spot next to the window).
There are a lot of greenhouses in this country, with heating and artificial lights, in which tomatoes, cucumbers and flowers grow.
I try to use my living room, with a large window at the south-eastern side, as my 'greenhouse', but I see I can't grow plants from seeds there without added artificial light. (I use a 'daylight spectrum' lamp) The plants would be very 'leggy' and weak, aphids and other pests and diseases would attack them.

Here I am, the moment I've been waiting for all morning.  It is officially Sunrise.  Right now, this moment as I start to write this post.

I look outside and it's gray and dark.  I don't need a flashlight, but then again, I seldom do even at night this time of year.  

However, if I open the door to the henhouse, the chooks won't even bother coming down off the roost.  It's that dark.  It will be at least half an hour before the chickens think of getting up.  And we're on the top of the ridge, the part of the yard that gets the most daylight hours.

Inside the house, it's barely light enough to get from one room to another without banging into anything.  But to make coffee or find my socks, I'll need to turn on a light.

Two days ago, at this time, I was outside, the chickens were up and happy, running around, no electric lights needed.  Two days ago it was fairly bright weather. Two days ago, the chickens went to bed at 5:30pm, yesterday they were in bed by 4:40.  Chickens seem to be a very good indicator of day length.


Inge Leonora-den Ouden beat me to it.  It's the weather.  Most of the winter, our weather is overcast and gray.  Thick clouds or fog blanket our coast from about the end of October to April.  Even though the sunrise/sunset gives us about 8 hours of daylight, we get a fair bit less.  Like The Netherlands, we are approaching the time when electric lights are needed even in the middle of the day if we want to do something as frivolous as cooking or cleaning.  

Yet, my plants still grow.  They grow much larger leaves this time of year.  I wonder if it's a coping mechanism.  

Joe Ruben wrote:I suggest that anyone seriously interested in just how powerful day length, and thus latitude, have been on humans, might want to take on a great wintertime read of Guns, Germs, and Steel by Jared Diamond (biologist).  It's readily available used!

And, for me, was a fascinating read.

Ah! To be around 35* to 45* N.

If you're in Southern Colorado, you are between 35* and 45*!  For example, Alamosa is 37*.  

So we have weather, height, latitude (as in angle the sun rises at different times of year - as well as actual daylength), and obstacles all affecting daylength microclimates.  

It would be interesting to see how this applies to an urban setting.

We haven't talked about twilight.  Twilight is much longer the further north or south we are.  Is twilight strong enough for some plants?  My first thought is no, they need direct sunlight... but then again, we do have plants that grow in strong shade.  Maybe some plants can use twilight?  


This shows us that we can create daylength microclimates in much the same way that we create temperature microclimates with wind breaks, swales, compost piles, &c.  This would allow us to grow light-sensitive plants beyond their regular range - maybe something to take into account when, let's say, growing cotton north of the 46th, or linen in equatorial zones.  

R Ranson wrote:So we have weather, height, latitude (as in angle the sun rises at different times of year - as well as actual daylength), and obstacles all affecting daylength microclimates.  

It would be interesting to see how this applies to an urban setting.

We haven't talked about twilight.  Twilight is much longer the further north or south we are.  Is twilight strong enough for some plants?  My first thought is no, they need direct sunlight... but then again, we do have plants that grow in strong shade.  Maybe some plants can use twilight?  


This shows us that we can create daylength microclimates in much the same way that we create temperature microclimates with wind breaks, swales, compost piles, &c.  This would allow us to grow light-sensitive plants beyond their regular range - maybe something to take into account when, let's say, growing cotton north of the 46th, or linen in equatorial zones.  

Creating microclimates is part of permaculture. I am trying to make some warmer parts in my garden, with more sunshine on it; I do it with Hugelkultur, creating Hugels (small hills) facing South (the North side is a vertical pile of concrete tiles). This year was the first year of my first Hugel, and it worked well. So I am creating a second Hugel now, parallel to the first one. To be able to plant young plants (transplants) on the Hugel early in spring (earlier than is considered normal here) I want to grow them indoors from seed, in the window-sil of the mentioned window. I hope coming spring will be somewhat sunny, and if not ... I'll use my daylight lamps

Yes, of course I am.  37n37

When it comes to plants and daylight, I've observed two main issues that keep some plants from thriving:  Too little light and too much light.

