Humidity (or the volume of water vapor in the air) has a HUGE influence on cannabis plants. It affects growth, health, resilience to pathogens, and, in turn, the quality and size of your yield.
Indoor growing makes managing humidity easier as it keeps YOU in control. With sufficient know-how and decent equipment, your relative humidity (RH) levels can be optimized for healthy, explosive growth. Getting to grips with humidity will also help prevent some of the more common problems faced by rookie growers such as mold and stunted growth (we’re talking about the the plants, not the rookies). To understand why humidity is so important, we need to answer a few basic questions. What is humidity? How does humidity affect transpiration? Do you keep RH the same through seedling, vegging and flowering? How do you monitor and control humidity in the grow room, and why does it matter? As with most learning experiences, understanding the why leads to a far deeper understanding of the how.
What is humidity and how does it affect cannabis?
Humidity, airflow and temperature (HAT)make up the golden triangle of optimizable environmental conditions. They help with cell flexibility, turgidity and many other processes. Get the humidity part right and you’re on the way to amazing results. Get it wrong and you’re on the path to major problems. But, what is humidity?
Humidity, basically, refers to the level of water particles suspended in air. That said, there are different ways of describing humidity: absolute, relative and VPD.
What is Absolute Humidity (AH)?
This describes the amount of actual water vapor in the air at any given point in time, regardless of other factors. It’s calculated in grams per cubic meter (g/m3).
What is Relative Humidity (RH)?
Relative Humidity describes the ratio of actual water vapor to the maximum amount it can hold in a given temperature-pressure environment. It is expressed in percentage (%). Compared to absolute humidity, RH is more relevant to cannabis growers, given it takes into account additional environmental factors.
Warmer air has a higher saturation point. Raising air temperature without changing moisture content will cause relative humidity to fall. Inversely, cooler air lessens the maximum amount of vapor the air can hold, resulting in a rise in relative moisture.
What is Vapor Pressure Deficit (VPD)?
Measured in units of pressure, VPD is RH, air temperature and leaf temperature in a single value. Instead of controlling these factors separately, VPD looks at how one variable affects the others. It’s a more accurate measurement than both absolute and relative humidity, and is often utilized in greenhouses and large-scale operations.
VPD can be computed as follows as per the ASCE Standardized Reference Evapotranspiration Equation:
T is the temperature in degrees Celsius, and RH is relative humidity:
Saturation Vapor Pressure (es) = 0.6108 x exp (17.27 x T / (T + 237.3))
Actual Vapor Pressure (ea) = RH / 100 * es
Vapor Pressure Deficit = ea – es
A simpler way of understanding VPD is with the formula VPD = [(100-RH)/100)*SVP using the following VPD value chart as reference:
|Temperature (°C)||RH (%)||SVP||VPD|
If you want to dive really deep into it, you’ll learn that temperature and relative humidity isn’t simply about staying within a recommended range. It’s also about keeping your VPD levels optimal, creating equilibrium between the moisture pressure inside the cannabis leaves and the grow room.
How does RH affect transpiration in cannabis plants?
Plants sweat during a process called transpiration. Roots absorb moisture that gets distributed around the plant, helping with photosynthesis and other functions. Any excess moisture evaporates from the surface of the leaves.
Transpiration is crucial for three reasons.
- When the plant draws in moisture, it brings in nutrients from the soil, too.
- Water is vital for photosynthesis.
- Transpiration, like sweating, helps to cool the plants.
Why is humidity important for marijuana plants?
Excess humidity reduces the ability of plants to transpire (and can promote the spread of pathogens). With this comes an inability to take in water and nutrients through the roots. If humidity is too low, the plants will sweat too much and soon become dehydrated. Excessively low humidity forces the stomata to close (in an attempt to preserve moisture), resulting in a slowdown of photosynthesis. In either case, it’s bad news for your cannabis plants.
