These thoughts have been cooking in my brain the past year and I haven’t really had time to put it in text. This is purely a discussion and model on paper, not reaffirmed through live, multiple sample, multi-variable experiments. Bonsai is largely anecdotal which there is nothing wrong with. Something is to be said about the multi-generation, accrued experience that refine techniques and allow bonsai to be practiced at the highest levels where it is today. But anecdotal knowledge is just that. A personal account or observed response to a set of conditions experienced by that individual. While it can bear truths and contain value, we should be hesitant to extrapolate such knowledge quickly without a BIG contextual and considered asterisk that says BUT. 

In the midst of a state wide, maybe historic, heat wave I wanted to talk about watering the foliage in hot weather. It’s a contested statement of whether you should water foliage or not, with observations of the foliage getting burned. The commonly attributed reason for such is that water droplets create magnifying glasses. This is unlikely to be the case as bound droplets to leaf surface are unable to create a centered and concentrated beam of light. Exceptions would be plants with very extended stem/leaf hairs or very waxy or hydrophobic leaf surfaces that result in suspended or more spherical water droplets.

Both in my discussions with other professionals, even my Oyakata in Japan, had told me not to water foliage when the temperature is very high. Personally I’ve observed some small amounts of burning post foliar watering on a hot day, but not consistently and not on everything. Also the tree may have just been normally sunburned, it’s hard for me to conclude it was from the water. And I probably won’t run an experiment to see what conditions will burn my bonsai. This really made me think about under what conditions this may be held true and if it has any relevance or warranting concern for bonsai practitioners.

My thought for a long time was that the most efficient mechanism for a plant or foliage to cool itself is via transpiration of water via the stomata into the air. Water within a leaf typically maintains at or near saturation (think 100% humidity), which is typically higher than ambient conditions. Because of the difference in vapor pressure, water is then transpired out of the leaf and through that process pulling heat out of the plant. When we coat foliage with water, we’re not just cooling the foliage but changing the mechanism by which heat is transferred. 

a mountain covered in clouds and trees

Let’s consider all the major mechanisms for heat transfer in the model of a leaf. Firstly, there is radiation which is generated from the sun hitting the leaf surface, some of which is reflected. Heat can then be transferred via conduction or migration between other solid/liquid interfaces. There is convection occurring with the air and water. And lastly evaporation or transpiration from the stomata. 

We can say for a steady state system where energy in = energy out

The First Law of Thermodynamics | Physics

 

Thermal Energy gained = Thermal Energy loss

Absorbed radiation = reflected radiation + convection + conduction + evaporation

 

When foliage is covered with water, for the covered stomata bearing surface area we are effectively halting transpiration which relies on a vapor pressure difference of water in the leaf vs outside of the leaf. With “ 100% ” water on both sides nothing is then transpired. Heat then must rely on existing methods of convection (heat transfer via the air) as well as conduction+convection through the leaf water-droplet interface. 

What we must discern is whether the heat transferred by means of conduction and convection through water is greater than that of the transpiration through the now covered stomata. Does the plant cool faster with water droplets coating the leaf or are existing mechanisms of transpiration and convection more efficient? If the latter is true it means your plant cools faster with wet foliage. If not it means by watering your foliage you actually decrease the rate of cooling in the leaf, which conditionally could be detrimental to its health. Exceeding temperatures of 42C or 107F many temperate species may experience degradation of metabolic processes, denaturing of proteins, and other irreversible damage.

I’ll be completely honest in that I only took one course in thermodynamics in college and I forgot almost everything. Fortunately for me, the energy model of a leaf has already been modeled and we can vet this idea with hypothetical numbers and existing literature. This article consists of my thoughts after affirming a concept with existing literature. 

I will skip defining all the variables and just review the relevant concepts and factors. As previously discussed, a leaf absorbs thermal radiation from the sun. While some is reflected, this absorbed heat exits the leaf through 2 primary mechanisms. Convection via the leaf air interface and transpiration through the stomata. 

Convection - Definition, Heat Transfer, Types, Examples, and FAQs

An assumption I will make here is that the difference from cooling by air is negligible from cooling by water. This is not necessarily true, as convection by water is significantly more efficient than air. However there is more to factor such as wind speed and temperature of ambient air and water. This starts to complicate this discussion, so we’ll simplify this case study and purely evaluate how impactful transpiration is. 

Firstly to restate, Heat in = Heat out

So the thermal energy or radiation from the sun causes the leaf temperature to increase. This accrued heat can be released by means of transpiration, convection, and minimally reflected off the leaf. I will reference literature where the leaf temperature is evaluated with and without the presence of transpiration, at different values of total absorbed radiation defined by Q in W/m^2. Just think of the last part as different sun intensities. Here is the defined model for reference:

Anyways we’ll just review the relevant parts. 

First let’s define how hot the leaf gets if the leaf was in a vacuum and could not cooled by convection or transpiration. 

