Relative humidity, usually abbreviated as RH, is one of the most important climate factors in cannabis cultivation. It influences how much plants transpire, how stable the microclimate in the crop remains, and the degree of risk for mold, slow development, or loss of quality. Especially for cannabis, RH is relevant not only in the grow room but also after harvest—that is, during drying, curing, and storage. Current cannabis research also shows that excessive humidity outside of sensible VPD ranges can significantly impair flower development, biomass, and cannabinoid concentrations.

What does relative humidity actually mean?

Relative humidity describes the proportion of water vapor in the air in relation to the maximum amount the air could hold at its current temperature. It is expressed as a percentage. The crucial point here: Warm air can hold more moisture than cold air. That is why RH is always temperature-dependent and not an absolute moisture value. This is exactly why the same moisture content can lead to very different RH values at different temperatures.

Why RH is so important in cannabis cultivation

Cannabis reacts significantly to the interaction of temperature, humidity, and air movement. If the RH is permanently too high, the climate in the crop becomes more humid, the plant transpires differently, the canopy dries out poorly, and the risk of undesirable microclimates increases. A recent study even showed that increased canopy-level RH outside of optimal VPD thresholds can delay flowering, reduce biomass, and lower cannabinoid concentrations. Conversely, air that is too dry can put unnecessary stress on plants, especially in early development phases or with sensitive plant material.

Correctly classifying RH in the individual phases

Instead of naming a single "perfect humidity," it makes more sense to differentiate by development phase.

Germination and early young plants

In the early phases, a higher humidity is usually preferred compared to later in flowering. A recent study on hemp seeds found the best germination at 20 °C and 65 % RH. For Cannaseuse, this means in practical terms: early phases generally benefit more from a more humid, non-drying-out climate than from dry air.

Vegetative phase

During the growth phase, cannabis is often maintained under moderate to higher RH. A recent review article cites 27 °C and 70 % humidity as favorable conditions in the studied system for vegetative growth. These values are not identical for every strain and every setup, but they clearly show the direction: during the veg phase, the air can generally be more humid than in late flowering.

Flowering phase

With the onset and especially throughout the course of flowering, RH is usually reduced step-by-step. The same review article cites 22 °C and 50 % humidity as a favorable target value for flowering, while the Frontiers study clearly shows that RH that is too high during flowering can become problematic. Especially in dense stands and compact flower structures, a drier, cleanly circulated climate is usually safer than high humidity.

Why RH should never be thought of without temperature

Many grow texts treat RH as an isolated target value. In practice, however, it is always linked to temperature. This is exactly why VPD is so important in modern grow understanding: an RH that works well at one temperature may already be too humid or too dry at another. The current cannabis study on canopy-level RH phrases this very clearly by connecting negative effects not simply to "high humidity," but to humidity outside of optimal VPD thresholds.

RH when drying cannabis

After harvest, RH becomes a direct quality factor. A review of post-harvest processes describes that, in cannabis practice, 18–21 °C and 50–55 % RH are frequently used for slow, gentle drying. These conditions are intended to help control the drying process rather than drying out flowers too quickly or leaving them hanging for an unnecessarily long time in a climate that is too humid. Oregon State also cites the same guidelines for slow drying.

RH during curing

Curing is no longer just about getting rid of water, but about stabilizing the flower in a controlled manner. A review cites 18 °C and 60 % RH for two weeks as advantageous curing conditions in the referenced work. At the same time, the same source emphasizes that curing in the cannabis industry is not yet uniformly standardized. For Cannaseuse, the cleanest statement is therefore: Curing requires stable, moderate conditions and does not benefit from strong fluctuations or excessive heat.

RH during storage

For storage, literature emphasizes a cool, dry, and stable environment rather than a single universal percentage. A review describes storage at 18 °C and 45–55 % RH for already manicured buds, while other works investigated controlled storage at 18 °C and 60 % RH. This shows that the exact RH targets for storage are not completely uniform in the literature. However, there is a consensus that cool, dark, and controlled conditions are important if cannabinoids and terpenes are to be preserved as long as possible.

