Summer watering to cool roots

[Maros]

Andrew Robson routinely advocates thoroughly soaking trees, foliage, benches and the ground around the benches multiple times per day during unusually high heat events, Rakuyo and
Crataegus had little to no casualties during our insane heat dome event in 2021 when Portland reached a ridiculous 116F, while Mirai suffered what Ryan has described as catastrophic losses.
I personally have been watering everything every few hours when temperatures are above 95F for the last few years even though I'm aware they are shut down and not "using" water and have seen
no root rot or fungal issues.
Just my 2 cents, take for what it is worth

I remember Andrew saying that in a video. In my case, with 98% of my collection consisting of deciduous trees, I approach it similarly to Andrew. Up until 30C/86F, I water twice a day. If it is hotter than that and I'm home, I water the trees to cool down pots and the surrounding area. For the second year, I'm susing 50% shade cloth on part of my garden, and it makes a big difference, I have to admit.
I understand situation could be different for conifer-centric collections.
From my experience, since I'm really watering a lot, I would agree with Jelle; it takes a lot of water to kill a tree in well-draining modern substrate. In those crazy hot summers we are experiencing in the last decade or so, I would be very careful in giving any advice on watering (especially on restricting it on hot days). I believe more trees die due to underwatering and heat damage than the other way around.
Many people still believe that watering during the middle of the day, in summer, with 35°C, is somewhat harmful, magnifying effect of water droplets is real, etc.

 

[shohin_branches]

I think people often forget that root rot is caused by a bacterial or fungal infection in the rootzone and not by water. Dead roots and moisture just gives the pathogen a place to reproduce. I have saved a spruce with root rot by doing a July repot into bonsai soil and using horticultural sanitizer on it (physan 10) but that tree was still in the nursery container and not in bonsai soil.

Root Rots on Houseplants

Root rot is a general term for any disease that causes the deterioration of a plant's root system. Learn more about root rots and how to avoid them in this factsheet.

hort.extension.wisc.edu

 

[Bonsai Obsessed]

This might be dumb, but I've thought of putting some white duct tape on my black nursery pots and/or white cloth loosely around the soil/pots on days when you could easily cook an egg on a car hood. I fear the black pots are too hot for the roots on the hottest days.

One of my cats is mostly white with black spots. She loves basking in the sun. When I pet her, the black spots are super warm, while the white is not.

You’re absolutely correct on covering dark pots in the sun! I’ve done temperature testing and the difference is dramatic. I’ve used foil as well as damp white towels to cover dark pots, temps can drop up to 40 degrees which is huge.

 

[shohin_branches]

I think people often forget that root rot is caused by a bacterial or fungal infection in the rootzone and not by water. Dead roots and moisture just gives the pathogen a place to reproduce. I have saved a spruce with root rot by doing a July repot into bonsai soil and using horticultural sanitizer on it (physan 10) but that tree was still in the nursery container and not in bonsai soil.

Root Rots on Houseplants

Root rot is a general term for any disease that causes the deterioration of a plant's root system. Learn more about root rots and how to avoid them in this factsheet.

hort.extension.wisc.edu

Correction: Physan 20
This is what I get for posting before coffee

 

[TomP]

We are consistently in the mid 90s here in middle Georgia. It is humid. My shimpaku don't seem to be suffering with fill sun all day, but I do worry about the ones in smaller, 6" shallow pots. I did lose some random branches last year. Should they be under shade cloth?

I do not put my shimpakus under shade here in Alabama and they thrive.

 

[BrianBay9]

Correction: Physan 20
This is what I get for posting before coffee

I suspect the roots start dying due to lack of oxygen, THEN the infection sets in. Frequent watering adds oxygenated water and does not contribute to dying roots. Pooling water or substrate that retains too much water leads to roots consuming all the oxygen available, then dying.

 

[rockm]

Not sure what the point is.

