What is Light Stress in Cannabis Plants?

If you’re new to growing cannabis, understanding the signs of light stress may be difficult. This is namely due to the fact that light stress (or light burns) look similar to other issues your cannabis plant may struggle with, such as nutrient deficiencies.

In order to differentiate from other problems, here are a few things to keep in mind:

• The first signs of cannabis light stress will appear near the top of the plant, where leaves are closest to the light. The tips of the leaves may start to point upwards and have very mild discoloration.
• If the problem persists, your leaves will start to yellow, appearing lighter in color than other leaves on the plant.
• The tips of the leaves will also be yellow, but the veins within them will still appear green.
• The leaves may start to curl upwards (as a means of protecting themselves). On top of this, they may turn crispy and/or develop burn (brownish) spots.
• Your cannabis buds may appear bleachy or turn a bright white. If this happens, the buds have lost their potency and are no longer good for consumption.

The signs of cannabis light stress largely resemble that of nitrogen deficiency. For this reason, some new growers may become confused about the true problem at hand.

In order to differentiate the two, it’s important to consider how the leaves are reacting. If they fall off, this is a nitrogen deficiency problem – light stress WILL NOT cause cannabis leaves to fall off the plant. On top of this, nitrogen deficiencies will cause yellowing to start at the bottom of the plant rather than the top.

With an understanding of what light stress cannabis plants look light, it’s even more important to know how to prevent it from happening. As mentioned, light stress occurs when cannabis plants are too close to their light source.

So, a simple solution would be to move the light source away from the plant. Right?

Unfortunately, there are a number of reasons your plant can be experiencing light stress. Here are a few prevention measures to keep in mind:

• If you believe your light source is too close to the plant, perform a simple hand test. This involves placing your hand under the light at a canopy level. If this placement is too hot for your hand (or causes irritation), then the light source is likely too hot for your plant.
• If you’ve moved your cannabis plant recently, you’ll want to ensure it continues to receive the same amount of light. Changes in the lighting environment can also cause light stress.
• When purchasing new grow lights, be sure to do your research. Different types of grow lights require varying distances from the plant.
• If you want to be extra cautious, you can buy a LUX or PAR meter.
  • During the vegetative stage, the recommended lux levels are 30,000 to 70,000 lux/m2. During the flowering stage, the recommended levels are 50,000 to 90,000 lux/m2.
  • As for PAR levels, the recommendations are 200-400 PPFD for seedlings, 400-600 PPFD for vegetative, and 600-900 PPFD for flowering.

In order to recover a plant that’s struggling with light stress, here are a few things to consider:

• Move the light further away to avoid stressing out the plant. You can incorporate a lux meter, PAR meter, or do a hand test in order to determine whether or not your new light positioning is ideal for your plant.
• If you’re unable to move lights further up (vertically), you can incorporate plant training techniques, such as stress training or topping your cannabis plant.

Keep in mind that if you detect the problem early enough, you’ll likely recover plants within a one to two week timeframe.