You did it! You just grew the most gorgeous cannabis plant. It smells better than a batch of freshly baked Blueberry Muffins, and its sheer, natural beauty is beyond words. You are certain that the sight of it alone could bring world peace. You bask in the glory that is Mother Nature’s gift to humanity: a fully matured cannabis plant. OK, so what’s the next step?
Kill your plant.
Yes, it is true that in most cases you have to kill the plant at this stage. The preferred euphemism of this stage is harvest. Harvest is an important time in the crop’s cycle as this is the culmination of all the blood, sweat, & tears you put into cultivating your beautiful plant. Basically, all you need to do at this stage is to cut the stem at the crown of the plant, so you can get ready for dry and cure. Perfect! Blog post over. That’s all we need to know, right? Well, not exactly. I’m here to tell you there’s a bit more to know about harvesting your cannabis, especially about the WHEN to harvest, which we’ll get to a little bit later.
We must remember why we are cultivating cannabis in the first place. When you were focusing so much on the right temperature, humidity, soil moisture content, rootzone fertility, etc. during the flowering stage, you were actually just optimizing the amounts of cannabinoids and terpenes in the flowers. We all generally know what the cannabinoids are responsible for when we consume cannabis, but the terpenes are very, very important as well.
What Makes Terpenes So Important?
First off, the terpenes are what provide the aromas and flavors of cannabis, and there are a lot of them (roughly 120 known in cannabis). They have other effects that most people don’t realize as well. For example, it’s primarily the terpene profiles that give cannabis the “sativa” or “indica” feel. Cannabis that has a higher concentration of terpenes such as α-pinene and limonene tend to be categorized as sativas. Both α-pinene and limonene are bronchodilators, which means that you get more oxygen per breath after consuming these terpenes. This generally gives an uplifting response.
Terpenes such as (−)-β-caryophyllene, linalool, & eugenol have been shown to be muscle relaxers. Hence, they are typically found in higher concentrations for cannabis cultivars typically associated as indicas. Additionally, the most abundant of the terpenes found in cannabis, β-myrcene, has many health benefits including anti-nociception, anti-inflammatory properties, and most interestingly β-myrcene decreases the resistance of cannabinoid movement across the blood-brain barrier. This is especially interesting because consumption of β-myrcene prior to medicating with cannabis means that the patient can actually consume less cannabis and still feel just as medicated. Using less cannabis while still maintaining a medicated state is a great property for people who want to prolong their cannabis stash as long as possible and to avoid going dankrupt.
Keep Your Terpenes.
I’m sure at this point, you’re thinking, “Sweet. Thanks for the biochemistry and physiology lessons, Dr. Robb. What does all this have to do about harvesting our crops?” Well, I wanted to emphasize the profound importance of terpenes in cannabis. You definitely don’t want to overlook that. Also, I don’t want you to waste all that time and energy of cultivating your big, beautiful crop only to result in a flower lacking nose and flavor because you were careless with your terpenes. Harvest is where your primary focus switches from terpene production to terpene retention. Harvest and post-harvest biology have more to do with terpene retention than pretty much anything else.
What do I mean by terpene retention? The terpenes found in cannabis are VOCs, otherwise known as Volatile Organic Compounds. Let’s break that down. Compound: a chemical substance consisting of molecules with different atoms in it. Cool. That pretty much explains anything in the plant. Organic: a compound that has carbon in it. Sweet. So once again, this could almost describe everything in the plant. Volatile: describes a compound’s tendency to vaporize at normal temperatures and pressures.
BOOM! This is important, and here’s why. The terpenes are volatile, which means that they can vaporize and off-gas. You can test this yourself by simply walking by a lavender or rosemary or cannabis plant. They tend to smell. The aromas that you are sensing are VOCs, and in this case, terpenes that have vaporized from the plant.
Now imagine walking by said plants on a hot day vs. a cold day. In which situation do you think the terpene aromas would be more pungent? If you said hot day, then you are one step closer to understanding optimizing cannabis harvest.