My point is, A singular water event is not an issue for trees in open substrate. It takes much longer to kill off your roots.
Once the pot is saturated in a storm, it is saturated. It does not matter whether that is an inch, two three or a whole bucket of water. When I water, I fully saturate the pot, similar to a big storm, And it will drain and the tree will take some up. Do not water and in a day of two the trees rootball is back to normal.

Half an hour drive north they received over 2 inches in one hour last night, which is where they got the brunt of the summerstorm. Our drainage system is built for 1 inch of precipitation an hour, so this causes localized flooding. Big difference, hete it is so flat it does not accumulate in one spot, so flooding stays most of the time marginal and confined to areas directly hit and/or the river floodplains (Where one is not supposed to build houses so all good). Last night just

Yeah once a lot is saturated it’s saturated but HOW LONG CAN IS BE SATURATED? Thats kinda the issue aint it? Three days of completed saturation four days five days? A lot of that saturation is mitigated with drainage BUT constant downpours don’t allow any soil to really dry

That can lead to issues. Sorry I’ve seen it here. May not happen in Germany it does here

 

[Vince Sartain]

My two cents, I am in hot dry Southern California and I've taken a bit of a different approach. I pot trees, over 150, with about a 70% inert and 30% composted pine bark. I also have many that are in their original black plastic nursery pots with nursery soil. I also have them in under 40% shade cloth. I use an automatic spot spitter irrigation system that also delivers a weak solution of fertilizer with every watering and I only water once a day at noon for 5 minutes. I also use an inexpensive wifi enabled moisture meter system as well as check my system everyday in case of failure of some sort. I check roots roughly once a month on a few of my finicky species. I cut back watering to every other day in fall and remove the shade cloth and reduce watering to every 2 days in winter, As far as the "cool/hot roots" issue I'm not sure I get it. The nursery trade here in Southern California has a bazillion plants in black nursery pots with nursery soil and no shade at all and as far as I can tell from all the ones I've talked with they are watering once a day, some with automatic systems and some are hand watering. And they seem to have no issue with over or underwatering or root heat issues. My point in chiming in here is, granted there are species that are more susceptible to various issues but as a whole I think too often we overcomplicate things! BTW, I probably have over 50 different species.

 

[leatherback]

HOW LONG CAN IS BE SATURATED? Thats kinda the issue aint it? Three days of completed saturation four days five days?

Absolutely no need to shout and get angry here. However, I would recommend reading back in your arguments where initially you started this whole discussion with saying that a singular rainstorm after watering already causes issues as the rootbal can remain wet for 18 hours..

As for how long? Last year we pretty much did not have more than 2 days in a row with less than 3 hours rain pretty much all summer. So all my pots were soaked. All the time. Did not have to water for weeks on end. May not happen Fairfax, it sure does here.

I am done. No sense in having a discussion is someone every songle post changes position and arguments.

 

[ZombieNick]

This thread took a weird turn. Don't worry about overwatering in Palm Springs with the mix you are using, that will not be an issue for you. The real issue is, constant watering is not the ideal solution for hot roots. I don't recommend misting or watering over the top of the trees either, because that causes it's own issues, but I am sure people will disagree with me on that. In Palm Springs I have to assume you are already under 70% shade cloth, if not, you probably should be. I think you were right to reach out to PBS for advice. While well-intended, advice from people who aren't experienced with your extreme environment may be misguided.

 

[nuttiest]

Yeah once a lot is saturated it’s saturated but HOW LONG CAN IS BE SATURATED? Thats kinda the issue aint it? Three days of completed saturation four days five days? A lot of that saturation is mitigated with drainage BUT constant downpours don’t allow any soil to really dry

That can lead to issues. Sorry I’ve seen it here. May not happen in Germany it does here

I think for me, for the trees that really should have had a repot in spring but dd not, it is something like 36 hour when I start seeing trouble... anything with rearranged soil , a soil cave, or drooping leaves goes under one of the umbrellas. No room? switch them out for others in 2 days, after 10 days mini monsoon take some out of the pot and just plop them on the benches. If you are lucky and the wood dries up, it will aid in soaking up some water, A week ago it finally ended, but went on for 22 days straight.