A batch of cannabis can have an acceptable moisture percentage and still tell you surprisingly little about how that water will behave during storage.

That is the practical difference between moisture content and water activity.

Moisture content tells you how much water is present in the flower. Water activity—usually written as a_w—helps describe how available that water is to interact with the product and its surrounding environment.

For a commercial post-harvest or QA program, those are two different questions. Treating them as interchangeable can hide useful information about drying consistency, storage stability and microbial risk.

What is moisture content?

Moisture content is the amount of water contained in a material, usually expressed as a percentage of its mass.

In cannabis flower, moisture content can be useful for evaluating drying progress, finished-product consistency and weight. But the number does not describe how strongly that water is bound inside the plant material.

Two samples containing a similar percentage of total water do not necessarily have identical water activity. That is why there is no universal formula that lets a commercial operator take a moisture-content percentage and reliably convert it into a_w for every flower lot.

Moisture content answers: How much water is in the flower?

Water activity answers a different question.

What is water activity?

Water activity describes the relationship between water in the sample and water vapour pressure at equilibrium. ASTM defines a_w as the partial vapour pressure of water in the product divided by the vapour pressure of pure water at the same temperature.

The scale runs from 0 to 1. At equilibrium, an a_w of 0.62 corresponds to approximately 62% equilibrium relative humidity in the sealed headspace.

That relationship is particularly useful after harvest because flower is constantly exchanging water vapour with the air surrounding it until equilibrium is reached.

Why cannabis producers should care about water activity

ASTM developed cannabis-specific guidance for water activity because a_w can provide useful information about both product quality and storage conditions.

ASTM D8197 specifies a recommended 0.55 to 0.65 a_w range for dry cannabis flower intended for human or animal use. The standard associates values above that range with increased concern for undesirable microbial growth and values below the range with greater risk of physical damage and breakage during handling and storage.

That benchmark is useful, but Canadian operators should keep an important distinction clear: ASTM D8197 is a voluntary consensus standard. It is not a Health Canada statutory a_w limit for dried flower.

Health Canada’s good production practices guidance requires licence holders to control contamination risks and gives water activity as an example of a measurable critical limit that can be used in hazard-control programs where appropriate.

Water activity is therefore best treated as one process-control and QA variable within a broader validated system—not as a substitute for finished-product testing or microbiological controls.

Moisture content and water activity should work together

A commercial drying program becomes more informative when both measurements are used for what they are good at.

Moisture content can help answer whether water has been removed consistently from one batch to another. Water activity can help answer whether the remaining water is in a state that supports the storage condition your QA program is targeting.

Consider three hypothetical flower lots that all meet the same moisture-content specification. If one lot measures 0.58 a_w, a second measures 0.63 a_w and a third measures 0.69 a_w, the moisture percentages alone have hidden a meaningful difference.

The point is not that one reading automatically releases or rejects a lot. The point is that a_w data gives the QA team another signal to investigate before the material moves farther downstream.

That may lead you back to drying-room loading, flower density, environmental gradients, container dwell time or sampling practices.

Water activity can expose drying-room inconsistency

A drying room is rarely perfectly uniform. Dense locations, corners, rack geometry, plant load, airflow patterns and proximity to supply or return air can all create different drying conditions within the same room.

That is why a room-average temperature and relative-humidity value cannot, by itself, prove that every part of the batch reached the same endpoint.

Growhaus covers this in more detail in Cannabis Drying Room Design: Airflow, Load and Environmental Consistency. Water-activity measurements add a product-level check after that environmental-control step.

A useful QA investigation might compare a_w by drying room and rack location, cultivar or lot, harvest date, dry duration, flower size or density, packaging line and storage interval.

Patterns are usually more useful than isolated measurements.

Sample handling matters

Water activity is temperature sensitive, which means inconsistent sample preparation can create noisy data.

Your SOP should define more than simply “measure a_w.” It should define how samples are selected, how much material goes into the sample cup, sample conditioning, instrument verification, measurement mode, acceptance criteria and how results are recorded against a lot.

Consistency in the measurement method is what allows the data to become useful for trending.

Where should water-activity testing happen?

For commercial cannabis operations, consider a_w measurements at several control points rather than treating the meter as a final-release instrument only.

Near the drying endpoint

Use spot measurements during process development to better understand how room conditions translate into the actual flower.

Before bulk storage or curing

Confirm that flower entering storage is inside the operating range established by your QA team.

Before packaging

Check whether holding, trimming or processing has shifted the product.

During packaging validation

Evaluate how the packaged system behaves over time.

During investigations

Compare lots associated with microbial failures, excessive dryness, weight changes or quality complaints.

The objective is not to collect more numbers. It is to connect measurements to decisions.

What about humidity-control packs?

Two-way humidity-control products and water-activity measurement solve different parts of the problem.

A meter tells you what the a_w is. A humidity-control system is designed to help manage the moisture environment inside a closed package or storage container.

ASTM D8197 discusses salt-based relative-humidity control sachets as one approach that can help maintain cannabis flower within its recommended storage range under appropriate conditions.

Growhaus currently carries the Rotronic AwEasy Bluetooth Water Activity Meter along with commercial humidity-control formats including ViVi Fresh 62% RH bulk packs.

But a pack should not replace measurement, and measurement should not replace process control. Packaging material, headspace, flower mass, initial a_w, storage temperature, pack size and exposure history all matter.

Validate the complete system.

Water activity is one part of microbial control—not the whole program

High-moisture conditions can favour microbial growth, but microbial failures in cannabis rarely have only one cause.

Contamination can enter during cultivation, harvesting, drying, trimming, storage or packaging. If the process continually reintroduces contamination, simply changing an a_w target will not solve the root problem.

For a process-level approach, see Reducing Microbial Failures in Commercial Cannabis: Start with the Process.

Water activity belongs inside that system as a useful control and investigation variable.

The commercial value is repeatability

For a commercial producer, the best reason to measure water activity is not simply to generate another QA number. It is to reduce uncertainty.

A repeatable a_w program can help your team understand whether flower leaving one drying room behaves like flower leaving another, whether product changes during storage, and whether packaging is maintaining the intended environment.

That information can help connect cultivation, post-harvest, QA and packaging teams around the same measurable endpoint.

And when a batch behaves differently, the measurement gives you somewhere useful to start investigating.

A practical next step

If your operation currently relies only on moisture percentage, consider running a short correlation study.

Take representative samples from multiple lots and record both moisture content and water activity at the same process stages. Then compare the results against drying-room conditions, cultivar, storage time and finished-product outcomes.

You may find that moisture percentage tells one part of the story while a_w explains another.

Growhaus can help operators evaluate water-activity measurement, humidity-control formats and the surrounding post-harvest workflow—but the goal should always be a validated process built around your facility, product and QA requirements.

Technical references

• ASTM D8197 — Standard Specification for Maintaining Acceptable Water Activity Range for Dry Cannabis Flower
• Health Canada — Good Production Practices Guide for Cannabis
• USP General Chapter <922> — Water Activity

About the author
George Dickinson is co-founder of Growhaus Supply and a commercial greenhouse industry veteran with more than three decades of controlled-environment agriculture experience. He works with commercial growers on cultivation systems, facility challenges and practical operating solutions.