Stress for Success: Weighing the risks and benefits of HST

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Give a grower a few years of cultivating cannabis plants and they’ll likely hear rumblings of stressing out the plants…in a beneficial way, of course.

High-stress training (HST) is a set of several techniques aiming to add stress to plants so they can, say, improve yields or boost the level of cannabinoids. 

Think of this approach similar to how humans deal with stress: when plants endure pressure, they shift into survival mode and change growing behaviors.

The research is almost there to help prove HST is beneficial. Some studies have found that underwatering and creating drought conditions for cannabis plants can improve the cannabinoid concentration.

But it’s not a one-size-fits-all application. Which HST techniques are ideal to bring to the operation, and which should be left in the shed? Grow Opportunity spoken to several Canadian cultivators to learn more about topping, knuckling, temperature management and other strategies they say will improve the growing experience and yield. 

Top of the morning to you

Canopy management is critical for any grower dipping their green fingers into HST. Topping, a well-known HST technique, refers to removing of the top part of the plant to encourage more growth in the lower branches. 

By removing eight to 12 of the branchers to lower the height of the canopy, growers can find the right balance to how the plants will grow under light and how the branches will fill out the space to contact that light, says Will Fournier, part of Canna Canada’s product development team. 

“Cutting the apex allows the plant to grow wider rather than taller, too,” he adds.

Tyson Cramer, chief cultivator at Aurora Cannabis, says topping can be beneficial for growth but don’t jump into it without a trial period. “Certain Cultivars will react to topping differently from one another so perform some runs to pay attention to what happens when, say, the topping happens prior to the flip or four days after the flip.”

Cracking with control

When Daniel Vaillancourt, founder of consulting group 4trees Cannabis, looks back at which HST approach he enjoys leveraging, knuckling (also known as supercropping) is top of the list. By squeezing the stalk with the index finger and thumb in order to squish in slightly, this technique coaxes the branch to fall over and become horizontal, he says. “Over time it will point back up at the light and develop a large knuckle in that pressure spot, and it really increases cellulose levels and different hormones that promote growth,” says Vaillancourt.

He cautions that knuckling has to be done roughly one to two weeks before flowering, and it’s a sensitive operation: place too much pressure on the branch and it can snap off. 

Cramer says Aurora’s operations are too widespread to manually knuckle each plant, but he recommends it for smaller lots, as it’s something he’s seen that allows light to hit the top colas effectively. “Whether it can add THC levels, that’s not something I’ve seen concretely, but there’s potential there,” Cramer says.

Make a bet on netting

To bend branches successfully, another HST approach is attractive to some growers. “I net and trellis aggressively,” says Jawad Skieneh, master grower at Greenway Cannabis. 

He explains how the trellis acts as a support system and offers a way to stress out the plant in bending the branches back and allowing sunlight and supplemental light to hit every node and grow more laterally and vertically. “Light can then get at the whole plant. If the plant remains very vertical, the top can shade the bottom and create microclimates, creating an inconsistent grow,” Skieneh adds.  

The downside of netting is how overcrowded the grow can become, says Fournier. “Some netting operations will create overlapping of stems and leaves, so it can be messy, and could potentially lead to bud rot and other diseases, and also block airflow,” he says.

Stay cool

Adding stress to cannabis can also include manipulating the temperature in the greenhouse. Skieneh favours cooling down the operation with certain strains, which often “lead to higher terpenes profiles and more beautiful dense flowers,” he says.

Take into consideration the strain at hand, because certain flowers will naturally grow densely, whereas other strains will need that lower temperature near the end of the grow cycle. “You want to match the climate to your genetics,” Skieneh says.  

Another HST tactic is depriving plants of water to encourage the roots gobbling up as much oxygen as possible.  Vallaincourt says when this sensitive approach is done right, it can increase the THC levels in some strains. But use it smartly, he adds. “When you’re drying out your roots, you can kill rhizomes that are surrounding your roots, that never come back,” he says. 

Why darkness matters

An integral HST technique is light management, Vallaincourt suggests. “We create this artificial fall phase for the plants,” he says. 

Near the end of the plant’s life, he tends to knock off the lights completely, an HST approach that “brings out a lot of terpenes, boosts the THC levels,” he says. Now that terpenes are gaining wider acclaim for being the essential oils of cannabis, can any HST strategy work towards boosting their levels? Cramer has seen how adding sugars to soil can manipulate terpene levels in plants, “but it’s a practice difficult to do at scale,” he adds.

But where’s the science?

As anecdotally effective as HST techniques can be for cannabis growers, some experts warn how little concrete evidence backs up their claims. “Where is the academic literature proving that, say, topping works?” asks Mike Dixon, a professor at the University of Guelph’s School of Environmental Sciences. “Without having an actual experimental data set to refer to, I hesitate to pass judgement.”

Dixon recognizes the uphill battle researchers face. “Cannabis in Canada was illegal for so long, and it still is in the U.S., so it’s hard to get data to support theories.” Dixon is hopeful that as more studies analyze what stress does to cannabis, the stronger the case will be.

Still, no matter the approach taken, ensure data is tracked, “rather than just going by feel or intuition,” says Dixon. 

Skieneh formalizes the data-gathering process by always keeping a pen and pad on him when he tracks grows cultivated with HST. “You can look at things how much closer are the nodes today, and how can I get that beautiful crop again that I just grew?” he says. “Give yourself the tool to look back on what worked so you can replicate it later.” 

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David Silverberg is a freelance journalist and writing coach in Toronto who writes for BBC News, The Toronto Star, Leafly and more. He was also the editor of The Medical Marijuana Review.  DavidSilverberg.ca

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