My cannabis plants are looking yellow. How can I fix this?

When the leaves of a plant start to show changes in colour and shape, they’re telling us that something’s wrong. Learning the real reason behind this is essential for plants to recover so that they can continue growing as usual and without any additional surprises. There is a wide range of factors that can make a cannabis plant wither, but not all of them require the same course of action. Therefore, the first step is to always identify the origin of the deterioration.

The first rule of thumb is to keep calm and not to rush to remove all the yellowish leaves that you find. Remember that the leaves are the tool that the plant uses to collect the energy from the sun, so if you remove them you’ll be seriously affecting photosynthesis. And, if the plant is young, this can easily end in tragedy.

However, if the plant is mature (i.e. it’s in the last few weeks of flowering), there’s no need to panic. It is common for leaves to turn yellow and eventually fall off during this phase. This usually has to do with the final process of flushing the roots at the end of the flowering phase to eliminate residual nutrients, which can often lead to leaf wilting.

Yellow marijuana leaves: too much or too little water?

The leaves usually turn yellow when either the plants haven’t been watered for a few days or they are being watered in excess. Excess irrigation is a common mistake amongst new growers: it’s not that hard to flood the young plants unintentionally when they’re sitting in large pots. In this case it is best not to water all the soil in the pot but just a small circle around the plantlet, until it starts stretching. Then you can gradually increase the amount of water as it grows.

For a cannabis plant to thrive, it is key to provide it with the right watering cycle (i.e. with neither a shortage nor excess of water). To find out how the plant is accepting the amount of water that you’re providing it with, you can lift the pot when it’s dry and also when it’s freshly watered, in order to check the difference in weight.

But if you don’t fancy lifting heavy pots, you can always purchase a humidity meter and place it in the soil. In hydroponic systems you need to be even more meticulous with this, and check that programming systems, tanks, diffusers and pumps are all set up correctly from the very beginning of cultivation. And needless to say that everything needs to be checked with certain frequency to ensure its correct functioning.

Yellow leaves in indoor grows: lack of light or overexposure?

It is common to find yellow leaves at the base of the plants when they are fully grown. This has to do with a lack of light, which usually happens when the higher leaves start to provide shade to the lower leaves, which then start turning yellow and end up falling off.

This process is quite normal in outdoor plants, although if leaves start falling in excess you might need to consider moving the plant to a place where it can absorb solar energy more evenly. The solution is even easier in indoor grows thanks to the existence of all types of lighting technologies that make the energy reach all corners of the crop, thus improving productivity.

Yellow leaves and buds appear more often in indoor grows due to excessive light intensity. But here the opposite happens: it is the leaves and shoots on the upper parts of the plant that start showing symptoms. As the problem goes from top to bottom, the lamps need to be repositioned at the right distance for them not to burn the plants. It is best to follow the manufacturer’s directions to avoid this happening.

Yellow leaves during the flowering phase: lower or higher pH levels?

Yellowy leaves can be a warning sign of a fertilisation imbalance or unadjusted pH levels. When we don’t have the right pH level, plants can’t absorb all the elements and nutrients in the watering solution.

If you’re growing in soil, in conventional growing pots, pH levels must be kept between 6 and 7, whereas these need to range between 5.5 and 6.5 in hydroponic systems. Imbalance is more common in hydroponic cultivation: as plants don’t have the buffer of the substrate, these are much more sensitive to the effect of the water.

It is best to have a pH meter at hand to check pH levels regularly. There are also cannabis fertilisers that include pH-adjusting solutions.

Shortage and excess of nutrients in fertilisation regimes

Cannabis plants also need the right amount of nutrients according to the growth phase that they’re at. Although dosages are usually well described on fertiliser containers, it’s not uncommon to overdo it or to fall short when adding them to water. The leaves are a good mirror of these little slips, and their appearance will tell you which nutrient the plants are missing:

• When the leaves start to turn whitish on the tips, this is a sign of zinc deficiency.
• Yellow stains on the leaves on the upper part of the plant can be a sign of manganese deficiency.
• When the leaves turn whitish on the edges and veins, this is a clear symptom of a lack of magnesium.
• A shortage of sulphur usually translates into some parts of the leaves turning reddish-orange.
• Greyish or dead shoots reveal a lack of boron.
• If the leaves start to age and become yellowy, this can be due to a nitrogen deficit.
• Round, yellow stains on young leaves can indicate a calcium deficiency.
• When the edges of the leaves curl up and look as if they’re burnt, this can be a clear sign of a lack of potassium.
• If the lower leaves turn dark and look curved, the plant is exhibiting a serious phosphorus deficiency.

All these deficiencies can be overcome with fertilisers that are rich in the nutrient that the plant is missing. In order to apply these water-soluble solutions correctly, it’s important to flush the roots first. A good tip is to use a calendar to keep track of the feeding regime, and gradually increase the dosage to see how the plant responds. Don’t forget that a lack of nutrients is as dangerous as overfertilisation, which can eventually result in burnt plants.

Yellow leaves caused by pests and diseases

The change in the appearance of the leaves can also be a symptom of the presence of certain insects or fungi, or even a sign that the plant has a disease. Pests are usually recognised at first sight, as you can normally see the little predators moving around the plant. Microorganisms or fungi like mildew or powdery mildew are more subtle; they usually settle on the leaves, forming stains that later transform into a white powder, generally on the top part of the leaves.

The best way to prevent these setbacks is by isolating the growing space properly, both indoors and outdoors. This task is easier indoors as indoor grows usually entail grow tents. Nonetheless, indoor grows also require a disciplined hygiene regime. Outdoors, you also need to be careful with pets, as they often carry plagues and insects.

Ultimately, any anomaly in the colour or appearance of the leaves reveals an underlying issue which, as long as it’s treated with some knowledge, should still lead to a successful crop.