For Q = 800 W/m2 the leaf temperature is 75.2 C

For Q = 600 W/m2 the leaf temperature is 51.1 C

For Q = 400 W/m2 the leaft temperature is 19.8 C

Now let’s add in convection, while omitting the effects of transpiration. By using the following formula we can see how only convection, or transfer of heat via the leaf to air interface, influences leaf temperature. As a brief note at higher wind velocities (V) and smaller leaf diameters (D) both transpiration and convection rates are increased. This is not relevant to what we are vetting, but the successive charts map the leaf temperature influence at varying V/D ratios.

Anyways we can observe that convection definitely plays a role in cooling leaf temperature. This effect is increased for small leaf diameters or high wind velocities. We can also make another inference. When sunlight is very low such as in the evenings (Q = 400), the ambient air temperature is higher than the leaf temp. This results in the leaf temperature increasing.

And finally we can incorporate the effects of transpiration. We have to draw some additional assumptions about the relative humidity or humidity outside of the leaf, as well as resistance to water leaving the leaf. In this model rH was defined as 50% and r = 100 s/m

This is too many numbers for some people so just just focus on the relevant columns. The temperature difference with and without transpiration for V/D = 1

Notice there is an observed 40% additional reduction in leaf temperature for the highest observed Q value. The relative difference between transpiration and convection is diminished as Q decreases. Another pertinent note from the original chart, that when V/D is high or conditions are very windy or leaf diameter is very thin/small the relative difference between transpiration and convection is also diminished.  

So what does this mean? At higher temperatures, transpiration plays a significant role in cooling of the leaf compared to solely convection by air. This suggests that at extreme weather conditions, let’s say in excess of 100+ F if we water our foliage with water, conditionally this may result in heat being trapped LONGER in the foliage. Assuming the non transpired heat is significant enough, this can attribute to damage in the foliage or burning! So maybe, just maybe as the reality of climate change/instability is readily observable and more hobbyists experience extreme heat watering the foliage may not be a good idea. Perhaps utilizing things like shade cloth to protect foliage or insulating pots with towels/cloth to prevent them from heating in the sun is a more reliable method. And if you are going to water the foliage of your plants anyways, DRENCH them so you can utilize the cooling effects of convection by water to the fullest and immediately cool the foliage. Misting plants in 100 degree weather is probably the worst option, as micronized droplets will quickly adjust to ambient air temperature so you get less cooling benefit and you also prevent transpiration. ***Additional notes in the end

Additionally I noted that for thinner leaf types or in windy conditions the effect of transpiration is diminished. The referenced article discusses this in regards to boundary layer resistance of air around the foliage, I won’t discuss this in my post but you can read it yourself if interested. There is a secondary implication I wanted to state that as the plant loses water/turgidity and the internal vapor pressure of water is reduced, convection plays a more significant role compared to transpiration. Trees also have an internal mechanism to close the stomata if the water deficit is too high. This is problematic as the tree loses 1 mechanism to cool itself. 

But in average temperate climates, especially if you are not experiencing extreme heat this concern is less relevant. I will still water the foliage of my plants in low 90 degree weather with no observable detriment. Anyways, just food for thought. Think critically, and clearly. In reality, we are all just dabbling and making inferences from our observations.

For people who just only read the conclusion here is my synopsis:

In 100+ temperatures on temperate species do not mist the foliage of your trees. If you do water the foliage, drench the foliage and ensure the water coming out of your hose is not warm or hot. Sub 100 temps it is highly unlikely that watering your foliage will burn the leaves. In hot weather it is likely more effective to keep trees well watered and roots cool as well as utilize shade cloth, so that the plant can continue to transpire and cool itself.  

Up next I’m going to feed my bonsai an infusion of HB-101, decomposed cat food, while whispering to my trees everyday and tell you how healthy it will make my trees. 

 

Reference: See chapter 11

University of Washington Center for Quantitative Science, Updated by the TrEnCh Project. (2020b, November 23). Physical Processes in ecosystems https://bookdown.org/huckley/Physical_Processes_In_Ecosystems/

 

***Regarding some of my comments, I hypothesized that in misting the foliage in high temperatures there could be an ill effect where small water droplets (quickly adjusting to ambient temperature conditions) landing on the leaf prevent transpiration while not providing significant enough cooling benefit. This would result in a net decrease rate in loss of heat which could cause the foliage to burn.

I posted this article on my social media and received some great comments from Addison Galambos to point out some holes in my statements. First is that there is still evaporative heat loss from the water on the foliage. Evaporation is a similar mechanism to transpiration however as a physical process its not subject to the resistance of water moving through the plant tissue and is more efficient. Water would evaporate, drawing heat from the leaf, perhaps minimally by natural convection from the air, as well as direct radiation from the sun. It would be interesting to compare the rate of heat loss compared to transpiration (which is primarily transferring heat from out of the leaf) vs evaporation which is more efficient, involving higher local amounts of water, but also is transferring heat from 2 additional sources not just the leaf.