Nope, citrus does not require a number of dark hours. In fact, they need all the light they can get.
Sj, do you give your AV's shortened days? Wow, I've never heard of that..I give my AV's as much light as possible and they bloom all winter..I think a monthly dose of fertilizer and Superthrive help w/blooming in mid-winter. AV's and Hibiscus are the only plants that get feedings in winter.
Citrus get a break from food in winter. If I lived in Fl or Ca I might consider feeding yr round, but because of lack of sun they're not fed. It's only early Oct and the days are dark already..it feels like night right now. I've still got a few more plants to bring inside..to top it off we had another rain storm early this morning. Now the remaining plants that's to come indoors are soaked..It never fails..LOL

Anyway, give citrus as much light as possible..Toni

Hi Toni,
I have my violets & streps under lights 12 hours per day year round. They do need a daily resting/dark period. It's easy to tell when violets get too much light - they "let you know" by their appearance....the clever things! The room they're in does not get much additional light. They all bloom year round. I constant feed them using 1/4 tsp of dyna-grow per gallon of water all year long. I don't alter that by the season. Here's to blooming flowers! Stephanie :)

Hi Stephanie..I haven't had a strep for yrs..how about Gloxinias? Do you grow them, too? I know they go through a dormancy period..but my AV's get sun yr long..Everyone has their own ways of growing plants; we do what works best for our plants, right?
For instance, I prefer bottom-watering AV's then top watering..how about u?
What type of citrus do you have? Do u have a website? Toni

Toni Toni Toni - Please consider what you're saying before you're so free with instruction? "Nope, citrus does not require a number of dark hours. In fact, they need all the light they can get." This kind of stuff can have a real effect on the satisfaction people get from certain aspects of the growth cycle.

Actually, plants have the capacity to "count" the number of hours of light that strikes photosynthesizing surfaces, and leaves are responsible for doing the counting. They contain a protein pigment that's sensitive to light (phytochrome). This pigment occurs in a form sensitive to visible red light and another sensitive to far-red light. Molecules of this pigment convert from one form to the other, depending on the type of light it absorbs. However, during the dark phase, the form sensitive to far-red light gradually reverts to the other form, so the length of dark period, determines the ratio of the two forms. This is how plants "count" the number of hours of darkness, and it is because of phytochrome's conversion from one form to another that the plant is able to detect the end of the dark period.

The meat of the issue is: Only when a plant's darkness requirement is met will the leaves release certain plant growth regulators which travel from the leaves through various tissues to buds, stimulating some of the buds to switch from leaf to flower production. So Toni, if you want flowers/fruit (the aspects of the growth cycle alluded to above), don't forget to turn out the lights ...

Al

I remember reading an article about oranges not having their nice color due to the lack of winter chill. This was in Florida and I guess they didn't get enough winter nights where the temps dropped enough. Keep that in mind if you leave the lights on all the time. Like everything else, I think moderation is best. I think there are no rules set in stone, cause what you say doesn't work, another person will sware by it. To each his/her own.

Al, I'm not talking about leaving plant lights on 24/7. I do use additional lighting on for citrus and they do just fine..many are in bloom and fruit now.
My advice to Steph was she didn't need to place a citrus in a dark corner a certain time of the night..Like one would a Kalanchoe or Poinsettia. Toni

It happens all the time, Toni. You're busted. At best, what you said was presented so it could not possibly be taken the way you say it was intended. I don't follow you around to see what you are offering up here, and I have no axe to grind with you personally - I think you're a perfectly lovely person with a lovely soul. However, I cannot be the only forum contributor that feels a need to counter misinformation when I see it. If you feel a need to offer advice in areas where you have little knowledge, there is always going to be risk that you'll be gently corrected. Remember: "First, do no harm."

Al

Hello Al,

Thank you for the information you have provided here. This is my first season to bring in my citrus trees from outside. I really want them to at best thrive and, at worse, maintain till I take them back outside in the spring.

I have a lot to learn. I have read (and re-read and re-read) your post above, about citrus "counting the hours" of light, etc. If you could comment on my specific set-up, I would be most appreciative. In my home situation for example, I have a meyer lemon in a room where there are windows and skylights, but not much direct sun as we back to woods and have many trees around us (there will be a little more when the trees lose their leaves, but still not a lot). The meyer lemon (which has 5 nice-sized lemons on it and 100+ buds), for example, is under a 200 watt compact fluorescent fixture. In that scenario, how many hours per day should the light be on? I also have a pink variegated lemon (all foliage) under it's own 200 watt compact fluorescent light. Would it get the same number of hours?
Thank you, Stephanie

Hello Al,

Thank you for the information you have provided here. This is my first season to bring in my citrus trees from outside. I really want them to at best thrive and, at worse, maintain till I take them back outside in the spring.

