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What To Do If Your Cannabis Plants Have Yellow Leaves

Chlorosis is a problem that all gardeners have to face at some point, and that includes cannabis growers. The term designates a lack of chlorophyll and usually results in leaves turning from a succulent green to a lame yellow. Fear not, however, because the issue can usually be resolved once the cause of the discolouration has been identified.

Here are a few common causes of yellow leaves in marijuana plants.

Nutrient Lockout

Most people assume that yellow marijuana leaves signal a lack of nutrients in cannabis plants, and they’re often spot on. However, in the majority of cases, the problem isn’t underfeeding but nutrient lockout, whereby environmental conditions prevent plants’ roots from being able to absorb the minerals in the growing medium.

Most commonly, this occurs when the soil pH slips out of the optimum range. As a general rule, soil should be maintained at a pH of 6.0 to 7.0, while hydroponic systems need to remain at pH 5.5 to 6.5. Large fluctuations in acidity can reduce the bioavailability of nutrients like nitrogen, which means that no matter how much feed you add, your plants simply aren’t going to develop that verdant glow.

Luckily, all this can be avoided relatively easily using a simple pH meter and a bottle of “pH Up” or “pH Down” solution to measure ph level.

Nutrient Deficiency

Sometimes, yellow leaves really are a sign of nutrient deficiency. A lack of nitrogen in the soil, for instance, tends to cause leaves to begin yellowing around the edges before the chlorosis spreads to the centre. Nitrogen deficiency is a common reason for yellow marijuana leaves.

Brown spots are a good indication of a potassium deficiency. As well as lack of calcium.

Insufficient iron or magnesium can also strip your plants’ leaves of their green glory achieved through photosynthesis, so it’s important to make sure you’re feeding your crop with the right amount of nutrients.

On the flipside, using too much plant nutrient can lead to nutrient burn, so don’t get too excited!

Temperature

While cannabis strains like Blue Dream are famous for changing colour when grown in cold temperatures during the flowering stage, most cultivars really don’t like it when the mercury drops too low or rises too high. Anything colder than ten or hotter than 28 degrees Celsius is sub-optimal and can cause green leaves to turn yellow.

If growing indoors, this variable can be easily controlled by adjusting the thermostat, although outdoor growers may need to go to a little more trouble to keep their plants comfortable. A greenhouse, for example, can help protect plants from cold temperatures, while extra shade keeps plants cool in the heat. This helps avoid stunted growth.

Watering

Plants that receive too much water sometimes develop yellow leaves due to a lack of oxygen around the roots. On the other hand, if you’re underwatering your plants, yellow leaves will probably be the least of your worries.

It doesn’t take a genius to work out the solution to this one. Ensure you’re also always using filtered water, cannabis plants don’t lie water with a ph imbalance.

Lighting

When making use of a grow room, it’s essential to keep your grow lights at an appropriate height. Hang them too low, and they can burn the tops of your plants (light burn), causing the upper leaves to turn yellow or brown through heat stress.

A light source that is too high, on the other hand, may not deliver any precious photons to the lower leaves, causing them to droop without enough light.

Root Rot

Waterlogged soil creates an oasis for fungi and bacteria, some of which may attack a plant’s root system and cause them to rot. When this occurs, the plant will look very unhealthy, with droopy, yellow leaves.

The best way to prevent this is by ensuring your growing medium doesn’t stay wet for too long. Drainage and air circulation are your two best tools to control this key variable. It goes without saying that overwatering should also be avoided.

Pests

Humans aren’t the only cannabis lovers out there, so it’s important not to create an environment where pests can thrive. There are all sorts of critters that love weed plants, and if you give them a chance to establish themselves, then you might have a real infestation on your hands.

Spider mites, for instance, feast on cannabis leaves to the detriment of the overall plant. This can cause the remaining leaves to turn yellow, alerting the grower to the problem. While the pesky arachnids can be killed with aromatic oils like neem or cinnamon oil, sometimes the only way to get rid of them is by pruning the affected foliage.

Fungus gnats, meanwhile, are another common pest that tends to move in and lay their larvae in soil that is kept continually wet. The best way to eliminate the invaders is to refrain from watering for a few days, allowing the soil to dry out. This will prevent the larvae from hatching and cause the gnats to move on.

On the whole, chlorosis can be seen as a general indication of something wrong with your plants. Checking the environmental conditions can help you diagnose the issue more precisely and reveal the appropriate solution. Your weed leaves will thank you.

Cultivation information, and media is given for those of our clients who live in countries where cannabis cultivation is decriminalised or legal, or to those that operate within a licensed model. We encourage all readers to be aware of their local laws and to ensure they do not break them.

This post is also available in: French

Ben Taub