I am hoping Millet or one of the other experts sees this post and can answer the following question. Sorry if it duplicates another post. I see repeatedly that hand misting citrus does no real good but that using a humidifier is good as well as providing pebble trays under citrus pots. I have two Meyer lemons that have had issues with WLD for the past two winters. Since I have no supplementary heating or lighting (but have a good southern-facing exposure), I have tried to cut the light to my trees to provide better "balance" - as millet likes to say. But my question is why do citrus thrive in places like Arizona/California if there is so little humidity in the atmosphere. Granted they get huge amounts of light and the climate is warm almost year-round. Is humidity overrated or should I try to get a humidifier for my Meyer Lemons and invest in old-style Christmas lights? My WLD problems are less than last year but I would like to try to mitigate the leaf drop this winter while waiting for spring. Thanks!

[this post cross-posted at UBC Botanical Garden & Centre for Plant Research]

As written on UBC... The presence, or absence, of humidity has nothing to with WLD. WLD in containerized citrus is caused by the tree's foliage setting in the direct rays of the sun, causing the surface of the leaf blade to become hot (easily 100F+). If this happens while the root zone is cool the tree cannot cool it self down. With root zone temperatures at or below 55.4F, citrus roots are not able to function, and therefore cannot send water to cool the tree's foliage through transpiration. Without the transpiration of water from the leaf blade, the tree cannot cool down, and therefore to get back into balance discards its leaves. Even at temperatures above, but near, 55.4F winter leaf drop can still take place. To protect a citrus tree form WLD keep the tree's root system at least at 64F. If the tree still drops some leaves, raise the root zone temperature up to 70F. CHRISTmas lights work well to raise the temperature of the medium. Even the smaller Christmas lights will work. Citrus appreciate higher humidity, but still generally do well in lower humidity. Citrus will appreciate a humidifier, but it is not absolutely necessary. Citrus grown in higher humidity, generally produce fruit with thinner smoother peels, and higher juice content. Good luck with your tree. - Millet (1,108-)

I just took measurements and my soil temps are roughly in the 60-65 degree F range. It is now sunny but it has been extremely cold outside lately (15 degrees F or less) and the plants are up against a window. We get long stretches of cloudy overcast weather here in IN in the wintertime. I may try getting the Christmas lights, keeping the vertical blinds closed more to filter the sunlight, and have thought about fertilizing at half strength.

I believe you said you fertilize year-round. I have been using florikan Dynamite 18-6-8 fert with minor nutrients (Home Depot). It is slow-release and I sprinkle it on the surface of the soil. Should I be fertilizing and should I try to scratch the fertilizer more into the soil? I did not want to damage the surface roots of the lemons but it seems that the polymer-coated fert takes months to dissolve into the soil. I'm wondering if my trees aren't getting sufficient nitrogen as the leaves are yellowing in places. Or is the yellowing/leaf drop all WLD-driven?

Thks.

Ingardner...
In Arizona, Texas and other desert growing areas, like morocco, egypt, tunesia and parts of israel, palestine, greece and as found by recent studies in many oasis of the arabian deserts, Citrus plants suffer not anytime the possibility of water uptake.
So a good irrigation provides water to the rootzone, ensuring optimum water uptake capacity by the roots.
The roots itself, mainly are in a light, sandy soil, which is usually light alkaline, well drained and highly aerated. And the soil usually cools only in the top areas during the nights, the deeper areas remain above critical temperature levels, so promote still a well water uptake by the roots.
During the day, the top soil will be hot, to hot for proper water uptake, but, the deepers soils remain cooler, so still permit optimum water uptake by the root system of the citrus plant.

So, as you see: The soil and irrigation make water uptake nearly all times optimum.
The high radiation of sun light and the overall low air humidity, will only force the plant to increase the evaporation of water. This is the most effective cooling source. Because evaporation of water has a very high energy value, making it possible to hold the leaves below critical temperatures... And there is another trick... The leave face-down side is light silvery and togehter with the citrus oils a very shiny and refective surface.
So citrus curls it's leaves, outside longitudially upwards. This holds a little humidity above the leaf surface, brings the reflective side to appear, thus reducing the solar power to the lamina, and thus reducing evaporation by photosynthesis, bringing more power to water evaporation.
Reducing photosynthesis reduced enrgy absorbing and so heat producing by this process, and the process needs less water. This now will increase evaporation power and so the cooling effect on the leaf surface.

That's why the trees trive well in even desert areas. And that's why the air humidity does not realy count..... if soil water levels and soil temperature/root temperature and plant health permit...

