If you keep the temp at a minimum of 50, why do you want the lights? Are you trying to keep them above 64? If so, then it seems that a heater in the greenhouse would be easier(but not cheaper).What I'm saying is, if they are not freezing, they are not dying. If you are looking for a festive look, use the lights. Scott

Scott,

Yes, I'm trying to keep the soil temp up above 64F, or at least above the 55F that I understand to be needed for water and nutrient uptake.

My observations last winter were that with a dip in temps, the citrus drooped despite adequate soil moisture. Then, without watering, the citrus would perk up by later in the day as the GH temp rose. I suppose that there could be other reasons for this happening, but the "55F required for water uptake" rationale seemed reasonable to me.

If I was trying for that temp, I would try something as discussed in the heating cable thread. Heating the GH would be pricey$$$ I would bury the pots and try to warm the medium that surrounds them. Best of luck! Scott

SB, as your plants are inside a 50 degree night time greenhouse you should only put the lights around the plant container. As you seem to already understand 55 degees F. is absolute zero for citrus roots. Also, it is important to understand that the soil temperature in the container will be lower than the surounding air temperature because of evaporation. So, a greenhouse kept at 50 will usually not keep your trees soil at that temperature. Because you have a greenhouse you can get 12 months/year of growth out of your trees by keeping your soil temperatures at or above 64F. Any where around 64 - 70 is ideal. If you wrap the lights with bubble wrap it will, of course, be a lot more efficient, but the system might work either way. Be sure to purchase a soil thermometer so you will be able to keep tract of the medium's temperature. I really do not know if you need to leave them on 24 hours a day. The main thing, if you wish to get good growth, is to just be sure to maintain the required soil heat. You need not worry about photoperiotic problems, they will not exist. Also, lights are a LOT CHEAPER method of heating than raising the temperature of the entire greenhouse. I use 20-ft. long 18" wide plastic heating rolls that are regulated by a thermostat. I just roll them out and set my trees on top. Each thermostate will operate four 20-ft. rolls. I set the thermostate at 64 and it turns on when the soil falls below that temperature. Lastly, by keeping the tree's growing medium between 64-70 your trees will benefit from an additional 6 months of growth, and you will elimainate the possibility of any winter leaf drop. Take care. - Millet

I've been thinking about the Xmas lights idea, and I read the (huge) recent post on the heating cables. One thing I don't understand about the Xmas lights as a heating source is how many feet of lights are you using per 16" pot (for example)? Are you mummifying your pot in lights? Or maybe one loop every couple of inches of height?

Also, the heating cable thread was, I think, specificly mentioning soil heating cables. As an alternative to the Xmas lights, couldn't you use the heating cables meant for preventing water pipes from freezing?

Greg

Stressbaby..since you're in MO, I'm assuming you have a heater running, right?
This winter is going to be a b...well, bad w/gas prices the way they are..I've a gas heater and a back up electric.
Sun heats up an area in winter..what size is your gh?
Mine is only 8x12 but fits quite a few trees and smaller plants.
I've an alarm that beeps if temsp dip below 45...As for leafdrop, I don't have any problems even though my gh is on the cool/cold side in winter. And my trees continue to grow, slowly, but surely. Citrus flower and fruit, depending on the types..
How long have you had your gh? Toni

Here in south Orange(!) County in CA we often drop into the mid to high 30's during our 2 or 3 months of winter. Last year we hit 31.7 one night. My citrus trees (25 different varieties) are never damaged from these cold temperatures and produce abundant harvests every year.

I don't know why you should be worried about keeping citrus tress above 50 or 55 degrees. In fact, its the cold temperatures that give my oranges and tangalos their beautiful orange color. I suppose they stop growing when the temperature drops, but they certainly don`t seem to need to grow all the time.

