To Top or Not to Top?

Apr 7

1. Hormone Response in Cannabis Plants

Canopy management and uniformity is one of the most important factors that help growers balance quality and yield. Many growers use the strategy of topping to promote the growth of lower branches and create an even canopy in an effort to maximize yield and quality. But is it always necessary or advisable? I see many growers at large scales that seem to top without ever even considering if it is the best strategy, as it is just part of their standard SOPs. Topping can be an important tool, but it is best used when you fully understand the science behind it and what is going on hormonally inside the plant. By analyzing the benefits/drawbacks and proper techniques of planning and implementing topping, we can approach canopy management in a more informed and scientific way to optimize our grow techniques.

Let’s first talk about the profound hormone response that occurs when the plant is topped.

Plants have two primary hormone classes that impact plant morphology significantly- auxins and cytokinins. When a plant is growing unimpeded, the plant has auxins that signal back and forth to promote both vertical growth (apical dominance) and to promote root growth down into the substrate. In this way, it creates ‘awareness’ of its height relative to its root mass to generate a plant that grows in proportion to itself.

When the plant is topped, the cytokinins come into play- promoting the axillary buds to grow. In this way, the plant responds to the impeded growth, and promotes new apical tips to grow, hopefully in a place that isn’t blocked or subject to predation. The plant is constantly balancing the ratios/types of auxins and cytokinins to achieve a balanced growth morphology. Genetics plays a huge role here too, and you could say that most plant morphology traits are manifested from the plant’s natural interplay between these two hormone classes.

As a tissue culture scientist, allow me to tell you that these hormones are VERY powerful and have a lasting effect on the plant…even in TINY doses (I’m talking a couple micrograms…)

Topping causes a rush of cytokinin activity, and as you may have observed, there is a significant pause in any growth for a period of 3-7 days after topping. This is because the plant is reallocating resources within its tissues in response to the cytokinins towards the axillary buds and away from the apical bud. It is only after 10-14 days that the plant fully recovers and begins rapid growth at these new bud sites.

One thing I PREACH when I consult with growers is to NEVER TOP WITHIN 2 WEEKS OF FLIPPING TO FLOWER. If you top within too close a time frame to flipping to flower, you force two incredibly profound hormonal changes in the plant to occur in close proximity. I have seen a lot of weird growth, herms, popcorn buds, health issues, nutrient deficiencies, etc. occur from topping and flowering at nearly the same time. Give the plant some time to recover from the topping and wait 2 weeks! Plus, you miss one of the most productive times in the plant's growth cycle to place your 7 day growth pause…keep that pause in Veg and time your vegetation period accordingly to accommodate this. Now enjoy the full benefit of the rapid growth of the first 2 weeks of flowering.

Properly elongated and angled axillary branches are the key to getting the maximum number of Quality Tops- a metric we will use to plant our canopy.

2. Apical Dominance, Branch Angle, Topping Response and Genetics

Last week, we talked about the hormone dynamics that occur when a plant is topped to better understand what is happening inside the plant.

This week, we are going to talk about how genetic predisposition impacts the way that plants respond to topping, and how we should take these traits into account when planning our canopy. Growers should always be targeting an organized canopy where all of its buds remain in a band of space that receives the optimal light levels, physical support and air movement, which will vary for each facility. So when we grow cannabis plants we want to ensure that after the stretch phase, the buds all stay in that narrow band of light and air movement, which is often where we place trellising infrastructure as well.

Apical Dominance

Apical dominance is one of the most important traits that influences our decision to top and most plants can be placed into one of three general categories- apically dominant, average and basal dominant.

Apically dominant plants that heavily favor vertical growth in their top bud can create challenges for growers as they get too tall. Tall plants tend to have variable quality since their top buds receive a lot more light than their lower buds. These plants should usually be topped, sometimes more than once. Another solution is to plant these varieties more densely and flip them to flower after only a short veg. This profile is often associated with high stretch.

Good candidates for topping
Denser planting
Shorter veg time
Typically high stretch

Average plants are a mix between these two extremes and may or may not need topping. Oftentimes, if plants have adequate spacing and light levels during vegetative growth, they will exhibit more favorable morphology and be less likely to require topping.

Topping strategy depends on veg time/lighting
Variable stretch (and can be more highly manipulated by crop steering)

Basal dominant plants grow their axillary buds almost as fast as their apical buds, forming more of a candelabra shape or broom shape. These plants usually don’t need any topping at all and may benefit from lower planting density and longer veg times to maximize yields. This profile is often associated with low stretch.

