How to increase THC when growing cannabis; Everything you need to know

How to increase THC when growing cannabis

How to increase THC when growing cannabis is a very valid question. One that many growers will ask themselves at one point or another!

The real question is: How to make sure you don’t do anything that affects a plant’s potential THC percentage.

Because buds will only ever reach the THC content that they’re genetically able to. Not more. But there are many things that can cause plants to end up with less THC than what is possible under optimal conditions.

All the ways to increase THC have to do with inviting the plant to produce more resin. Cannabinoids aren’t the same thing as resin, but they do reside in the resin of cannabis flower. As such, more resin usually means more THC.

Genetic predisposition, or why to start with a high-THC strain

Let’s start with something crucial to reaching the highest possible levels of THC in your harvest.

It all begins with choosing a high-THC strain. (And good quality seeds.)

When the aim is to grow buds that are loaded with THC, starting with a high-THC strain is hands-down the most important piece.

While cannabinoid levels can be influenced through growing techniques, timing, or other tricks, genetics determine whether the plant is able to achieve the coveted sky-high THC percentage. You won’t end up with more THC in buds than the amount the plant or strain is genetically capable of generating.

No big mystery here. The only way to get this right is by starting off with good, strong genes a.k.a. the best possible seeds or clones. Marijuana plants can not generate an infinite amount of THC out of thin air. As with all living creatures, their genetic code is the blueprint of what they are capable of.

Overall plant health

Obvious, perhaps. But in light of some of the more wild suggestions you might find online that allegedly help increase THC when growing, it seems worth mentioning that, first and foremost, happy, healthy plants are good. Extreme deprivation is not.

If you’re comfortable flirting with light conditions and simulated drought, go for it – however, tread lightly when experimenting with these techniques.

Do think twice before subjecting your plants to the strange approaches you might see mentioned online. It sounds boring, but the main goal to strive for is still a healthy, thriving plant.

It is not recommended you feed your plants sugary soda drinks, or let them die of thirst near the end of the flowering cycle, or boil the roots, or even give them LSD. Those ideas are all hogwash.

While a little stress may cause the plant to produce more resin, too much stress has a detrimental effect on the plant as a whole, including its THC levels!

Lights / UV’s

We don’t need any more proof that plants depend on sunlight to grow.

On a small scale, I’m sure you’ve marveled at the way in which seedlings turn to the light! It is quite impressive how quickly they do that, too. On a larger scale, the tropical regions on earth flaunt an abundance of lush, booming, and blooming vegetation.

Indoors, a top-of-the-line lighting system is one of the main investments of professional growers.

Inundating your plants with light is crucial to make them grow large and strong, but does doing so also increase THC production?

This could be the case.

Sunlight is, of course, hard to beat. If you have access to an abundance of sun, use that to your advantage!

With indoor grow lights, getting the same effect takes some fiddling around and experimenting, not to mention throwing some good money at it. Under the right circumstances, it is possible to increase resin production with artificial lighting also.

Why does (sun)light boost trichomes?

To humans, the threat of UV rays consists of causing skin cancer (at worst) and aging (at best), but in the case of cannabis, UV light may be a helpful ally in getting the most sticky, frosty kolas possible.

If cannabis plants produce more resin to protect themselves from the light, then an increase in intensity there should potentially lead to even more resin production. The damaging effect of an overdose of sun/light is, in this case, a “good” thing, since it results in more densely crystal-covered buds.

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Yield vs. potency

There are two things happening simultaneously here. Lots of light equals thriving, large plants. Bigger plants should yield more buds, even if those buds aren’t necessarily higher in THC.

However, if the amount of trichomes/resin on the flower increases, those buds will be more potent. In that case, they’re well placed to achieve their highest potential in THC!

Light seems to be the best strategy to pursue if you want to grow some high-THC weed. And if you’re not in a good place to catch a maximum of the sun (such as Hawaii, for example), then an artificial set-up could be a worthwhile investment.

Techniques that have certain effects (but not necessarily on THC)

The following strategies are used to increase a plant’s yield or growing speed and, admittedly, they do often result in bigger plants. Larger plants typically mean larger kolas and/or more of them. But as mentioned earlier, a larger yield does not necessarily mean more THC – as in, more potent buds.

Whether the concentration of THC in those buds is affected by the following techniques is debatable. Some growers say yes, others say no, and it is rather difficult to prove either way.


You may have heard about the use of CO2 when growing marijuana. However, in relation to potency and THC levels in the flower, CO2 is not your best bet. What’s more, the alleged effect on potency and THC has to do with a decrease in CO2, not the addition of.

