Downfoemythang
Well-Known Member
Anyone ever experience drops of water on a leaf when checking in on their plants in the morning? Not alot of water, just a drop or 2 on a leaf.
Unexplained Water on the leaves
- Thread starter Downfoemythang
- Start date
M4A1
Well-Known Member
Yeah I have. It was due to my exhaust fan not being on all nigth long. Here's my thread and some pictures of what it looked like the moring of it happening.
https://www.rollitup.org/general-marijuana-growing/206280-did-rain-inside-my-grow.html
https://www.rollitup.org/general-marijuana-growing/206280-did-rain-inside-my-grow.html
Downfoemythang
Well-Known Member
Thanks for that info. I was looking at the water like WTF!!!!? Where could this have come from?
the big d
Active Member
Its condensation, I think. Had the same issue with condensation on my lid and dripping down. You should clear it up or the lights will magnify through them water beads and burn your leaves, not to mention the high humidity there could be. I added a fan to move a little air and it did the trick.
It's dew. During the light cycle, your plants are transpiring (releasing water into the atmosphere), but the humidity doesn't condense because the temperature is high enough to hold the water (As air temperature increases, its capacity to hold water increases as well). However, once the lights go out and the temperature drops, the room hits a critical point called the "dew point" (where the water content of the air exceeds the air's capacity to hold it). At this point the water condenses to form droplets like the ones you saw on your leaves. The dew that you see on your lawn on a cool morning is caused by the exact same phenomenon.
Generally, a little bit of dew isn't going to hurt, but it should be discouraged. Too much of it, especially in late flowering, can cause problems like mold and/or bud rot. There are two things you can do to reduce or eliminate this potential problem. 1) Keep your extraction fan(s) and oscilating fan(s) going during the dark cycle, this will ensure that excess humidty is being removed and air is being circulated. 2) Use a dehumidifier to keep the relative humidty below 50%.
Generally, a little bit of dew isn't going to hurt, but it should be discouraged. Too much of it, especially in late flowering, can cause problems like mold and/or bud rot. There are two things you can do to reduce or eliminate this potential problem. 1) Keep your extraction fan(s) and oscilating fan(s) going during the dark cycle, this will ensure that excess humidty is being removed and air is being circulated. 2) Use a dehumidifier to keep the relative humidty below 50%.
Downfoemythang
Well-Known Member
That was informative. How the hell do you +rep someone?
Progression
Member
Heat is also a factor in that excess heat shuts down transpiration altogether, which isn't good, either. Transpiration is natural; typically about 99% of the water absorbed by the plant is lost through transpiration--only 1% is used in photosynthesis.
dbizzo
Active Member
vapour pressure deficit(VPD) is what your experiencing. Earl explained it well on another forum, here--
Thanks for the sticky frmrgrl, I am deeply honored.
I have gone into my grow room and been puzzled by a phenomena that I didn't understand. There would be water standing on a leaf that was under HID light and I was very bewildered why this was happening. I knew I had not allowed any water to spill on the plant, and I knew that my roof wasn't leaking, hell it wasn't even raining outside. So, where did this mystery water come from? Obviously the plant was transpiring water through the leaf and puddling on the top. But why? The answer is due to low vapor pressure deficit in my grow room.
If your interested in increasing your plants ability to grow in an enclosed environment, and you want to avoid diseases, and provide the best growing environment for your girls, then you need to be concerned about VPD.
So what is VPD?
Vapor pressure deficit, in ecology, is the difference between the actual water vapor pressure and the saturation of water vapor pressure at a particular temperature.
Unlike relative humidity, vapor pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation.
Plants lose moisture by transpiration from their leaves into the surrounding atmosphere.
The less moisture they lose, the more they like it.
We tend to think that the higher the relative humidity, the moister the air, the better it is for our plants, but that is only true up to a certain point.
What I am trying to show here is that relative humidity does NOT relate directly to the rate at which transpiration of water from the plant occurs.
Changes in relative humidity are not proportional to the rate of plant moisture loss.
How come?
The moisture holding capacity of air is measured in units of pressure, and there are two important measurements concerned with figuring out how much moisture a given block of air can potentially absorb.
First is the saturation vapor pressure (SVP): think of this as the maximum amount of water vapor a given block of air can hold.
