Cannabis offers cues when it’s time to harvest – along with historical data and testing

Just Released! Get realistic market forecasts, state-by-state insights and benchmarks with the new 2024 MJBiz Factbook member program, now with quarterly updates. Make informed decisions.

Image of Nic Robertson of 4Front Ventures in an indoor cannabis cultivation room

Nic Robertson looks over the canopy at a 4Front Ventures cultivation facility in Massachusetts. (Photo by Marvin Sandoval)

(This story is part of the cover package in the August issue of MJBizMagazine.)

Nic Robertson, vice president of operations in Massachusetts for 4Front Ventures, said commercial growers have moved away from eyeballing the color of trichomes and squeezing buds to gauge moisture content in favor of scientific methods to evaluate when cannabis flower has reached an optimal point for harvest.

“Over the last seven or eight years, I have been proven wrong by my aesthetic view of how mature I think that plant is at that time,” Robertson said.

“Getting those test results and showing higher levels of CBGA – the precursor to a number of other cannabinoids – than I would have expected indicates that that plant could have gone another few days or another week before it truly reaches its maximum potential.”

Today’s cannabis cultivators have more scientific approaches to decide when their crops are ready to harvest, such as rigorous data collection, frequent testing and tools such as microscopes and moisture meters.

History as a guide

“Once we first grow something, we figure out how long it takes to finish. Is it eight weeks, nine weeks, 10 weeks?” said Kevin Sparks, lead cultivator for Insa’s operations in Pennsylvania.

Knowing the window when a strain historically finishes helps cultivators plan their harvests months in advance – or even at the start of the growing year, Sparks explained.

Robertson agrees that keeping a detailed history is important, adding, “We are always learning from the crop.”

If a strain from the previous crop finished on day 58, then the strain from the ensuing crop likely will finish on or very close to day 58, assuming “everything goes to plan for the next crop,” Robertson said, “and there were no stressors that would cause the plant to finish early or cause the plant to delay and finish late.”

That’s a big “if,” however.

“Oftentimes those crops are not exactly duplicated. Certain things are out of our control,” Robertson said.

In commercial settings – especially large or medium-sized grows where production schedules require crops to be flipped five or six times per year – cultivators often must decide between the perfect and the good.

“Every harvest that we do is based on the harvest that is right before and the harvest that we’ve scheduled right afterward. So, we only have a small window of time that we can say, ‘OK, we’re going to pull these plants a little early, or we’re going to give them a few extra days without that domino effect being introduced into the rest of the schedule,” Robertson said.

“Some connoisseurs might raise their nose to this, but over time, I have noticed there’s not a massive difference in a two-day window.

“If it works better for our labor scheduling to harvest on a Friday rather than having to call 25 people in on a Saturday, I’m harvesting the room on Friday.”

 Sampling and science

For some cultivators, another important method to help decide when plants are ready to be harvested is sampling – or pinching off a small piece of flower and testing its cannabinoid profile and potency at that point in time.

Marco Malatrasi, chief cultivator at Florida-based Fluent Cannabis, uses samples to help determine the “ripening curve” for his crops and evaluate how plants in a certain crop are progressing toward harvest.

“For the ripening curve, you need quantitative measure. From your batch, you pull down one plant in week seven, eight, nine and 10, if you have it … taking samples all through the ripening process as you get closer and closer to that harvest date,” Malatrasi said.

He added that you don’t need to sample whole plants but, rather, small buds from different parts of the grow room (back, front, center, edges) and different parts of the plant (top, middle, bottom).

“I’m testing for what’s going to end up in the jar,” Malatrasi said.

He does the sampling twice a week because “big changes” to the cannabinoid profile and potency can occur in only three days – and changes are more likely as you get closer to the end of the cycle.

By testing those different samples, you can see “how they are doing chemically” Malatrasi said.

Everything that we’re looking for is in the ripening curve and finding that sweet spot of when to harvest. … If you still have a lot of CBG, a precursor to THC, you’re probably still early. If you have more-than-usual CBN, you’re running late,” Malatrasi said.

His preferred measuring instrument is high-performance liquid chromatography (HPLC), but that can be expensive. Less expensive tools exist, such as Purpl Pro, a handheld potency tester, and a tabletop potency-testing apparatus from Orange Photonics.

Using these instruments, growers can study marijuana flower’s cannabinoid profile.

