So one of my questions is, For those who grow with LED, how high do you keep your room temps to promote aspiration? (I'm also running 1500ppm co2). THe concept is fairly new to me, as I'm an old-school HPS farmer, and these are way different.
Running Co2 with low temps is sort of but not a total waste of gas and money. I'm told that with the new LED tech running a lot cooler than HID that when using LEDS one has to assure the enviro has higher temps to utilize the co2 more efficiently.
And here is a snippet to support my claim,
LEDs
It’s a common misconception that
LED grow lights cannot compete with HID purely in terms of yield. Actually LEDs simply require an increase in ambient temperature for comparable results with lower energy consumption and heat generation. LED-based light sources differ from sunlight and HID not only in their duo-chromatic, photo-synthetically tailored spectral output, but also in the fact that they produce virtually NO IR. So a plant growing in a room with HIDs at 78f will actually exhibit the metabolism of a plant at 83-85f, while the same plant in a room with LEDs at 78f will only have a 78f metabolism rate.
- Tip #1 -So when you flower with LEDs, you must raise the room temperature 5-7 degrees f higher than you would run with HID, with all other conditions equal.
This is the main reason why LEDs have historically appeared to not perform as well as HID in flower. If you optimize all conditions for the unique requirements of LEDs, yields and quality will equal those seen with HIDs. (See the post “Tips and Tricks for using LEDs” for more) And remember, when running higher daytime temps for LEDs, the night temps (or Nutrient tank temps) must be correspondingly lowered to maintain <75f in the root zone, as well as your nutrient ppms reduced to offset evaporation.
TEMPERATURE, CO2, & WATER TRANSPIRATION
Temperature effectively acts as the plant’s throttle. The hotter the plant gets, the faster it grows, to a point at which it STOPS growing because one or more related systems can’t keep up. The rule of thumb is that plant growth doubles for every 10 degrees Fahrenheit (10f) increase in temperature, if all other factors are optimum. (
http://www.sjsu.edu/faculty/watkins/CO2plants.htm)This point at which plant growth stops is determined by a complex relationship of Temperature, CO2, and Water transpiration.
As the temperature goes up, CO2 requirements also go up. Contrary to popular opinion, university studies have shown plants can grow healthily with Co2 levels exceeding 20,000ppm! BUT BEWARE: CO2 levels above 3000-4000ppm are potentially harmful to HUMANS, and extended exposure above 5000ppm can be fatal. I have personally spent short periods of time (3-5 minutes) at
accidentalCO2 levels above 10,000ppm with no ill effects, but I HIGHLY recommend that no one run their room above 3000ppm, andALWAYS observed your CO2 levels BEFORE entering a CO2 enriched grow room.
The widely accepted optimum daytime (light period) temperature for cannabis at atmospheric CO2 levels (360ppm) is 78 degrees. This, however, is based on sunlight or HID lights which are rich in
Infra-Red radiation, as mentioned previously. Formal university research into the effects of enhanced CO2 and higher temperatures has not been conducted specifically on cannabis, but their performance has been extensively tested on various
C3 plants (95% of all plants are C3 including cannabis).
While benefits among different plants vary widely, they have all demonstrated consistent and dramatic increases in photosynthetic activity and growth with elevated temperature and CO2 levels. Also, the
medicinal properties of plants have been shown to increase with CO2 supplementation. Again no data exists in this regard for Cannabis. I have observed excellent results at an 83-85f room temp running 2000 ppm CO2 (which is as high as the controller will go.)
- TIP #2 – To maximize the results of the elevated CO2 levels (and minimize wasted CO2), the temperature must increase proportionally. General rule: starting at ambient 78f/360ppm increase your temp 1 degree F for every 300ppm of CO2 enrichment.
But as important is the fact that, the higher the CO2 levels go, the higher the point at which the plant shuts down into “protection mode” due to extreme temperature, and this can be especially helpful in hot summers. So it begs the question,
what is the upper limit for temperature, if CO2 and all other factors are optimized? Is there one?
Unfortunately, yes. There is one critical factor that provides a practical upper limit to running higher and higher room temperatures and corresponding CO2 levels, and the resulting higher plant metabolic rates. That factor is the root-zone temperature. Average root-zone temperatures should NEVER exceed 75f, and in fact are optimum around 65f-70f.
The root zone needs to stay below 75f because dissolved oxygen saturation in water dramatically decreases above 70f, regardless of the amount of air bubbling in your tank. This drop in oxygen saturation decreases root growth and health, and encourages the growth of anaerobic (non-oxygen) root pathogens. All
beneficialbacteria and fungi (exp. Mycorrhiza) need
oxygen. Almost all plant
pathogens are
anaerobic and prefer
low or no oxygen conditions. And while drying the root zone out by under-watering does increase oxygen to the roots, it is extremely risky to let the root zone get too dry when running high temperatures because if the
turgor pressurein the plant drops below the safe threshold (i.e. wilt), the hotter the air is, the faster the plant will be irreparably damaged or die once wilting begins.
For wet hydroponic/aeroponic systems, your root zone temperature is basically a function or your water/nutrient tank temperature, so in this case the root-zone temp can be controlled by employing a nutrient tank chiller. But for soiless or soil-based grows in pots or other mediums (rock wool etc.) if kept moist, the root zone has a fairly high
thermal mass and the root temperature will generally oscillate around the average temperature (halfway point) between the highest light period temp and the lowest dark period temp. The larger the root zone thermal mass (pot size, tank size etc..) the less variance (oscillation) the root zone temp will see from the average.
For example, if you run your light period air temp at 90f, your dark period air temp will have to drop to 60f to achieve an average temp of 75f in the root zone. And remember, 75f is the
high limit, and an average of 65-70f is
ideal. (With soiless media, you can also employ a nutrient tank
chiller to cool your nutrient tank to 60f to 65f, to allow for higher dark period air temperatures, but one study has suggested that a 10 degree f differential for day to night temps was optimal for growth and yield, and
enhances flowering. )
It is highly recommended that if you decide to run your light period room temps above 85f in a quest to increase your yield, that you implement a
probe-type soil thermometer permanently in the root zone to allow you to actively monitor, on-going, the root zone temp in at least one sample plant, preferably the hottest one. For pots, place the probe at least 6 inches down and 1” from the side of the pot and log it every day at the beginning and the end of the lighting period, to see the amount of root-zone temperature swing, and to insure your root zone temp does NOT exceed 75f.
