How to Top Cannabis Plants: The Method & The Science Behind This Old Trick
by Henrique Dias on May 16, 2021
Here at BudTrainer we have trained countless plants and broken more branches than we can count. We've also researched everything there is to be researched on the topic, and even got a college degree in commercial cannabis production to learn a bit more.To help you, we gathered all of our knowledge and put it together in a way that is clear for you to understand, and easy for you to apply.
Experience level: beginner
When to start: in the final pot, after the 5th set of leaves appears
When to stop: 1 week before the flowering stage
Auto-flowers: it is not recommended topping autoflowers since they don't have enough time to recover
What you need:
Scissors or garden shears with a thin tip
Advantages of topping cannabis plants:
Higher yields & less popcorn bud
More top colas instead of a single one
Lower profile cannabis plants that save you space
Super easy to do and almost impossible to screw up
Why is topping such a popular cannabis pruning technique?
You get multiple top colas instead of just one. You also get higher trichome density per cola, and ultimately, larger yields. However, there is no research that shows the difference yet.
Your cannabis plants grow from a Christmas tree shape that is tall and thin into a bushy shape that is short and wide instead - occupying a lot less space but filling a lot more volume.
If you are planting outdoors, topping your plants keeps them lower to the ground and less prone to falling during high-wind days.
- Topping cannabis plants is best implemented while also defoliating in order to increase airflow within the plant. Read more about the science and the method of cannabis defoliation in this post.
Topping Cannabis Plants: The Science
1. Survival & reproduction
Cannabis in the wild (Image from Cannabis & Glass)
Just imagine a field with thousands of cannabis plants next to each other that are all fighting for survival and reproduction rights. In order to survive, every cannabis plant must access water, carbon dioxide, and light. And in order to reproduce, pollen from the male plants must meet the flowers from the female plants.
There are two important things to note here: access to light and pollination. You may have noticed that cannabis plants grow tall and skinny rather than short and bushy. This is because they want to stand out from the crowd and get as much access to sunlight as they can.
Additionally, female cannabis plants need to pollinate. For this to happen successfully, their buds need to be far up in the sky as they can be. This makes them exposed and increases the chances that male pollen will land on them via the wind. Alternatively, if the plants are trained off to the sides and the have a lot of shorter colas, the probability of pollination significantly decreases.
For the sake of survival and reproduction, then, we can conclude that the female cannabis plant is not so concerned about the quantity of buds - it’s a far better bet for it to have 1 amazing and exposed cola that will guarantee her reproduction rather than 100 smaller colas that will not catch light, or pollen.
2. Breaking the pattern
However, when growing cannabis seeds at home, we kind of defeat the plant’s two purposes of survival and reproduction. Here is why.
For starters, we purposely want to stop our female cannabis plants from reproducing, otherwise they would end up giving us a bunch of seedy weed. And we are also not worried about its survival, because we can give our marijuana plants lots of space, water, light, and love.
In a nutshell, the whole purpose of cannabis plants having that single tall and exposed cola gets defeated the moment we don’t need our cannabis plants to worry about reproducing and surviving anymore.
So what if we could make our cannabis plants go from 1 tall cola into something like 10 or 20 shorter colas? Instead of growing cannabis plants like a Christmas tree, what if we could make our weed plants grow like a bush full of mini Christmas trees?
The good news is, we definitely can and it’s called topping! When you top your plant during the vegetative stage, you are basically changing its anatomy to grow into multiple large and juicy colas, but using basically the same amount of time. If you're doing large-scale cannabis cultivation, topping cannabis plants not only increases your yields by huge amounts but also guarantees a more even distribution of cannabinoids. Not to mention it helps keep your marijuana plant nice and short. Just ensure that you don't wait until your plants reach the flowering stage.
In order to learn the technique, though, we first need to learn how marijuana plants grow tall in the first place. If we can learn how they do it, we can then learn how to interrupt this natural pattern.
3. Auxins and apical dominance
There are natural growth hormones produced during the vegetative stage in the apical growth tip of almost every plant called auxins. Auxins are responsible for cell elongation, and are the main driving factor helping the plant and the lower branches grow toward the light. This is why when you bend a branch sideways it starts to grow back up; i.e. you basically shaded the bottom part of the branch and forced the bottom cells to grow more than the top cells, which is what causes the distortion. This diagram explains it better.
Apical dominance. Image from Better Bonsai.
Auxins are mainly present in new growth sites (i.e. old hardy branches don’t have it) and they are produced in higher concentrations in those growth sites that are closer to the light.