“Principally, you’re looking for CBN and CBG, because one’s a precursor and one’s after the fact. That’s cueing my potential. For example, if I do a reading at week seven and I have a certain THCA percentage, that’s great, but how do I know it’s going to increase? Well, that’s where I’m looking at the CBG percentage,” Malatrasi explained.

It’s also important to look at trichome density. For that, Malatrasi likes to use a microscope and a so-called Secchi disc.

“You can use them in the field. They’re good, thorough and quick, which is important when you’re harvesting a few thousand plants per week,” Malatrasi said.

When inspecting trichomes, cultivators favor those with big, bulbous heads and – in terms of extraction purposes – those that have long stems that make them easier to knock off.

It’s also important that there be a high density of trichomes on the flower and that their color be what growers typically describe as “milky.” Too clear, and they are underripe; too milky or amber in color, and they are getting overripe.

For research and development purposes, Malatrasi often will “sacrifice” one or two plants by letting them become overripe because it tells him when “the plant starts to fall off.”

But taking samples to test before harvest might be complicated in some states because of their seed-to-sale tracking systems and compliance requirements.

“We have to track each individual plant separately, so it becomes really hard to, say, take one little chunk off the plant and then weigh that and then track the rest of it separately,” Robertson said.

“It creates this compliance risk, and it’s just a nightmare of data for us to maintain that. So, we harvest the plants full, and we dry those plants in a full-plant hang that are still individually tracked by RFID (radio frequency identification) plant tags.”

Jushi’s evaluation processes at all of its cultivation sites include “harvest walks” throughout the harvest room, bringing together leaders from the site’s cultivation and processing departments “and even maybe a manufacturing individual,” said Ryan Cook, executive vice president of operations at Jushi.

“These reports guide us … so Josh (Malman, Jushi’s head cultivator) and I can have conversations weeks in advance of any harvest and generally know where things are headed,” Cook said.

“We have our own analytical labs and our own microbial labs inside the facilities, so we have the ability to test far in advance. That allows them to gauge the potencies weeks before we’re prepared to harvest.

“We’ve got full-on harvest prediction planners, where we can see from the clones that I took this week when that plant is going to be harvested. And ideally, we’re working in a way that we say, ‘Hey, in 16 weeks, I need half of that product going to flower and half that product only to extraction,’” Cook said.

“We are able to make some adjustments at the end of cycle near harvest based on measurement, but we try to make those determinations earlier.”

 Wrapping up

While scientific measurements are a much larger part of cannabis harvesting than they used to be, the old-school tricks that once informed growers aren’t necessarily obsolete.

Whether you’re using an HPLC machine, a microscope or jeweler’s loupe to examine your plants, you are still looking for the same visual cues.

“When you know it’s done, it’s usually by trichrome production,” said Frank Golfieri, chief cultivator for Insa’s Massachusetts operations.

Flower to be used for fresh-frozen extraction is best harvested slightly on the early side, when the trichomes are clear to a little milky. Earlier means that there are more terpenes because fewer terpenes have been lost to heat.

If you harvest for flower, wait a few days longer, when the trichomes are milkier, Golfieri recommends.

All of Jushi’s cultivation facilities are equipped with digital microscopes that can take high-resolution photos of trichome size, shape, color and density.

“The age of the plant and what the trichomes look like is absolutely something we’re going to look at before we harvest,” said Malman, Jushi’s head cultivator.

“We’re looking at pistil retraction near the end of the cycle. You normally see these big, white fuzzy pistils. … As those start to get older and more mature, they will start to retract down into the calyx.

“Sometimes they change color … orange or brown. Sometimes they don’t, they just fully retract. Those are visual cues that we look for.

Jushi also watches “the color of the flower as we’re dropping that temperature near the end of the cycle. Both in a daytime and nighttime period, you start to see more purples, more anthocyanins coming out in the plant. So that’s generally a cue for us,” Malman said.

Golfieri agreed: If (the hairs) are all white, it’s still too early to cut down. You want them to be orange. And the flower should have a velvety, sticky feel. If the hairs are red and some new white ones are sprouting up, that means you’re feeding it too much nitrogen,” he said.

Off-gassing aroma also is a sign that plants are ready to be harvested.

As the end of cycle approaches, cultivators should lower flower-room temperatures “to preserve as much of that off-gassing as possible,” Malatrasi said.

“If you already have off-gassing, then it’s probably time to get the plants harvested. I want off-gassing to occur in the dry room.”

Omar Sacirbey can be reached at