Yellow Leaves on Your Cannabis Plant? Causes and How to Fix Them

The most common reason a cannabis leaf turns yellow is stress. Whether it's bad watering, too much heat, off pH, a nutrient deficiency or a pest infestation, yellow leaves outside of late flower are a sign of an unhappy plant and should be dealt with as soon as possible. To understand the science behind it, start by looking at what's inside a typical leaf and how that ties into the plant's overall health.

Leaves are made of plant cells. Plant cells contain organelles called chloroplasts. These chloroplasts hold pigments that absorb different wavelengths of light. Most of those pigments are green chlorophyll, but there are others in the mix. The most notable are yellow and orange carotenoids, hiding underneath the dominant chlorophyll.

Chlorophyll's job is to absorb sunlight and turn it into energy through a process called photosynthesis. When plants have plenty of resources, chlorophyll thrives. But when plants get stressed, chlorophyll starts breaking down, revealing the yellow carotenoids underneath.

Carotenoids can't turn light into energy directly through photosynthesis; they have to pass it along to chlorophyll to finish the job. So while a few yellow leaves won't shut down photosynthesis, heavy or untreated yellowing can stunt the plant or kill it outright.

Why do cannabis leaves turn yellow?

When your plant is in flower, yellow leaves are considered a natural process, triggered both by the plant's aging (senescence) and by the soil running out of nutrients — especially nitrogen, which drives chlorophyll and therefore the green color.

Many growers find that keeping nitrogen low toward the end of flower gives better bud production and a cleaner, more defined flavor, and some report a bump in potency too, since excess nitrogen late in flower tends to push leaf growth instead of buds.

Still, while yellow leaves are common in flower, they're not normal at other stages. So it's important to check where your plants are in the grow.

Yellow leaves during veg

In the vegetative stage, cannabis leaves turn yellow when the growing medium runs out of nitrogen. As it gets used up, the plant taps its "reserves," moving nitrogen from older leaves to new growth. That causes yellowing that starts at the bottom and works its way up, reaching new shoots in severe cases.

pH problems and poor nutrient uptake are known to cause this symptom too, and cannabis plants are heavy feeders, so running out of nutrients is a really common problem. Here are a few tips:

• At this point you'll often need to transplant, especially in the plant's first months. A ton of growers around the world make the mistake of waiting for the lower leaves to yellow before doing the first transplant.

• A simple, effective fix in these cases is to transplant into a bigger container with a mix rich in compost, worm castings or even amendments like blood meal, at about one teaspoon per five liters (roughly 1.3 gallons) of growing medium. The great thing is you can also combine it with beneficial bacteria and products that boost nutrient uptake.

• On the synthetic side, you'll find a wide range of fertilizers with different nitrogen concentrations that make feeding easy. With so many concentrations to choose from, you can pick one that fits the plant's age, your soil type and your grow plan.

Within a few days you'll see new growth coming in darker green, while the yellowing stops spreading. That's a sign the plant is getting its nutrients and putting out healthy new growth.

What should you do when cannabis leaves turn yellow?

The first thing to do when yellow cannabis leaves show up is check the pH of your growing medium (soil, water, etc.). That's because pH that's out of balance, whether too high or too low, can lock out nutrient uptake.

Watering problems

Once you've measured and adjusted the pH,  take a look at your watering schedule. One of the most common causes of yellow leaves on weed plants is overwatering or underwatering. Overwatered plants have leaves that look swollen and droopy, while underwatered plants look thin and brittle. Poor drainage can also lead to overwatering, so always grow in containers with drainage holes.

Nutrient deficiencies

This is often the number-one reason leaves turn yellow. These are the elements that cause chlorosis (yellowing from lost chlorophyll) when they're not getting where they need to go:

Nitrogen

This is one of the most common causes of yellow leaves on a weed plant. You can catch it in time if you notice the leaves turning pale green before they go yellow at the edges. The symptom usually starts on the lower leaves and slowly works its way up.

If you don't treat it in time, the stems take on red or purple tones and the leaves start curling and dying.

It usually shows up in the growth stage, and to fix it you need to check whether your growing medium is good quality and whether you're using the right fertilizers. It can also happen that you've overfed and the plant can't take up the nutrients it needs.

Potassium

Potassium is a key mineral for plant development, playing a role in photosynthesis and in how the stomata work. If your plant is short on it, you'll see yellow leaves along with brown spots starting to form on them.

Calcium and magnesium

If the leaves start yellowing at the end of flowering, like we said, there's no reason to worry. At that point, the plants are putting all their energy into building buds, so the rest of the plant doesn't get as many nutrients.

If it happens at the start of flowering, though, there's a good chance the plant is short on calcium or magnesium.

Yellow cannabis leaves can also come from a lack of other secondary nutrients and micronutrients like zinc, iron, manganese or sulfur.

You'll find plenty of products to fix yellowing leaves, but the most recommended is CalMag, a calcium-magnesium supplement.

Light burn

If yellow leaves show up at the top of the plant (specifically, closest to the light), your plants are probably dealing with light burn, not a nutrient deficiency. Light burn or heat stress in cannabis can happen in a temperature-controlled room just as easily as in a hot one if the leaves get too close to the lights. The best way to fix light burn is to move the plant away from the light, usually to somewhere between 20 and 50 cm (8–20 in), depending on the wattage and type of lighting. 

On top of that, it's important to keep all your environmental factors in check, and to watch for pests indoors and out, to keep yellow leaves off your cannabis plants.

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