Disclaimer:
This article will discuss using some pretty high RH levels, so I want to make it clear this discussion only applies to growth stages before you get significantly dense flowers which usually happens around week 5. After about week 5, your focus should shift towards lowering humidity (raising Leaf VPD) to prevent Botrytis and PM.
Understanding the Air/Leaf temperature differential is key to unraveling the forces that drive transpiration.
As you probably already know, the VPD relationship is a way of understanding how environmental pressures act on a plant to either stimulate or suppress water movement up through the plant and out through the stomata, in the process called transpiration. This movement of water is critically important for two reasons. First is that it brings up nutrition in the water, and second is that it removes heat through evaporative cooling. That loss of heat causes the temperature of the leaves to drop below room temperature and the faster the water moves, the farther the temperature drops.
When transpiration is functioning at maximum rates, this can cause temperature differences of up to 9C or 16F.
The type of light sources in the room determine how much of a difference there is between the room temp and the leaf temp. Some light sources like HPS, emit invisible radiation that warms up the surface of the leaves. This causes the net temperature change to be reduced. LED lights emit very little radiation so you tend to see the full temperature change under this type of light source.
Because of this, when you measure the leaf temp, you can expect to get different ranges of temperature difference depending on whether your room uses HPS or LED lights.
For example:
- HPS – the maximum Leaf / Air temp differences can be roughly 5F, or 3C difference.
- LED – the maximum Leaf / Air temp differences can be roughly 16F, or 9C difference.
How does transpiration lower the temperature?
The general idea is that the faster the water flows through the plant, the more heat is removed and the cooler the plant will become.
On the surface this sounds simple, but in the real world the way it ends up working is an extremely complex interaction of internal conditions within the plant, and external environmental variables that drive evaporation outside the plant.
Variables that affect the leaf/air temperature difference
Each one of the variables below (and probably many others as well) has the potential to cause changes in plant health, or external evaporation forces – both of which can change the speed of transpiration.
Fixed Variables – things that stay the same
- Light fixture type (LED, HPS, Fluorescent, Magnetic Induction etc)
- Container size and material
- Irrigation delivery style
- Planting medium type
Mobile Variables – things that change throughout the day
- Air Temperature
- PPFD
- Air circulation speed
- Relative Humidity
- Irrigation frequency and duration
- Plant size
- Plant health
- Cultivar variation
- Plant water use
- EC in the planting media
- Root health
- Plants may also actively control moisture loss by closing the stomata in cases of low humidity.
Each of the variables above can change the health and vigor of the plant as well as affecting how fast evaporation happens in the area around the plant.
All these factors combine to influence the speed of transpiration in ways that are impossible to predict.
So, what’s the deal with measuring plant happiness?
Think of the speed of transpiration as if it is a measure of plant happiness.
The faster the plant transpires, the happier it is in the current environment.
You can actually measure plant happiness with a simple infra red thermometer by simply comparing the room air temperature, to the leaf temperature.
The farther the leaf temp is below the air temp, the happier your plants are.
Here’s how you do it.
Start by picking your target air temperature somewhere in the optimal photosynthetic range of roughly 76F-86F (or 25C-30C), then adjust the RH until you achieve a starting Leaf VPD of 1.0 on a VPD chart. This will get you in the ballpark.
After an hour, measure 10 leaves and get a temperature average. Compare that to the room temp. If you are lower than the room temp, you’re doing good. If you are higher than the room temp, that’s not good.
Now begin lowering the Leaf VPD by raising the RH, then wait a couple hours and measure leaf temperatures again. The temperature change will tell you if you’re going in the right direction.
If the leaves are cooler than the air – good!
~
If the leaves are the same as the air – bad!
~
If the leaves are warmer than the air – super bad!
Keep lowering the Leaf VPD by raising the RH and you’ll eventually see the leaf temperature drop below the room temperature. You may need to increase the humidity far more than you expect but you’ll see the air and leaf temperatures getting farther and farther apart until you reach the optimal speed and then after that it will drop back down if you go above the optimal humidity.
The highest point will be your optimal temp/RH setting for that day, but it may or may not work again tomorrow, depending on all the unknown variables.
Actual Farm Results to Compare
The following examples show two very different air temperatures that achieved the same result of maximum transpiration.
Even though the temperature difference in the upper farm was FAR greater, remember there is a completely different range of what you can expect depending on the light source used in the room. The upper room used LED lights so max transpiration is a 9C spread. The lower room used HPS lights so the max transpiration creates a 3C spread. Even though the numbers are different, the two room actually have an equally high transpiration rate.
The truly interesting part is that the leaf temperature (in red) was essentially the same in both examples.
This is clear evidence in support of the concept that you must run your room warmer when using LED lights.
