by Silas Sativarius
Perhaps the biggest mistake most growers make is thinking that some magical nutrient combination will have a dramatic impact on their plants growth rate. It’s easy to understand why—nutrient manufacturers all insist that their products will “maximize yields” or “boost your buds” and the like. Unfortunately, the vast majority of these claims are overrated. While the perfect nutrient combination can significantly increase growth under the right circumstances, the reality is that a plant’s growth rate or metabolism will only go as fast as the weakest link in a system of several key factors. These factors that influence plant growth rate, in order of importance, are:
- Water / Humidity/ Roots
Over the next few weeks I will discuss each of these topics separately in detail, but they are so completely interdependent that it is impossible to fully understand them in pieces without a proper overview. So this first installment will provide a fairly detailed outline of the very complex relationship between Temperature, Co2, water, and plant metabolism. Bear with me here, because though it’s long and a bit complicated, it’s very, very IMPORTANT!
LIGHT & INFRA-RED
Both sunlight and HID light sources produce copious amounts of Infra-Red (IR) radiation. IR is effectively heat, but it only exhibits that heat when absorbed by something. This means that IR can pass innocuously through the air having little effect on its temperature because air is transparent and absorbs little IR. But when it strikes the leaf of a plant, it is absorbed and it heats the leaf. There is some evidence to suggest that IR closest to the visible range can enhance the red band absorption of chlorophyll, but IR’s effect on the plants leaf temperature and metabolism are more pronounced. As an example, if your room temperature is 78f using HIDs, the IR striking the leaves will increase the effective leaf temperature by 5-7f. This means that while you THINK your room is operating at 78f degrees, the plant is actually seeing 83-85f. However, this IR induced leaf heating decreases from the top to the bottom of the plant, concurrently with the visible light.
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 accidental CO2 levels above 10,000ppm with no ill effects, but I HIGHLY recommend that no one run their room above 3000ppm, and ALWAYS 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 beneficial bacteria 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 pressure in 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.
- Tip #3 – Lower dark period air temperatures or nutrient tank temperatures enough to keep root zone temperature below 75f. Use a probe type soil thermometer to monitor soil zone temps on-going.
As the temperature and metabolism rate increases, the amount of water the plant has to transpire to cool the plant, retain the critical internal pressure, and transport the increased nutrients necessary for the higher growth rates, must also increase. The increase in overall metabolism and photosynthesis will inherently drive more water transpiration, but if the ambient relative humidity is too high, the water being transpired out of the leaf pores (stomata) will begin to condense on the leaf (and create very high humidity microclimes) and create resistance to transpiration, as well as create a perfect moist environment for pathogens like mildew. So as temperature/transpiration requirements increase, it is helpful to decrease relative humidity to assist in the evaporation of the water transpired from the leaf AND increase air movement in the room.
A good rule of thumb is if your ambient temperature is above 80 f, your humidity must be kept around 50%. (REVISED) Below 45% RH root evaporation becomes a problem.
As you increase temperature and/or decrease the humidity, you INCREASE root zone water evaporation when using a soiless or soil based media. Evaporation in this context represents water that evaporates from the root zone without being used by the plant. This evaporation will leave behind either higher dissolved nutrient levels (ppm) in the root zone moisture than originally supplied at feeding; or if allowed to fully dry, the unused nutrients become exposed to air and oxidize forming salts (the white stains on your pots) which are harmful to the root system and will raise the root zone PH–not good. So as temperatures go up, and/or humidity goes down, root zone evaporation goes up, and with it the potential for salt buildup or nutrient toxicity.
- Tip #4 – With higher temps and/or lower humidity, nutrient ppm levels provided should actually be reduced moderately and/or you should water with plain (non-reverse osmosis) PH adjusted water or RO with 1/3rd strength ppms periodically to keep those root zone ppms from rising.
Filtered tap water is actually better than Reverse Osmosis (RO) in this context because it can supply your plants some extra calcium and magnesium missing in RO water. Also, pot / media covers or other ways to minimize evaporation can be very helpful when running high temp and/or low humidity conditions.
- Tip #5 – Include EDTA or other chelation agents throughout your grow to prevent salt buildup and enhance mineral availability (many high-end fertilizers already come with EDTA included). Evaporation is an especially important consideration when using LED grow lights because as mentioned previously, they lack Infra-red (IR) radiation, and thus require higher ambient air temp levels to produce similar metabolic rates to those found using HIDs.