Too Little Light
It's been mentioned several places that most plants have trouble growing with less than 10 hours daylight.  They don't suggest that includes twilight, so I'm assuming they mean sunrise to sunset.  According to that very nice daylight calculator that Eric linked to above, I have less than 10 hours of daylight between the first week of October and the second week of March.  

Plants need sunlight to do their thing.  Oversimplified, they need light to grow.  Without light, they can't absorb carbon from the air.  Here's a quote from Johnny's selected seeds:

Most plants do not grow when day length is less than 10 hours. Even if the temperature is kept within the optimum range — for example, in a climate-controlled greenhouse — most plants will just sit dormant until the magic 10 hours of light per day arrives.

Notice it says "Most".  I'm curious what plants do grow in the winter.  Given that it's seldom very cold here (some years we do get snow, but that seem to be less and less often), the temperature is fine for many cool weather plants to keep on growing.  From my own observations, these are the plants that keep growing in the winter and what changes as the days get shorter.

Kale - leaves get bigger
Cabbages
Chard - slow down in Dec and Jan, only to take off in Feb.
Fava beans - focus on root growth in the winter, but still produce leaves.  I have one plant this year that is producing pods and still flowering.  I don't think it knows it's winter.
Barley, oats, & wheat
Chickpeas
Lentils
flax
snow peas
soup peas
weeds - they grow like crazy with no regard for sunlight.  Especially the heart-seed weeds, I think it's shepherd's purse
miners lettuce
lettuce (with the red tint, not so much the green stuff)
chickweed
brussels sprouts
dock
onions


What other plants have you observed growing in the winter?  How much of "plants don't grow in the winter" is based on temperature more than daylength?  Did you notice, most of these plants are common staple crops in traditional Northern European cultures?  How do we use traditional breeding techniques to create landraces of these plants that thrive even more in low light?  Would any of these do better in our daylength microclimates?  

This is just the growth I can see.  What's happening under the soil?  Does overwintering pulses and grains really make for stronger plants?  I think so, but I'm curious what other's experiences are.  

to R Ranson

Thanks for that post!  

It helps me plan.

I'd give you an apple if I could.

This winter has been great for observing daylength rather than temperature.  It's been crazy warm so far, with our low temps far warmer than our usual highs.  We've had a frost, a very light frost, but no freeze yet.

Things that are growing in my garden this winter that shouldn't be:
roses - flowering and setting seed, but no big shoots of growth.
sunflowers - sprouting in the soil
amaranth - sprouting in the soil, and seedlings growing very slowly - only the red ones though, no green.
a maxima squash is re-growing from the roots.  It set fruit, but because it's so wet, and the chickens keep getting out, the fruit doesn't mature to full size before it starts to rot.
Beans - warm weather poll beans - still producing tiny leaves, looking the worse for wear, but some flowers.
Beans bush - got dug up last week because I wanted more space for garlic.  Still producing leaves.  No flowers or fruit
Beans runner - pods and leaves still growing slowly, looking a bit ragged.
buckwheat - flowering, setting seed, germinating, growing - but not thriving.

These are all plants that grow only in the summer here.  That's the official position.  Most of these don't go in the ground until May or after.  These are all plants I was lead to believe are very daylength sensitive - especially to short days.  Yet here they are producing growth in what is roughly an 8 hour day.  
So why is this?

I have one plant this year that is producing pods and still flowering.  I don't think it knows it's winter.  

I think I mentioned that if wee don't have a freeze I am going to have fall Boysenberries. Normally they have there first blooms the first week of March. same day length and temperature we are having now. So when they went dormant because of the heat and drought and then rain, temperature and day length were the same as spring they formed buds and bloomed.  Normally the decreasing temperature wood have stimulated shedding the remaining leaves and making the buds dormant.  This Year most of the rain storms have come at nite fallowed by clear days. We have an alltime record of consecutive days in November with temperatures above 60F. Our over night lows have been average highs.

Equatorial tropics remain around 12 hours of day and night but part of the day is blocked by rain storms so less than 10 hours may be adequate. So they can successfully have tropical greenhouses in Colorado at high elevations where they get bright winter sun. So light is a factor to plan in your micro climate.

I've also wondered if Moonlight is the right spectrum to have an effect on plants.  Can they use this light to grow?

Another random question: if we use technology to augment other kinds of microclimates, what technology would permaculture feel is acceptable to alter day length?