Caagary, your thread is certainly correct. You live in California and can grow outdoors year around. If a northern grower, who has to grow citrus in a cold climate, such as stressbaby, only desires to get 5 or 6 months of growth from her trees, than yes, she can do whatever she wishes concerning temperature. However, when a person has gone to the expense of a greenhouse, then why would'nt they want their trees to attain the growth available to them? By keeping the soil temperature around 64F the tree grows 12 months a year. I have grown 52 different varieties of citrus trees in a large Colorado greenhouse for the last 20 years, and I get all the fruit color plus a full years growth from my trees. I could keep my greenhouse temperatures cool at 40 degrees, from October through March, and just let all of my trees set there in a "dormant" state and lose all the potentional growth that I could have had, but if I wanted to do that, then why would I bother to build a greenhouse? It all depends what you expect from your trees. Take care. - Millet

as a resident and grower in OC (Calif.), and a follower of Millet's counsel and co-design discussions, and ideas of my own, i have found soil warming (in SoCal) to be quite rewarding. it's a lot of fun to have a tree acting like it's in the middle of Spring during the "dead" of Winter. soil warming gives lots of new growth during normally slow months. If a grower is willing to protect it, it gives someone a lot to talk about half way through Winter. :-)

Plus, a SoCal grower doesn't necessarily need a greenhouse. So, in addition to Millet's observations, soil warming makes a lot of sense for SoCal growers who don't mind working a bit extra in Winter. :-) regards. -vince

I'm going into my second winter with a 20' x 23' glass GH heated with two 24K BTU vented propane heaters. Bananas, bougs and the rest seemed to do just fine with a min temp of 50F, so I didn't want to increase the GH temp just for the citrus.

I've purchased a soil temp thermometer. I think I'll wrap the containers with lights and cover them with bubble wrap and monitor the temps to see what kind of temp differential I have between soil and ambient air temp. I'll post results, if anyone is interested.

Thanks again, everyone.

StressBaby - have you accounted for the aeration needs of citrus roots in your method? Will the bubble wrap be opened up near daily so the roots can breath? A lack of soil/root aeration will produce a pretty quick (and negative) response from your trees. Last Winter, I once left my citrus roots relatively "sealed" in the name of warmth retention, and they did not like it. in 7 days, 2 of 5 containerized trees were sending new roots straight up into the air trying to get more oxygen. Be advised. With all the warming designs and variations and such, I still think the biggest hurdle to successful warming is achieving successful oxygenation *with* the warming. Trees roots demand more oxygen with warmer temps. It gets interesting. -v

I intend to wrap only the sides of the container. The tops and bottoms will be left open.

SB, how many bougs do you have? Are they deciduous? The reason I ask is my gh drops down to 50 or a little warmer on the coldest nights of winter..I'm torn between setting my bougs in the gh or my home. Daytime temps vary.
I find citrus growing in cooler winter temps do better than those growing in hot, stuffy,(house) conditions. Especially after a summer outdoors. The only time I have leaf drop problems is when I leave citrus out too late in the season, temps drops, then brought in a warm house. This is one reason I prefer bringing citrus and a few other tropicals inside in early Sept..This year I didn't have a chance to do so, so I'm sure many citrus leaves will fall..We've had 45F at night last wk..Good luck, Toni

What type of Christmas lights are being used? Are these the small lights that supposedly do not get hot or are they the old fashioned lights that look like night light bulbs? The reason I ask is that I have had to move my 5 citrus trees in the house for the winter. Our nighttime temps are getting into the low 40's. The daytime temps are still warm enough, 80's+, however I do not get home from work until the temps have started dropping, so I have moved them inside. I have prepared the room they are growing in with extra lighting and humidity and the house temp will be set at 68 for the winter. Last winter I had a lot of leaf drop due to not enough light and possibly too cool temps. I wish to keep the leaf drop at a minimum as most of the trees have fruit that should be ripe sometime around December. The trees I have growing are as follows. Trovita orange, 2 Myers Lemon, Key Lime, Calamondin. The Calamondin and the Myers lemon are both loaded with fruit and are still flowering.