What happens when RH is too high

RH that is too high means more than just "slightly more humid air." In the plant crop, it can delay flower development, degrade the microclimate in the canopy, and impair the formation of important compounds. The current Frontiers paper showed, among other things, delayed flowering, lower biomass, and lower cannabinoid concentrations at increased canopy-level RH. After harvest, excessive humidity also increases the risk that drying and storage will not proceed cleanly.

What happens when RH is too low

Air that is too dry can also degrade the plant climate, especially in early phases. If the air is too dry, the transpiration load increases, young plants or freshly established plants are more prone to stress, and the balance between water uptake and transpiration becomes more difficult. This is precisely why early phases are generally kept more humid than late flowering or storage phases. This conclusion is supported by the different RH ranges mentioned in studies for germination, vegetation, and flowering.

How to better keep RH under control in your grow

RH control works most reliably through the interaction of several factors: measuring, moving air, exchanging air, and actively humidifying or dehumidifying as needed. Precisely because RH is temperature-dependent, individual measures are often not enough. A hygrometer alone only measures, a dehumidifier alone does not solve poor air circulation in the crop, and air circulation alone does not automatically lower the absolute moisture in the room. For Cannaseuse, the most important principle is therefore: RH should always be understood as part of the entire climate system. This classification is supported by literature on RH, drying, and VPD.

Why RH is more than just a number for Cannaseuse

At Cannaseuse.de, it's not just about strong genetics, but about the conditions under which genetics can truly show what they are capable of. RH influences how calmly a plant grows, how safely dense flowers make it through the flowering period, and how much aroma is actually preserved after harvest. This is exactly why humidity is not a minor detail, but a part of the quality foundation—in the grow just as much as in the jar.

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FAQ – Frequently Asked Questions about relative humidity in cannabis

What does RH mean for cannabis?

RH stands for relative humidity and describes the percentage of water vapor in the air in relation to the maximum possible moisture at the current temperature.

Why is RH so important for cannabis?

Because it directly influences transpiration, microclimate, flower development, and the risk of moisture problems. A recent cannabis study shows that RH that is too high outside of sensible VPD ranges can negatively affect flowering, biomass, and cannabinoids.

Which RH makes sense in the vegetative phase?

In the veg phase, a more humid climate is often maintained than in flowering. A recent review article cites 70 % RH at 27 °C as a favorable condition in vegetative growth.

Which RH makes sense during flowering?

During flowering, RH is usually kept lower. The same review article cites 50 % RH at 22 °C as a favorable target value, and current research warns against RH that is too high during flowering.

Which RH makes sense when drying?

A cannabis post-harvest review and Oregon State frequently cite 18–21 °C and 50–55 % RH for slow drying.

Which RH makes sense during curing?

A review cites 18 °C and 60 % RH for two weeks as advantageous curing conditions in the referenced work, but simultaneously emphasizes that curing is not yet fully standardized.

Is RH alone enough to properly control the climate?

No. RH must always be considered together with temperature and, ideally, VPD. The same RH can act very differently depending on the temperature.

What happens when RH is too high?

RH that is too high can delay flowering, lower biomass and cannabinoid values, and make drying difficult after harvest.

What happens when RH is too low?

RH that is too low increases the evaporation load and can stress young or sensitive plants in particular. This is one reason why early phases are usually kept more humid than the flowering phase.

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Conclusion

Relative humidity in cannabis is not a small detail, but a central control element for plant climate, flower health, and post-harvest quality. Early phases usually benefit from higher RH, flowering generally requires drier conditions, and during drying and curing, a stable moisture regime often makes the difference between clean quality and avoidable loss. The decisive factor here is not only the RH itself but its interaction with temperature, air movement, and VPD.