Here’s a list of processes essential to plant life and potential effects due to changes in humidity:
|Body Processes of Plant||Increase in VPD (Low RH)||Decrease in VPD (High RH)|
|Transpiration rate||Transpiration rate increases||Transpiration rate decreases|
|Bulk flow of water and plant stress||Water utilization increases, inducing stress||Water utilization decreases, reducing stress|
|Nutrient intake at the roots||Nutrient uptake increases||Nutrient uptake declines|
|CO2 uptake and stomata opening||Stomata closes, reducing CO2 absorption||Stomata opens, increasing CO2 uptake|
What is the optimum humidity for a marijuana plant?
The optimal humidity level for cannabis depends on the growth stage. Generally, it should fall between 30% to 80%.
|Recommended Humidity Levels for Varying Stages of Cannabis Growth|
What is the optimum humidity level for cannabis seedlings?
Seedlings and young clones have weak, underdeveloped root systems. To help them absorb moisture, relative humidity should be kept high: between 65% and 80%. During this stage, you should aim for a temperature of 77°F during the day and 69.8°F at night.
What is the optimum humidity level for vegetative cannabis?
As young plants continue to develop their root system, their ability to absorb water improves. You can now lower RH to 40% to 60%, with temperatures of 69.8 – 82.4°F during the day lowered to 64.4 – 75.2°F at night.
What is the optimum humidity level for flowering cannabis?
By the time the plants enter the flowering stage, their root system should be fully developed. You can now drop RH to 40 – 55%. This can also help keep your buds free from mold and mildew. Keeping temperatures between 68 – 78.8°F helps keep air moisture at ideal levels.
What is the optimum humidity level for late-flowering cannabis?
Tweaking humidity levels at this stage isn’t absolutely necessary, but it can help improve the yield, flavor and appearance of the plants. Lowering RH to 30 – 40% and temperatures to 64.4 – 75.2°F slightly stresses the plants, encouraging more resin production. This can lead to more potent buds, crusted with sparkly trichomes.
How do you maintaining optimal humidity when growing cannabis indoors?
Sustaining proper moisture levels becomes all the more critical when growing indoors, where simulating perfect conditions is a must.
How do you monitor humidity levels when growing cannabis indoors?
A good way to measure the moisture content of a grow room is with a hygrometer, but there are other gadgets that can help.
- Hair Hygrometer. These devices use human or animal hair to measure humidity, as they tend to contract or expand in response to moisture. While they’re quite cheap, they’re only reliable when measuring mid-range humidity (30 – 80%).
- Dew-Point Hygrometer. These determine humidity by measuring the temperature at which vapor starts to condense. They’re very accurate, but not cheap. Some dew-point sensors can also measure VPD.
- Psychrometer. Also known as the Wet-and-Dry-Bulb Thermometer, these measure air moisture by comparing the evaporation rates of wet and dry surfaces. They’re less pricey than dew point devices but can be tricky to operate.
- VPD Sensor. These aren’t as popular as other types of measuring devices and they tend to come as components of bigger humidification systems.
- Humidistat. Able to monitor and maintain favorable moisture range, these are humidifier/dehumidifier units with built-in hygrometers. When they sense a change in humidity, they automatically restore appropriate levels by increasing or decreasing moisture in the grow area.
- Humidity Controller. Similar to a humidistat but a lot cheaper, humidity controllers also adjust moisture levels. However, they need to be plugged into an external humidifier or dehumidifier, and multiple sets may be necessary to maintain a large space.
Monitoring the amount of moisture in the grow room requires day-to-day logging and adjusting. Humidity is sensitive to fluctuations in temperature, so checking moisture levels during the light and dark cycles is vital. Any drastic changes to RH should be immediately noted and acted upon.
How do you control humidity levels when growing cannabis indoors?
When controlling humidity, it’s important to remember that factors inside and outside the grow room can contribute to the amount of vapor in the air. The following measures can be followed to maintain humidity levels within an optimal range.
Insulation helps maintain a constant temperature inside the grow room. One of the most common ways to seal a grow room is by lining the walls with rigid foam boards, creating an additional barrier against the outside environment. It helps prevent external elements causing fluctuations in humidity.