Speaking of the ones flowering, should I pollinate these trees by hand or are they self-pollinating? Or should I just let the blossoms drop un-pollinated until spring?

TIA for any advice.
Andi

Andi, your trees are self-pollinating. Since your trees were out in summer, I'm sure our friendly bees helped..Toni

Thanks Toni.
What type of Christmas lights am I to put around the pots? OR is the house temp of 68 going to keep them warm enough? Most of the lights sold these days are of the miniature nature, even the ones sold for outside use. I want to keep the soil warm but do not want to cook the roots or burn the house down in the process.
For the supplemental lighting I am using 3 60 watt grow bulbs and a single fluroscent light tube. I am hoping that this will provide enough light along with the east and south facing windows. I have also placed a cool mist humidifier in the room with the trees. This goes through about a gallon of water a day. For the time being the windows are open during the time I am home and closed when I leave. This should exchange the air enough.

Some of the blossoms have not opened yet. Should I just let these drop?

If you or anyone else has any ideas that I have not thought of please let me know. These trees are special and one has been brought back from what seemed certain death.
Thanks again
Andi

Andi, I do not use Christmas lights or any other lights on citrus soil..I believe citrus should remain cool in winter. Not cold, but cool..
Before buying citrus, I purchased books and read online articles that all say it's best to keep citrus on the cool side during winter months.
If you've ever went to a commercial nursery, you'll note all citrus are in the cooler parts of the gh.
Sounds like you're doing everything possible..humidifier, opened windows for fresh, circulating air, and supplimental lighting.
I'd let the blossoms drop on their own..they don't last long to begin with, so might as well enjoy the fragrance while they last.

What happened with the citrus that was near death? Toni

The citrus that was near death was suffering from root rot caused by over watering. It had lost all its leaves and was starting to have twig die back. I tossed the old soil that was sodden on the bottom where the roots were. I trimmed the roots back to where they were no longer mushy. Pruned the top 2/3 of the leafless branches. Dipped the roots that were left in a 10% chlorine bleach solution. Repotted in fresh clean soil mixed with a LOT of perlite and sand. The tree responded almost instantly and within 2 weeks there were new leaf buds. The tree is now a glorious fully leafed 2ft beauty. Hopefully next spring I will have blossoms that will form into sweet Trovita oranges. I had tried ST but the tree was still failing.

Hopeful,

I typed bougs when I should have typed boug. I have one rather big one which completely defoliated two years ago when we brought it inside the house. Last year in the GH there was no leaf drop and it bloomed all winter. Here is a pic from last year, it is 50% bigger now, even after hard pruning last spring:

{{gwi:632215}}

When you say cool soil temp in the winter, what kind of temps are you talking about?

Stressbaby, your boug is beautiful. And I love your gh..I wish mine was a big as yours. I see a palm in a blue pot, and what may be a ficus next to it..Not sure about the ficus though..
Cool temps: anywhere from 55-65. Toni

Millet's right. This morning with GH temps at 68-69F, the citrus container soil temp is 62-63F, a diferential of 6 degrees. Fascinating. Lows outside in the 30s for the next few nights...I'll try the lights and report back. SB

SB, are pots sitting directly on the ground? If so it's likely to be cooler which is why there is a 6 degree decrease. I keep my citrus on top of upturned pots so they don't sit on the ground. (in the gh, not indoors)
Last winter I tested my soil using 2 guages, and soil temp was the same as air temp. Toni

The pots are sitting on upside-down clay pans, the kind that go under a terra cotta pots.

I watched the temps all day and my first conclusion may have been premature. By about noontime, the differential was zero, the soil temp was equal to air temp. This morning, before the sun was up, soil temp was the same as air temp as well. I wonder if the differential in the morning was just the lag in soil temps as the container warmed from sunlight and ambient air. In this case it should be warmer than outside air for a few hours in the evening. I'll check that and string up the lights this weekend.

