I am currently doing extensive testing in the interest of identifying all the areas that need to change to get HID yields using LED’s in terms of grams / per square foot of canopy, not just per watt. I will be posting those results in this forum, but what follows are some tips and tricks courtesy of the folks at California Lightworks (CLW) that can help optimize your results using LED grow lights.
PS.. A special heads up to all my peeps, CLW is giving away LED growlights for 3 months on the CLW Facebook page!
Tips & Tricks for Optimizing LED yields
LIGHT – A Flat, Even Canopy
Light Intensity is the only aspect of plant growth that is not dependent on other factors. You want to provide the same optimum level of light to as much of the FLOWER structure as possible, whenever the lights are on. This is particularly important with LED’s because by nature they are less diffused, i.e. more directional (how directional of course depends on the optics systems used) and so while far more energy and chlorophyll efficient; they are also more prone to shading. When Flowering with LED’s, expert growers report that best results are achieved using Sea of Green (SOG) or SCROG techniques, (just Google SCROG). But in short, you want the flowering canopy DEPTH to be no more than 12-16” with all parts of all flowers seeing the light if possible. All flowers if any below this level should be pruned by end of the second week of flower (to push plant energy into the higher more optimal flowering tops,) and higher large sun leaves – if they are shading lower flowers – should be gradually (1 leaf / plant / 3-5 days) pruned by the 4th week to insure all remaining flowers have maximum light exposure. The top of the canopy should be in general a consistent 16” to 24” from the light source. The more consistent the better.
TEMPERATURE – Run Hotter
Temperature is the primary metabolic regulator in plants. Basic rule, the hotter it is, the faster they grow. But as the temperature / plant metabolism increases, plants require more water, CO2, oxygen (roots only) and nutrients in the correct ratios, and the plant will only grow as fast as the weakest link. 78 degrees Fahrenheit (78f) is the widely accepted optimum temperature for most plants at atmospheric CO2 levels (360PPM). But this number is actually misleading, because this recommendation was based on tests done with Sunlight or HID lighting, both of which have very high levels of InfraRed light (IR).
When a leaf absorbs IR, it heats the leaf creating actual leaf temperatures of 5-7 degrees higher than the 78 degree air temperature. Thus with light sources rich in IR, an air temp of 78f is functionally 83-85f to the plants metabolism. LED’s create no IR, so to achieve similar metabolic rates found in HID or greenhouse conditions at 78f air temp, LED growers need to run the room air temp at 83-85f. Air temp can also be dropped 3-5f in the last 2 weeks of flower to enhance ripening, but remember, humidity goes up when air temps go down, so be prepared.
Air temps higher than 85f can be run successfully with 1000 ppm (or higher) CO2 supplementation. The higher the temp, the higher the CO2 necessary (NOTE: above 3000ppm CO2 is dangerous to humans.) But running the air temp higher than 85f generally REQUIRES proportionally cooler dark period air temps to maintain an average root zone temp below 75f. Oxygen saturation in water drops off rapidly above 75f, so allowing the root zone temp to get much higher than 75f will starve the roots for oxygen no matter how much oxygenation you provide to your nutrient tanks. Also, root pathogens prefer high-temp, low-oxygen conditions. So running room air temps above 85f can be challenging for all but the most experienced growers. If attempted, it is recommended that you implement a root zone probe-type thermometer in your warmest pot/container/media to help you monitor and take steps if necessary to maintain the root zone at or below 75f. The probe should be placed at least 4” down into the media and 1” from the outside of the pot. Cooler nutrient tank (60-75f) temps can also help maintain the root zone below 75f without having to change the dark period air temp. But nutrients should never be cooler than 60f at the time of watering. Unfortunately, every grow environment is unique, and there is no substitute for personal experience, so start low, i.e. 83-85f air temp, and gradually work your way up over successive grows.
NUTRIENTS / HUMIDITY – Use less Nutrients
As room temperatures get higher, water evaporation from pots/media increases, which then increases nutrient concentrations in the dryer regions of the media. This process can create toxic levels in the root zone if not addressed. So with the higher room temps required for optimal LED results, expert growers recommend reducing nutrient concentrations slightly (around 10-20%) to compensate. This does NOT apply to true hydroponic systems, only soil or soiless medias, (peat, rock-wool etc.) An alternative to lower nutrient levels is to water with plain RO PH adjusted water 1 out of every 2-3 watering cycles to balance the root zone nutrient levels. This is extremely important with air pots i.e. fabric or other highly aerated pots with soiless media. But in general, the number one mistake inexperienced growers make is OVER FERTILIZING. Every element has to be balanced and optimal to create optimum yields.
Higher relative humidity (RH) levels will also reduce pot nutrient evaporation, with optimum RH levels around 50% (60% max.) This is somewhat dependent on the strain of plant and its mildew tolerance. In the last 2-3 weeks of flower, however, it is recommended that humidity be kept below 50% (40% min.) to retard Botrytis flower molds. Higher humidity levels also inhibit two-spotted spider mite growth.