Factors that influence growth and development ( Temp and VPD)

Temperature
Plant growth and development is
primarily influenced by temperatures
at the growing points of plants (i.e.,
roots and shoot tips). When we are
discussing temperature, it is important
to understand that plant temperature
(not air temperature) drives physiological
responses in plants. Air temperature can differ by as much as
10° F from plant temperature, depending on your light source (HPS,
MH, or LED), light intensity, humidity, and air speed. For example,
HPS lights emit a large percentage of their energy in the infrared (IR)
range (800nm–1000nm) which is not photosynthetically active yet
significantly increases plant temperature. As a result, growers need
to decrease their air temperature set-point to counter the additional
radiant heat.
All crops have a species-specific base temperature, at which growth
and development will not occur. Above the base temperature, growth
and developmental rates increase with temperature until an optimum
temperature is reached. Above the optimum temperature, plant
development decreases. Light intensity primarily influences
the rate of photosynthesis, while plant temperature primarily influences
developmental rates. Net photosynthesis under increased PPFD will increase as temperatures
approach the optimum temperature for
the species of plant you are growing;
however, the optimum temperature
for photosynthesis depends on the
concentration of CO2 ; it is
important to understand that as you
increase temperature, you will also
change the morphology of the plant by
increasing developmental rates. The
ratio between light intensity and temperature
is known as the photothermal
ratio. If you choose to grow at warmer
temperatures, you need to ensure that
you are providing an adequate light intensity, or you may produce
plants that have increased internode distance, small stem caliper, and
an overall spindly growth habit.
The difference between day/night temperatures (DIF) will also significantly
influence plant morphology. For example, if your day/night
air temperature is 75°/65° F you have a +DIF of 10° F, which will promote
stem elongation of most crops. Alternatively, if you have a warmer
night temperature 65°/75 °F (day/night) you will have a -DIF, which
will suppress stem elongation. Depending on the growth habit of
your crop, you will need to find a balance between temperature and
light intensity to achieve your desired plant architecture.

RH and VPD
Relative Humidity (RH) is the amount of humidity present at a
given temperature and is expressed as a percentage. When air is
completely saturated, it has a RH of 100%. Temperature and RH are
the two main variables that influence water movement within a plant.
Evapotranspiration is a process plants use to cool leaf surfaces. As
the temperature of a leaf increases, plants will pull more water from
the growing media. Water is evaporated from the leaf surface and
as a result the leaf temperature decreases. Increasing the
temperature in your controlled environment will reduce your RH,
causing an increase in transpiration rates and water demand, while
decreasing your temperatures will increase RH, causing decreased
transpiration and water demand.
A good tool to use when growing
in a controlled environment is vapor
pressure deficit (VPD). VPD is a good
indicator of plant stress brought about
by either excessive transpiration
(high VPD values) or the inability to
transpire adequately (low VPD values). When the VPD is too low (humidity too high) plants are unable to evapo¬rate enough water to en¬able the transport of mineral nutrients (such as calcium), and in cases where VPD is extremely low, water may condense onto the plant and provide a medium for fungal growth and disease. Table 8 provides VPD val¬ues based on temperature and humidity. Generally, you will want to grow your plants in the optimum VPD range. However, during establish¬ment growth (especially vegetative cuttings), optimal VPD is around 0.3 - 0.5 kPa, which is outside of the opti¬mal range in our VPD table.

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copy and pasted from fluence bioengineering. didn’t feel like writing it all…

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Keep this stuff coming. Pure gold. Thank you.

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hmm what is VPD? I used a VPD calculator and input my temp of 85 and RH of 55 are these good results? what is ideal VPD?

SVP: 4.111 kPa
VP: 2.261 kPa
VPD: 1.850 kPa
HD : 13.249 g/m

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Depends what stage of growth your in ! Low vpd for seeldlings , med vpd for growth and higher vpd for flowering!

