Yellow leaves on cannabis plants: how to treat and prevent them

The appearance of yellow leaves on your cannabis plants can be the first sign of a serious problem. However, not everything is lost if you know how to react quickly and appropriately. In this article we explain how to detect the causes of this symptom as quickly as possible, and how to act fast so that your plants recover all their splendour.

The process which turns the leaves yellow is called ‘chlorosis’, and it occurs when chlorophyll is not properly produced by the plants. Chlorophyll is a green pigment that provides plants with their characteristic colour. It is an essential element for plants to carry out photosynthesis, produce food from light, and keep healthy. Therefore, the more chlorophyll the plants lose, the less food they can produce.

Although the yellowing of the leaves represents a natural process in the last stages of flowering, chlorosis is a warning signal in any other case. Since it can happen for various reasons, proper diagnosis is essential to identify or discard the most common causes.

Reasons why yellow leaves may appear in your marijuana grow

The most common causes of chlorosis may be connected to human error (for instance, nutrient deficiency), and can usually be identified, fixed, and prevented relatively easily.

On other occasions, the origin of the yellowing of the leaves may be related to an environmental factor. Controlling this in indoor grows is much easier than outdoors, where it can become a real challenge.

Here are all the possible causes of chlorosis so you can identify the one that may be affecting your grow:

pH levels

Maintaining the correct pH levels of the irrigation water is essential in cannabis growing. This should range between 5.5 and 7 (depending on the type of grow and the phase of the cycle).

It may seem obvious, but it’s really important to remember to measure the pH levels before each watering session; especially if you’re using certain growing mediums like coco coir, because any error in this regard would be paid dearly. If the pH levels are too high or too low, they can prevent the plants from absorbing nutrients, leading to the appearance of yellow leaves. When this happens, flushing the roots with the right pH levels is highly recommended before resuming the use of fertilisers.

Nutrient deficiency

The amount and type of nutrients that you should give your cannabis plants varies according to the stage that they’re at. The appearance of yellow leaves in your grow could be a sign of inadequate fertilisation:

• In the case of nutrient deficiency, the leaves turn light yellow. You’ll need to increase the amount of fertiliser so they can recover their functions and their green colour.
• In the event of over-fertilisation, the leaves take on deep yellow hues and become curved. When this occurs, you need to flush the roots with plenty of clean water before fertilising again.
• Sometimes, it may be due to the lack of a specific nutrient, so it’s vital to identify it as soon as possible. The deficiency of certain nutrients can lead to yellow spots that appear in different shapes and sizes. Remember that a lack of nitrogen is more likely to occur in the growth phase, whereas a potassium deficiency is more likely to happen during the flowering stage.
• The area where the symptoms of chlorosis appear is also key to identifying the cause. For example, if symptoms start on the top part of the plant or in the new growth buds, it is likely to be an iron deficiency. If the lower leaves (and not the top leaves) show signs of chlorosis, this is likely to be due to a lack of magnesium.

Irrigation volume

Watering your cannabis plants properly is essential. The amount of nutrient solution that you need to use will vary according to the type of substrate, the environmental conditions, the size of the plants, and their stage of life. The appearance of yellow leaves may signify that you’re not watering your grow correctly:

• Overwatering: Yellow leaves may indicate the beginning of root rot, so you’ll need to try to accelerate the water absorption and evaporation processes by slightly increasing the temperature and/or decreasing the ambient humidity levels.
• Lack of irrigation: In this case, the plant is warning you that it requires immediate nutrient intake, so you’ll need to provide it with abundant irrigation as soon as possible.

Lighting

Indoor growing requires a uniform distribution of the light, always keeping the right distance between the plants and the lamps:

• If plants are too close to the lamps, the apical leaves may turn yellow. This is a sign that they’re starting to burn due to an excess of light.
• If the light doesn’t penetrate into the plant well enough, the lower leaves may turn yellow due to the lack of light. If the plant feels that it takes more energy to keep those leaves alive, it will absorb as many nutrients as it can from them before letting them drop off.

In both cases, you’ll need to reestablish and maintain the right distance from the lamps, as well as to ensure a good distribution of the plants within your grow tent or grow room. You should also use pruning or training techniques to increase the lighting under the canopy so that the lower leaves get more light.

Pests and fungi

There are numerous pests that can affect cannabis plants by directly attacking their leaves, or even their roots, thereby causing chlorosis.

• When pests affect the leaves (for instance thrips, mulch flies, or some fungi like mildew or powdery mildew), it’s important to apply the right treatment directly on the leaves to solve and prevent the appearance of a new attack.
• If the pest has affected the roots and the leaves are subsequently turning yellow, the problem will probably be more serious, and it’ll take longer to sort out. Therefore, it is best to use biological prevention products, such as mycorrhizae, to avoid getting to that point altogether.

Temperature

The right temperature for the development of cannabis plants ranges from a minimum of 18ºC at night to 28ºC during the day. The plants’ vital functions may become blocked in colder or warmer climates, which would facilitate the appearance of yellow leaves. Given this situation, you must monitor the ambient temperature indoors, and try to protect your outdoor grow in extreme temperatures.

Super Silver Haze x Lemon Skunk x Ruderalis

Lemon Haze Auto

If you are looking for an energizing and refreshing marijuana experience, Lemon Haze Auto is the perfect choice for you. With a high THC content (+18%), Lemon Haze Auto offers a very citric and refreshing taste. It is very easy to grow, making it suitable for beginners and experienced growers alike.

Chlorosis in the final stages of flowering

Yellow leaves aren’t always a warning sign. On the contrary, they represent a natural process that occurs during the final stages of flowering, when the plant concentrates its last ounce of energy on the flowers, whilst gradually stopping absorbing nutrients and carrying out photosynthesis in the leaves. In fact, when this doesn’t occur, it is usually a sign that the plants still contain residual nutrients, which can affect the crop quality negatively in terms of aroma and flavour.

As you can see, knowledge is the best tool for a cannabis grower. Now that you know the possible causes of the appearance of yellow leaves, you’re better prepared to prevent it from happening. With the right care and attention, especially when detected in the first stages of growth, the yellow leaves on cannabis plants won’t necessarily have a negative effect on your grow. In fact, they can also prepare you for an even bigger harvest!