S Tonin wrote:Your question got me curious, so I tried googling.  I didn't come up with much past the "carbon during the day/ oxygen at night" plant biology basics, but I did find this from Cambridge University: http://www.thenakedscientists.com/HTML/questions/question/1769/ .  The part I found most interesting was this:

"I’m sure many of you are familiar with the folding of leaves that we see in the clover growing in your lawns and lots of plants in the garden fold up their leaves at night. Darwin was interested in this and thought that it was to do with the leaves trying to maintain their heat balance at night. What we think is happening now is that the leaves are trying to avoid moonlight so as to prevent their circadian rhythms being disrupted by those very light intensities because they certainly do respond to moonlight."

It kind of makes me wonder how much old agricultural lore about planting, cultivating, and harvesting during certain moon phases might be from generations of growers observing changes in the plant triggered by moon cycles.  I'd really love to see if there's any correlation between when plants begin to flower or when fruit ripens with the phase of the moon (and how it might be affected by day length, and if that contributes to a good year or a bad year for certain crops).    

The folk lor in my fathers family was that bracken ferns were to be mowed during the full moon because they grew at night during the full moon and therefore the roots would continue to send up stored water and food which would bleed out of the cut stem. It worked bu how much was due to the moon light verses cutting on a regular schedule where the ferns were still growing but had already put considerable reserves into the growth.

Another random question: if we use technology to augment other kinds of microclimates, what technology would permaculture feel is acceptable to alter day length?

If permaculture is based on sustainable use of resources rather than depletion of resources for proffet.  Then the sustainability of the technology, which can vary radically by region, should determine. Where I am winter's short day and overcast rainy days is also the source of the electricity I might use to extend my light quotient for my winter food harvest.  So with more generating capacity than what is being used this time of year and relatively inexpensive price compared to what is produced from fossil fule, I use electric lights  to extend my production but that may not be as ethical in another circumstance.  

Sunrise, right this moment.  The weather is lightly overcast and the world is bright.  The clouds seem to be amplifying the sunlight which means, it was light enough to wake up the chickens over half an hour ago.

That's almost an hour earlier than yesterday.  

I declare, that where I live, weather has a huge influence on daylength - at least from day to day.  But I'm wondering, do weather trends have such a huge influence on plants and their 'understanding' of daylength?  
In the summer, we have almost no clouds, in the winter, we have almost every day overcast.  What effect does this have on plant growth?

I'd guess that on a cloudy overcast day, plants would be able to do less photosynthesizing then on a sunny one. However, on a hot sunny day, there can be more light then is needed, and the leaves can overheat, shutting down photosynthesis. This will vary depending on the plant. I'd think that here in Colorado, with an intense sun and no clouds, a food forest could have more layers then one in Britain with constant cloud cover, since here in Denver there is too much light for one layer to utilize.

Latitude plays a part in this too; the angle of the sun has a big effect on how concentrated the light is.

To show how my garden looks in this time of year, which plants are still growing (and which aren't anymore) I made this video and put a link in the thread about my garden:
https://permies.com/t/48736/Permaculture-small-Dutch-town#513004
When you're there at youtube, you can see my other video too, on the 'typical Dutch weather', and more on my garden.

(my videos are not edited and made with a tablet, so they do not look professional)

I took some photos this afternoon.  The weather here has gone down below 10C the last two days, so I think winter is nigh.

Pole beans - still producing new, but stunted and yellow, leaves



Here's my cover crop of barley, lentils, flax, favas and weeds.



A fava bush still in full bloom




Cauliflower and cabbages growing strong.  





Amaranth seedling among the leek



The beat, chard, amaranth and summer beans have all slowed down their growth, but still put the effort in.  The leaves that are there are darker red than normal, or pale yellow.

Of all the things in the garden right now, the chickweed is growing the fastest.  

These plants are still growing in short daylength.  A lot of places I've seen say that plants just stop when the days get too short, but these ones haven't stopped... they just slowed down a bit.  Some plants don't mind short days.  This is good news.  Now to start playing around with different varieties to see what does best in my conditions.  

Of all the things in the garden right now, the chickweed is growing the fastest.  

During the 1950's the winter cover crop was chickweed.  It was a sandy soil in the bottom of a natural swale that emptied into the bay that was fertilized from the dairy goat barn above. The carpet of chickweed would be one foot thick by spring and we rolled it off with a horse drawn spring tooth harrow. and it would compost itself at the top of the garden.

