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Humidity and Marijuana Plants

Mar 11, 2020
Gardening Tools & Equipment

Humidity and Marijuana Plants

Humidity is crucial where cannabis plant health is concerned. It’s a measure of the amount of moisture vapor in the air, and it’s surprising how many growers overlook this important factor. Humidity prevents or facilitates water evaporation in plants.

If humidity is low, your plants will absorb more water and nutrients, because evaporation speed increases in a drier environment. With that said, dryness isn’t always a good sign. If evaporation is allowed to continue too fast for too long, it will put an enormous strain on a plant. The marijuana plant will close its stomata and slow evaporation, in turn stunting its own growth.

That’s why it’s important to monitor the humidity level in your growing environment at every stage in your crop’s growth. For instance, a plants’ flowering phase requires less humidity than is needed in the vegetative stage. The smaller a plant’s roots are, the more moisture it needs. The best way to measure a grow room’s humidity level is with a hygrometer; it should be about 70% in the beginning and it should drop by roughly 5% per week until it’s at 40%. Then, it can be safely kept at that level.

In this guide we’ll discuss:

  • Temperature and humidity
  • Cloning
  • Seedlings
  • The flowering stage
  • Humidity schedules
  • Humidity increases
  • Humidity decreases
  • Measuring humidity levels
  • Watering
  • Outdoor cultivation and humidity

Read on to learn how humidity affects the success of a cannabis crop.

Humidity and Temperature

There’s a strong correlation between humidity levels and temperature. A room’s temperature determines how much moisture the air can hold. When the temperature is at about 68° F, for instance, the air can hold about 7.2mL of water for 100% humidity. At a temperature of 32° F, the air will hold only about 5mL of water. That’s why the air is drier in the winter than in the summer. With the right indoor cultivation setup, ventilation won’t be an issue. Proper ventilation not only keeps the plants from gathering excess moisture, but it also releases airborne humidity.


Like seedlings, clones at first have small roots. They can’t absorb much water, and they should hold on to what little moisture they do take in. It’s crucial to slow the evaporation process, and to do it, you’ll have to maintain a high humidity level. 70% humidity is a great place to start.

If the indoor temperature is approximately 72° F and there’s fluorescent lighting and ample humidity, your cuttings will do well. If you’re using 600W HPS lamps, strive for a 30% humidity level. By following these guidelines, you’re more likely to produce strong, healthy plants.


A seedling functions a little differently than a clone cutting does. They absorb water rather quickly, considering their root size. Don’t remove any seedling leaves, as they’re crucial for light and water absorption. It’s a good idea to start out with 60% relative humidity (RH) and gradually reduce it to 40% RH.

The Flowering Stage

During the flowering stage, your plants are at their most mature. Their roots can absorb more nutrients and water during this time. However, mold is a big concern, so you’ll need to keep the room at a low humidity level. The older a plant is, the more susceptible it is to bud rot and other forms of mold.

Humidity Schedules

You can precisely follow these guidelines to ensure maximum growth and good plant health. Keep in mind, though, that seedlings and clones have different humidity requirements.

  • If you’re cloning plants, keep the humidity level at 70% for the first two weeks. Then, as the flowering phase begins in week three, gradually decrease it. Taper it off by 5% each week during weeks four through 11.
  • On the other hand, seedlings should start off with only 60% humidity. In the third week (the first week of the flowering phase,) the plants should be at 55% humidity. During weeks four through seven, drop the humidity level back to 50%. Keep it at 45% during the eighth and ninth weeks, and for the last two weeks, keep the humidity at 40%.

Humidity Increases

If you’re growing a garden indoors, it’s not too hard to increase the humidity level. It’s possible to start by spraying water on the walls and the flowers, but eventually, you’ll have to move the lights farther away from the plants to slow down evaporation. Another easy option is to purchase a humidifier, which turns water from a liquid into a vapor. You can also leave open water containers in the grow room.