On hotter days, your cannabis crop is going to have a stronger smell, and this simply has to do with the heat increasing the kinetic energy of the terpene molecules, which gives those molecules a greater propensity to volatize. The warmer the temperature…the more terpene volatization…the more terpenes will be in the air…the stronger the smell. That’s great that your cannabis smells so good on a warm day, but the more terpene molecules in the air mean that there’s less of it in the flowers. We want to retain as many of those molecules inside the plant once you harvest because now that the plant is dead/dying, it will no longer be replenishing any terpenes that are lost via volatilization.
Now that we all know more about terpene volatility; how can we use this information to better harvest our cannabis? Well, let’s look at how other terpene-heavy plants are harvested for some hints. Who here among us has not been to a commercial lavender harvest? No one, right? You may remember while witnessing the commercial lavender harvest that it started in the early morning. This is done with terpene retention in mind.
Early morning tends to be the coolest part of the day, which means more terpenes will be retained in the plant after harvest. In cannabis, we see similar harvests. For indoor growers, you can pump the AC or turn off all or most of your grow lights (just enough to see). Greenhouse growers can pull shade cloth to cool off the greenhouse. Outdoor growers can do the same thing their lavender cultivating colleagues are doing: harvesting in the morning (or another part of the day or night when it’s cool outside).
Can I Harvest Already?
We’ve discussed when to harvest during the day to optimize terpene retention for a better cannabis crop. What about when to harvest during the season to optimize/maximize cannabinoid production? Lucky for us, cannabinoids don’t have the same problems with vaporizing at normal temperatures and pressures that terpenes have. THC is not going to off-gas on a sunny afternoon (unless that sunny afternoon is approaching 315°F). Typically, knowing when to harvest with the cannabinoids in mind depends on the genetics of your cultivar. Some cultivars can be harvested as early as 45 days after you set the photoperiod. Some can go as long as 70-80 days. On average, we typically see the harvest date is set as 60 days after the photoperiod, but this is more a rule of thumb than a hard-set law that must be abided by. To get a better idea of when specifically, we should harvest, we just need to look to the trichomes.
OK, But How Do I Check My Trichomes?
The trichomes on the female flowers are where the cannabinoids and terpenes are primarily concentrated. When you look at them under a microscope, they look like little translucent water towers or mushrooms. The top, bulbous portion of the trichome will turn from clear to milky or cloudy in color.
That milky white coloration is indicative of THC loading. You want to make sure the plant is fully matured, which in turn maximizes THC content. As the plant ages, some of the trichomes’ globular tips will turn an amber or brown color. This is more indicative of THC degrading to CBN. Some people try to avoid any amber trichomes, whereas some people wait until around 20% of the trichomes have turned to amber. The reason why a cultivator would wait for some of the trichomes to become “over mature” is because the trichomes don’t mature all at the same time, so if you’re allowing for some “over maturation”, you give time for the late-blooming trichomes to catch up with their THC loading. I recommend using a jeweler’s loupe to get a closeup view of those trichomes as your plant approaches that 60 days to maturation mark.
I hope this helps my fellow cultivators in knowing when to harvest. Once that plant is cut down, it becomes a game of terpene retention. Harvest is the first step to the most important part in all of the cannabis production: drying/curing. We will be tackling that one next month but until then…
 This is just nerdspeak for it helps kill pain; aka it’s an analgesic.
 THC’s boiling point at 1 atmosphere of pressure is 314.6°F. Yes, it can volatize before then, but this is the temperature where there’s enough kinetic energy to initiate boiling off.
 If you remember from one of my earlier blog posts, I prefer to use the term cultivar rather than strain. Strains refer to bacteria. Cultivars (short for cultivated variety) are used for plants.
The Schedule For The 9-Part Series
Part 1 – Introduction: Meet Dr. Robb (June 19)
Part 2 – Cannabis Veg Cycle (July 17)
Part 3 – The Flowering Cycle (Aug 21)
Part 4 – Harvest (Sep 18)
Part 5 – Dry and Cure (Oct 16)
Part 6 – Trimming (Nov 20)
Part 7 – Assembly Required (Dec 18)
Part 8 – Maintaining Mom’s and Genetics (Jan 15)
Part 9 – Cuttings and Cloning (Feb 19)