Second measurement is the difference between the amount of water vapor actually in a given block of air and its SVP (i.e., the maximum amount of water it could absorb).
This difference is called the vapor pressure deficit, or VPD. Think of VPD as the water sucking power of the air, because it is actually the VPD that interests your plants, not the relative humidity.
At face value, VPD (sucking power) seems to be the same as relative humidity - because relative humidity is the ratio of the actual vapor pressure in the air to the SVP.
Its not the same, because the SVP of a given block of air increases exponentially as the air temperature rises - the higher the temperature, the greater the amount of water vapor that air can hold.
Rather than giving a physical explanation of why humidity and VPD are different measurements, because I'll get out of my depth in about two seconds, just look at how the VPD (sucking power) changes at various temperatures if the relative humidity stays the same at 75%:
Cpa represents how much water the atmosphere can absorb
VPD calculation is an improvement over relative humidity (RH) measurement alone because VPD takes into account the effect of temperature on the water holding capacity of the air, which roughly doubles with every 20°F increase in temperature.
Rather than giving a relative measure of the water content of the air, VPD gives an absolute measure of how much more water the air can hold, and how close it is to saturation.
For example, a typical 100' long x 30' wide x 10' high greenhouse with 80% relative humidity has about 14 lb of water in the air at 50°F, while 70°F air holds about 28 lb of water at the same RH.
This is reflected in the VPD values of 0.036 psi (0.25 kPa) and 0.072 psi (0.50 kPa), for the lower and higher temperature conditions, respectively (see Figure 2). Thus, VPD can be used to identify healthy air moisture conditions for plant production, while taking into account different temperature levels.
How does VPD compare to relative humidity?
Figure 1 shows how VPD relates to the customary thinking about humidity. Higher VPD means that the air has a higher capacity to hold water, stimulating water vapor transfer (transpiration) into the air in this low humidity condition. Lower VPD, on the other hand, means the air is at or near saturation, so the air cannot accept moisture from the leaf in this high humidity condition.
Figure 1. Vapor Pressure Deficit (VPD) enhances or inhibits the crops ability to transpire.
Therefore, vapor pressure deficit is a useful way to express the vapor flow in the system, both for condensation and transpiration.
Higher VPD increases the transpirational demand, influencing how much moisture from plant tissues is transferred into the greenhouse air. Consequently, VPD is being used to predict crop water needs in some commercial irrigation systems.
In contrast, very low VPD indicates closer proximity to the dew point, meaning harmful condensation can begin to develop.
Using the canopy temperature to determine VPD gives the best indication of condensation risk, showing particularly how close the canopy is to the dew point.
Looking at the temperature and vpd on a graph, you can see how the vpd is increasing exponentially as the temperature rises, while the relative humidity remains constant:
Here is another good article with a graph. VPD
VPD values run in the opposite way to RH values so when RH is high VPD is low.
If humidity is too low (i.e. high VPD), the stomata on the leaves tend to close in order to limit transpiration and prevent wilting. This closing of the stomata will also limit the rate of CO2 uptake and hence limit photosynthesis and consequently plant growth. Low humidity also reduces turgidity (water pressure within the plant cells) and this in turn also restricts growth. Blossom end rot in tomatoes and capsicum can also be attributed to low humidity (high VPD).
Conversely, if humidity is too high (i.e. low VPD) the stomata will fully open but even so the plants will be unable to evaporate enough water to carry minerals into the plant and so again, growth will be impeded and mineral deficiencies (particularly calcium) may occur. In addition, the plants may exhibit soft growth, fungal problems and mineral deficiency symptoms.
It is frequently stated that VPD more closely matches what the plant "feels" in relation to temperature and humidity and therefore forms a better basis for environment control. Unfortunately, VPD is extremely difficult to determine accurately as it is necessary to know the leaf tissue temperature. Attempts to measure leaf temperature reliably on an ongoing basis have often ended in disaster. One of the problems is that the plants leaves are in differing amounts of sun with some leaves in full sun, some in partial sun and others in full shade. This makes the concept of "leaf tissue temperature" particularly complex.
VPD can be used to identify disease-causing climate conditions. For example, several studies that explore disease pathogen survival at different climate levels reveal two critical values of VPD.