Humidity Decreases

As your plants enter their flowering stage, you’ll need to gradually decrease the humidity level. Extractor fans work well for this purpose, as does dropping the temperature by bringing cold air in. To ensure reliable, consistent action, it’s best to use a dehumidifier. These machines turn water vapor back into liquid and hold it or drain it away. A larger unit is preferable as it reduces the time spent checking and cleaning the holding tank.

Even with indoor gardens, outdoor humidity has certain effects. For instance, if it is rainy and warm, it may be good to slow down or turn off the extractor fan. Be sure to avoid sudden temperature spikes.

Tracking and Measuring Humidity Levels

As mentioned previously, you can measure your room’s humidity with a hygrometer. Simply place it above your crop in a well-ventilated location. Hygrometers are inexpensive, ranging from $5 to $10, and wired units work best because you won’t have to turn on a light to check the humidity level. These units monitor levels over time so you can easily track humidity highs and lows.

Energy Efficiency

Humidity is a long-standing factor in the agricultural process, and cannabis cultivation is no exception. Though it’s most closely linked to temperature-related problems, humidity’s effects are just now getting the recognition they deserve. When growing cannabis, humidity control methods vary widely.

The most crucial thing to focus on when buying dehumidification equipment is its efficiency. The most straightforward measurement of a unit’s efficiency is its energy consumption/water removal or cost/water removal ratio.

Watering Your Crop

Unsurprisingly, water leads to humidity increases. During your crop’s growth stage, this isn’t an issue. However, as the plants bloom, it may present a problem. To minimize humidity increases, only water the plants when the grow room’s lights are on, so the water evaporates faster. Avoid spraying the buds, as it may lead to bud rot.

Outdoor Planting and Humidity

If you’re cultivating cannabis outdoors, there’s little to worry about as far as outside humidity is concerned. You’ll naturally get more moisture when the plants need it, and the humidity level will drop when light and temperature are decreasing during the plants’ late-summer flowering stage.

With that said, at the end of the summer there may be enough morning dew to cause mold growth. Therefore, when you’re checking your crop in the morning, it’s important to remove as much of the dew as possible. If you can easily move the plants, try to plan so they’re kept out of the rain.

Optimal Humidity for Auto-Flowering Plants

The right humidity level for auto-flowering cannabis strains depends on several environmental factors. Plants should be kept at a relative humidity of 40% – 70% during the pre-flowering stage; any higher than that, and the plants may suffer. Water in the stems and leaves won’t evaporate at such high humidity levels, which will stunt the plants’ growth.

Because pre-flowering is such an important time for auto-flowering strains, it’s crucial to maintain an ideal humidity level. You’ll have to regularly adjust the humidity as the plants begin to flower, as high moisture will increase the chances of mold growth and plant infection.

Excessive moisture causes mold to grow in these plants’ dense buds. If you’re not careful, the mold will spread quickly. Mold is virtually undetectable during the pre-flowering and flowering segments, but it quickly spreads throughout the plant. Without quick action, an infected plant may not be salvageable.

How Humidity Affects Cannabis Plant Growth

Like all other plants, cannabis plants adjust their stomatal openings based on air humidity and VPD (vapor pressure deficit). High humidity is a real problem because it inhibits the plants’ absorption of water, compromising the crop’s quality. Similarly, if humidity is too low and transpiration is elevated, the plants will close their stomatal openings to prevent wilting. However, this slows photosynthesis and overall growth.

As briefly mentioned above, the two functions that are closely related to airborne humidity and crop performance are photosynthesis and transpiration. We’ll discuss these factors in the sections below.

  • Transpiration: through this process, plants take in water through their roots and emit water vapor through porous leaves. The hotter and drier the air is, the faster the plant’s transpiration rate will be. However, transpiration and moisture deficit aren’t directly related. In extremely dry air, a plant can only transpire so fast before it starts to wilt.