Today is sunny.  A few minutes ago, I took a photo facing west.  We can see the sunlight descend along the trees.  It was taken at 22 minutes after official sunrise



as you can see, the hill in the distance is bright and sunny.  However, the trees close to my home only have their tips enlightened by the sun.  I'm watching the sun, creep along the trees.  To the west-northwest, the trees have more sunlight than the ones directly west.  

It is now 44  minutes after the official sunrise.  The light has just reached the kitchen garden and is about to touch the house.  

We don't normally get strong sunlight this time of year, so it's very interesting to see how much time it takes for the sun to reach us in the morning.  But interesting.  

I came across this article and video about the relation between day length and temperature on plant growth.

The article gives much mor detailon Mercola.com

Short days have their challenges.  

However, my biggest trouble in my garden is the summer - the days are just too darn long for many plants.
Some plants like cotton and cowpeas are sensitive to long days and they won't flower or set seed until the day length is less than x number of hours.  By then, it's too late.

So while I'm planning my summer garden, I'm wondering how I can create shorter days in parts of the garden.  I'm guessing shade, but morning or afternoon shade, I don't know.  Any thoughts on this?

R Ranson wrote:I took some photos this afternoon.  The weather here has gone down below 10C the last two days, so I think winter is nigh.

Pole beans - still producing new, but stunted and yellow, leaves



Here's my cover crop of barley, lentils, flax, favas and weeds.



A fava bush still in full bloom




Cauliflower and cabbages growing strong.  





Amaranth seedling among the leek



The beat, chard, amaranth and summer beans have all slowed down their growth, but still put the effort in.  The leaves that are there are darker red than normal, or pale yellow.

Of all the things in the garden right now, the chickweed is growing the fastest.  

These plants are still growing in short daylength.  A lot of places I've seen say that plants just stop when the days get too short, but these ones haven't stopped... they just slowed down a bit.  Some plants don't mind short days.  This is good news.  Now to start playing around with different varieties to see what does best in my conditions.  

What variety is the red cabbage you have shown in your pic?  

Um....pointy one.

The seed us from Baker creek seeds rareseeds.com
Possibly this one

Thanks!  

R Ranson wrote:Short days have their challenges.  

However, my biggest trouble in my garden is the summer - the days are just too darn long for many plants.
Some plants like cotton and cowpeas are sensitive to long days and they won't flower or set seed until the day length is less than x number of hours.  By then, it's too late.

So while I'm planning my summer garden, I'm wondering how I can create shorter days in parts of the garden.  I'm guessing shade, but morning or afternoon shade, I don't know.  Any thoughts on this?

I had good success with putting my planting barrels on the east side of the house.

A valuable tool for farmers is the Degree Days system in which you calculate a number based on temperature patterns over a set value. Degree days are tracked by ag extension offices which makes accessing local calculators very easy.

Degree days have more usefulness IMO than day length for determining plant activity such as bud and fruit sets, and most importantly managing an IPM system.

https://en.wikipedia.org/wiki/Degree_day
http://ipm.ucanr.edu/WEATHER/ddconcepts.html
http://jenny.tfrec.wsu.edu/opm/displaySpecies.php?pn=-50

Don Man wrote:A valuable tool for farmers is the Degree Days system in which you calculate a number based on temperature patterns over a set value. Degree days are tracked by ag extension offices which makes accessing local calculators very easy.

Degree days have more usefulness IMO than day length for determining plant activity such as bud and fruit sets, and most importantly managing an IPM system.

For me Degree days are pretty easy to use.  I love them and I'm glad you brought them up.

It's possible you aren't so far north as I am.  I'm above the 48th, which means day length is a big influence in my garden.

Degree days are a good indicator of how well a plant will grow here, but not a good indicator if we can get a harvest off them.  

Even in the greenhouse, peppers, cowpeas, and cotton won't set (or in come cases bloom) even when the degree days are perfect.  It's pure day length that's the problem.  Once the day length is less than 12 hours, the fruit/seed often doesn't have enough time to mature before frost/winter.