Personally, I have not experimented with CO2. To be clear, we’re talking about adding CO2 to the grow room here and increasing the amount of CO2 around the plants. The substance does cause plants to grow faster and with more foliage but it has no effect on the concentration of cannabinoids.

Allegedly, lowering CO2 when plants begin to produce resin can cause them to produce more ethylene. Ethylene is a hormone that encourages the ripening of seeds in general and stimulates resin glands in the case of cannabis. Less CO2 means ethylene production goes up and the plants get the message to put all of their energy into resin production.

Using additional CO2 when growing is yet another expense. One that makes sense for professional growers, to grow more plants in the same space and time frame, due to a faster rotation. For those who aren’t in charge of a large operation, the usefulness of such an investment is debatable.

So, for one, the addition of CO2 has nothing to do with THC and potency. Secondly, if you’re growing ‘au naturel’ and not adding any CO2 during vegetation, then there isn’t much to lower either.

Longer vegetation time

Rumor has it that a longer vegetation time can increase weed’s potency. I haven’t come across any solid proof that this is the case. If you’re in control of the light and you let plants grow larger in a vegetative state before switching to flowering, there’s simply more ‘plant’. Logically, there may be a larger yield.

Let’s say, for argument’s sake, that longer vegetation does indeed lead to more potent buds. It can be hard to rule out what causes increased THC levels in such a scenario. Maybe it’s the time in veg, maybe it’s those beaming grow lights the plants have been under or a combination of both.

As of now, a longer vegetation time does not appear to have a direct effect on the buds’ potency. The prospect of a bigger yield is a fine reason to try this, though!

Lower humidity levels

There is no real data to support the idea that lower humidity levels increase the amount of THC in buds.

With that said, it is commonly accepted that keeping things on the dryer end during flowering may cause the plant to produce more resin as a way of protecting itself from dehydration. The more resin, the more THC, so it seems worthwhile to be on top of this aspect of growing with a simple tool: the hygrometer.

Even if it can’t be proven that lower humidity helps buds become more potent, it certainly won’t hurt. Nothing good happens to cannabis flowers in high humidity! (See point two below, about mold.)

It should also be noted that monitoring humidity levels does not equal falling into the extreme. Subjecting plants to a massive drought would not be healthy either. The main concepts when it comes to humidity are:

  • Seedlings need a higher humidity and, as plants grow, they’ll gradually need less. Here‘s an article by Robert Bergman about the correct humidity levels at all growth stages.
  • During the flowering stage, too much humidity can put your plants at risk of developing mold. It’s useful to be aware of this risk and what to do about it, especially since visible signs of mold usually mean it’s too late for the crop in question. Check out: How To Prevent Mold When Growing Cannabis.
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Humidity is an entire topic of its own when it comes to growing cannabis and a very interesting one. It may not have all that much to do with THC and potency, but it’s an important part of growing those happy, healthy plants.

Lower temperature

Lowering the temperature to increase THC levels in cannabis flower is yet another claim with a certain tenacity.

It makes sense on some level. The arrival of the fall and winter mean it is time to produce the strongest possible offspring and plants operate in perfect synchronicity with the seasons.

The main issue with this logic is that there are many potent strains with a very high THC percentage (or at least the genetic potential thereof) that do not so much depend on the arrival of colder weather. Those tropical strains seem to do just fine without.

So does lowering the temperature really lead to increased resin production and higher THC levels in buds? It’s a concept in need of further testing, that may or may not be an ‘urban myth’.

If you want to try this strategy, keep in mind that the idea is to stimulate plants to pour their remaining energy into resin production, not freeze them to death.

Harvest at the peak of THC production

On to harvesting time, which is something you can actually work with!

There is a prime time to harvest, and if you can get this part right too, your buds will be well on their way to contain the most THC they’re genetically capable of producing!

A cannabis plant’s THC production peak varies depending on the strain. Within the same strain, cannabinoid and terpene levels can vary quite a bit throughout the flowering stage. Different cannabinoids reach their peak production at different times, so in order to harvest at the “right” time, it is good to know which cannabinoid you’re after specifically. In this scenario, that would be THC.

The cannabis flower will be less potent and taste more earthy due to low cannabinoid and terpene levels. Waiting longer to harvest gives the trichomes ample time to develop. But the longer you wait, the more highly intoxicating and sedative your flower will become. This is especially true for indica strains, but even sativa strains can become sedating. Additionally, if you extend the flowering stage too long before harvesting, THC will begin to degrade.

