The heat from the bulb caused massive burning everywhere it touched. If a plant’s leaves directly touches the lights, it leaves “burns” from the heat of the bulbs. This plant was green and healthy through the vegetative stage under an LED grow light, but the leaves started dying soon after flowering started (even though that distance had been fine in the vegetative stage). This is also very common with LED grow lights with just read and blue diodes, without any diodes in the green spectrum.
These plants seem apparently healthy, but the top leaves keep getting lighter and lighter, in this case from a 600W HPS that was kept just under a foot (30cm) away. The leaves slowly turned yellow over the course of a few weeks, getting light burn even though the temperature was a comfortable 75°F (24°C). These yellow leaves were caused by an LED grow light that was too close. If you don’t realize it’s light burn, the symptoms are inexplicable! A mild case of marijuana light burn is often mistaken for a nutrient deficiency or a pH problem, but if you look closely, the symptoms are concentrated directly under the grow light. Light bleaching is most common with high-power LEDs and HPS grow lights because these can be brighter than the sun. Basically, bud bleaching is what happens when buds get too much light, kinda like how hair can get bleached if you spend plenty of time in the sun. Except a “sun-burnt” bud is often less potent, and may have lost it’s “cannabis” smell!
Buds which have been bleached tend to be low potency or even have no potency (no available THC or other cannabinoids). Therefore you should avoid light-bleaching your plants at all costs! Sometimes light-bleached cannabis will get mis-labeled as “albino cannabis” or “white cannabis” but the truth is that the white color is not healthy, so this is not a desirable trait (even if it looks pretty cool). Often though, light burned buds look like they’ve been burned. LED-burnt cannabis buds – notice how all the tiny “sugar leaves” have turned yellow or brown. In this case the LED-burn caused the leaves closest to the LED to turn red. Although the buds smoked pretty well anyway, they definitely weren’t as pretty as they could have been! The leaves too close to the LED grow light turned yellow and wilted. For some reason, cannabis plants seem a lot more prone to light burn after they start flowering. Another example of a bud that has light burn from a too-close LED. Solution: If your marijuana plants are getting too much light, try removing some of the lights or moving your grow lights further away from the tops of the plants. If you can’t move the light further away, bend your plants over so the tops are further away or if your plant is still in the vegetative stage you might even consider cutting off the top of the plant to remove some of that height. Reduce power of grow lights and/or move them further away from your plant (How far away do I keep grow lights from my plants?) It is unlikely for your plants to get “light-burned” from the sun when growing outdoors, and they definitely can’t accidentally grow into the sun. Outdoor plants can show signs of light stress if plants were used to shady conditions and moved into direct sunlight without time to get accustomed to the brigher light levels. It also may be possible in extreme high light conditions if the plant is unprotected but in general cannabis plants like a lot of light. When making changes to your plant’s environment, it’s best to make changes relatively slowly if possible. For example when moving a cannabis plant from indoors or outdoors you might consider giving the plant some shade for a few days before moving it into full sunlight. When learning how to grow cannabis, it’s best to try to keep things at a comfortable temperature at all times for optimal growth. If it’s too hot for you, it’s probably too hot for your plants. Outdoors, it’s a lot harder to control temperature, but there are steps you can take to protect your outdoor plants from the heat including supplementing with sea kelp, partially covering them and making sure they’re well watered. Quick Summary: Nutrient burn or fertilizer burn is one of the most common beginner cannabis growing problems. The yellow or brown leaf tips are caused by too-high levels of nutrients at the roots, which disrupts the flow of water through the plant and causes the symptom of burnt tips on leaves. (Nutrient burn is often called “Nute Burn” in the cannabis growing community). When the roots take in more nutrients than a cannabis plant can use, the overabundance causes problem with water flow in the plant, triggering brown or bronze “burns” on the tips of your leaves.
If nutrient levels are not lowered, the burnt tips start traveling inwards and the ends of leaves start becoming crispy and twisted. As nutrient burn progresses, the tips start getting bronze, crispy, curled and sometimes twisted. Although you can stop nutrient burn from getting worse, the burnt appearance won’t go away on the leaves that were already affected. Nutrient burn is most common when feeding cannabis too-high levels of bottled nutrients and especially chemical or mineral nutrients. This is what you’ll find in most non-organic plant food. Mineral-based nutrients can help increase cannabis growth rates and yields compared to organic-based nutrients due to the fact that these nutrients are so easily absorbed by the plant roots. The plant doesn’t have to do any “work” to get the nutrients. The downside is your plant can easily take in more than it can use if the nutrient levels are too high.
Nutrient burn affects the tips of marijuana leaves. You may just see it on one or two leaves, or it can appear all over the plant.