Ideally you need actually leaf temps but for good estimation 2-3 degrees lower are what your leaves are coming in with regard to ambient room temp

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Not sure what the conversion are for those values but seems like your RH is in the ballpark for flowering however your temps seem a little high if that’s the stage your in right now

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ok i’ll check my leaf temps with my IR thermo, i’m in late veg just waiting to be confident i’ve eradicated the thrips before i flip em to flower. i’m using LEDs so i keep my ambient temp higher as the lamp doesn’t provide as much heat to the leaves

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1.850 KPa looks a tad to high for late veg box in upper right are your stages info!

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@MadScientist can you split this convo in to the VPD thread from my 1st q for me, sorry bro! :wink:

@Tinytuttle yes but at the moment i’m only able to control one of these factors which is humidity, so basically the temperature im at right now (which is 32 on the highest leaves, 28 on majority of canopy) the only way to bring it to the acceptable range would be raising my humidity to around 70-75 and actually keeping it there for the entire flowering … because i’ll be upping my wattage and increasing the temperature thru out flower … i think i’m gonna have to stop exhausting in to the room but it’s getting cold enough to see your breath so the side of my house will just be blowing hot air non stop which could be suspicious no? i mean i’m legal but i mean for robbers etc… i might just need an AC?

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Come again, please, i got lost…

Edit, you want me to join them together @legalcanada?

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to move them, i posted in the wrong thread originally to ask about VPD i didn’t see this thread before posting

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Done good sire. Your wish is my command!

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vpd is vapor pressure deficit. vapor pressure is basically the relationship between temp and RH and the pressure it creates. vapor pressure deficit is the difference between the vapor pressure and the pressure the plant creates during transpiration.
think 2 forces pushing on each other.
so if your temp is high and RH is low there is less pressure in the air so the plant transpires faster.
and the opposite is true low temp and high RH slow transpiration in the plant.
ideal range is from .45 - 1.25 kPa with a target being .85 kPa, however most plants grow well between .8 and .95 kPa.

side note: it appears the chart i posted is not in kPa. apologies.

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going off TTs chart above i’m around 1.45 in late veg but the plants seem to be doing OK

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Plants will do OK outside the vpd range, but staying within it or as close as possible you will see increased growth rates, more time between waterings, and a lot happier plants in general.

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Like Dewb said earlier in the thread plants will do fine outside the range but for optimal growth the Vpd needs to be in the sweet spot. I queued in on your post section highlighted … so in following natures sign when coming in to flower ( your increasing temp part) in fall, temps are decreasing not increasing when keeping your 40-55 Rh and keeping it a little cooler conditions you’ll hit your sweet spot!

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ok i’ve realized having an AC is unavoidable lol thanks for your help guys :slight_smile: i’ll hit that sweet spot alright :smiley:

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found this in my notes on vpd

here’s one for all the math people
needed:
ambient temp
relative humidity
canopy air temp

now the fun
to compute directly from temperature:
(do this for air temp and canopy temp)
vpsat = e^A/T+B+CT+DT^2+ET^2+FlnT
where
vpsat is saturation vapor pressure in kPa (kilopascal)
A = -1.88 x 10^4
B = -13.1
C = -1.5 x 10^-2
D = 8 x 10^-4
E = -1.69 x 10^-11
F = 6.456
T = temp of air in K (kelvin)

Temp in K = temp in C (celsius) + 273.15
Fahrenheit to celsius = ( F* - 32) x .5556

then we need the partial pressure of the vapor

vpair = vpsat x RH/100 (RH being relative humidity)

and finally the vapor pressure deficit which is the difference between our answers

vpsat - vpair = vpd
or if we have the canopy saturation (which why we did all that math for both air and canopy)
vpcanopy - vpair = vpd
(more accurate because we are looking for that balance between the pressure the plant creates and the pressure in the environment.)

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I was just trying to learn more about this…Thanks Dewb…

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