The fan leaves on the lower part of my cannabis plants are turning yellow. Is this normal given they are in day 27 of flowering?

Yellow Leaves During Flowering Not a Cause for Alarm

During the final two weeks of the flowering cycle (days 42-56 for an eight-week flowering cycle), it is natural for the lower section of a cannabis plant to turn yellow or discolored.

In fact, cannabis plants will and should lose some of their pigmentation. This is a good indicator the plant has been provided with the appropriate ratio of nutrients for blooming and the plant is putting its energy toward flower production during the final weeks.

Generally, cannabis growers don’t need to worry about yellowing leaves on the lower section of the plant during the later weeks of flowering.

However, if the yellowing is happening during the vegetative stage or in the first few weeks of flowering, an examination of the environmental conditions, the pH of the solution and/or medium, and the nutrient concentration will lead the horticulturist to the cause of the discoloration.

The most common causes for yellowing leaves on the lower section of a cannabis plant are nutrient deficiency, over- or underwatering, and/or temperature.

Common Causes for Yellow Cannabis Leaves During Veg

Nutrient Deficiency

Generally, the discoloration or yellowing of the lower section of a cannabis plant is caused by a macro-nutrient deficiency. Most commonly, the yellowing of leaves on the lower fan leaves is due to a lack of nitrogen. However, this does not always mean the grower is not supplying the garden with adequate nitrogen.

In some cases, the deficiency is caused by the pH value of the medium or nutrient solution. If the soil or hydroponic solution’s pH value has fluctuated out of the desired range (6-7 for soil, 5.5-6.5 for hydroponics), particular nutrients will become unavailable for absorption, regardless of how much of that nutrient is present. A relatively inexpensive pH meter is a valuable tool a cannabis cultivator can use to ensure the pH value is within the desired range.

Over- or Underwatering

Over- or underwatering cannabis plants can also lead to discolored or yellow leaves. Overwatering will reduce the amount of available oxygen around the roots, potentially creating the perfect breeding ground for pathogenic anaerobic microorganisms. If the yellowing on the plant’s leaves is caused by overwatering, you will likely notice a slight droopiness in the plant’s leaves and possibly copper or brown colored spots on the yellowing leaves.

Underwatering can also cause a cannabis plant’s leaves to yellow. Underwatering is less likely to be the cause because to get to the point where the plant is stressed enough to turn yellow, other tell-tale signs of underwatering would be obvious, such as wilted foliage.

Temperature

Last, but not least, temperature can play a role in the yellowing of leaves on a cannabis plant. If the ambient temperature fluctuates out of the desired range (60-80°F), yellowing of leaves may occur. \

During the lights off period (night cycle), the temperature should not drop below 60°F. For the lights on period (day cycle), the temperature should not exceed 80°F (the exception would be when supplementing CO₂) . Temperature fluctuation above or below the desired range, especially when occurring multiple days in a row, can be the culprit of yellowing leaves.


Once the cause is properly identified, a horticulturist can take action and quickly rectify the problem. I hope this answers your question.

Keep on Growing,
Lee G. Lyzit

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