Manage Air Circulation
A ventilation system prevents stagnant, humid air from attracting molds and fungi. However, it must be set at the right levels. Fresh air can lower the temperature of the room, causing transpiration to slow down. Gentle airflow can help keep vapor in the air, allowing plants to absorb the moisture, while a well-pruned plant will help a plant transpire efficiently and correctly.
Use Humidifiers And/Or Dehumidifiers
Humidity can be manipulated with humidifiers and dehumidifiers. Smaller humidifiers work just as fine, but larger ones mean you won’t need to refill the tank several times a day. Sizing a dehumidifier, on the other hand, is a little bit trickier. An important thing to remember is that plants transpire 97% of absorbed water. Estimating the total water input in the garden will help identify the right device.
For instance, 30 plants that receive 2 liters of water each per day means a water input of 60 liters. The dehumidifier should be capable of pulling this amount of moisture from the grow room.
Overwatering can lead to stagnant water and drowned roots. This adds to the humidity of the room as the plants release more vapor without absorbing any. To prevent this, ensure proper drainage and provide just the right amount of water.
Reservoirs in the grow room may also increase humidity levels. Make sure they’re covered.
Water At Appropriate Times
Watering the plants when the temperature in the room is still low causes transpiration to slow down, resulting in lower humidity levels within the grow room. It’s essential to consider the timing of watering to make sure it will not cause sudden spikes or drops in humidity.
Have large Companion Plants
If your growing environment is too dry, try adding large plants to the grow room. These will transpire and gently raise humidity levels. The water vapor released will help feed the smaller plants, too. Companions should only be kept until the early vegetative stage of cannabis, as excess moisture in the later phases may encourage pathogens.
Controlling temperature when growing indoors.
Humidity is closely related to temperature. Temperature monitoring and control is essential to maintaining ideal moisture levels.
Install An Air Conditioning Unit
Air conditioning units have adjustable thermostats that makes them extremely convenient. However, installing an inappropriately-sized AC unit can lead to unwanted fluctuations in temperature. Make sure your AC unit is a good fit. Their capacity is usually expressed in British Thermal Units (BTU). To find out how much BTU a grow room needs, consider the total area of the growing space.
|1 – 1,200||5,000|
|1,200 – 1,600||6,000|
|1,600 – 2,000||7,000|
|2,000 – 2,400||8,000|
|2,400 – 2,800||10,000|
|2,800 – 3,200||12,000|
|3,200 – 3,600||14,000|
|3,600 – 4,000||15,000|
|4,000 – 4,800||18,000|
|4,800 – 5,600||20,000|
|5,600 – 6,400||22,000|
|6,400 – 7,200||25,000|
Renew Air Supply
Using fans and vents with bigger tubes or higher velocities will remove more warm air and bring in cooler air faster. Ideally, the air in a grow room should be replaced every 1 – 3 minutes. Installing exhaust systems with intake and outtake fans should also help give plants a steady supply of cool, fresh air, preventing the grow space from overheating.
Have Sufficient Heat
Employing a heater is the most obvious and reliable choice to keep a cultivation area warm. However, you’ll find HID and HPS grow lights will give you much more than enough heat! Always make sure there’s adequate space between lights and plants to avoid burning. Meanwhile, when germinating your cannabis seeds or rooting seedlings or cuttings, a heating mat can come in handy, too.
Indoor humidity control for marijuana plants
Ensuring your plants receive the right levels of moisture is key to achieving a successful grow. Generally, plants in seedlings and vegetative stages require moderate to high levels of humidity, to encourage the formation of robust roots. As the plants grow and enter the flowering stage, humidity levels are lowered to help prevent mold and fungi. Humidity is further decreased at the late-flowering stage, forcing the plant to produce potent resin glands.
Needless to say, consistent monitoring and control is essential. Buy a good hygrometer and thermometer and record your numbers twice a day (light and dark). Be attentive and observant and you’ll be richly rewarded.