Greg asked a question that you guys blew off and I want to know too..

Couldn't you use the heating cables meant for preventing water pipes from freezing?

I have a feeling that they won't get warm enough - in the 60° range..

Millet, where do you get plastic heating rolls? Do they come with the thermostat or is that extra?

Farslayer, It all depends on the upper heat level that a heating cable is capable of delivering, but generally no, you cannot use the common water pipe cables. Normally they do not get warm enough. The system that I use, and have for the last 8 - 10 years is manufactured by the Flexwatt Corporation. The heating rolls are made out of plastic and come in 5-foot, 10-foot or 20-foot lengths. The widths are 10" and 18" wide (I believe, as I have not measured them in years). As they are plastic you can water containers on top of the heating rolls, even when they are opperating without any problem. Just roll them out and set you trees on top and you are ready to go. The thermostats (actually a transformer) can be set from 40 degrees to 100 degrees F. Each thermostate can opperate 4 heating rools. They are a little expensive, but everything of commerical nursery quality generally is. As someone said in another thread, you get what you pay for. I opperate two complete full systems, and they have worked without fail for 10 years. I purchased them from the American Cay Works & Supply Company of Denver, Colorado. 303-534-4044. American Clay Works (named because they used to manufacture clay pots) is a large greenhouse and nursery supplier. Take Care. - Millet

SB, is there a way you can prop them up higher?? Toni

Actually, if the greenhouse was constructed correctly, a greenhouse floor should not be cold or even cool. This is especially true of smaller hobby greenhouses. In a properly built greenhouse the floor should be a very valuable heat sink and a source of warmth. When a greenhouse is being built, a foundation trench 4" wide and 4' deep should be dug around all four sides. This then is filled with the common foil coated insulation sheets such as Panalara or Celetex. By insulating in this manner the soil inside the greehnouse is insulated from the outside. This enables the greenhouse floor to continually absorb heat which therefore becomes a good heat source for the greenhouse. The soil in my greenhouse is always at or near 70 degrees. I grow my Papaya's in the greenhouse soil and they do well because of this. It is unwise to construct a greenhouse in cold areas without insulating the foundation area.- Millet

BUT, heat rises and cold air settles downward. As the heater cycles off and on, temperatures throughout the gh are not always consistent.
Raising plants/elevating off the ground puts them in a position where temperature is more constant..This works for me..I cannot speak of others' gh's. Toni

Toni, what you say can certainly be correct, and the problems you are talking about in the above post are certainly present in some greenhouses. However, again if the greenhouse is correctly built, your concerns about hot and cold temperature zones within a greenhouse sould not be a problem. I'll give my greenhouse as an example. I have three medium/large greenhouse fans located near the peak close to the greenhouse roof, one near the north end, one in the middle of the greenhouse, and the third fan near the southern end. These fans constantly blow all the warm air that rises up to the ceiling back down to the plant zone,in a constant cirulation. I also have fans near the floor of the greenhouse that circulates the air horizonally around the greenhouse. The temperatures are pretty much constant within the greenhouse. The only cooler area would be right next to the vents. When a greenhouse is built, heated, and designed properly temperature zones should not be much of a problem. Now I agree with you that growing smaller house plants or bedding plants up off the ground and on greenhouse benches would be a better situation. But not because of a lack of warmth (my greenhouse floor stays around 70) but because it is more convenient. All of my trees and larger plants set directly on the floor and do well. Most all problems due to internal greenhouse clilmatic situations can be traced to faulty, improper or the direct lack of good greenhouse design. A properly designed greenhouse is of paramount importance, to insure an environment for quality plant culture. Take care. - Millet

I've been watching soil and air temps the last couple of days. There is as much as a 6 degree lag in warming in the morning. There is as much as an 8 degree lag in cooling in the evening. By about 1:00pm the soil temp has risen to ambient air temps and first thing in the morning soil temp is equal to air temp as well.