As water transpiration increases, (on a dry, warm, sunny day, a leaf can transpire evaporate 100 percent of its own weight in water. ) water uptake from the roots proportionally increases, and this can put strain on the root capacity, and make the plant more sensitive to water stress (wilt.)
- Tip #6 – Anytime room temps are increased, they should be increased slowly, no more than 2 degrees per week to allow the roots enough time to expand their capacity.
Allowing plants to get slightly dryer than normal prior to increasing temps can also stimulate root growth, but remember, the hotter it gets, the more consistent the root zone moisture has to be maintained to avoid the potential for wilt which can stunt growth temporarily or even permanently. It only take ONE wilt episode to make a serious impact on yield once a plant is in flower, and as the buds start to ripen and get heavy, branches will often break if root moisture levels get too low. So letting pots or media get excessively dry to stimulate root growth should only be used very carefully in the vegetation stage, NEVER in flower.
- Tip #7 – use various root stimulators / enhancers the last week of veg and first week of flower to help maximize root capacity and health before flowering begins.
Also, when roots are aggressively growing, foliage/flower growth will slow as plant energy is redirected to the roots, and in flower your growing time is limited. Therefore it is best to get your roots slowly acclimated to the higher temps in vegetation mode, reaching the max temp set-point you will run at the latest, by the end of the 2nd week of flower when most of the root development and pre-flower stem growth is finishing but the actual flowers are just beginning. You do NOT want to stress the plants in any way during flower.
- Tip #8 – For the last 2-3 weeks of flower, room temperatures and CO2 levels should be reduced back down to ambient levels (78f / 400-450ppmm) to encourage and enhance ripening which triggers an increase in flower resins and aroma production.
Lowering temperature during ripening will also help reduce evaporation, which becomes increasingly problematic late in the flower cycle, because salt build-up is cumulative and thus most prevalent (and always detrimental) towards the end of the flower cycle.
- Tip #9 – Watering (not flushing) with plain water or 1/3 strength nutes more frequently, i.e. +-50% of feedings, during the ripening stage is strongly advised.
Flushing additives containing EDTA (Final Phase etc..) or other chelation agents can also help breakdown and prevent salt buildup. But Warning, Adding significant EBDT to a root mass saturated with high ppms will break down trapped salts creating more ppms, as well as making the existing ppms far more absorbable, and it can KILL a plant. So always flush thoroughly with tap water (or very low strength nutes) first before using high levels of EDTA. (I will discuss this more in the the upcoming “Root Zone: article)
Also, it is a common misconception that a “final flush” (i.e. eliminating all nutrients for the last week or two) improves the “taste” of the final product by reducing “minerals” from the buds. The amount of “minerals” (technically they are colloidal elements) at any moment traveling in a plant regardless of the feeding levels is microscopic compared to the weight of the plant, and the vast majority of those miniscule levels are carried in the xylem (plant vascular system) found in the stems and leaves, NOT the flowers. So the only real, functional benefit of flushing is it reduces chlorophyll levels, i.e. it yellows out the plant, and this will improve the taste, reducing the need for curing because it is the green chlorophyll that causes the harsh taste often wrongly ascribed to “minerals.”
However, if you reduce the chlorophyll, you reduce the plants energy source and with it all aspect of growth including resin. Bottom line: flushing to the point of significant plant yellowing will improve taste, but it does it at the possible expense of resin and fragrance production, so you may be better served to flush only enough to prevent salts, and remove the chlorophyll later during the drying/curing process.
- Tip #10 – Keep plants green all the way to harvest. (see pic above) Eliminate chlorophyll after harvest with a carefully controlled, slow 60deg F / 60% RH drying / curing procedure, instead of using aggressive flushing (nutrient starvation) to yellow (pre-cure)plants.
It is clear that you can achieve substantial benefits through increasing temperature, (especially with LEDs,) in conjunction with proportional CO2 enrichment. But with increases in ambient temperature and CO2 come correspondingly higher demands for optimal root zone conditions, nutrients level and humidity level control. Therefore, as you run your temperatures higher, the complexity of your grow increases exponentially. For this reason, it is HIGHLY recommended that all changes be done in very small increments, and that each change be tested through an entire harvest cycle before the next incremental change is made. But if done carefully and scientifically, tremendous gains in yield and quality can be realized.
To those who grow, WE SALUTE YOU!