Conversely, if the air is humid, a plant won’t take in much water from the soil, which means it won’t absorb much fertilizer, either. This poses quite the problem, as inadequate intake of certain elements will cause nutrient deficiencies. Furthermore, low water absorption is linked to high growth medium pH, which limits the availability of micronutrients. Typically, such problems are encountered during winter and in early spring, when outdoor temperatures are low. They can also occur during the hottest, humid parts of summer.

  • Photosynthesis: by this process, cannabis plants fix water and carbon dioxide within themselves to produce sugars used for growth and energy. When the humidity level is normal and the temperature is high, the stomata open to bring in carbon dioxide and initiate photosynthesis. If the growing environment’s air is too dry and the plants are starting to wilt, the stomata will close, slowing photosynthesis and stunting growth. The quality of your cannabis crop depends on whether conditions are optimal for photosynthesis, and humidity has a crucial role in the process.

It’s essential to maintain a constant humidity level in an indoor growing environment, but it shouldn’t be so humid that it approaches the dew point. If temps are at or right below the dew point, the air can’t hold moisture and condensation will build, covering leaf surfaces. When there’s water on a plant’s leaves, it greatly increases the chances of disease while inhibiting water and nutrient uptake.

Sometimes it’s difficult to maintain a temperature that’s above the dew point, especially during the winter when there’s not much air exchange. Dripping is a concern even when the relative humidity is enough, as it increases the risk of disease and leads to uneven drying. The use of heaters and fans helps, but there may still be enough humidity to cause condensation. Infrared heating systems may reduce condensation because they increase the temperature of surfaces such as plants and growing media, but they don’t affect the air.

Tips and Tricks for Indoor Cannabis Growers

If you’re planning to cultivate cannabis for personal or medicinal use, there are a few things to keep in mind, such as air conditioning, lighting, and humidity control. By monitoring these factors, you’ll ensure adequate airflow and maintain greater control over your cultivation effort. Not only will you maximize the overall yield, but you’ll also minimize threats from pests and disease. Most new growers know how to control room temperatures, but they often struggle to maintain the right relative humidity. Here are a few tips for indoor humidity control.

  • Insulate and seal the room: room insulation and sealing are the building blocks of a successful cannabis cultivation Builders often use various insulation materials to create barriers between grow rooms and the elements. If you’re in an area with low outdoor humidity, you’ll still need to insulate your grow room. With proper insulation, you’ll stop wind, sunlight, and humidity from affecting your crop.
  • Control the room’s temperature: when cannabis is grown indoors, the weather is less of a concern. However, the climate inside the grow room will still affect the crop. Lighting is crucial; while many are tempted to use high-powered lights, that may raise the temperature indoors. Some growers mistakenly buy AC systems that are improperly sized for the environment, leading to frequent temperature shifts.
  • Monitor air movement: humid air may hold more water, but it is also lighter than the surrounding air. Therefore, it always rises to the top of the room. In the meantime, CO2 (a component crucial to plant growth) stays near the bottom. If you’re striving for a great crop, proper airflow is important, and it must come from the top, bottom, and sides of the room.
  • Ensure proper drainage: in nature, water that moves is healthy. When water stops moving, it becomes stagnant. Molecular oxygen levels decrease, providing the perfect breeding ground for anaerobic bacteria. Stagnant water also increases a grow room’s humidity level. With proper drainage, you’ll prevent water stagnation and minimize the presence of these harmful bacteria. If you’re growing cannabis hydroponically, be sure to keep water reservoirs covered.

Many times, growers assume that they’ve perfected their indoor humidity control strategies. However, when they find that the grow room’s relative humidity is too high, those thoughts are dispelled. By following these tips, you’ll find it easier to keep your indoor garden room at the right humidity level for optimal growth.

