I am about to get 3 days of 100+F temps with 40-60% humidity in Mississippi. I am concerned about some of my more heat sensitive trees (deciduous and struggling trees). I am wondering if it would be wise to pull them inside during the peak afternoon hours? I put up a shade tent to block the sun from late morning onward. Primary concern is my cryptomeria. Seems 1 day at ~95F has already caused a few brown needles. Checking trees 3-4 times a day to ensure they do not dry out as well. I have aluminum foil on all of my black nursery containers as well. Any advice is appreciated.
My cryptomeria is browning pretty quickly too and its been extremely hot here in Mexico. I wasn’t sure it was the heat or what since the 2 previous cryptomeria sprigs i’ve had eventually met the same fate and completely browned out and I haven’t figured out what it was. I was even shy to put it in shade in case it was overwatering that was hurting it.
I see that you have a fence on two sides of your tree area and fairly deep shade. How much breeze do you get?
Trees cool themselves via transpiration. This is driven by the ability to evaporate water vapor from the surface of the leaves and photosynthesis which uses water. if you don’t have much breeze the relative humidity in the semi-enclosed space will most likely be much higher than relative humidity out in the open which will slow evaporation. The deep shade will dramatically reduce photosynthesis and further reduce the use of water.
I would recommend that you replace the solid cover of your canopy with 50-60% shade cloth to let in more light. I would also set up a fan to move the air around. An oscillating fan set on low that blows across the trees or a box fan that blows under the bench at a moderate setting to suck air down and away from the trees.
In a very dry climate such as the desert we have the opposite problem - evaporation is faster than tree can transport water to the leaves, particularly if it is breezy/windy. The shade cloth helps a bunch, but it is sometimes necessary to mist or overhead water to temporarily give the tree a break having to transport so much water so that it can rehydrate the leaves. In addition, that extra water evaporates from the leaves cools them. My hottest, driest days fall into that category so I water the leaves at least once during the day without worrying too much about the pots - trees with a large canopy and small pots also need the second or third pot soaking.
We get a fair bit of wind but you are correct the fencing does cut it down quite a bit. We are moving in about 3 weeks so I wasn’t able to think of a quick and sturdy way to put up shade cloth. I could try replacing the canopy cover with shade cloth though…
I can try to get a fan going to move some air. Not sure if I have anything large enough though.
Sounds like you are almost more concerned with the trees staying too wet?
Thanks for your suggestions!
Actually, I am concerned about the trees transporting enough water to the leaves for cooling. A small part of the cooling is due to the cooler water in the pot being transported to the leaves. A much, much bigger part is the evaporation of that water. For example, if the pot is 20C (36F) cooler than the leave then every gram of water that makes that journey absorbs by 84 J (10 Calories) of heat from the stems and leaves. The same 1 g of water will absorb 2265 J (541Calories) - nearly 27 times as much. Therefore, to cool our trees we have the maximize the tree’s ability to evaporate water from the leaves/needles.
That makes complete sense. I wonder if there is a break point of reducing ambient temp through shade vs increased transpiration with higher light/wind intensity. For example a properly watered tree in full sun at 110F vs one in full shade. The ambient air temp would probably be similar if not the same but the total radiation the plant is subjected to would obviously be much higher in full sun. I would think this is not a simple linear relationship.
Shade cloth generally comes in about 30% to 80% density. I think most of the professionals use about 50-60% density when they put it up for their bonsai.
I swapped out my solid canopy cover for some 40% shade cloth. Ill monitor and see how the trees react. If any trees start to show heat damage would it be best to pull them back to full shade or potentially indoors? I know i have heard Ryan say pulling trees indoors (a/c space) can be counter productive but he did not elaborate.
I have trees in two locations - Phoenix and western WI. each require different things. the shade cloth is a really good idea rather than the solid shade you now have. I use 30% in Phoenix which is very low for Phoenix and most people use 50-60% there. it depends on where in the yard you are at. my backyard faces north so not the same as a south facing backyard in Phoenix. I have issues with humidity so rely on pavers to store and evaporate water. the wind and fan needs should be evaluated before you just do that. get a humidistat and look at humidity in that area. suspect Mississippi is pretty humid but are you having a dry year? weather is so wierd this year. observe the air flow at your trees. depending on what directions you have that little corner in, it may not be that much of an issue. I use a similar set up in Phoenix only with a wall on the west side and don’t have issues with air flow. don’t do things without verifying they make sense for you. that is my advice.