Pruning in the heat, leaves are swamp coolers?

[hemmy]

During stretches of 100F+ summer heat, is there pruning work that could be done on juniper, deciduous, and broadleaf evergreens?

Obviously in some places, those temps are just called “summer” and I guess the work must go on. Although stressing the tree through wound healing or foliage mass loss might be compounded during heat waves. I’ve seen 94F cited as the point where photosynthesis slows down.

 

[hemmy]

To expand, trees move water through capillary action driven by transpiration at the leaf stomata. The cooling effect of evapo-transpiration keep the leaves cool. I’ve seen it cited that 5–10% of water transport is photosynthesis and the remainder is for cooling. It seems to me that the water molecules being carried up to the leaves in the xylem would be adsorbing heat and releasing heat during the evaporation. I didn’t find much on heat transport in the xylem.

This presentation:
http://www.sfu.ca/~jstockie/papers/Talks/potsdam12.pdf
states:
“A parametric study of heat transport indicates that:

Sap flow has a minor effect on temperature distribution, concentrated near the roots.

Solar heat flux introduces an upward ‘shift’ in temperature.”

I’m not sure exactly what the 2nd part means, but I take it all to mean that transpiration doesn’t have much cooling impact on the root mass and evaporation from the soil surface is a larger factor in heat loss.

Thoughts?

 

[hemmy]

All of this, because I’ve been busy and my J. prostrata semi-cascade has become a landscape shrub. I’d like thin it out and cut back to branch outlines. But we are finally getting the summer heat with 7 days of +102F. We have had a pretty mild summer with only a couple days above 100F in my yard. Thanks to the heat dome being firmly parked over OK and TX.

My thought is to wait a week as sub-90F temps are forecasted. But if the needles are only cooling themselves, then what harm does removing the foliage mass do to the branches, xylem network, and roots?

Energy replacement through photosynthesis might be delayed until cooler temps. But a few days shouldn’t matter if the temps are cooling?

 

[Deep Sea Diver]

Interesting ideas.

Reviewed the presentation. It’s a good start to model the sap flow, especially of the freeze side of the equation. (Yet this is a fairly complex situation to model)

However the other side off the model (heating) does not appear to model transportation at extraordinary high temperatures such as you describe, except to briefly mention cavitation (and the need to find a way to unite the models)

The heating side seems to lack the effect on this model of a trees’ stomatal closure mechanism at high heat, nor the slowing and interruption of flow due to cavitation in the vascular vessels.

Getting back to your question. A couple points to consider…

a. Photosynthesis slows at the extraordinary high temperatures you describe.

b. It’s due to combination of a number of things. Among these: Stomata closure, cavitation, biochemical process ‘breakdown’ at high temps, etc

c. Pruning in these conditions may result in greater loss of fluid, vascular flow interruption in affected portions of the tree, perhaps resulting in loss of overall cooling capacity, slowing energy acquisition, cooling and healing of wounds, and die off in discrete portions of the tree(s)

d. Another point to consider is the study is modeling a broadleaf species (maple), vs a conifers which is evolved to withstand higher temperatures than most maples.

e. On the other hand waiting until temperatures moderate would turn the situation described in c.. above around, resulting a better. healthier result.

So short story, it appears there is a lot to be lost by acting now and a lot to be gained by waiting.

Good luck,
DSD sends

 

[hemmy]

However the other side off the model (heating) does not appear to model transportation at extraordinary high temperatures such as you describe, except to briefly mention cavitation (and the need to find a way to unite the models)

The heating side seems to lack the effect on this model of a trees’ stomatal closure mechanism at high heat, nor the slowing and interruption of flow due to cavitation in the vascular vessels.

Thank you, those are good points.

So short story, it appears there is a lot to be lost by acting now and a lot to be gained by waiting.

Agreed! Plus the humidity is +55% so the heat index was breaking 110F. Another good reason to put off being outside!

 

[Japonicus]

@hemmy will you be updating this thread with the process and results
or does it have its own dedicated thread?

I am wrapping up my juniper work (sort of) for the season as we are getting closer and closer
to frost and the end of the growing season. I am still removing down growing shoots.
Nothing on top during this current heat wave, but will resume some top branch work once back in the 80's.
What I thinned in July is back budding nicely now. So + 5 or 6 weeks from today if I were to thin,
gets me to the end of the better part of the growing season. What's a young bud to do till Spring?
Resist the wind and transpiration I suppose.

Nice semi cascade

 

[hemmy]

will you be updating this thread with the process and results
or does it have its own dedicated thread?

I’m going wait a week to thin and clean, assuming the forecast holds for temps back below 95F.

And probably not, and it does not. But I guess I should start one. It gets neglected, because I’ve just never been happy with the branch pad definition and silhouette.

Thank you for the compliment.

 

[Shibui]

Pruning in the heat does not appear to cause problems, except sometimes sunburn when shading foliage is removed and previously shaded bark or foliage is suddenly exposed to full sun.
1. The branch you pruned off does not draw water to provide evaporative cooling but it's gone so doesn't need cooling any more. all remaining foliage still draws water so continues to cool themselves.
2. Loss of photosynthesis is not a huge problem. All plants have evolved to store more energy than they need to cope with regular loss of foliage - wind, browsing, etc, etc. Temporary losses can be managed - otherwise we would never be able to prune.
3. Trunks do not need cooling or get cooling from water transport. It's only when the water evaporates that cooling occurs. Some thinner bark species can get sunburn branches and trunks, not because of reduced water transport but by sun shining directly on previously protected bark that has not developed proper sunscreen. Juniper has thicker bark protected by old, dead layers so sunburnt bark is not a problem.
4. Juniper foliage is generally sun hardy so sunburn on suddenly exposed foliage should not be the same problem it is on more tender species like maples.
5. Avoid the heat for yourself but no need to worry too much about the junipers.