I have a lot to learn. I have read (and re-read and re-read) your post above, about citrus "counting the hours" of light, etc. If you could comment on my specific set-up, I would be most appreciative. In my home situation for example, I have a meyer lemon in a room where there are windows and skylights, but not much direct sun as we back to woods and have many trees around us (there will be a little more when the trees lose their leaves, but still not a lot). The meyer lemon (which has 5 nice-sized lemons on it and 100+ buds), for example, is under a 200 watt compact fluorescent fixture. In that scenario, how many hours per day should the light be on? I also have a pink variegated lemon (all foliage) under it's own 200 watt compact fluorescent light. Would it get the same number of hours?
Thank you, Stephanie

Al, what you say is right about phytochrome and plants in general, but do you have specific, published information to indicate that citrus induces flowering by that method? I tend to doubt it, since citrus trees can flower year-round. Almost any stress (change in temperature, dry weather, etc.) can pop them into bloom, regardless of season of the year.

So while I can't prove that they don't need dark nights, I do know that they are not obligate short-day plants like, say, a chrysanthemum or poinsettia would be.

Al, I really don't understand you. Are you saying Citrus are short-day plants? If so, I disagree. Citrus grow in full sun, or as much light possible yr round..One reason people would like growing citrus in greenhouses and additional lighting in their homes.
I don't know where you're getting your information, but in every plant book I've read on citrus, they state to place citrus in the brightest spot possible during winter months. I've NEVER read a book that advised people to place citrus in a dark spot while over-wintering..Ever.
As I stated to you before, I am no chemist, but only grow plants as a hobby and advize on how I grow said plants..(of all types) And they do quite well considering I'm in Il. I am pleased w/the turnout.
Why do you say I'm busted? Anyone is free to view my website and look at pictures then decide for themselves.
If you have a problem with me, you should write to me via an email. GW is not a place for disputes in rude manner like saying I'm busted. Toni

Al:
Your knowledge of citrus culture is way up there. You are at different level.
There is no use to argue with someone at a different level of knowledge like us here.
You can't win.
BTW I saved all your answers to this thread and to other forums for future use.
I learned a lot from you.
Thanks.

Girls & Boys,I go away for several months and when I come back you guys take to fightening. First of all...I would have asked this question differently.Look at the wording here.I certainly can see why this is confusing.***Suzanne

Hey Suzanne, Long time no see..
Nothing ever changes..(S) Same ol', same ol'.
So, how have you been and are you here to stay? Toni

Hell-o to you Toni...I am elusive in my hobbies so I'm here and then I'm there.I do peek in from time to time.I have just taken care of all of my citrus for the fall & winter mode. Moved 5 of my 9 citrus back into the greenhouse before the frost catches up with me.I also had 22 Brugmansias to trim back and also get into the other greenhouse.I've been busy.I was just admiring my Bearess Limes & Meyer Lemons & the baby Satsumas and figured I should pop into the forum and see if I have missed anything. And found this discussion heated.And I must contribute my 2 cents also.My citrus sleep nightly,No lights just heat this time of year.Mine only get daylight through the fall and winter they are in a greenhouse and I get baskets full of lemons & limes.Though,I must say that my grapefruit,satsumas,and cara cara's would appreciate I'm sure more artificial light through these dark months daily. Everyones situation is different.This question addressed could have had 2 different meanings, being active or a dormant state. I have to add also that I feed my potted citrus 4 times a year ( a granular) citrus food and Miracid 3 times a year.I would imagine food & light would be different in a "in house" situation. ***Suzanne

Suzanne, you'll have to post some pics..the last time I saw your plants I was amazed.
Your Brugs always fascinate me. They do so much better than my lone peach flowered brug.
It gets dark here so early that I feel my plants need a few hours additional light..not only only because of the time of yr, but because since Aug we've had nothing but rain and clouds. So added light it is. When I say rain, I mean a lot. This has been one dark summer here in IL. I'm surprsed my HIbiscus trees bloomed as much as they did. And they, like citrus, need light.
Well, it's great seeing you back..Toni

