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27 May 2021

Why Cannabis Leaves Turn Yellow

Cannabis plants are capable of communicating any stress and nutrient deficiencies they are experiencing. It is up to the grower to observe their crop and develop an eye for plant health. One of the most common indicators of plant stress is chlorosis or yellowing of leaves caused by a lack of chlorophyll (the green pigment critical to photosynthesis).

Chlorosis is caused by several factors and is often an indicator of a more significant issue. When cannabis plants lose their pigmentation, it means they are less able to synthesise light into the sugars they need to survive and grow. The grower who identifies chlorosis in their plant’s leaves should not panic and instead, take steps to diagnose their affected plants.

Common reasons for cannabis leaves turning yellow can be attributed to poor growing practices or nutrient deficiencies. The conscientious cultivator who troubleshoots their operation and takes decisive action will be able to remedy leaf chlorosis relatively quickly. Read on to find out more about the leading causes of cannabis leaf yellowing.

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Poor growing practices: pH, watering, temperature

Chlorosis can result from negligence on the part of the grower just as much as it can be caused by pests and unpredictable environmental conditions. The most common causes of yellow leaves due to poor growing practices are nutrient lockout caused by pH imbalance, water stress from over or under-watering, and temperature problems. The grower can resolve these issues with a critical assessment of their growing style and their plants’ growing conditions.

The nutrient absorbing capabilities of cannabis plants can be hindered despite the availability of nutrients at the level of the root system if the grower is not careful about maintaining the optimal pH of their growing medium or substrate. This can cause marijuana weed leaves to turn yellow, as nutrients such as nitrogen, zinc, and magnesium are needed in the production of chlorophyll. This is known as “nutrient lockout”.

The grower can solve nutrient lockout by measuring the pH of their water and/or nutrient solution using a pH meter or testing kit and then increasing acidity or alkalinity until it is between the pH level of 5.8 and 6.2 (no higher than 6 for hydroponics). Not only will optimal pH maximise nutrient absorption and prevent leaves from turning yellow, but it will also save the grower money and resources by preventing waste of nutrient feed.

It would benefit the grower to pH their water as well, as pH can fluctuate throughout the year depending on rainfall and any treatments applied by the water supplier.

Yellow leaves can also indicate stress from over or under-watering. Water-stress-induced chlorosis is usually a secondary indicator, meaning that to get to the stage of yellowing leaves, the plants will have been stressed for some time. The easiest way for the grower to diagnose over-watering is by observing their plants and growing medium.

Over-watered plants will exhibit firm, droopy leaves even after watering. Leaves will feel “heavy” and “full” as they are experiencing an oversaturation of water. Curling is common and not limited to the leaf tips, but rather the entire leaf. Over-watering may also cause root rot.

One way for cannabis growers to diagnose over-watering is by lifting their plant pots. If the pots are heavy to lift, especially two or three days after watering, then it is likely that the plants are being given too much water. Another method for growers using soil is to dig under the surface (up to the first knuckle after the fingertip) and to check how moist that layer is. If the soil is quite wet after several days, there’s a good chance it has been overwatered.

Growers can get over-watered plants back on schedule by letting their plants dry out over a few days while making sure to regularly scout their plants for any water stress-induced illnesses or pests. When restarting the watering schedule, keep track of the moisture level of the top of the growing medium. The goal is to water cannabis plants every two to three days.

Yellow leaves can also be the result of under-watering, but growers will be able to tell that their plants have had little water since they will notice the droop or even wilting before any chlorosis sets in. Even though cannabis plants prefer soil that dries out a little bit between waterings, it is best to give them the right amount of water to keep them in good shape.

Marijuana leaves can also yellow in response to heat stress. This is solved using a thermometer to keep the growing environment between 18C and 28C. The grower can also implement fans in their grow rooms to promote air circulation and make sure no hot spots develop as a result of grow lights. Heat mats or even rugs can be used to make sure plants stay warm in the evenings.

Cannabis leaves turning yellow

Nutrient deficiencies

Cannabis plants need nutrients of different kinds and amounts as they progress through their lifecycle. Any deficit in these nutrients will result in stunted growth, susceptibility to viruses, and even death, and all of these outcomes will be preceded by leaves that turn yellow. While yellow leaves can indicate any number of deficiencies, the location of the yellow leaves is just as necessary for the grower to diagnose what their plants are missing.

A grower suspecting nutrient deficiencies should pH their nutrient solutions before making any changes to the feeding programme to ensure their plants aren’t suffering from nutrient lockout because of incorrect pH.

Yellow leaves lower down on the stem or at the bottom of the plant may indicate a nitrogen deficiency. Nitrogen is a mobile macronutrient, meaning the plant can transport it to areas of need. Yellow lower leaves are lacking in nitrogen as the plant is getting insufficient amounts from its roots and is transporting nitrogen from older leaves to the new growth that needs it. This can result in green leaves but also stunted growth and lower yields.

If lower leaves appear green and healthy, but newer fan leaves higher up in the canopy are showing signs of chlorosis, the plant may be suffering from a magnesium deficiency.

Magnesium plays a vital role in the production of chlorophyll, and marijuana plants suffering from a deficit of magnesium will be unable to photosynthesise.

Iron deficiency can also cause newer leaves to turn yellow at the base of the leaf. One way to tell this apart from other deficiencies is that iron deficit will also cause new leaves to develop entirely yellow rather than turn yellow over time. These leaves will slowly turn green as the plant gets older.

There are many reasons why cannabis plants leaves turn yellow, and it is up to the grower to diagnose exactly why. By starting with seeds bought from reputable sources such as Marijuana Grow Shop, growers can grow their crop with confidence that yellow leaves are merely the result of the plant’s development and age (leaves will yellow and fall off as the plant reaches maturity) rather than any deficiency caused by the plant’s genetics. MGS offers a jaw-dropping selection of high-quality seeds from the world’s most passionate growers and reputable seed banks, including a selection of vigorous feminised seeds, on-schedule autoflower seeds, and exclusive regular seeds.

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Post author
Martin is a production horticulturist with experience in commercial cannabis cultivation and sustainable farming from his time with Emerald Cup Award-winning farmers Esensia Gardens in northern California's Emerald Triangle.
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