> The presence, or absence, of humidity has nothing to do with WLD.
For once Millet, I don't agree with you (I mean about citrus ).
1) in WLD the leaves fall green. They are not burned. Hence the main factor is not temperature.
2) the leaves that fall first are the leaves inside the canopy, not the one that are in the direct sun.
3) in green houses we don't have WLD (or much less) although the roots keep very cold for months and the leaves have direct sun for days, but the air is saturated because green houses are closed in winter.

I myself consider that the main factor in WLD is lack of water and not high temperature.

Sylvain,
yes and no. You'r re right, but there are some points... maybe I can help you little.

WLD isn't temperature influenced, if this would, well, in serveral growing areas around the world would have increases leaf drop during winter times, but the do not have.
Also it ist not light influenced, because if only light would be the factor, there had to be an increased leaf drop measureable betwen the equatorean areas and the more mediterran growing areas more far from the earth mid belt... but even here: No, that isn't.
And it's not only humidity influenced, because in many areas the humidity is very low, even on cold nights or cold days, but even here: Not WLD will occurs.

It's the play of all growing factors together.
Sure, we must find, that a high humidity can or will certain decrease WLD, how much I am unsure, but I am properly sure it has to or will reduce this effect. But it won't abandon it or make it unhappen!
Citrus, as many other plants, are able to do some water uptake by the leaf. This may not work realy good, but will help the trees in high stress to quickly establish a better condition, so if after a drought the rain occurs, the uptake by the rootsystem often takes a while, so the plant can uptake water by the leaf, just to ensure a better tugor pressure, and thus establish much quicker a normal habit status again.
We use this mechanism in spraying our plants with nutrition solutions, because togehter with moisture, the plant can takeup some nutrients, so they were much more quicker available were they were need, as if they had to pass trought the uncertain factors in the soils, to the root system and now being upset to the canopy. For many reasons, this does often take a time to long, for being realy helpfull.
In fact, if we have a tree suffering a iron deficiency. We all know, that now applying iron to the ground will often be worthless, because if the soil isn't that good, all applied iron will get fixed in it, so be unavailable for the root system and all iron applied is worthless for the plant. But spraying will help, even ins soils prone to iron fixation. Here the iron nutrition solution will direct hit the affected leaf surface, the iron will be taken up by the leaf and the iron deficiency is less or cured.

So in high humidity environment we can consider, that the demand of water from the root system is more less, than in hot arid conditions. Because the citrus plant, as most C3 plants, isn't realy effective at photosynthesis, so more than three quarts, some scientists claim 7 parts of eight, of all water uptaken by the root system is used for evaporation.
So think about: if we asume a large Citrus tree can uptake 1000 litre of Water the day, more than 750 litres will be evaporated, and only less than 250 litres are used for photosynthesis... and if the claim of some scientist are true: more than 800 litres are avaporated and only less than 200 are used...
So in a pot, one litre the day, and now think, that most of that will be evaporated, and isn't used by the plants metabolism....

Okay. Citrus, as most other plants reduces the inside leave area first. If you watch carefully: Apples do so, pears, cherries and peach trees do the same und look at the forrest: Usually most oak and beech trees do the same.
Because the inside leaves get more an more shaded. After rebuilding those shaded leaves to shade-leaf areas the plant realy knows, when those leaves get ineffective. Usually with new growth these ineffective leaves will be properly replaced and that's why the are abandoned. Alos usually the inside leaves are commonly the older leaves, and at the outside, closer to light and air, new shoots emerge and here we find the younger leaves.
So shading leaves inside the canopy is a typical process, found on many trees, and if I grown an apple in a pot, well, even here the inside or closer to the stem leaves will be shed first under certain circumsances - because it's normal, and has nothing to do with less light conditions.
And because plants are living beings, it's that they won't do things allways after intelligent decicisions, often they do because they have to.
Like a thirsty to death man coming to an oasis... do you realy guess, he will order a glass of champaign or a cool beer? No, he will even slurp the dish water bucket to the bottom!! He acts on demand, not on intelligence.
Think on this, if you once work in your own oasis: Take care for those guys, because drinking the dish water may finally kill them - serve a good beer, even on your own bill...