Dear Toni,
Like you, I like to give my citrus bright light in winter so they can make more food. I usually grow them in a poorly heated porch with supplemental lighting. My only problem is that they tend to bloom heavily in the winter, but fail to set fruit, possibly due to the cold. This year I will try some in my kitchen window to see if they are happier in a warmer place. So far, my Otaheite orange is the most fruitful of all. My 4 inch plant is loaded with fruit in various stages of development! No other citrus has been this fruitful for me. The fact that they don't taste very good is less important since they are so beautiful!
My outdoor Brugmansia is about ten feet tall and loaded with fragrant bloom. I leave it out all year with only a little pine mulch. It resprouts in the spring in its sheltered south facing spot by my house in my zone 7b garden.

Palmfan, I agree, most citrus will do okay at 40 degrees..of course 50 would be better, especially at night, but for ppl like us who live in cold climates, we do what we can.
You're BRugs are 10' and growing outside??? Wow, that's really impressive..Do they freeze back in winter? You really have to tell me your secret..LOL I know Datura's can deal w/lower temps, (relative of Brugs) but never heard of a Brug living outside in NJ..do you have pics? I'd love seeing your 10' trees..You're giving me ideas now..LOL..Toni

This may be a little late to chime in.
I don't understand Al's post. Am I to assume He's talking about ALL plants needing certain time in dark?
Maybe he should visit where I used to live in Alaska.
NO NIGHTS during the summer. We grew some of the biggest prize winning vegetables on record. BTW, My neighbors had citrus in the greenhouse and the 24/7 daylight made them happy as a lark. I'm no expert, but gosh, if something works well, don't ignore it.

Tom

Tom, that's exactly my sentiment..what works for some, may not work with others. We experiment, try different tactics, and whichever works fine for us is what's important..Toni

Actually, before I started using this forum for advice I did search out some biology on plant responses to environmental factors. One of the things that stood out to me the most was what Al talked about, and I read this on my own before joining gardenweb. I agree with Al, this is a plant response in all plants not just citrus trees. Requirements of each plant will vary based on their genetics but the mechanism is still the same. They respond differently to red and far red wavelengths. He doesn't mean that they require "darkness." If the plants detect less red wavelengths, then it triggers a hormone response for flowering. You can still expose a plant to light and make it flower, that is why so many led grow lights specifically add more red wavelengths than blue. It's the amount of a particular color that determines a response for flower production, not the amount of darkness needed

A good way to test this with our own eyes would be using poinsettias that do require darkness for flowering. Put 1 plant in full dark, 1 under far red wavelength only, 1 under red wavelength only (the red that's closer to the blue side of the spectrum). That would be pretty cool to see

Dr Malcomb,

So nice to hear you again. i learned so much from you; and you are a kind teacher, unlike me who is a world renowned curmudgeon

Hey, I thought I was the world renowned curmudgeon!

If LED were a rope you would give your citrus enough light to pull it out of a deep dark hole. I think all the points above have merit but they are just looking at it from a different angle. Most northern citrus indoors are deprived of light and the more the better if your goal is to actively grow during the winter months, but 24/7 light is not good either.

Two interesting articles I found researching this subject

https://www.cropking.com/blog/light-greenhouse-how-much-enough

https://www.gardenista.com/posts/13-things-nobody-tells-you-grow-indoor-citrus-trees-lemon-lime/#:~:text=Yes%2C%20citrus%20trees%20can%20have%20too%20much%20light.,a%20vegetative%20state.%E2%80%9D%20Think%20of%20them%20as%20sleep-deprived.

The ideal for Cannabis is suppose to be 12/12 flower and 18/6 veg. Looking for ideal ratios for Citrus.

My suggestion is if a citrus tree is brought indoors and you are using a 12/12 schedule and the tree is dropping leaves, particularly when the lights are turned off than I would start to increase the length of light. Keep an eye on leaf temperature for signs that the plant is getting stressed.

Unfortunately comparing one persons light to another's light setup is like comparing apples to bananas. The hours of sunlight in the wintertime in zone 8 or 9 is very different than the amount and quality sunlight in zone 4-6 and it depends on the month.

When you read articles on citrus you really have to parse the information and weigh and measure objectively because their situation may not be the same as your situation.