I tried the lights today. These are the commonly available, small Xmas tree lights you can get at Walmart. The first problem I encountered was trying to wrap a tapered pot evenly. When I realized that Millet's heat came only from the bottom, I realized that I didn't have to solve this problem...I just wrapped the lights around the base, rather than wrapping them evenly around the container. I just put one strand on one plant, and it went around perhaps 12-14 times. After 5 hours the wrapped pot was 1-3*F warmer near the surface than the other pots. I then checked the temp in the bottom (by sticking the thermometer in one of the drainage holes). The wrapped pot was 5*F warmer in the center of the bottom of the container, as compared to the other pots.

At this point I'm thinking that the perimeter of the wrapped pot was likely warmer still, and I didn't want to over do it the first night and "cook" my citrus roots. So I spread the strand out evenly among three pots, using 4 wraps at the base of each. Lows tonight in the 30's, so I expect the GH to be down to low 50's by morning. We'll see...

Millet, when I built the GH I put rigid styrofoam insulation around the perimeter against the foundation wall from the footer to ground level. I have 4 HAF fans, so I think the temps are pretty even, except, as you say, right in front of the intake shutters. I do have about 30 linear feet of raised beds, 24" wide and about 20" high. They also act as a bit of a heat sink and the soil temps are a little higher there than in the containers. My citrus trees right now are on upside down pans on top of these raised beds, so the containers themselves are 24" off the ground.

Again, any advice is appreciated. SB

SB - I can't speak to greenhouse dynamics, and won't. Therefore, I'm completely irrelevant at this point in discussions. However, your work truly fascinates. Can you tolerate a SoCal perspective, for SoCal readers?

With one "warmed" and outdoor Fall/Winter under my belt, and benefitting from influence from countless parties here, would it be fair to "type" soil warming into three general observations (perhaps 5 by time I'm through - LOL)?

1. The first might have to be whether the warming method is internal or external to the container. You are working with external, as many do.

2. The second, as people observe - is that heat does rise, and this can play very significantly into one's design. However, for a grower who's primary cold "front" is the top of the container (an outdoor grower with a planter, perhaps?), this can produce a great deal of heat loss out of the top of the container, as heat rises.

But doesn't pertain to you (with the greenhouse). Thanks for tolerating a SoCal digression. :-) FWIW - I also used a "soil" thermometer to take readings and analyze my design shortcomings real-time. My biggest loss of heat was from the top, but this wasn't in a greenhouse, but in an outdoor planter.

3. Aeration - already accounted for.

4. Foliage protection, already accounted for.

5. With X-mas lights specifically, I would just quote those practitioners from last Winter, as it may not be emphasized well enough yet. That is - X-mas lights for soil warming (and foliage warming, for that matter) have to be the small incandescent types, not the LED or neon style, as the latter give off virtually no heat.

6. With tall containers, a SoCal grower (without greenhouse) probably has to consider "wrapping," whereas any greenhouse grower probably has the option of either or both. Will watch.

Please continue with updates and findings, particularly in the "wrap vs underlay" discussion. Circumstances have "biased" me toward wrap in the past, but I was scrambling for any solution last Winter, not the best as is thie hope this Winter, however divergent our circumstances. :-) Thanks! You're kicking butt. -v

Good point, V, about the lights. I guess I didn't realize that they made LED Xmas lights. And you are welcome to talk SoCal soil warming anytime, just don't talk SoCal football. :-)

This morning, with GH temp of 53*F, surface soil container temps were 55-60*F and bottom temps were 60-64*F. The taller containers had differentials of 5-6*F from bottom to top. The shorter, squattier (sp?) container had a differential of zero. The container that had the lights on it alone yesterday evening was a couple of degrees warmer than the other two. Again, this was with four loops of lights around the bottom of each container.