  • Properly size the dehumidifier: for most growers, a top-quality dehumidifier is at or near the top of the list of must-buy equipment. However, too many novice growers try to economize here, and it ends up costing them more in the long term. Don’t make the same mistakes! A home dehumidifier won’t work because it’s not designed to handle the amount of moisture found in the average grow room. Furthermore, they’re inefficient and they use too much energy. For many cultivators, the cost of a commercial unit may seem high, but it’s a worthwhile investment.

It’s important to remember that plants will transpire about 97% of the water absorbed. Therefore, it’s crucial to buy the right size dehumidifier so it pulls enough moisture from the air. If you have a large indoor garden, you may need multiple dehumidifiers. Design your system so that if a unit malfunctions, the others will continue working. It may seem costly at first, but this strategy will help you save on power when compared to the use of a residential dehumidifier.

Controlling Humidity in an Outdoor Cannabis Garden: Some Easy Tips

Even if your climate is ideal for cannabis cultivation, you’ll still have to balance your electricity usage and your spending to adequately control humidity. Proper climate control is crucial in a marketplace where marijuana prices are dropping but energy and labor costs are on the rise. Here are three easy tips to control humidity in your outdoor growing environment.

  • Ensure good airflow. Experienced growers often spread plants apart, preventing them from touching one another. Though such a layout may result in a smaller overall yield, it’s better than losing an entire crop due to moisture-related diseases such as bud rot.
  • Use a data logger to continuously record temperature and humidity in the growing space. With a data logger, you will be able to gather data and use it during the decision-making process. For instance, if you’re noticing that the area’s humidity stays at 60% or higher, it may be time to invest in a dehumidification system.
  • Don’t use too much electricity on lights. Many problems in large-scale marijuana cultivation operations arise when growers use all their available energy on lighting. With this, there won’t be enough electricity left to run a dehumidification system. When you use too many lights, you may trip breakers, overload circuits, receive fines from the local fire department, and sacrifice much-needed dehumidification and air conditioning. Be careful, and don’t jeopardize the entire crop in order to increase the final yield by using extra lights and extra plants.
  • Plan and properly design your outdoor growing facility: Though some growers learn by trial and error, that can get expensive very quickly. There are numerous resources, both online and in print, that will help you design, build, and use your outdoor cultivation area properly and efficiently. After all, it’s cheaper to do it once (and do it right) than it is to go back and re-do the entire garden!

Don’t Forget About Ambient Humidity

There’s more to humidity than what’s happening in the cultivation area. Ambient humidity is the moisture encountered outside your grow room or outdoor garden. In an air-conditioned space, the humidity level stays around 30% because the air conditioner removes most of the moisture from the air. In heated areas, the humidity may be even lower as the heating unit removes more moisture.

In the time it takes to prepare buds for consumption, they may lose a substantial amount of moisture. Without a good way to adjust and monitor humidity in the growing and storage area, your buds will dry out too quickly. When storing your prepared cannabis, a humidity control product will allow you to quickly replace lost moisture in buds and in the air, so you’ll enjoy consistent quality every time.

Relative Humidity and its Effects on the Drying, Curing, and Storage of Marijuana

If you want a high-quality cure, it’s important to store your harvested crop in an area with a relative humidity that’s in a favorable range. Again, a humidity control product will help you maintain the ideal humidity level inside your storage area. From there, the biggest concerns are light and temperature. A good humidity control system will adjust levels based on changing temperatures, but it’s still important to keep your harvested cannabis flowers away from high heat. This prevents decarboxylation and minimizes the activation of cannabinoids. As many in the cannabis sector suggest, store your buds in a dark, cool place.

In Conclusion

Though you’ll have to consider many factors as a cannabis cultivator, humidity is one of the most important. With too much humidity, your crops will suffer from bud rot, mold, and other diseases; with too little moisture, they may wither and die. With the tips in this guide and a functional humidity control system, it’s possible to grow a green, lush, and flavorful crop. Check out some tasty strains at!


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