Yellowing appears most on the parts of the plant closest to the light. Yellow leaves do not pull out easily, even if the whole leaf is dead Light burn often takes a few weeks to develop and is most common once the plant is past the 6th week of the flowering stage (when plants aren’t making many new leaves to replace old ones). Cannabis light burn usually affects the top leaves closest to the grow light instead of affecting the plant evenly.
Light burn is when your leaves are working too hard for too long, causing them to die early. Even if the temperature is in a good range, your plant can still get light burn if the grow light is too close. It’s kind of like how skiers can get sunburned even in the freezing temperatures because of all the sunlight reflecting off the snow. Light burn is most common with powerful lights like HPS/LED/LEC. It’s also common when switching to new bulbs (which are stronger than old bulbs) or when there is no glass between the bulb and your plants. Some plants are more sensitive than others, and you may have one plant suffering from light burn while the others are fine. That can make it harder to diagnose the problem since some of your plants are thriving in the same environment!
Light burn symptoms can be different from plant to plant, but they always seem to happen mostly to the parts of the plant that are closest to the light. 5.) Temperature Problems (Heat Stress / Cold Shock) Yellow or burnt leaves near the light General yellowing of upper leaves Leaves start “turning up” at the edges, or forming “tacos” How Do Growers Get It? If you put your hand where your plants are and hold it there for 30 seconds, is it too hot to be comfortable? If it’s too hot for you it’s likely too hot for your plants. Although relatively rare indoors since most growers struggle with heat instead of cold, a temperature under 50°F (10°C) can also cause pale or yellow leaves. Some plants will even die if it hits freezing temperatures! Placing grow containers directly on concrete in a basement can kill them with cold overnight! This poor plant was decimated by a heat wave – it went through several days of 100°F+ temperatures! Luckily the buds were still great 🙂 Too much heat can cause the edges of leaves to curl upwards and make “tacos”. Sometimes extended periods of high temperatures causes spots and other odd symptoms in addition to yellowing. This plant was exposed to temperatures under 40°F (5°C) at night, causing all the newest growth to turn so pale yellow it almost looked white! Yellowing in between the veins on leaves, often located lower down on the plant. A magnesium deficiency is almost always caused by incorrect pH though if you’re using heavily purified or soft water (such as RO – reverse osmosis – water) you may need a Cal-Mag supplement to make sure your plant is getting enough magnesium. It should be in the 6.0-7.0 range for soil growers and 5.5-6.5 for everyone else. If a Magnesium deficiency persists, consider getting a CaliMagic supplement that is made for plants (you should always add Magnesium and Calcium at the same time because these two nutrients work together in the cannabis plant). With a magnesium deficiency, the yellowing happens between the veins of the leaves, while the veins stay green. Sometimes Triggered by Old Age / Natural Senescence / Light Deprivation. It’s actually normal if you only see these symptoms on a few leaves at the bottom of the plant that are no longer getting any light. The plant eventually “gives up” on old leaves if they spend days or weeks without light, which often happens to the lowest leaves at the plant gets bigger. If this is the case, the leaves often seem droopy, limp and tired. These leaves don’t “stick straight out” like normal leaves because the plant isn’t wasting resources by putting energy into them. This is most common when using relatively weak grow lights like fluorescent lighting or CFLs, since the light doesn’t easily reach the bottom of the plant. Therefore this symptom is only something to worry about if it’s happening on leaves that are still getting light, or if you’re seeing the symptoms on many different leaves instead of just an occasional leaf here and there.
Iron deficiencies are unique because the yellowing always affects the newest growth; it does not happen to older leaves that are already green. Unlike most other nutrient deficiencies that cause yellowing, yellow leaves from an iron deficiency will usually turn green, starting from the outside edges and working inwards. Unless you are using RO or very purified water, an iron deficiency is almost always caused by incorrect pH.
This is because cannabis needs very little iron, and most sources of water already contain trace amounts of iron. The pH being too high or too low is the most likely the cause of this problem. Bring your pH into the correct range and iron deficiencies will just go away. If using purified water or water that doesn’t contain much natural iron, you may need a Cal-Mag supplement which includes iron like CaliMagic.