I have been applying Millet's advice on winter leaf drop to my seedlings in a variety of ways and my seedlings are actually still growing.

I put them in my shop at night anytime the temp is suppose to go below upper 50's-- which is pretty much every night now. I leave them in the shop with no lighting on days that the outside temp is below 60 (like the last 3 days). I move them outside in full sun when we have warm days like last week.

Today I actually put them outside while the soil temp was in the upper 40's but kept them misted to cool the leaves until the soil temp got above 60. Then they stayed out the rest of the day. No wilt, no leaf drop.

I had 1 or 2 leaves drop before I realized that the early morning sun was getting to a few pots on the end of the table-- I have since blocked that low angle sun. Several seedlings started a new flush last week.

I think Millet is absoluly correct-- the leaves overheat and dehydrate when exposed to sun while the roots can't function. If you supply the cooling (ie misting)-- no problem. In fact I think that cooling is probably the major benefit of adding humidity indoors as it will increase the heat capacity of the air.

One additional test I might try one day is to use a fan for cooling the leaves while a plant with cool roots is exposed to sun.

Skeet

Skeeter, interesting experiment, thanks for posting.

Do post back if you try the fan. My prediction would be that the fan will worsen the leaf drop, because increased water loss due to transpiration will more than offset any cooling effect.

If leaf overheating is the reason that the leaf uses transpiration and evaporation and if the fan can provide the cooling by conducting the heat to air --the leaf may not need to transpire and may not increase leaf water loss. It is possible, however, that since the leaf may have to open the stomates to conduct photosynthesis it will loose water in that process and since the roots will be inactive and can't resupply the water, the leaf drop problem will still occur, eventhough it did not when cooling was supplied by misting which also supplied water.

Skeet

Today I set 2 plants out around 9 AM-- Air temp was 58 and it was a little windy (10- 15 mph), soil temp (measured near the center of the pot) was slightly above 50 and rose to approximatly 58 by 10 AM, humidity was 75%. One plant was misted anytime the leaves had dried. The other plant was left alone. It was partly cloudy -- so the sun was not continuous, but it was out at least 50% of the time. Both plants had a new tender leaf that was only partly developed. The young tender leaf on plant that was not misted did begin to cup upward, but did not wilt. The misted tree never showed any signs of stress.

I believe the wind and the clouds kept the leaf cool and prevented any severe stress from overheating the leaves and cause wilting. The duration was a little short due to the rapid warmup. Next time I get a calm, cold but sunny day, maybe I will take a couple of the small key lime seedlings and expose one to a fan while one is just exposed to the sun.

Skeet

Most WLD occurs following a sunny day. WLD is a problem with plants placed inside a cool room. and the foliage placed directly in the rays of the sun coming through a south facing window. Therefore the foliage has no breeze nor mist to cool the leaf surface. - Millet

Skeet, 70 to 80 percent of the tree's roots in round containers are unfortunately growing (circling) near the outer part of the container near the containers walls, and not in the middle of the pot. However, this is not true for trees growing in Air Root Pruning containers. Therefore taking the root temperatures about one inch inside the edge of the container will give you a better reading of the actual temperature of the roots. -Millet

Hi Millet,
I believe your explanation of the WLD, I was just trying to explore the factors affecting the leaf heating and cooling. I clearly saw the effects earlier this fall with droopy leaves in the early sun when the containers had been left out overnight and the roots were cool. However, I think these simple test show that keeping the leaves cool by a variety of ways (darkness, misting or a gentle flow of cool air) all work to prevent WLD.

Most of these seedling range from 2 to 6 in tall (5 to 15 leaves), but I have them in gallon pots (I know I probably overpotted), So right now I think most of the roots are still near the middle.

I haven't heard you say much about humidity, but I think that the major role of humidity in WLD is that it increases heat capacity (the ability of air to cool or heat by contact). However, it may have a direct role in reducing dehydration of the leaf as well.

Skeet