When studying horticulture, we were taught that it is the hours of dark that affect the plants' growth patterns.
This is a quote from the University of Arizona:
Light duration or photoperiod refers to the amount of time that a plant is exposed to sunlight. When the concept of photoperiod was first recognized it was thought that the length of periods of light triggered flowering. The various categories of response were named according to the light length (i.e., short-day and long-day). It was then discovered that it is not the length of the light period but the length of uninterrupted dark periods that is critical to floral development. The ability of many plants to flower is controlled by photoperiod.

Read here more about the effects of light quantitiy and light quality on plants:  https://cals.arizona.edu/pubs/garden/mg/botany/environmental.html

R Ranson wrote:

Don Man wrote:A valuable tool for farmers is the Degree Days system in which you calculate a number based on temperature patterns over a set value. Degree days are tracked by ag extension offices which makes accessing local calculators very easy.

Degree days have more usefulness IMO than day length for determining plant activity such as bud and fruit sets, and most importantly managing an IPM system.

For me Degree days are pretty easy to use.  I love them and I'm glad you brought them up.

It's possible you aren't so far north as I am.  I'm above the 48th, which means day length is a big influence in my garden.

Degree days are a good indicator of how well a plant will grow here, but not a good indicator if we can get a harvest off them.  

Even in the greenhouse, peppers, cowpeas, and cotton won't set (or in come cases bloom) even when the degree days are perfect.  It's pure day length that's the problem.  Once the day length is less than 12 hours, the fruit/seed often doesn't have enough time to mature before frost/winter.

Interesting you mention having a hard time with peppers.  I'm sitting just above the 61st and have a similar problem because of our extremely long days of summer.  Jalapenos are typically the only pepper I plant.  I've found that if I start my plants fairly early (Marchish)  indoors then pot them up to 2-3 gallon pots move them to the greenhouse as soon as the temps permit and keep them in the greenhouse for the duration of the season I will get a crop.  They usually come in late in the season, but I do get peppers.  Nothing like the abundance of my garden when I lived down south before we moved to AK, but enough for our needs.  I've tried other "typical" more southern crops with limited success and at times, depending on the crop, abundant frustration.  Gardening in AK has been and still is a fun learning experience.

Not trying to hijack the thread, but I'm interested in the effect of natural vs artificial light and day length on people.  Seems to me like it is connected, since we and plants are living things.

When I lived in Alaska I knew lots of people who had problems with Seasonal Affective Disorder (or SAD).  It's caused by not enough natural light.  Symptoms include mostly depression, lack of energy and lack of ability to concentrate.  Nowadays they sell full spectrum lights that we can have in our homes, or at least set by for a while every day and they often work like magic.

Before we had a fancy name for it and it was officially recognized, people living in the north figured out ways to deal with it.  The traditional cure for cabin fever (now called SAD)was to get outside for at least a couple of hours every day, snowmobiling, skiing, dogsleds, snowshoes, checking a trapline, anything to get you out, moving, while the sun was out.  I lived in a small village there when I was in high school.  Much of the work was seasonal and it was generally recognized that men who's work ended when the snow flew needed to run a trapline in the winter even it didn't make much money.  It was a question of health.  Those who didn't have something to get them active and outside during the short days would often end up in bad shape by spring (drinking, drugs, depression, etc.).

The last few decades I lived in Alaska, I worked in an office. I had only my lunch break to get out in the sun, I never noticed myself getting depressed as we neared winter solstice (not very self aware according to my wife), but by late February and into March even I noticed I couldn't get the grin off my face as the days lengthened.  Kids went to school in the dark and came home in the dark.  People paint their houses bright colors because we need the color (in the winter it seems everything is white or black).  

I had a couple of roommates years ago who spent a couple of years in Iceland.  They told me that the buildings used to be all dark or drab (stone or concrete) and, like all of the arctic, their suicide rates were much higher in the winter.  When the govt went around and painted the buildings bright colors the suicide rates immediately dropped.  

It's been pretty well documented that kids in classrooms with lots of natural light do better than kids in classrooms with artificial light.  

I read some interesting stuff on the effect of fluorescent lights on squash.  As I recall, squash grown under one kind of fluorescent light produced only male, while squash grown under a different kind of fluorescent light produced only female blossoms.

Artificial day extension can be tricky as indicated not only brightness but color is involved.  Plant look green because that is the color they do not use.  Winter light tends to have the red infrared wavelengths filtered out. probably why Squash don't put out female blossoms until the sun gets warmer.  This fall I extended the season with daylight bulbs and ruby red heat lamps. adding the heat lamps immediately caused the cherry tomatoes to start blooming again.
As I have read blue into ultraviolet stimulate vegetation growth and orange into infrared stimulate blooming and fruit production.
You can find pictures of grow operation where the plants are black because they use no green light to save on energy costs.