- Air temp: 30C / 86F
- Leaf temp 21C
- RH 50%
- LVPD 0.7
- Temp difference 9C
LED Lights – Warm Room
This room was running high temperature with a very low Leaf VPD which achieved the highest transpiration rate I’ve seen.
- Temp: 25C / 77F
- Leaf Temp: 22C
- RH: 52%
- LVPD: 1.2
- Temp difference: 3C
HPS Lights – Cool Room
This room achieved the same temperature spread which indicates the same transpiration rate as the room above. However this was achieved while using a very moderate temperature which is at the bottom of the recommended temperature for cannabis.
Is temperature differential the same as Leaf VPD?
Leaf VPD is a combination of Air Temp, Humidity, AND the Leaf/Air temperature difference, so this is a relationship showing how all three aspects combine together. When you have all three aspects combined in one Leaf VPD number, you can find a theoretical number that will give you a good transpiration rate, but it doesn’t include the influences of all the unpredictable variables in the list above that are impossible to include in any mathematical formula.
The theoretically proper Leaf VPD numbers are close to meaningless when compared to actually measuring transpiration to see what the plant prefers.
Live measurement of leaf temperature difference automatically includes the influences of any unseen and unpredictable variables as it provides a very exact look at how fast your plants are transpiring.
Is there a perfect Leaf VPD?
I’ve always been told that there are very specific LVPD ranges for each stage of the grow cycle. Perhaps those numbers are well intended best guesses among plant scientists, but this process provides a very direct way to see exactly what numbers the plant itself actually prefers. This is the plant talking back to us, and the results do NOT match the old LVPD numbers we’ve been given.
While the numbers may be wrong, the concept of Leaf VPD holds true as it is the single most important mover of transpiration.
This reinforces the commonly held concept that using Leaf VPD is a great steering mechanism for transpiration, but it also reveals that the relationship between the two is not as predictable as we’ve been told – mostly because it doesn’t account for the influence of all the unpredictable variables in the room.
General concepts:
- There seems to be both a ceiling and a floor to this concept. If LVPD goes above roughly 1.4, that appears to be the ceiling and transpiration begins slowing down RAPIDLY immediately after passing above 1.4 Leaf VPD.
- If LVPD drops much below about 0.5, this also seems to cause a crash in transpiration ie, the “floor.” However, some farms get super high transpiration at numbers way down in the 0.6 to 0.7 range – which is impossible according to humans.The exact upper and lower tipping points are dependent on what air temperature you start with, as well as influences from all the other things in the list of variables.
- In Veg, the highest functioning rooms I’ve seen run high RH using LVPD numbers as low as 0.6.
- Night time transpiration is just as critical as in the day because the plant must get O2 in through the stomata and emit CO2. The same critical line of 1.4 LVPD seems to hold true both day and night.
- Transpiration drives like a Cadillac – it smoothes right over little spikes in the environment and hardly seems to notice until the change becomes a trend.
- PPFD and CO2 seem to have very little noticeable effect on transpiration speed, although they obviously have effects on plant growth, so they might have a longer term contribution to transpiration rates, but not anything you can notice changing on a daily basis.
- Normally, temp and RH are connected, so changing temperature will also change RH. However, if you manipulate RH externally by either adding or subtracting actual moisture (which is different from RH) using a Dehu or Humidifier to alter the actual moisture levels in the air.
- Optimal Temp/RH combinations that are on the high side of the temperature range are where you can achieve the fastest transpiration and optimal photosynthesis, but I would caution to always start out in a more moderate temperature range and work up slowly by raising your target temperature a couple degrees at a time.
- Leaf temps in the 21-23C range seem to be the optimal, but getting that requires higher air temperatures which depend on the type of lights.
There is no single recipe for success.
Looking at the long list of variables, and the examples from two farms (above), you can see that its possible to achieve high transpiration rates in a wide variety of conditions, but it is not so simple that someone can simply tell you to use ___ temperature and ___ LVPD and everything will be fine.
This process is a method of experimentation. You make a small change, and then use this process to measure the temperature difference and that tells you if you went the right direction, or not.
This method is a tool you can use to drive transpiration using temperature and humidity very much like the brake and gas pedals in a car.
Please leave a comment and let me know if this works for you.
Interested in working with the author?
If you have any interest in implementing this concept to elevate transpiration on your farm, get in touch and I would be happy to work with you as an independent consultant.
Email: Glen
To give credit where credit is due….
This concept is derived from my work as a farm consultant through Neatleaf where I’ve spent the past three years using the Spyder and Dashboard features to view farms all over the US.
The ability to compare the environmental conditions on roughly 25 farms at a time is the only way I could have seen this pattern emerging.
The Neatleaf Spyder is an absolutely incredible and revolutionary tool that brings plant science concepts together with solid data that you can use to manage your own farms remotely from anywhere in the world.
I’d be happy to tell you more about the system and provide an introduction to the owners if you’re interested in implementing this tool on your own farm.