How to Increase THC Levels in Your Cannabis Home Grow

Francis Cassidy September 25, 2020 2 comments

We’ve consulted the best to come up with the tips you need. 

Cultivators continue to breed for ever-higher levels of THC. But, the average home grow doesn’t have the same abilities as a large-scale farm. Well, good thing we’ve got some tricks. Here’s how the average grower might increase THC levels in a home grow.

The levels of THC in modern cultivars are higher than those attained naturally. It’s the result of tireless decades of artificial selection and breeding on the part of cultivators. Every hybridized cannabis plant went through a process where the male of one variety was crossed with a female from another. By selecting for specific phenotypes, breeders gradually began to define the characteristics of the new cultivar through a continual refinement process.

Breeders strengthen the genetics to ensure healthy and stable descendants through backcrossing, a process that involves pollinating the new variety with itself or a parent. This ensures it’s homozygous and leads to enhanced stability, where each seed contains the same genetic sequence every time.

Through a process of trial and error and gradual improvements, breeders develop new cultivars with the characteristics they desire. Many of these cultivars now produce extremely high quantities of THC – above twenty percent. Some of the most successful cultivars go through years of growing, crossing, and backcrossing all in an attempt to define the phenotypes and ensure consistent and healthy genetics.

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Why Genetics are King When it Comes to THC

For home growers, genetics are the primary driver of THC levels in cannabis. For individual cultivars, there is a ceiling for THC levels, when growers follow proper growing practices.

Basically, there’s little point in aiming for THC levels of twenty percent with a cultivar that produces fifteen percent. No matter how good a grower you are, you’re unlikely to pass the ceiling set by the seed’s genetic profile.

The essential first step for those chasing high THC content is to obtain clones or seeds of high-THC cultivars from a reputable source. Some of the common varieties that exhibit high levels of THC include Bruce Banner (twenty-three percent), Original Glue (twenty percent), and White Fire OG (twenty-one percent).

How to Increase Levels of THC in Cannabis

With solid genetics selected, here are some home grow tips for those who want to maximize the genetic potential for increased THC content.

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how to increase thc content represented by cannabis plants with a scrog net above them

Help The Plant Divert its Energy into THC Production

While genetics play the most important role, that doesn’t mean that growers can simply pop a seed in the ground and return in two months to reap a bountiful harvest.

Experienced growers know that to produce the most potent buds, the cannabis plant must be kept healthy at all points during the grow cycle. This involves proper training, preventing pest infestations, and meeting the plant’s nutritional needs. Any energy that the plant diverts into combating external issues takes away from the energy it can use to produce highly potent buds.

How to Increase Levels of THC in Cannabis: The Importance of Light

The landrace cultivars that exhibit the highest THC levels all have one thing in common; they evolved high up in equatorial climates with long days and abundant UV light.

A study published in Economic Botany (1983), attributed high levels of UV-B radiation to increased THC production in cannabis.[1]David W. Pate. (1983). Possible Role of Ultraviolet Radiation in Evolution of Cannabis Chemotypes. Economic Botany, 37(4), 396-405. Retrieved June 2, 2020, from The reason for this is quite simple. The THC-rich resin in cannabis flower is a protective coating that defends the plant from the harmful effects of excessive UV radiation. This means that cannabis growing at higher altitudes outdoors is likely to produce higher amounts of THC. It’s merely a protective mechanism that translates to enhanced medicinal benefit.

We can’t all relocate to the Hindu Kush mountains on the Afghan-Pakistan border to grow high-THC cannabis, but we can increase the light intensity in an indoor grow room during the flowering phase. This will help the genetics express their ability to produce THC to a greater degree.

Could Drought Increase THC?

It may sound counterintuitive, but science suggests that by making plants thirsty in the late flowering stage, growers may achieve higher THC levels.

A study published in HortScience (2019), investigated the effects of targeted underwatering, or controlled drought stress on cannabinoid production. Researchers hypothesized that by inducing controlled drought conditions, growers may enhance the potency of the yield.[2]Caplan, Deron & Dixon, Mike & Zheng, Youbin. (2019). Increasing Inflorescence Dry Weight and Cannabinoid Content in Medical Cannabis Using Controlled Drought Stress. HortScience. 54. 964-969. … Continue reading

Researchers grew eight plants in total. Accordingly, on day thirty-nine, they separated these into two groups and subjected one group to the drought conditions. Once harvested and cured, researchers noted that the drought conditions provoked a twelve percent increase in THCa levels. This finding occurred with other cannabinoids too, with CBDa quantities found to be higher in the plants subjected to drought.

The explanation provided by scientists is that as the plants sense a stressor, they produce more of their secondary metabolites that include cannabinoids and terpenes.

how to increase thc content represented by cannabis plants with yellow turning leaves

Harvesting Cannabis

The optimal harvest time is when terpene and cannabinoid levels peak. Those chasing the highest THC levels need to time this window to perfection. The signal lies within the trichomes—those fine, hair-like appendages found mostly on the buds.

By using a magnifying glass or a microscope, check the trichomes for both their shape and color. Trichomes should be well-rounded with a noticeable mushroom head, while the color should transition from clear or milky in appearance to a brown or amber tone. This represents the peak of THC production and is the ideal time to harvest.

THC will begin to degrade when this window expires, so it’s critical to time the harvest correctly to preserve maximum THC levels.

Old Wives Tales

There are many old wives tales bandied around the internet that suggest ways of increasing THC content. These include decreasing CO2 levels during the flowering phase and decreasing temperature by ten degrees Fahrenheit to enhance resin levels. Sadly, neither of these techniques are scientifically validated, even if that doesn’t stop some growers from trying.

Why Cannabis Isn’t all About THC

THC is the primary driver of potency in cannabis. However, there’s much more to cannabis than THC levels. Further, terpenes play a huge role in enhancing the medicinal benefits of any cultivar. When a consumer combines these with cannabinoids like THC, there is an accentuated benefit.

So, what should those who take a more calculated approach to medicating do? They may find that certain terpenes, when combined with THC, will enhance its effects further beyond what a percentage point gain may ever achieve.


Author avatar

Francis Cassidy
Francis Cassidy is a freelance writer who writes on a variety of topics. With a particular focus on the cannabis industry, he aims to help ensure the smooth reintegration of cannabis back into global culture. When not writing, he's to be found exploring his new base in British Columbia, Canada. You can follow his other works including his photography on his blog

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  1. Mark Morales

    This article is a complete misrepresentation of the study. The paper warns that this was done on a specific and uncommitted strain. The soil and nutrition was very specific. It even states this is not likely to be the case or even viable in home grows or production settings but requires more controlled studies.
    It’s scary an article like this is out there spreading false information under a medical website.

    • Jennifer Grant

      There is no warning. I see no problem with the way the author has summarized the studies. Don’t be scared.