Studies show that fungal pathogens survive best below VPD (<0.43 kPa).
Furthermore, VPD is most damaging below (0.20 kPa).
Thus, the greenhouse climate should be kept above (0.20 kPa), to prevent disease and damage to crops.
How do we calculate VPD?
Well this is a problem. You need to know several temperature values and use a formula to calculate VPD. This is probably not something you are going to do everyday. There is a chart but I am not able to copy it into the post, so I will try to extrapolate it here.
I reached a couple of preliminary baseline conclusions.. assuming the target Kpa is
.9 and that leaf temp is 3ºC below air temp..
if the temperature is 85F, humidity should not be higher than 62%
if the temperature is 80F, humidity should not be higher than 58%
If the temperature is 75F, humidity should not be higher than 53%
If the temperature is 70F, humidity should not be higher than 47%
the rule of thumb seems to be a target temp of 80F should have just under 60% humidity.
An interesting observation is
that as temperature drops by 5 degrees F,
required humidity also drops by 5 %
so, 80F @ ~60%, or 70F @~50%, are good ballparks..
Here is a VPD calculator if you have all the temps to plug into it. VPD calc
Sometimes, when you think you have a nute deficiency, you are really just experiecning VPD which can mimick Mg deficiency.
Here are a couple of examples of what high VPD will do.
I just live with it.
.
Edited by Earl (03/28/08 09:06 PM)
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Thanks for the sticky frmrgrl, I am deeply honored.
I have gone into my grow room and been puzzled by a phenomena that I didn't understand. There would be water standing on a leaf that was under HID light and I was very bewildered why this was happening. I knew I had not allowed any water to spill on the plant, and I knew that my roof wasn't leaking, hell it wasn't even raining outside. So, where did this mystery water come from? Obviously the plant was transpiring water through the leaf and puddling on the top. But why? The answer is due to low vapor pressure deficit in my grow room.
If your interested in increasing your plants ability to grow in an enclosed environment, and you want to avoid diseases, and provide the best growing environment for your girls, then you need to be concerned about VPD.
So what is VPD?
Vapor pressure deficit, in ecology, is the difference between the actual water vapor pressure and the saturation of water vapor pressure at a particular temperature.
Unlike relative humidity, vapor pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation.
Plants lose moisture by transpiration from their leaves into the surrounding atmosphere.
The less moisture they lose, the more they like it.
We tend to think that the higher the relative humidity, the moister the air, the better it is for our plants, but that is only true up to a certain point.
What I am trying to show here is that relative humidity does NOT relate directly to the rate at which transpiration of water from the plant occurs.
Changes in relative humidity are not proportional to the rate of plant moisture loss.
How come?
The moisture holding capacity of air is measured in units of pressure, and there are two important measurements concerned with figuring out how much moisture a given block of air can potentially absorb.
First is the saturation vapor pressure (SVP): think of this as the maximum amount of water vapor a given block of air can hold.
Second measurement is the difference between the amount of water vapor actually in a given block of air and its SVP (i.e., the maximum amount of water it could absorb).
This difference is called the vapor pressure deficit, or VPD. Think of VPD as the water sucking power of the air, because it is actually the VPD that interests your plants, not the relative humidity.
At face value, VPD (sucking power) seems to be the same as relative humidity - because relative humidity is the ratio of the actual vapor pressure in the air to the SVP.
Its not the same, because the SVP of a given block of air increases exponentially as the air temperature rises - the higher the temperature, the greater the amount of water vapor that air can hold.
Rather than giving a physical explanation of why humidity and VPD are different measurements, because I'll get out of my depth in about two seconds, just look at how the VPD (sucking power) changes at various temperatures if the relative humidity stays the same at 75%:
Cpa represents how much water the atmosphere can absorb
VPD calculation is an improvement over relative humidity (RH) measurement alone because VPD takes into account the effect of temperature on the water holding capacity of the air, which roughly doubles with every 20°F increase in temperature.
Rather than giving a relative measure of the water content of the air, VPD gives an absolute measure of how much more water the air can hold, and how close it is to saturation.
For example, a typical 100' long x 30' wide x 10' high greenhouse with 80% relative humidity has about 14 lb of water in the air at 50°F, while 70°F air holds about 28 lb of water at the same RH.