I'll leave the lights alone for a few days and see what happens...it looks to me as if each pot may eventually benefit from another couple of wraps around the bottom.

Benney, needs to chime in concerning Christmas Lights. He was the person that first thought about them (at least I think he was). He used them successfully all last winter. Besides, Benney is a electrician and understands lights vs. heat/watts. - Millet

My question is, after reading Millet's response, if everyone's gh is correctly erected, why then is it necessary to use Christmas lights?
I go to nurseries during winter months and have never seen any additional heating instruments on plants. It'd cost an arm and a leg. Prices would be jacked-up because of electrical overhead.

I'm not saying using lights is wrong, it may help..but for the life of me, I cannot figure out why ppl are doing this w/citrus that are growing in their homes/appts.
The average house temp is 68-72 degrees or higher. Toni

If one wrapped x-mas lights around there container an say the soil temp was maintained at 70 degrees F. Could the air temp. be held at a lower Temp, Say 45-55 degrees without harm to the tree, or would that be cause of WLD

Or would there be any harm to it at all by doing so? By maintaining optimum root temp an cooler air temp would the tree still grow? Tom

Tom, you have brought up a really great point. I just yesterday e-mailed onewebfoot an article on that very situation. Vince has been doing a lot of research/design on the value of citrus soil heating. The relationship between temperatures and flowering has to do with the tree's foliage. The relationship between temperatures and the tree's transpiration system has to do with the growing medium/roots. This is not understood by a lot of people. Good observation! -Millet

Millet, could I trouble you to explain your last post a bit more? I'm confused.

Are you saying that foliage temps, not soil temps, regulate flowering? Are you saying that the tree may grow, but not flower, if soil temps are OK but air temps are I thought Tom was asking if the foliage, like the roots, would shut down at

Tom, this is my opinion, but I think there's a better chance of a plant undergoing leaf drop when citrus is brought in a hot, stuffy, heated house after being outdoors.
My gh sometimes drops down in the 50's and stil grows, bloom/fruit..though I must admit at a much slower pace..then again, there's very little sun and we all know lack of sun sure doesn't help plants of any type, grow. Toni

Forgive me, folks, if I don't understand this correctly, but if WLD occurs when transpiration losses exceed supply from the roots (like when the soil container is cold from being outside, and the foliage is suddenly placed in a sunny window or warmed by higher ambient air temps), then to some extent the foliage must have been shut down when it was outside. Otherwise, the leaf drop would have occurred outside when the temps decreased, not just when the plant was brought inside to warm up.

If this is the case, what is "absolute zero" for the foliage? Assuming adequate soil temps, how low can air temps go before growth ceases from shutting down the foliage? Maybe Toni's growth (and mine last year) is slower in winter because the citrus spends some percentage of time under 55*F where the growth is zero...

And if this is the case, maybe the lights really only function to eliminate that 6 degree lag in soil temps in the morning, when the ambient air temp is up to 60*F and photosynthesis and transpiration have begun, but where the roots cannot help out because the soil temp is still 54*F.

Sorry for all the questions, but I feel like I have a lot to learn.

Absolute zero for foliage is different for different varieties. Anywhere from 32 to 15 degrees.Consider absolute zero for foliage to be when the leaves freeze to death. Absolute zero or 55.4F pertains to citrus roots. At 55.4F citrus roots cease to function. You are correct WLD is not a serious problem for trees grown outside. However for trees grown indoors in an artificial situation WLD can be a problem as many, many hobbiest can attest. You seem to generally have a good understanding of the problem. When a citrus tree is placed in a cool room and the foliage is directly in front of a southern exposure a problem can occur. Direct sunlight can easily raise the leaf surface 18+ degrees above the ambient air temperature, depending on the duration of time the tree is subjected to these conditions. If the tree's roots are at 55.4F degrees or lower the citrus roots cannot and do not function. They cannot sent the required water up to the foliage for evaporation to cool the leaves. Even at temperatures closely above 55.4F the root's ability is many times insufficient to properly cool the tree. Many times WLD happens after a very sunny day. WLD rarely happens after cloudy days. Stressbaby, your correct in your thinking. The value of the lights for a tree in the situation that I described above is to keep the growing medium's temperature high enough so that the roots can operate. If your greenhouse conditions are such that the temperatures can keep the roots approximately at 64 degrees then you should not need the extra protection that lights can afford. Unless you are only trying to obtain the maxium growth out of your trees all year around. An example an orange tree in Florida might take 10 years from seed to produce fruit, but the same tree in the tropics might only take half the time. BTW nice home and greenhouse. Your an interesting person, I'm glad you joined the citrus forum. Take care. - Millet