So, back to WLD.
What now if correlated on temperature. First nothing.
Appearently the photosynthesis works better in cool conditions. And if we think about above, we understand well, why...
Because the photosynthesis produces heat. Solar radiation is used to power the metabolism of the plant, to produce, for what other living beings need food for: To get the source of all power -> sugar
Plants build from water and Carbondioxide sugar... and that sugar is the energy source for all growing and living activity.... Other living things eat, and uptake sugar by food. Plants do not. They get their sugar by photosynthesis, so do not realy need this as a source...
So the photosythesis uses solar radiation, but the range of solar radiation is very broad. Photosynthesis only needs a little. Other parts of the range will be reflected or absorbed... the absorbed parts of the radiation range will at one hand power the pothosynthesis, other parts of this absorbed range are transformed into heat... this heat will heat up the leaf lamina, and th lamina will be the cooling surface. So wind helps, to keep the lamina cool and the work of the photosynthesis properly... also the evaporation of water will now cool down the leaf surface, just to ensure a proper leaf temperature for photosythesis....
So if the air is cool, there will be less water need for evaporation - cooling the leaf surface. So the photosynthesis works much more effective and the plant can power photosynthesis at full strenght, with less water...
So temperature affects the economy of photosythesis and the water demand of the plant. In cooler condition, less water is need, even in full light conditions. Good to know...
So what's the backdraw?
Well, Olsen found in a field trial at the Texas Citrus Reaserch Station once, that cold roots will only affect the trees little, because the well established root system is able to get water from the depth soil areas by tap roots. But in a pot this get's worse: If the pot get colds, the roots get cold.
Citrus has some tresholds for operation, if we want to call that so. So for optimum living condition, it has to be 30° C, and 32 to 34° C in the ground. Consider: Roots optimum at over 30° C....

Roots are the most important thing in water supply to the plant. Without uptake of water and delivery to the canopy, well, the tree will dry and die.
So for optimum root activity there is a temperature over 30° C need, that's the point, were maximum growth and maximum water uptake is given.
The other point to know is, that below 20° C the curve of root activity quickly drops, so root activity below 20° C is strongly restricted and at the point of 12,3° C the root activity is nearly zero. Zero means: Water uptake to the lessest point... this will be enough, Prof. Castle guessed, for some photosynthesis, but never for any real transpiration activity.
Prof. Dr. Syvertson, who measured photosynthesis activity, also guesses, that for a proper metabolism this will not deliver enogh water.

So and here comes WLD, the imbalance of the system.
Photosynthesis works better in coler condition. According to Dr. Syvertson photosythesis in Citrus can start at 9° C air temperature and will do well there. It will work properly up to 20° C, with only slight evaporation and if temperatue raises, the evaporation will now strongly increase, to ensure a good leaf temperature for photosynthesis.
Consider now: 10° C air temperature - good for photosynthesis.

But as we discussed above: Root activity at 10° C - zero, so nearly absolute no water uptake

If we now consider, that the plant is able to throttle the photosynthesis, as most plants are able to, this won't have any affect to the leaves.. nothing will happen. The system is in balance...
But as solar radiation increases, as more heat radiation will hit the leaf blade and heaten up the leaf...
What's now needed, not to get cellular damage by over heating? Of corse: Water!
The leaf has to evaporate water to cool the surface and avoid damage by heat... But: There is no root activity, so where should the water come from? How should the evaporation for cooling be done, if no water is available in the canopy from the root system?

In free planted trees this never will happen, because tap roots usually will extend down and find usually allways areas from were water can be taken up, except many very shallow growing stocks.

This first point in WLD is, that cold roots won't uptake water, while high solar radiation will heat up the leaf blade.
The next point is more critical: The onset of heat in the greenhouse or conservatory. If the sun heats up the air of the conservatory or the greenhouse, the demand of water will strongly increase, because higher leaf temperatures occur much more quickly by a lower delta T.. So the leaf much faster needs water for evaporation, as in cool air conditions.
Unfortunately the pot, with the high mass of insulation potting mix won't follow that temperature change that quick as the air.
So the air heats up mach quicker, than the pot, so the leaves get much quicker warm than the roots... so here the imbalance starts.
That's even well documented by Johann Chr. Volkamer in his second book. He strongly recommends to place the trees in muck beds. Because the muck keeps the roots warm, and thus the trees thrive much better in the modern 'orangeries', as in the more dark, cold and badly insulated usually wodden shelters.
Also in his first book, he recommends, to let the sun shine onto the pot, rather the canopy, and shelter the canopy by a little roof...
So, what is found is once, 300 years before, is now strongly involved with WLD.
Because the solar radiation heats up air and leaves, the photosynthesis starts, and with it the heat of the leaf increases... that might be good, if air temp remains low, but if the air temperature increases too, the imbalace air <-> root temperature can occur and lead to WLD.
Because in if water for evaporation is need, but the cold roots cannot uptake, the trees quickly suffer a water stress, and this leads to leaf drop.
And here the most prone, to such process, leaves are abandoned first: The older, shade-leaves... because their higher potential for less light conditions will be much lesses be able to deal with this imbalance, thus those leaves drop...

