There are several different reasons why your pot leaves are turning yellow. A variety of factors cause chlorosis, the technical name for a reduction of chlorophyll that results in yellow leaves. This isn’t a definitive list; however, it’s always important to properly diagnose an issue before attempting to solve it. So here are four reasons your weed leaves are yellowing—and how to deal with them properly for a heavier harvest.
Not Enough Light
During photosynthesis, leaves take in light and carbon dioxide (CO2) and convert it into plant energy. Without enough light, leaves will begin to yellow and eventually slow growth to a standstill. Common incandescent house bulbs are severely insufficient, and fluorescent lights must be kept quite close to plants to be remotely effective.
The Fix: Increase the amount of light the plant is getting. This could mean lowering an existing grow light to the proper level above your plants’ canopy or investing in a stronger lighting unit. I highly recommend using HID (High Intensity Discharge) lighting, such as MH (Metal Halide) or HPS (High-Pressure Sodium) lighting for growing pot plants indoors. LED (Light Emitting Diodes) and Compact Fluorescents are a decent, if not perfect, alternative if heat or power usage is an issue.
Over or Under-Watering
Marijuana plants like a wet-dry cycle for their roots. Over- and under-watered plants will droop and soon show the telltale signs of chlorosis.
The Fix: Stop watering over-watered plants and increase watering for under-watered ones. Sounds easy, but it’s one of the most common mistakes beginner growers make. Lift your containers if you can to get an idea of what they feel like when soaked and how much less they weigh when dry.
pH or potential hydrogen is the measurement on a scale of 1-14 of the acidity or alkalinity of a soil mix or nutrient solution, with 7 being neutral. Soil pH should be kept between 6-7, while hydroponic pH should be 5.5-6.2.
Fluctuations outside these parameters will lead to nutrient lockout, preventing your roots from being able to take in food. Often misdiagnosed as a deficiency of nitrogen or iron, an undetected pH imbalance can compound problems further when more nutes are added. This creates an over-abundance of plant food in your root zone that your plants cannot absorb.
The Fix: Use a pH meter to measure the level of acidity or alkalinity of your soil and nutrient solution. Adjust using pH up or down accordingly. Bear in mind that these solutions come in concentrated form, so add them sparingly to raise or lower pH incrementally.
If all other factors are in balance—light, water and pH—then the most likely culprit is a lack of food for your plants. Nitrogen and iron are the most common deficiencies that cause yellowing leaves, but it could be any number of macro or micronutrients as well.
The Fix: Water with a nutrient solution high in nitrogen. Plant food bottles typically display NPK ratio on the labels. N is for nitrogen, P for phosphorus and K for potassium. Choose the nutrient with a higher number at the beginning. They’re labels will say “Grow” or “Vegetative” as opposed to “Bloom” or “Flowering.” If you decide you have a lack of iron, foliar feed with chelated iron. You should see your leaves greening up within a few days.