When: after your plant has a total of 5 or 6 nodes

Topping is probably the most popular pruning technique available. It involves cutting your main branch right after a node (pair of leaves + growth sites), so that the remaining growth sites can turn into stronger branches. This allows cannabis plants to “bush out”, since topping breaks the tendency of the plant to grow into a Christmas tree shape. Topping is highly recommended, and if you haven't yet you can learn how to top in this article. 

When: after 4 to 8 weeks from planting your seed

At this point, many leaves and small branches will have appeared, inhibiting your plant's ability to absorb CO2 and light in the center of the canopy. This is why it is important to defoliate and prune the middle of your canopy at this point, giving your plant the structure it needs to continue growing. On top of this, it is also important to start training your marijuana plant at this stage, if you haven't yet.

When: after 4 to 8 weeks from planting your seed

Pruning and defoliation often go hand in hand. It is much better to prune at the same time as you are defoliating because they are both stressful activities for the plant.

1 week before harvest time, your cannabis plant will have put on most of its weight and will start focussing on filling up its trichomes with cannabinoids. This is when most of your pistils are brown and the trichomes are going milky and amber. It is recommended for you to do one last defoliation (no pruning) at this stage for a few reasons.

1. It helps uncover those lower buds and sugar leaves that have been shaded by a big fan leaf for the last 4 to 6 weeks. 

2. It causes a mild stress on your plant and forces it to allocate all of its resources to the buds, which results in more trichome and cannabinoid production. 

3. It saves you over 50% of the time trimming because most of your fan leaves will have been already removed.

Our sun is nearly 100 million miles away, which means that it doesn’t matter if your plant is a few ft taller or shorter - the bottom and the top leaves will feel no difference in light intensity (as long as they are not in the shade). However, when it comes to indoor lighting, the intensity of your grow light decreases a lot as you get farther away from the source - from 5 to 10 times weaker every foot of distance away.

What this means is that training cannabis indoors is more about optimizing light distribution at the top, such as maintaining a flat canopy and defoliating the bottom of the plant (like the plant on this diagram). When growing outdoors, however, you don’t need to worry so much about this - just make sure to choose a spot in your backyard that has the most amount of sunlight, all day long, and you can have leaves from top to bottom (just not in the middle!).

Airflow is essential for plants to absorb CO2 as it helps circulate the air released by the leaves (which is full of oxygen) and renew it by air that is full of CO2. For this reason, it is equally important to train indoor and outdoor marijuana plants to maximize airflow around the leaves and inside of the canopy.

The main difference, however, is that outdoors plants have the extra pressure from pests while indoor plants generally don’t. This is why we highly recommend defoliating outdoor plants at least once a month to clear out all old leaves that are turning yellow, not catching any more light, or that are blocking airflow to the middle of the plant. As long as you don’t create pockets of clustered leaves, you should be safe from the pressure of pests!

Autoflowers only have around 30 days in the veg stage before they automatically go into the flowering stage (AKA “auto-flower”), so it is best to avoid defoliating as much as possible, and only cut yellow or brown leaves, or bud sites that are really small and in the middle of the canopy. 

It is also not recommended to defoliate more than 10% of your autoflowers’ leaves at once, and definitely not recommended to top them, otherwise the stress can be too much and the growth will be stunted.

The only time when it is recommended to defoliate your autoflower cannabis plants is 1 week before harvest (the pre-harvest defoliation mentioned above). This allows you to avoid too much trimming after you harvest, and forces all of those trichomes and cannabinoids to come out due to the added stress at the end of the cycle.

One of the main purposes leaves serve is that of catching light and using it to convert water and CO2 into oxygen and sugars through photosynthesis. In this context, then, leaves serve primarily as solar panels and the more exposure to light they have the more photosynthesis they will be able to contribute to the plant. Conversely, if leaves are completely covered or shaded, they will not be able to photosynthesize. It would be the same as putting one solar panel on top of another - the bottom panel won’t catch light.

This is why it is important to prune leaves and branches that are shaded by 2 or more other leaves on top of them, or that are touching one another. 

Transpiration is the process by which water vapor and oxygen (for us to breathe) leave the plant through the stomata - these tiny little openings under the leaves - while CO2 comes in from the air, thus allowing the photosynthesis process to happen. In fact, 95-99% of all the water that plants consume through the roots go back out into the air through the leaves while only 1%-5% stays in the plant and gets converted into sugars. This means that if your plant consumes 1L of water per day, it is "vaporizing" around 950-990 ml back into the air.

Another purpose that leaves serve is storing the nutrients they just absorbed from the water, so that they can use them later. Nutrients like N, P, K, and Mg are mobile, meaning that your plant can remobilize them from the old leaves to the new ones if they are suffering deficiencies. 

For example, when the leaves at the bottom of your plant start to go yellow, they are simply "donating" their stored Nitrogen to the new leaves, shoots, and flowers at the top. 

That’s why having a certain amount of green and healthy leaves is always important - it helps your plant remobilize those stored nutrients to its most vital parts. If you strip your plant naked like some folks do, you leave no buffer for it to mitigate nutrient deficiencies.

Microclimates are little pockets of air in different parts of your growing space that are different from the overall climate you set. For example, you can set a temperature of 26C/79F and RH55% in your fancy exhaust fan, but if you measure the middle of your canopy you will end up with a cooler temperature and higher RH.

Microclimates are most common on the corners of your growing space, the underside of your canopy, and in the middle of the canopy where it’s humid. Since these areas are usually more shaded and have less airflow, they make the perfect environment for pests to thrive in and the worst environment for photosynthesis to happen - this is why it is so important to defoliate clusters of leaves in the middle of your canopy and avoid microclimates.

Plants don’t just absorb CO2 during photosynthesis, they also absorb oxygen “by mistake” in a process called photorespiration - where instead of producing sugars, they consume sugars - becoming smaller instead of bigger. 

Have you ever noticed how small and light-green the leaves inside your canopy are? That’s because there is too little CO2 and so the leaves are respiring more than transpiring, i.e. consuming more than producing.

In other words, when you have a microclimate your leaves are consuming all of the CO2 available around them until there is no more, at which point they are forced to increase photorespiration and decrease photosynthesis.