I've had some good results playing with the Sun Seeker software to calculate the effects of shadows on day length for any particular spot:

https://permies.com/t/60433/advice-front-yard-edible-landscape#517559

R Ranson wrote:
Notice it says "Most".  I'm curious what plants do grow in the winter.  Given that it's seldom very cold here (some years we do get snow, but that seem to be less and less often), the temperature is fine for many cool weather plants to keep on growing.  From my own observations, these are the plants that keep growing in the winter and what changes as the days get shorter.

Kale - leaves get bigger Survives, no growth until march
Cabbages reds and Whites (January king) died end january
Chard - slow down in Dec and Jan, only to take off in Feb. Dies right down in October to just roots. now in end Feb beginning to regrow
Fava beans - focus on root growth in the winter, but still produce leaves. Do not germinate until april
Barley, oats, & wheat Stop in October, never timed restart
Chickpeas not grown, too cold in summer
Lentils again too cold in summer
flax to be honest I didn't know it was a peranial
snow peas can not overwinter
soup peas
weeds Blessedly free, nothing grows not even in the greenhouse untill April
miners lettuce This years experiment
lettuce (with the red tint, not so much the green stuff)
chickweed Dies November, comes back from seeds in May
brussels sprouts Died december!
dock Dies down over winter, goes Early november back May
onions The one I forgot outside died, frozen and rotted


What other plants have you observed growing in the winter?  How much of "plants don't grow in the winter" is based on temperature more than daylength?  Did you notice, most of these plants are common staple crops in traditional Northern European cultures?  How do we use traditional breeding techniques to create landraces of these plants that thrive even more in low light?  Would any of these do better in our daylength microclimates?  

This is just the growth I can see.  What's happening under the soil?  Does overwintering pulses and grains really make for stronger plants?  I think so, but I'm curious what other's experiences are.  

Here where I live (Northern Denmark 57N) nothing grows overwinter. some things such as winter wheat survive, but they do not put any growth on, not even weeds do. I doubt it is temperature related as it's not cold, -8C was the coldest this winter but most of the time it was +2 to -2 I'm originaly from the Uk and I'm trying to find things that can overwinter, like cabbages, sprouts, swede etc, but so far everything I have tried goes into winter in November looking good, and then come the end of December or January, it is dead. Not even in the greenhouse which sits 2-3C warmer on average do any weeds grow. No one even tries to overwinter anything outside other than Kale and even that is gone by January, I'm finding this really annoying as I love purple sprouting broccoli, but I can't keep it alive over winter!

Of course it's not just day light length that is important (in december that's just 6 hours here) but also strength, which is poor. it's normally cloudy and even when it isn't the sun is at such a low angle it doesn't do much. on sunny days this winter with a outside temperature of -3 the greenhouse was sitting at 0 to +1 (wood and glass un-insulated) in summer the temperature difference inside and outside is over 10C with the window and vents open. It is also apparent when one reads about food forests.
Multi layers really do not work here, My garden was abandoned for 3-4 years so I can get a good feel for what will grow under an apple tree from looking under the 5 I have. the answer is not much, one other layer can survive. I have a nice wavey alder tree, very light canopy and under that grows redcurrents (a good woodland plant) under them, nothing, bare earth, not even ground elder will take up residence there, there's some lesser celendine in the spring. and those redcurrents are two weeks later to fruit than the ones just next door that are not shaded, this is for me a good thing as it extends the season on them! the local woods, tend to be mixed species 50-100 year old some more like 20. they are generaly single layered, with occasional bramble patches. clearings grow raspberries, but they grow long stragley and do not produce much, they survive is about all that can be said.
I think that light intensity is probably as important as day length. perhapse some measure like Watts per Meter might show a correlation to what can be grown.

The discussion on shade effecting day length is interesting, I had never thought about it in those terms, rather I just avoid planting things I know like sun or heat anywhere where they may be shaded.

Oh going back to the original questions, chickens, yes if it's not raining, if it's raining mine never get out of bed, i think they fear they will shrink. Mine just (3 days ago) started laying again after winter. they stopped in the beginning of october and now are restarting the end of february, most certainly day length related.