In a nutshell, this is why a lot of plants grow like a Christmas tree - the auxins produced at the top growth tip are what promote this tall growth while keeping the side lower branches smaller since they are farther from the light.
4. Shifting the focus
If most auxins are concentrated at the growth tip, wouldn’t it make sense to cut that top off, completely remove the upper growth, and let them be redistributed toward the lateral branches?
The correct answer is yes.
If you cut the top of a cannabis plant off during the vegetative stage, all of the auxins that were being produced there are suddenly removed. Here, the goal is primarily to invert the classic Christmas tree shape by manipulating your plants' growth patterns.
But since the plant still needs to grow, it starts producing auxins elsewhere. This gives the lower lateral growth room to assume dominance.
When a cannabis plant loses its apical meristem (the main top) it shifts the production of auxins to the lateral branches, almost as if they each became an independent plant themselves. In fact, this sudden production of auxins in the lateral branches forces them to grow out and become the plant’s new tops. This prevents your plant from reverting to vertical growth.
Before topping; after topping; 3 days after topping.
Low-Stress-Training (LST) and apical dominance: Another way to shift apical dominance and achieve lateral growth dominance is with a technique called Low-Stress-Training, where you bend the top of the weed plant horizontally and tie it to the ground or the pot, forcing the side branches to be taller than the main top itself, and thus be more exposed to light. This, in turn, shifts the apical dominance to the side branches and encourages horizontal growth. You can conduct this gentle training technique during the flowering stage to change the shape of your plant. For more on LST, feel free to read this article.
FIMing: FIM stands for Fuck I Missed, which is basically short for a fucked-up topping. When topping cannabis plants, you remove the entire top growth of the main stem, leaving instead two new grow sites above the second or third node you cut. Because your plant's node has two growth tips, topping doubles the number of the dominant growth tips. With a FIM, however, you cut your top in half instead of entirely - leaving another 1 to 4 extra nodes on the main stem of your weed plant. This leads to a messy growth of 3 to 8 new nodes, and is useful only if you have a smaller space to grow in. Unlike topping, FIMing doesn't slow down vertical growth. However, the disadvantage of FIMing is that you never know exactly what you are going to get - it comes in all forms and shapes.
FIMing (Image from Your Weed Coupons)
Topping Cannabis Plants: The Method
When & How
To increase the number of growth tips your plant can support, you should top after your seedling has started showing the fifth node and before it starts showing the 7th. The best time of the day to top is before the plant goes to sleep, so that it has the whole night to recover.
In order to top your plant, all you have to do is cut the tip of the main stem after the third or fourth node like in this video below. And you are done!
BudTrainer PRO TIP: when topping a cannabis plant for its first time it is recommended that you remove the growth sites from the first node (with the smallest set of leaves) because they never develop into strong branches. If this can’t be done on the same day because those growth sites are still too small, you can wait a few days until they come back out, and then remove them, like we did in this video.
How to top your cannabis plant after the third node
Leave a stub behind
Make sure to leave a ¼" stub behind so that you don't run into splitting issues in the future. On the plant below, for example, I didn't leave a stub behind and it split exactly in the spot I had topped it.
Remove small growth sites
Whenever you top cannabis plants, you can also choose to remove any weak growth sites that you don’t want to develop into a full branch in the future.
Weak growth site (or shoot) on the right side of the node
Top multiple times
Every time you top your plant you are basically multiplying the number of colas and growth tips it will have. And you can top a plant as many times as you want. For example, if you top it once and leave 2 nodes behind (like in this previous video), this means you will have 4 new branches developed (2 growth sites per node). If you then top all of them again and leave another 2 nodes behind on each new branch, you will have 4 branches x 4 = 16 new branches! You can continue topping marijuana plants to ensure the lower branches receive more light and multiply your plant as much as you want.
The plant below, for example, was topped twice - the first time at the third node (I removed the first node though, leaving only 2). The second topping was done two weeks later at the third node of each of the 4 branches. However, this time I left only 1 node behind on each of the 4 branches, which gave me a total of 8 branches in the end. This particular plant training technique is called a manifold and you can learn it in this article that I wrote.
Only top before flowering
We like to give our plants at least 2 days from the last topping before we switch it to flower. Even better is waiting 1 week, and then making the switch. If you have an autoflower, make sure to only top it if it flowers in 4+ weeks, or it will not have time to recover.
That's it, folks! You are now officially ready to go cut those tops off your plants. Make sure you do it at the right time and with clean tools. You will be set!
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