48 Hours Of Darkness Before Cannabis Harvest – Dank Or Dumb?

The balance between darkness and light may sound like the premise for a superhero movie, but it’s actually an everyday concern for cannabis growers. And while much has been written about the virtues of luminosity, less is known about the dark arts of weed cultivation.

For instance, some say that keeping plants in darkness for 48 hours before harvest time leads to larger colas, more trichomes and higher concentrations of THC and terpenes. However, whether or not this claim holds any truth remains a point of contention.

The Darkness And The Light

All photoperiod cannabis strains require a change in the light cycle to trigger the switch from veg to the flowering stage. More specifically, an increase in the number of hours of darkness signals that the growing season is ending and causes plants to prioritise flower production.

Indica strains generally have shorter grow cycles than sativa cultivars, and it’s up to growers to choose the right moment to alter the light cycle depending on which variety they are cultivating.

During flowering, a marijuana plant usually receives around 12 hours of light per day, mirroring the light conditions of the autumn equinox. Autoflowers aside, cannabis has evolved to reach its reproductive peak at this time of year, so it makes perfect sense for growers to maintain this photoperiod as harvest time approaches.

And yet, some insist on keeping their plants in complete darkness for a few days immediately before the chop. Such a practice sounds counterintuitive, and there are certainly a lot of sceptics. Still, there is some method to this apparent madness.

Why Keep Cannabis Plants In Darkness Before Harvest?

The idea of keeping cannabis plants in darkness for 48 hours before harvest is a classic example of stoner botanist lore. No scientific studies have been conducted to either verify or negate the efficacy of the practice, although there are numerous theories as to why the trick might be beneficial.

For example, some cannabis growers believe that 48 hours of darkness stresses plants into producing more resin. Most serious breeders don’t buy into this theory, although the idea is not totally without logic. After all, everyone knows that stress causes an increase in THC and terpenes, which is why so many growers rely on light-stress training (HST) techniques.

If you’re looking for a more reliable and widely accepted form of HST, then you may want to try lowering the temperature in your grow room by a few degrees during the last week or two of flowering. This technique can help to enhance trichome production and boost cannabinoid percentages. However, whether or not an extended dark period before harvest has the same effect is less certain.

Another hypothesis states that cutting the lights panics plants into producing more nugs on each of their bud sites. According to this theory, the sudden increase in hours of darkness signals that winter is approaching and that a plant’s days are numbered. To increase the chances of being pollinated before biting the dust, a marijuana plant grows as many flowers as possible to ensure it has the maximum number of pistils available for pollen collection.

Once again, however, this unproven idea is widely disputed and probably wide of the mark.

Science vs Stoner Science

Yet there is a more compelling reason to keep your plants in the dark at the end of their life cycle. Even if all of the above sounds unconvincing, most growers acknowledge that cannabinoid levels peak at night and drop during the day.

This is because compounds like THC help to protect cannabis plants from pests and other stresses like UV-B radiation from sunlight. During the day, these defences are in higher demand, meaning cannabinoids get used up and degraded when the sun is out but are replenished at night.

The same goes for terpenes, highly volatile compounds prone to evaporation under light and heat.

Like THC and other cannabinoids, these aromatic ingredients wane during the day and are restocked at night. Based on this natural chemical fluctuation, many growers believe that keeping the lights off for a few days allows cannabinoid and terpene levels to peak, ensuring their weed is as dank as possible when they harvest.

Does Keeping Cannabis In The Dark Work?

The logic behind this extended dark period is pretty solid, although there’s no hard evidence to prove this method’s effectiveness.

You’ll often hear growers citing a study which supposedly revealed that plants exposed to 72 hours of darkness before harvest had 30 per cent more THC, although the paper itself is either unavailable online or doesn’t exist. According to rumour, the study found that darkness had no impact on CBD or CBN levels but did cause terpenes to increase.

Sadly, no other academic research has been conducted on this topic, so it’s impossible to confirm or deny the efficacy of this approach. Regardless, many growers choose to harvest at the end of their dark period, before the lights go on in their grow room. Similarly, plucking colas at dawn gives plants a full night to refill their cannabinoid and terpene levels for an outdoor grow.

The same reasoning explains why most growers keep their plants in a cool, dark place during drying and curing. Minimising light and heat at this time prevents the degradation of any desirable compounds and ensures that the finished product is as pungent and potent as possible.

Handlers must obviously use a head torch to avoid damaging plants when harvesting in the dark. Leaving nothing to chance, some people choose to use a torch that emits only green light, as fan leaves don’t absorb this for photosynthesis and therefore doesn’t disturb the light cycle.

Such a precaution is based on chlorophyll itself being green and therefore reflecting green photons while absorbing blue and red light. While this is true, it ignores the value of green light, which becomes apparent once you dig a little deeper into the minutiae of cannabis cultivation. Green leaves may not use green photons for photosynthesis, but their omission weakens plants’ defences, highlighting the need to provide cannabis plants with the full light spectrum.

So, Should You Bother With 48 Hours Of Darkness Before Cannabis Harvest?

Ultimately, it’s impossible to say whether keeping your plants in the dark really enhances their strength, aroma or taste. With no reliable studies for reference, it’s up to each grower to conduct their own experiments, taking pics of their plants and documenting cannabinoid and terpene levels under different light regimes.

If this is your first time growing plants in the dark, keep in mind that mould can quickly set in when light and heat are restricted, so it’s essential to ensure your soil is reasonably dry before flicking that switch.