Field Guides

Why freshness matters: the cure, the dates and the label

By SŌMA 7 min read Updated

Cannabis flower is an agricultural product, and it keeps changing after harvest. Its THC slowly breaks down, one study found its aroma compounds falling faster, and how it is stored shapes both; what curing does to it has been little studied. Freshness is not printed on a New York label as a single number, but the package and its QR code tell you most of what you need.

What curing is

After harvest, flower is dried and then cured: kept in closed containers at a set temperature and humidity for a period, at the end of drying and before storage, so that it holds a steady moisture content. Methods vary. One research group, for example, cured flower for a month in the dark at 21°C and 60% humidity, in sealed glass vessels opened every 48 hours.

A 2022 review of postharvest handling calls curing an important step for quality and, citing a single earlier study, credits it with less throat burning when the flower is smoked. The research behind such claims is thin: the same review calls curing one of the most significant postharvest operations, but says it has been overlooked and not investigated properly, and that how curing method, water activity, potency and chemical profile interact is still not widely investigated. Read any claim about a particular cure as the producer's description, not as a measurement.

What fades, and how fast

Terpenes are compounds that give cannabis much of its aroma. In a one-year laboratory study of flower kept in the dark at four temperatures, terpenoid levels — measured in one CBD-rich variety — fell under every condition, and on average were down by more than half after four months. The cannabinoids changed differently. The plant makes them as acids, such as THCA; in the same variety these fell by 26% on average over the year, mostly by turning into neutral forms such as THC and CBD. In whole flower, THC in the study's THC-rich variety, and CBD in the CBD-rich one, rose over the year. (More on terpenes in What are terpenes.)

THC itself breaks down over months and years. It oxidises over time, and cannabinol (CBN) is its main breakdown product. One older four-year study gives a sense of the pace at room temperature:

THC lost from plant material stored in the dark at 20–22°C, in one study of material grown in 1991 and 1993
Time storedAverage loss of THCStandard deviation
1 year16.6%7.4 points
2 years26.8%7.3 points
3 years34.5%7.6 points
4 years41.4%6.5 points
Losses are of each sample's starting value. The plant material was dried in a drying barn at 50–70°C, coarsely manicured and kept in closed barrels in an air-conditioned vault, so the figures describe that material, not any one package today. The authors believe that the higher the THC, the faster it breaks down over the first one or two years. Source: Ross and ElSohly, Bulletin on Narcotics, 1997.

Ross and ElSohly believe that material higher in THC loses it faster. In a later four-year study of six high-potency herbal and resin products, samples at room temperature (22°C) lost THC and gained CBN significantly, in light or in darkness, and almost all of the THC had broken down after four years; temperature changed how fast that happened, and light changed both how fast and how much of the THC became CBN. In a two-year study, ordinary light was the greatest single cause of cannabinoid loss, and air oxidation caused significant losses too. How to slow all of this down at home is in How to store cannabis.

The dates on a New York package

A New York label must give the date of expiration of the unopened product and its proper storage conditions. OCM's guide to reading a label explains why the date matters: cannabis can change when it is exposed to air, so the lab certificate will not be accurate forever, and the expiration date tells you whether it still applies to a package you have not opened. The label rules ask for no date for an opened package.

Nor do they ask for a harvest or packaging date, so a label may not show one. Two other places hold dates:

  • The lab certificate the label's QR code or link leads to. Its collection date is the day the sample was taken from the licensee for testing; OCM's guide also points to the received and reported dates. They date the test of a batch, not your package.
  • The Metrc Retail ID record. Since March 2026, OCM has required a Retail Item ID QR code on each unit a dispensary receives; products a shop received before 1 April 2026 may still be sold without one, up to their expiration date. Our guide to the New York cannabis QR code shows how to read the record it opens, as the producer filed it, for the packaged and test dates.

None of these dates is a verdict on a package. They tell you how long the clock has been running; storage decides how fast it ran.

Reading freshness for yourself

  • An expiration date that has not passed, on a package that is still sealed.
  • A recent packaged or test date in the record the QR code opens.
  • A distinct aroma when you open it: in one study, of a CBD-rich variety, aroma compounds were down by more than half within four months.
  • Flower that is neither crumbling nor damp. ASTM's standard for dried flower sets its moisture range to prevent breakage below it and mould above it.
  • A package that has been kept closed, dark and cool.

Freshness is part of quality

Two packages sold under one strain name can differ in grower, batch and age (see Why the same strain can feel different). Age and storage are the part you can read and control: check the dates before you buy, and store what you take home well.

Sources

Frequently asked questions

Does cannabis go bad?

It changes rather than spoiling overnight. THC slowly breaks down, faster with light, air and warmth, and in one study of a CBD-rich variety aroma compounds were down by more than half within four months. Flower kept too damp, above a water activity of 0.65 (about 65% humidity in a sealed container), is at risk of mould. A New York label's expiration date applies while the package is unopened and kept as the label says.

What does curing do?

Curing keeps dried flower in closed containers at a set temperature and humidity for a period, so its moisture holds steady before storage and packing. One 2022 review, citing a single earlier study, credits it with less throat burning when the flower is smoked, but the same review says curing has been overlooked and not investigated properly.

Is there a harvest date on a New York label?

Not necessarily. New York's label rules require an expiration date for the unopened product, not a harvest or packaging date. The lab certificate gives the day its sample was collected, and our QR code guide shows how to read the packaged and test dates in the Metrc Retail ID record, as the producer filed it, and the harvest the package came from where one was entered: usually a date, on some records a harvest name. Not every record carries one.

How fast does THC break down?

There is no single rate. In one older study, plant material grown in 1991 and 1993 and kept in the dark at 20–22°C lost on average 16.6% of its THC in the first year and 41.4% after four years, with wide variation between samples; its authors believe that material higher in THC loses it faster at first. In a later study of six high-potency herbal and resin products kept at 22°C, almost all of the THC had broken down after four years. Light, air and warmth speed it up. In one study of a CBD-rich variety, aroma compounds fell faster than the cannabinoids did.

How can I tell if flower is fresh?

Check that the expiration date has not passed and the package is sealed, look up the packaged and test dates through the QR code, and when you open it, look for a distinct aroma and flower that is neither crumbling nor damp.

Corrections

  • We said a harvest or packaging date is one of the most useful pieces of information on a label. New York's label rules require an expiration date, not a harvest or packaging date; the guide now says which dates the label, the lab certificate and the Metrc Retail ID record give.
  • We said a careful cure lets harshness mellow and the aroma settle and deepen, and that a rushed cure leaves flower flat or rough whatever the genetics. Curing has been little studied; the guide now says what curing is and that the evidence for its effects is thin.
  • We said good genetics are a starting point and freshness is what carries them to you, and that matching by sensory qualities helps you recognise what good smells like. We had no source for either and removed them.
  • We compared flower to coffee and tea, listed a hay-like aroma as a sign of age, and said one strain differs between sources largely because of curing, freshness and storage. We had no source for these and removed them.
  • We said the aromatic terpenes are volatile and fade first, so the nose dulls before anything else. The one study we found that measured them over time did so in a single CBD-rich variety, in which they fell faster than the cannabinoids; the guide now gives that study's finding rather than stating a rule.
  • We said terpenes are the compounds that give cannabis its aroma, and, in the checklist and the FAQ, that in one study aroma compounds were down by more than half within four months, without saying that study measured one CBD-rich variety. Terpenes give cannabis much of its aroma, not all of it, and the finding is now given with its scope. A later check also found an older study of terpene loss in air-dried flower, which What are terpenes describes with its limits.