 

[Deep Sea Diver]

Pruning in the heat does not appear to cause problems, except sometimes sunburn when shading foliage is removed and previously shaded bark or foliage is suddenly exposed to full sun.
1. The branch you pruned off does not draw water to provide evaporative cooling but it's gone so doesn't need cooling any more. all remaining foliage still draws water so continues to cool themselves.
2. Loss of photosynthesis is not a huge problem. All plants have evolved to store more energy than they need to cope with regular loss of foliage - wind, browsing, etc, etc. Temporary losses can be managed - otherwise we would never be able to prune.
3. Trunks do not need cooling or get cooling from water transport. It's only when the water evaporates that cooling occurs. Some thinner bark species can get sunburn branches and trunks, not because of reduced water transport but by sun shining directly on previously protected bark that has not developed proper sunscreen. Juniper has thicker bark protected by old, dead layers so sunburnt bark is not a problem.
4. Juniper foliage is generally sun hardy so sunburn on suddenly exposed foliage should not be the same problem it is on more tender species like maples.
5. Avoid the heat for yourself but no need to worry too much about the junipers.

Wondering….

Do folks commonly prune bonsai down under in temperatures around 40C?

cheers
DSD sends

 

[Michael P]

Very timely discussion since our temperatures are above 105F every day with high levels of ozone, and overnight temperatures that never fall below the mid 80s. I would like to do some work on junipers and tropicals, but my intuition tells me not to. I would really like to hear from people who regularly experience this type of weather.

 

[Cajunrider]

Wondering….

Do folks commonly prune bonsai down under in temperatures around 40C?

cheers
DSD sends

I dunno about down under but down south, in the past 3 weeks the high temp during the day has been consistently above 100 F. I continue my work on pruning and shaping all my bonsai (BC, JBP, Mayhaw, all tropicals etc.) except for Japanese maples. Those poor JMs dry to a crisp despite being moved to shade and water often.

 

[SmallTreeGuy]

Very timely discussion since our temperatures are above 105F every day with high levels of ozone, and overnight temperatures that never fall below the mid 80s. I would like to do some work on junipers and tropicals, but my intuition tells me not to. I would really like to hear from people who regularly experience this type of weather.

Heard that! Dallas is smoking hot

 

[Michael P]

 

[Shibui]

Do folks commonly prune bonsai down under in temperatures around 40C?

I prune trees whenever they need pruning whatever the weather and temps.
Commercial orchards now do most pruning in summer down here. I have not heard of no prune advice in hot weather. If that was the case the orchard trees would probably never get done.

 

[Deep Sea Diver]

Hmm…. So sorry. Actually wasn’t interested in commercial grower operations, pretty familiar with these, nor generally pruning bonsai in the summertime at odd times as I do this regularly myself.

Yet as an inquisitive person, I am truly interested if folks commonly prune bonsai down under in temperatures around 40C? If so, which species can be pruned at these temperatures and are there any limitations/rules of thumb folks use when doing so?

cheers
DSD sends

 

[BrianBay9]

Perhaps a more relevant caution is do not defoliate in excessive heat. Doesn't apply to junipers, obviously, but to deciduous and broad leaf evergreens.

 

[roberthu]

Hmm…. So sorry. Actually wasn’t interested in commercial grower operations, pretty familiar with these, nor generally pruning bonsai in the summertime at odd times as I do this regularly myself.

Yet as an inquisitive person, I am truly interested if folks commonly prune bonsai down under in temperatures around 40C? If so, which species can be pruned at these temperatures and are there any limitations/rules of thumb folks use when doing so?

cheers
DSD sends

Late spring, early summer and late fall and winter are the only times I prune. Most of my trees are in development or primary branch development by the way.

 

[hemmy]

Perhaps a more relevant caution is do not defoliate in excessive heat. Doesn't apply to junipers, obviously, but to deciduous and broad leaf evergreens.

Yes! But why not? What’s the underlying mechanisms at risk?

Let’s say partially defoliating a trident in +100F, under shade cloth. Water transpiration will be restricted. Does thinning the leaf mass reduce the capillary pressure on the sap draw to the remaining leaves?

Junipers move sap more slowly, but why wouldn’t this same risk apply? Or is it the slow speed that prevents damage?

 

[BrianBay9]

Yes! But why not? What’s the underlying mechanisms at risk?

I don't want to risk tender, new buds popping in 100+ heat and getting fried.

 

[hemmy]

I don't want to risk tender, new buds popping in 100+ heat and getting fried.

Definitely a valid concern in your location with high temps and low humidity!

But given a 2wk response time for budding after pruning, I’d have to never prune after late May/early June in my locale to be sure to avoid a potential 100F heat wave which would be at the limit of the weather long range forecast.

Of course my +100F generally comes with 50% humidity. The hot drying winds tend not to come until late summer.