So humidity can decrease this symptome, because if will supress the evaporation, because the heat tranfer to the humidity of the air around the leaf is much better, than to arid air.....
But misting the trees won't work, because this will only rise the humidity for moments, but in a greenhouse, were high humidities can be provided, that may work fine, especially is air temperature can be evenly kept cool.

so, that's how WLD should be understand... WLD is an temperature imbalance, and water uptake - water demand imbalance influenced by the growing environment... and I found it at several orangeries in Germany, while I never encountered if in the lesser and cooler orangeries I visited... But many older orangeries, restaured and now again filled with citrus, many gardeners were sad of their trees, because suffering heavily from WLD (modern heating, controlled temperature, but no automatic ventilation for cooling).. Here shading, as Volkamer mentioned 330 years before helps, as even on such days a good ventilation to reduce air temperature ensures a good plant condition.

Sylvain, wrote....1) in WLD the leaves fall green. They are not burned. Hence the main factor is not temperature."............. Sylvain, I certainly don't see any connection at all with the above statement and WLD.

The burning of leaves has nothing to do with WLD. Leaves must transpire in order to cool down, LONG before they start to burn. WLD is the difference in temperature between the leaves and the roots. In fact, the greatest leaf fall normally occurs after a bright sunny day.

Further, sentence #3 is also not correct......"in green houses we don't have WLD (or much less) although the roots keep very cold for months and the leaves have direct sun for days, but the air is saturated because green houses are closed in winter."

Many trees in greenhouses also experience WLD. I see WLD in my own greenhouse when the root are cold, and the tree's foliage is in bright sunlight.

Sylvain, read this research and perhaps you will have a clearer picture of the causes of WLD.

http://www.steffenreichel.homepage.t-online.de/Citrus/WLD.html#English

Steve, I agree with a little of what you say, but some of the extrapolations that you try to make, I cannot agree with. Further, in trying to make your case, you are making the causes of WLD much more difficult then they really are. The cause of WLD is really quite simple. But I guess that is why they make Fords and Chevrolets.

Millet (1,107-)

I had WLD on my meyer for 2-3 years in a row. It happenned when the temp in sunroom was 40's at night & they got direct sun. I'm keeping it warmer this year.
Millet, why weren't you at expo?! How ya been?

Patty, I had planned on going to go to the expo, but as the days went by, I just never got around to actually purchasing an airplane ticket. Did you go? If I knew you were going I would have never missed it. I am going to spend a week in Charleston, next April with the American Ivy Society tour. Take care. - Millet (1,107-)

No I didn't go either, but I looked at the pics posted of it & only ppl I recognized was Stan & Todd. Say hi to Todd & hello's to Stan, Ned, Scott, Darrin etc.
Hey, post some pics of that 'Ivy tour'.

I appreciate all the good feedback from Millet & others on winter leaf drop. I have been reading 'Growing Citrus' by Martin Page who seems like a very authoritative source on citrus. He talks about WLD as well and suggests that plants suffering from WLD can be pruned in spring by 1/3rd (p. 67). "This will stimulate production of more buds and produce a more luxuriant bush. Left as is, you may end up with several bare branches, with young leaves at the tips." I do have a number of branches on my lemons/limes that have no leaves on them - from WLD this winter & last. Will they regrow leaves along the length of these bare branches or do I need to prune them back to stimulate leaf production. I've always thought one should prune these bushes sparingly, if at all, during the 1st few years. I'm not a horticulturalist like some on this thread so a less technical response would be welcome.

I have also read the book "Growing Citrus by Martin Page. It is just one of many texts on citrus that I have in my library. I don't like being negative concerning Mr. Page, however, I though his book was among one of the worst books on the culture of citrus that I have read. There is way to many areas of disagreement with Mr. Page's book to cover in this post. I even begin to wonder if Mr. Page ever actually grew a citrus tree. But to answer your question....if the leafless limb's cambium is still green, then the branch is still alive, and will almost always sprout new foliage in the spring. If it were my tree, I would leave it alone. The best to you and your tree. - Millet (1,100-)

Yes, let the tree, and see which buds will sprout, and then after bud break and new sprouts appear, you will find the best places were to set the prunning shears and what to cut away...

Much better than prunning the tree before bud break...

Great advice. Thanks so much to all the experts out there helping newbies like us. If you know of citrus books for citrus growing novices, would love to hear of them.