If memory serves me, I think drastic changes in light exposure can cause unexpected leaf drop in plants brought in without putting them in shade outdoors for a couple of weeks before bringing them in.

I think my other experiences with leaf drop include soil kept too moist and possibly too cold (somewhat drafty window sill in winter time), and for newer growth, allowing soil to get too dry.

RE: holiday lights for heat.

I can only imagine this to be a good idea so long as you maintain safety precautions (using an outlet that will trip should a short occur) and you use care when watering. My trees are currently living on top of my cable box where the air above the soil is roughly 80 degrees year round.

You may want to coil the strand of lights such that they cover the entire area under a pot with even heat. Since I have no fan, the warm air just rises constantly.

Consider using a short shelf under the plant. You can probably use a pie cooling rack (pretty cheap at the supermarket) with the strand spiraled below the rack. Once you find the right number of lights under your pot, in a still room/GH, the heat rising from the pots should maintain a heat minimum around the leaf system.

I experienced my first bloom with a 12" lime tree one year after being picked up at a rest stop in Florida kept on top of a cable box. It's roughly 18" now with one lime around 1.25".

Millet - in follow up to foliage temps vs blooming temps, could I quote you from April-05? I just wanted to flag the "850-hour" figure you cited, as it bore out. I didn't know all this when I started soil warming, but the trees sure seemed to follow these guidelines to the letter last Fall/Winter. Thanks for the helpful info/guidance along the way, and now. -v

Begin quote --- "Know that if you grow citrus trees inside a warm greenhouse during the winter months you will need to be sure to start following citrus flower bud induction conditions around November 1st for the coming year's bloom. Citrus grown in warm greenhouses need to receive approximately 850 hours of temperatures below 68F (between November 1st and Christmas) to set a good crop of flower buds. If the trees are continually grown at higher temperatures the buds will NOT develop as flower buds by rather as leaf producing buds. It is the 850 hours below 68F that differentiate the buds from foliar (leaf) buds to flower buds. After the 850 hours of below 68F temperatures, which is sufficient for flower bud induction to occur, then give the trees 7-12 days of temperatures greater than 75 to 80 degrees F. which will trigger flower bud growth and bud swelling. This will give you a good crop. Conversely, if you do not want flowers thus no fruit but would rather produce a lot of stem and leaf growth (to enlarge the tree) then keep the temperature above 68F. -Millet"

I'd be interested to hear how long leaf drop happens after bringing in a citrus, placing in south windows/gh, with cold conditions..Toni

To toss a monkey wrench into the conversation. :-) Could all or any of these techniques be applied to growing tomatoes during the winter in zone 5-6? Thanks.

Just a quick question-- I've read through the posts, and see that using Christmas lights in citrus trees can help keep them warmer during the cold nights.
I have several citrus trees, two planted in the ground, one in a large clay pot. We have temps here in So. Cal. down in the low 30's at night....

I bought the standard white Christmas lights (not LED) and strung them through the branches. It seems when I touch the bulbs individually, they don't seem to be warm. Do they actually get warm? If they appear not to be warm to me, are they helping keep the tree warmer at all?

Just curious....