This is reflected in the VPD values of 0.036 psi (0.25 kPa) and 0.072 psi (0.50 kPa), for the lower and higher temperature conditions, respectively (see Figure 2). Thus, VPD can be used to identify healthy air moisture conditions for plant production, while taking into account different temperature levels.
How does VPD compare to relative humidity?
Figure 1 shows how VPD relates to the customary thinking about humidity. Higher VPD means that the air has a higher capacity to hold water, stimulating water vapor transfer (transpiration) into the air in this low humidity condition. Lower VPD, on the other hand, means the air is at or near saturation, so the air cannot accept moisture from the leaf in this high humidity condition.
Figure 1. Vapor Pressure Deficit (VPD) enhances or inhibits the crops ability to transpire.
Therefore, vapor pressure deficit is a useful way to express the vapor flow in the system, both for condensation and transpiration.
Higher VPD increases the transpirational demand, influencing how much moisture from plant tissues is transferred into the greenhouse air. Consequently, VPD is being used to predict crop water needs in some commercial irrigation systems.
In contrast, very low VPD indicates closer proximity to the dew point, meaning harmful condensation can begin to develop.
Using the canopy temperature to determine VPD gives the best indication of condensation risk, showing particularly how close the canopy is to the dew point.
Looking at the temperature and vpd on a graph, you can see how the vpd is increasing exponentially as the temperature rises, while the relative humidity remains constant:
Here is another good article with a graph. VPD
VPD values run in the opposite way to RH values so when RH is high VPD is low.
If humidity is too low (i.e. high VPD), the stomata on the leaves tend to close in order to limit transpiration and prevent wilting. This closing of the stomata will also limit the rate of CO2 uptake and hence limit photosynthesis and consequently plant growth. Low humidity also reduces turgidity (water pressure within the plant cells) and this in turn also restricts growth. Blossom end rot in tomatoes and capsicum can also be attributed to low humidity (high VPD).
Conversely, if humidity is too high (i.e. low VPD) the stomata will fully open but even so the plants will be unable to evaporate enough water to carry minerals into the plant and so again, growth will be impeded and mineral deficiencies (particularly calcium) may occur. In addition, the plants may exhibit soft growth, fungal problems and mineral deficiency symptoms.
It is frequently stated that VPD more closely matches what the plant "feels" in relation to temperature and humidity and therefore forms a better basis for environment control. Unfortunately, VPD is extremely difficult to determine accurately as it is necessary to know the leaf tissue temperature. Attempts to measure leaf temperature reliably on an ongoing basis have often ended in disaster. One of the problems is that the plants leaves are in differing amounts of sun with some leaves in full sun, some in partial sun and others in full shade. This makes the concept of "leaf tissue temperature" particularly complex.
VPD can be used to identify disease-causing climate conditions. For example, several studies that explore disease pathogen survival at different climate levels reveal two critical values of VPD.
Studies show that fungal pathogens survive best below VPD (<0.43 kPa).
Furthermore, VPD is most damaging below (0.20 kPa).
Thus, the greenhouse climate should be kept above (0.20 kPa), to prevent disease and damage to crops.
How do we calculate VPD?
Well this is a problem. You need to know several temperature values and use a formula to calculate VPD. This is probably not something you are going to do everyday. There is a chart but I am not able to copy it into the post, so I will try to extrapolate it here.
I reached a couple of preliminary baseline conclusions.. assuming the target Kpa is
.9 and that leaf temp is 3ºC below air temp..
if the temperature is 85F, humidity should not be higher than 62%
if the temperature is 80F, humidity should not be higher than 58%
If the temperature is 75F, humidity should not be higher than 53%
If the temperature is 70F, humidity should not be higher than 47%
the rule of thumb seems to be a target temp of 80F should have just under 60% humidity.
An interesting observation is
that as temperature drops by 5 degrees F,
required humidity also drops by 5 %
so, 80F @ ~60%, or 70F @~50%, are good ballparks..
Here is a VPD calculator if you have all the temps to plug into it. VPD calc
Sometimes, when you think you have a nute deficiency, you are really just experiecning VPD which can mimick Mg deficiency.
Here are a couple of examples of what high VPD will do.
I just live with it.
.
Edited by Earl (03/28/08 09:06 PM)
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