Harvesting too early means less potent buds because the cannabinoid levels are still low. Trichomes really need that last chunk of time to develop. Waiting is good. Also, the longer you wait, the more sedative your buds. This mainly goes for Indica strains, but Sativa’s can become somewhat sedative as well.

On the other end of the spectrum, there is a point at which THC starts to degrade. What actually happens is that it dissipates and oxidizes, turning into CBN, another cannabinoid.

So knowing that both harvesting buds too early and too late can have you miss out on the maximum THC content, when exactly is it just right?

Helpful tools to pinpoint when THC is at its highest

Trichomes are tiny but there are a few tools that can come in handy to determine the sweet spot before you start cutting.

  • Digital microscopes come in various resolutions.
  • When using your bare eye, an old-fashioned – scratch that, timeless – magnifying glass with a handle can be helpful. (But not as helpful as the microscope!)
  • You probably won’t be acquiring one specifically for this purpose, but if it’s there why not grab a digital camera with a macro lens to get up close and personal with your plant. And if you already have a camera body, a macro lens may be worth pursuing. (I’ve found great used manual lenses on eBay that fit my modern camera body.)

Do the trichomes still look clear and transparent? Then it is too early. They should turn to a milky white first. Also, the white hairs or pistils on the buds need to have turned dark and curled up.

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Do you have the tools to make out the shape of the trichomes’ heads? Early on, they still have flat heads. Once the heads round out and look like tiny mushrooms, it means the trichomes are fully developed.

Drying and curing

Drying and curing is the ever-so-important last step. This part of growing cannabis is way too broad to cover here in its entirety. Anyhow, you’re here for the THC, right? Not to read about every aspect of drying and curing buds. 😉

In a nutshell, drying and curing may impact the look, fragrance, flavor, and even the terpene profile and effect of weed. But whatever amount of THC is there, is there from when it was still on the plant. Drying or curing a certain way over another does not miraculously add more THC to the buds.

However, there are things happening to the terpenes present during the drying and curing phases. Further ripening, degradation, and oxidation can impact the flower’s properties and the type of high users experience.

Ever wondered why exactly uncured weed tastes nasty? This is due to the presence of moisture and chlorophyll.

Without getting further into the nitty-gritty of these final steps, let’s just say it’s as good a time as any to give your buds the best possible care.

For example, mold can set in if buds aren’t aired out regularly. There may not be any magic tricks to increase THC levels during this stage of the game, added mold is a sure way to ruin whatever THC is present!

Bottom line: 5 Good reasons to aim for the highest THC in your plants

You’ve chosen to grow a certain strain, based on one or more of its traits. It could be the scent and flavor, how well adapted it is to your geographic location, a specific type of high it induces, or certain medicinal effects.

More than likely than not, their cannabinoid profile is at the top of what influences your choice of strains. The amount of THC in buds is a crucial characteristic to most (if not all) users and for good reason.

Simply because it makes sense.

Whatever the reason, and whichever strain you’re growing, it makes sense to aim for achieving its maximum potential. So how about “just because you can” as a valid reason?

Hello, Houston?

For a stratospheric high. Period.

Quality (for pro’s)

Quality and reputation, as a professional in the business. If you are growing for a dispensary, the quality of your product is obviously crucial.

When your GSC’s flower hits the shelves, it can’t be half as powerful as the typical GSC, or a competitor’s contribution. Most customers have user experience and expectations.

Quality (for all others)

Quality in general. Because you deserve the best, even if it is grown at home.

And might we add reputation among friends? It is rather badass to be known among your friends and like-minded folks for some above-average, or maybe even excellent, buds. Nothing like a little friendly competition to keep you sharp!

THC as medicine

THC is not merely psychotropic.

Marijuana is being talked about more and more, whether it be in news reports, editorials, or at the dinner table. Have you noticed how most often, CBD is portrayed as the medicinal component of cannabis and THC as its ‘downside’?

That is simply untrue. As far as actual medical research has shown, THC is in fact the most promising cannabinoid when it comes to fighting cancer.

Not to mention the relief it provides to countless medical users with other ailments. The research is still limited but it does show that THC isn’t just a fun molecule (though it can be that, too!).

Photo by Jose Luis Sanchez Pereyra.

By Felis Cannabis

Hi, Felis Cannabis here. This little corner of the web is my scratch pole. The legalize movement is growing, but not fast enough for me to give up the incognito status just yet. ;-) Let's keep at it! We should have the right to use any plant we choose.