When the smoke alarm goes off that means there’s smoke in the house.
When water starts dripping from the ceiling that means you have a leak.
Unfortunately, the early warning system built into cannabis plants isn’t that definitive.
One of the most common signals that there’s a problem during your grow is when weed leaves turn yellow. There’s obviously an issue to be addressed — but what is it, and what should you do?
We’d love to provide a simple answer and wrap this discussion up in a paragraph or two, but there are more than half a dozen different reasons why the beautiful green leaves on a cannabis plant might start to yellow.
Let’s take them one at a time, and figure out what can be done to fix the problem.
When to Worry About Yellow Leaves
Botanists have a name to describe leaves naturally turning yellow: chlorosis. That’s a fancy way to say that leaves are losing their chlorophyll and dying.
Chlorosis occurs at the end of a growing cycle as winter approaches. Think about trees’ leaves turning color and falling off; that’s called senescence, and it happens to cannabis plants, too.
If your weed leaves turn yellow in the last few weeks of the flowering stage, there’s nothing to be concerned about. It’s even more likely to happen if you flush your plants (to remove nutrient buildup) before harvesting them.
Yellow leaves that appear in the middle of a grow, however, indicate that there’s a problem that must be addressed.
Yellow Weed Leaves and Grower Error
No matter how careful you think you’re being with your marijuana plants, there’s always a good chance that you’ve done something wrong.
Those grower errors are among the most common causes of yellow weed leaves.
Both overwatering and under-watering can cause leaves to turn yellow. If you give your plants too much water, that lowers the amount of oxygen in the soil and essentially starves the plants to death. Not enough water and they can’t survive for long.
Tons of water dripping from the pot (indoors) or wet soil will obviously be one clue that you’re overwatering your plants, but the leaves will also contain so much water that they droop and swell up. It’s less common that people under-water their weed plants, but dry soil and brittle, thin leaves will be the tipoff.
It’s simple in most cases: adjust the amount of water you give your plants. In most cases, that will quickly correct the issue and the leaves will turn green again. If that doesn’t do the trick, you may be using poor-quality soil or the wrong-sized containers, preventing proper drainage.
Light Burn or Deficiency
Getting the lighting right in indoor grow rooms can be tricky. Leaves on young plants that aren’t getting enough light may turn yellow (leaves on older plants will turn brown). Leaves and buds on older plants that are closest to the grow lights may turn yellow if they’re getting too much light.
Leaves toward the bottom of young plants that are suffering from light deficiency will turn yellow first. Check the amount of space on the stem between leaves; if it’s longer than the width of a leaf, the plant’s probably not getting enough light.
Light burn normally becomes a problem well into the flowering stage. If the top leaves are turning yellow and they can’t be pulled out easily from the stem, that’s a sign that the lights are burning them.
Start by increasing the amount of light the plants receive (for light deficiency) or reducing it (for light burn). Just repositioning the lights will normally do the trick, but seedlings getting too little light may need stronger grow lights.
pH Imbalance in the Soil
Home growers don’t often think about issues like pH (potential hydrogen), which measures how acidic their soil is. They should because a pH imbalance makes it difficult for their plants to get the proper nutrients and causes what’s called nutrient lockout — making their leaves turn yellow.
In addition to yellow leaves, you may see “leaf burn” around the edges or stripes, patches, or spots on the leaves. Inexpensive testers can measure the pH in your soil; it should be between 6.0 and 7.0 on a scale that runs from 1-14. Make sure the testing kit also measures calcium carbonate concentrate levels which can cause high pH.
Either transplant the plants into the soil with the proper pH or flush the plants with water and then adjust the soil’s pH. You can add lime to increase the pH or aluminum sulfate to lower it. Garden shops also sell additives designed to do the job.
One other note: the pH of water can also affect plants. Check the pH of your water source to make sure that isn’t causing the issue.
A lack of sufficient nutrients can cause weed leaves to turn yellow, and there can be many culprits.
- Nitrogen Deficiency: Older leaves turn yellow, buds may appear earlier than expected or be fewer than anticipated.
- Sulfur Deficiency: Fan leaves turn yellow and fragile, and grow smaller than usual.
- Iron Deficiency: New leaves on the plant turn yellow, followed by the veins in older leaves taking on the same color.
- Potassium Deficiency: Fan leaves turn yellow and curl, and the leaves may develop brown spots or brown tips.
- Magnesium Deficiency: Yellow veins develop in fan leaves, which curl up after that.
- Zinc Deficiency: Leaves show yellow veins, become wrinkled, and turn to the side.
- Calcium Deficiency: Plants and leaves show stunted and distorted growth; eventually lower leaves curl and develop yellowish-brown spots.
Increase the missing mineral content in the plant’s nutrient mix. Also check the pH of the soil and water, because improper pH could be contributing to the problem.
Yellow Weed Leaves and the Environment
Some causes of weed leaves turning yellow can’t be easily prevented when you’re growing outdoors, but they may be fixable in indoor grows.
- Temperature: When it gets above 85° cannabis plants can suffer heat stress; when it gets below 50° the temperature can induce cold shock. Both can cause leaves to turn yellow. Better temperature control and airflow are necessary.
- Pests and Fungus: Fungus gnats and thrips attack weed leaves, and the damage can turn leaves yellow before they die. Leaf septoria begins as black spots on leaves, which will then turn yellow and wilt. Root rot can lead to yellow leaves, as can bud rot. Careful inspection to look for pests or fungus, followed by immediate treatment, is crucial if the plants are to be saved.
Weed Leaves Turning Yellow: FAQ
Q: Is the end of the flowering stage the only time you don’t have to worry about leaves turning yellow?
A: There’s one other case in which it’s normal. When a young weed plant has grown more than a few sets of leaves, the bottom ones will almost always turn yellow and fall off. As long as you’re only seeing lower fan leaves with three “points” or fewer turning yellow, your plants are fine.
Q: Will applying the solutions you’ve suggested save plants whose leaves are turning yellow?
A: In most cases, yes, as long as you don’t take too long to correct the issues. The only exceptions are serious environmental problems like bud rot or root rot, which must be caught very early in order to save the plant.
Q: Can’t I just cut off the yellow leaves?
A: You can, but the ones that turn yellow aren’t outliers. They’re symptoms of a systemic problem. If you don’t find and solve what’s causing it to happen, more leaves will turn yellow and the plant may eventually succumb to the underlying issue.
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- Bevan, L., Jones, M., & Zheng, Y. (2021). Optimisation of Nitrogen, Phosphorus, and Potassium for Soilless Production of Cannabis sativa in the Flowering Stage Using Response Surface Analysis. Frontiers in plant science, 2587. 
- Xu, F., Vaziriyeganeh, M., & Zwiazek, J. J. (2020). Effects of pH and mineral nutrition on growth and physiological responses of trembling aspen (Populus tremuloides), jack pine (Pinus banksiana), and white spruce (Picea glauca) seedlings in sand culture. Plants, 9(6), 682.