Article written by
Homegrown Cannabis Co.American Seed Bank and Cultivation Experts
Content reviewed by
Dr. Lewis JasseyMedical Director - Pediatric Medicine
Table of contents
The ideal humidity for growing weed is 70% for seedlings and clones, 40-60% for vegging, and 35-55% for flowering. The process of cannabis cultivation introduces a broad scope of moisture levels, all ensuring you get high yields of resinous buds.
When the air is too moist, you risk mold and mildew. The buds don’t get that delicious trichome coating when it’s overly dry. So, controlling humidity keeps your marijuana garden healthy and prolific.
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Ideal Humidity for Every Growth Stage
When discussing the optimal moisture levels for cannabis, we refer to relative humidity. This concept describes the ratio of existing water vapor to the maximum amount the air can hold at that temperature. You don’t have to grasp this concept in detail, but you should understand how heat affects it.
Hot air can hold more water molecules, so relative humidity decreases when there’s a temperature spike. As the air cools, it moistens. That’s why it’s necessary to keep both parameters in check.
The ideal humidity level and best temperature for marijuana vary with the growth stage. Seedlings need plenty of moisture, and the requirement gradually drops as the plant matures. Here are the figures to aim for throughout the cannabis life cycle.
Note: We share ranges instead of exact figures below. Cannabis doesn’t mind some variance, and sativa cultivars generally enjoy higher humidity than indica ones. Once you learn these general rules, read up on your chosen strain for specifics.
Seedlings or Clones
Seedlings and clones have small root systems and can draw a limited amount of moisture from the soil. They also rely on the leaves for water consumption, so they enjoy high humidity levels of 65-70% with temps of 68-77°F.
Vegetative stage marijuana absorbs water through the roots to power development. The acceptable humidity range in this stage is 40-65%. You can start at the high end of this spectrum and lower it by 5% weekly. High temperatures (70-85°F) help move moisture through the plant and supercharge growth.
Plants enjoy slightly lower relative humidity in the flowering stage. This figure should stay below 55% once they bloom, dropping by 5% each following week. The ideal temperature is somewhat under that in vegging (70-80°F).
Late flowering introduces a reduced need for water, and further decreasing humidity can improve yield size and bud flavor. Aim for 35-40% and 65-75°F in the final 14 days before the harvest.
Whether growing cannabis indoors or outdoors, maintaining the best humidity is vital to getting healthy and high-yielding plants. How do you monitor this figure?
Marijuana plants display visual cues of inadequate humidity levels:
- When it’s too high, wet spots form on the leaves. If left unchecked for an extended period, white powdery mildew and bud rot inform you of trouble.
- Low humidity makes plants appear limp and lifeless. They compensate by drawing more water through the roots, which may cause nutrient burn and yellow leaf tips.
Why wait for symptoms to appear, though? Gadget and gardening stores sell hygrometers that let you keep this factor in check.
Hygrometers measure the relative humidity of the surrounding space. They work indoors and out, although they’re more accurate in the former. Some even connect to smartphone apps to immediately inform you of fluctuations.
Proper hygrometer positioning ensures valuable readings:
- Outdoor growers should place their meters in areas protected from rain and wind. Ideally, choose a north-side location to avoid the sun-heated air affecting the results.
- Indoor growers should place one meter along the plant base, one at canopy level, and one above their plants. Since humid air rises, you should check if there are considerable imbalances in your grow room.
Outdoor readings change with the season, weather conditions, and time of day. Even indoors, you might see fluctuations due to the following conditions:
- Light usage: Heat-emitting HPS lamps enable the air to retain more moisture. As a result, humidity increases.
- Ventilation: The humidity drops as cold air enters your grow space. If it constantly circulates, you experience fewer spikes and a lower average.
- Watering: Relative humidity is slightly higher after you shower crops. The leaves perform transpiration and release moisture into the atmosphere.
Don’t stress about slight swings, but react if the reading remains outside the ideal level for too long.
How to Regulate Humidity
Humidity control is one of the cannabis cultivation essentials to master. The tips below will assist the cannabis growing process indoors and outdoors.
Expert tip: Choose durable strains with excellent genetics to learn the ropes of controlling humidity without compromising harvests. Visit our partner Homegrown Cannabis Co. to buy seeds and grow sturdy weed that doesn’t mind environmental fluctuations.
Regulating Indoor Humidity
Whether in a massive facility or a compact grow tent, indoor cultivation gives you complete control of the climate. Imbalances happen less often, and it’s easy to correct them.
Here’s what you can do if your space is overly humid:
- Cool the area using an air conditioner. Warm air retains water, and driving down heat has the opposite effect.
- Install a dehumidifier. These appliances draw air over a cooling system and extract excess moisture.
- Water during lights-off hours. Transpiration rates are much lower at night, so there’s no additional humidity spike.
- Boost the airflow. Air circulation expels excessive moisture. If you have exhaust fans, consider supplementing them with small ventilators.
What if the base humidity is too low for cannabis? Here’s how to raise it:
- Install a humidifier. These appliances disperse tiny water particles into the air, increasing humidity.
- Put open water containers around the plants. As the liquid evaporates, humidity increases.
- Mist the foliage. This temporary solution increases your plants’ access to moisture while you take care of the humidity.
Indoor plants flourish under controlled conditions, leading to high harvests of sticky buds. Stay on top of these readings and avoid disrupting them for the best results.
Regulating Outdoor Humidity
Growing outdoors gives you fewer options to maintain optimal humidity. You can familiarize yourself with the climate and check the forecasts to prepare for whatever conditions are coming, though.
Expert tip: Seedlings and clones should stay under a humidity dome until they develop at least three nodes. Young plants suffer immense stress when moisture is lacking.
What about vegging and flowering plants? They can take weeks to recover from overly moist or dry air spells, so prevention is better than cure. Here’s what you can do to maintain the ideal humidity if the weather is hot and arid:
- Water before sunrise or after sundown. That way, you prevent excessive evaporation over the hottest hours.
- Topdress the soil. Add a layer of mulch or peat moss to make your medium retain more moisture.
- Spray the foliage with water. Mist the foliage during scorching weather to moisten the plant and help it breathe.
- Hang damp towels around your plant. As the water evaporates, it’ll increase the humidity surrounding the foliage.
If facing a period of exceptionally high humidity, you should:
- Remove the windbreaks. Ensure as much circulation around your garden as possible, as cold air can carry less moisture.
- Avoid over-watering. Transpiration increases humidity levels, which you wish to avoid during this period.
- Defoliate your garden. Thin foliage releases less moisture and causes smaller humidity spikes. Consider pruning excessive leaves.
If growing weed in pots, move them indoors during droughts and overly moist weather. Outdoor environments offer less precision by design, so don’t stress over every slight oscillation. Act if the issue lasts for several days and your plants display signs of ill health.
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Frequently Asked Questions
Does humidity affect bud density?
Cannabis plants need several things to yield dense buds: excellent light exposure, high-quality nutrients, and low moisture levels. When kept in high-humidity environments, they might suffer mold and mildew and produce small, airy flowers.
What does low humidity do to weed?
Low humidity (under 40%) is harmful to vegging weed plants. They drink more through the roots and can suffer nutrient burn when the air is overly dry. Flowering cannabis requires less water and thrives under such conditions, though.
Do plants need humidity at night?
Plant growth doesn’t cease at nighttime: marijuana spends the dark hours using its carbohydrate reserves and strengthening its cells. So, don’t disregard what your hygrometer tells you during light-off periods.