One of my cannabis plants’ leaves are turning over to expose the down-facing side. The plant looks very healthy (lots of rich green leaves, no spots), and the other ones are growing normally. I thought that it was too close to the light, but I adjusted that and there’s been no change. Any thoughts? — Jacqueline S.

There are several reasons why a cannabis plant’s leaves will turn over. Some are serious issues, while others can be easily corrected. This can be a frustrating problem, especially when the plant seems healthy otherwise.

Perhaps the most common reason for leaves turning over on a cannabis plant is excess heat. Since you already adjusted the light, the intense heat from your artificial light source is probably not the culprit. However, if the ambient temperature of your garden is too high, the leaves can turn due to stress. Ideally, the temperature of your growroom while the lights are on should fall in the 68-80°F range and should drop no more than 10-15 degrees during the lights-off stage. Humidity could be a factor as well. Large swings in humidity levels can also cause leaves to twist, curl, or turn. Make sure you have consistent humidity levels in your grow area; preferably in the 40-50 percent range (vegetative growth can be a little higher).

Another cause of leaves turning, especially if it is happening in one specific area of the garden, is intense wind. Air movement in a growroom is crucial, but if a fan is too close or the air movement is too powerful, the plants can become stressed. If the plant you are having issues with is closest to a fan, I would suspect this is your problem.

Light leaks during the dark period can also cause leaves to turn toward the unintended light source. Make sure you have a completely dark growroom during the lights-off period. When the lights are on, underside lighting or reflective material on the underside of the leaves can both cause the leaves to turn toward the light. Side lighting is generally acceptable, but underside lighting or reflective materials should be avoided.

Some nutrient deficiencies can cause the leaves to twist or turn as well. Generally, this would be accompanied by discoloration or wilting. However, during the early stages of a deficiency, the plants may exhibit leaf turn. Deficiencies in calcium and/or magnesium are commonly associated with this, so be sure these essential elements are available to your plants. Also, make sure to monitor and adjust your pH level. A pH level out of the desired range can lock nutrients out even when they are being supplied to the garden. For soil, a pH range of 6-6.8 will ensure calcium and magnesium are available for absorption. In hydroponic systems, a lower pH (5.5-6.0) is preferred.

Residuals from foliar sprays have been known to cause the leaves to turn; especially if the plant is sensitive to the ingredients in the spray. However, issues from foliar sprays would typically be universal throughout the plants and not just in a small section of growth.

Lastly, insect pests, such as spider mites, can cause leaves to twist and turn. Examine your plants closely; paying special attention to the underside of the leaves for eggs and larvae. If a pest insect is the issue, a treatment should be implemented immediately to negate further damage.

I hope this answers your question and helps resolve your issue.

Keep on Growing,
Lee G Lyzit