No topping required
Average to less dense planting density
Longer veg time
Low stretch

For commercial facilities, it is extremely difficult to get good results from both squat plants and apically dominant plants if only one protocol (planting density, irrigation, veg time, topping, etc.) can be applied. Cultivators should choose their strategy and stay within a general range of apical dominance and stretch characteristics for the best results or consider the ways in which they can modulate their protocol.

Branch Angle

Branch Angle is also important for accessing whether or not to top. This refers to the angle at which axillary buds extend. Cultivars that have a high branch angle and grow almost horizontally create lots of problems for our canopy management and usually require excess work trellising. Conversely, plants that have a low branch angle typically rely little on trellising and respond well to topping. Some cultivars exhibit higher branching angle after topping- extending more horizontally than they would. Observe your cultivars branch angle before and after topping!

Also, notice how branch angle manifests itself into overall plant morphology- it is a super important and often overlooked trait.

High branch angle is an annoying trait and I select against this trait as much as possible! I grew an Oregon Pineapple clone that grew like this, and it was impossible to get a good canopy with it and the heavily laden branches just draped everywhere. The optimal response to topping for a given cultivar is that it promotes strong VERTICAL growth of the axillary buds. Remember: the goal is to make Quality Tops- ones that are strong enough to enter our band of optimal light and air movement.

3. Canopy Planning

In the previous two posts, we discussed hormones and genetic predisposition to topping response. Let’s bring all this information together to plan our optimal canopy and decide on one of Life’s greatest questions: To Top or Not to Top?

Each facility has different limitations on canopy, irrigation, propagation and labor that may influence what the optimal planting density should be, but often there is still some flexibility to increase or decrease density or decide to top or not top.

First, we should measure our canopy space. This may be slightly larger than our table size, but we should start by only analyzing a small test plot, and then we can extrapolate to the rest of the greenhouse later.

Since we know that each cultivar behaves differently, I would always plant topped and untopped crops and record their traits. This may only need to be with a few plants to get pretty solid data. The key trait to record here is canopy width. Depending on the cultivar’s branch angle, we may have more apically dominant plants that are very narrow, while more squat plants may have wider canopies. Also, plants with high branch angle will have wider canopies than plants with low branch angle. Then, we record how many quality tops we can count on each plant.

Our goal is the find the configuration with the highest number of quality tops that fit into the canopy space!

This process is fraught with experimental error, and excessive bottom pruning, aggressive topping, trellising and other factors can greatly influence the quality of this data. We are trying to get a set point here, so natural plant morphology is our goal- try to reduce the amount of intervention as much as possible for this test plot.

Canopy space / ((½ *plant width)^2 * 𝜋) = Number of plants

We then take that optimal number of plants and multiply by Quality Tops

Number of plants * Quality Tops = Quality tops/canopy

Whichever condition has the highest number of quality tops should determine our planting density and to top or not to top.

Example: An untopped test plot has a width of 40 cm with 6 quality tops while our topped test plot has a width of 60 cm and 8 quality tops. For a canopy space measuring 1.3m x 2m (2.6m^2)

Untopped: 2.6m^2 / ((½ *0.4)^2 * 𝜋) = 20.7 plants

20.7 plants x 6 quality tops = 124.2 quality tops/canopy

Topped: 2.6m^2 / ((½ *0.6)^2 * 𝜋) = 9.2 plants

9.2 plants x 8 quality tops = 73.6 quality tops/canopy

This example gives interesting results, showing that more untopped plants with fewer quality tops yields almost 70% more than topping. This math can be helpful but is in no way definitive. You will often need to change the number of plants to properly tessellate and evenly space into your tables, but this is still very helpful to make more informed decisions about topping vs no topping. You can also overlap plant canopies more than this model allows. I would expect the optimal layout for these results to land at 18 plants/table considering the shape of the table for untopped and maybe increase to 12 plants/table for topping, which would still yield 108 and 96 quality tops respectively.

Recap

There are MANY pros and cons to topping/no topping- it's a complex topic
Topping causes a “pause” and should be timed properly to avoid weird growth and optimize growth during early flowering
Every cultivar grows differently and responds to topping differently- observe these traits and take them into account when deciding whether or not to top
Optimal yield depends on the ratio of plant width vs quality tops and how they fit into your canopy
Every facility has different planting density and irrigation constraints- there is no definitive answer. Use this model, but also trust your instincts to make the best decisions

Not Top

Pros

Faster veg
Fewer viroid/disease entry points
No chance of wonky growth in first weeks
Less labor
Bigger buds

Cons

May require more clones
Less uniform test results/color since top cola is often quite tall compared to bottom

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