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The leaves are the most important organs of a marijuana plant, because this is where photosynthesis takes place. In this process sunlight causes the leaves to create sugars out of water and carbon dioxide. In addition, the leaves keep a water flow going inside the plant through evaporation and capillarity. This water contains nutrients the plant actively absorbed from the soil.

Function of the leaves

Marijuana leaves with mesophyll
Big leaf

The biggest part of the leaf consists of a tissue called mesophyll. These are the cells with chloroplasts in which photosynthesis takes place. The bottom part of the leaf contains many holes with air. These are closed off from the outside world by stomata. Substances are exchanged with the air through these stomata. The stomata let carbon dioxide, oxygen and water vapor through and are predominantly located on the underside of the leaf.

The leaf has clearly visible veins. These are the vascular bundles. They consist of xylems and phloems. Water and nutrients are transported through these.

A leaf that’s fully shone on by the sun is a lot thicker than a leaf that’s in the shadow. Leaves in the sun have more cells with chlorophyll. They also produce more sugars, because more photosynthesis can take place. When more light hits the leaf – increasing the light intensity – the leaves produce more sugars because the rate of photosynthesis of higher.

A leaf that’s usually in the shadow reaches its maximum capacity sooner than a leaf used to a lot of light. So think about this when you’re pruning a leaf. A common mistake is to prune the big leaves to provide more light to the bottom leaves in the shade. But you’re actually pruning the leaves that produce the most sugars.

Creating strong leaves

Marijuana strong and healthy leaves
Healthy leaf

It is therefore important to have strong and healthy of leaves that can produce lots of sugars. The more light your leaves get, the more sugars they can produce, so the faster they’ll grow. Give your plants fluorescent, HM or HPS lighting. And remember: the more, the better!

Marijuana leaves with heat stress
Heat stress

Also make sure that the leaves don’t get too warm from the lamp being too close, because this would close the stomata to prevent dehydration. If it’s too warm for a couple of days, the leaves will curl inwards to protect themselves against the heat. Unfortunately, leaves can’t recover from this once the temperature is lowered. Curled leaves can’t nearly absorb as much light, which is a waste.

Your leaves obviously need to possess sufficient water and nutrients, but that’s the case for the entire plant. Just like the temperature, humidity and airflow have to be right.

Leaf problems

If your plant experiences stress, has a nutrient deficiency or is affected by bugs, this usually is expressed through the leaves. Here are some examples;

You should therefore check your leaves every time you’re looking for abnormalities. Check to see if there are any bugs underneath the leaves. It’s always good to use a magnifying glass, because spider mites and thrips are so small they can barely be seen with the naked eye. See if there are spots or dots on the leaf or if there’s any damage. Also check the edges of the leaf and the entire leaf for discoloration.

If you notice anything, check this symptom checker. This contains examples of almost all problems that can arise when growing marijuana. I made this symptom checker back in the days for ilovegrowingmarijuana.com and I have little to add to it. That’s why it might look familiar to you.

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