humidity for seedlings

How Humidity Affects the Growth of Plants

Everything in an environment affects how a plant grows, thrives and reproduces. When growing plants outside of an environment in which they naturally thrive, climate control for plant growth is essential to maximize the photosynthetic process. By maintaining optimal relative humidity levels in a greenhouse and other growing environments, you ensure optimal plant transpiration.

The Affects of Humidity on Plants

Relative humidity is the amount of water vapor in the air relative to the maximum amount of water vapor that the air can hold at a certain temperature. If the relative humidity level is 75 percent at 80° F, this means that every kilogram of the air in the respective space contains 75 percent of the maximum amount of water that it can hold for the given temperature.

Relative humidity levels affect when and how plants open the stomata on the undersides of their leaves. Plants use stomata to transpire, or “breathe.” When the weather is warm, a plant may close its stomata to reduce water losses. The stomata also act as a cooling mechanism. When ambient conditions are too warm for a plant and it closes its stomata for too long in an effort to conserve water, it has no way to move carbon dioxide and oxygen molecules, slowly causing the plant to suffocate on water vapor and its own transpired gases.

As plants transpire, the humidity around saturates leaves with water vapor. When relative humidity levels are too high or there is a lack of air circulation, a plant cannot make water evaporate (part of the transpiration process) or draw nutrients from the soil. When this occurs for a prolonged period, a plant eventually rots. When surrounded by warm temperatures in low relative humidity levels, transpiration rates in a plant increase, reducing the need for a grower to fertilize it.

The Importance of Climate Control for Plant Growth

As seedlings grow or when a grower propagates plants from leaf or stem cuttings, the young or collected plants automatically close their stomata as a protective measure to prevent water losses. To support cuttings and young plants, growers often uses plastic tents or propagation chambers that increase relative humidity levels surrounding the leaves and ensure proper air circulation.

In addition to water and air, plants use light energy for the transpiration process, as it causes liquid water to turn to vapor (evaporation). Greenhouses often maintain relative humidity levels below threshold values during the day and night by controlling the water content in air to maintain a minimum transpiration rate in plants.

Climate control for plant growth is an essential consideration in regards to pest and disease management. When conditions are too humid, it may promote the growth of mold and bacteria that cause plants to die and crops to fail, as well as conditions like root or crown rot. Humid conditions also invite the presence of pests, such as fungus gnats, whose larva feed on plant roots and thrive in moist soil.

Managing the growth and development of plants involves manipulating a growing environment so light, temperature and relative humidity levels to promote photosynthesis, high yields and generative growth. Optimal transpiration rates vary by plant type, age and season, making climate control for plant growth necessary throughout the year. Polygon offers custom-made temporary climate control solutions that create the ideal growing environment by regulating and monitoring temperatures and humidity levels. Talk to a Polygon representative today to learn more about how temporary climate control works in conjunction with greenhouse lighting and irrigation systems to optimize plant growth.

humidity control

Indoor Cannabis Growing: Relative Humidity and Temperatures

The most refined techniques to grow cannabis become irrelevant when relative humidity and temperatures are not being controlled – learn more about these two major factors.


Final results of an indoor grow are greatly influenced by the way growers keep in control of parameters that influence their plants growth. There are two basic factors that can easily be forgotten when we’re busy thinking of other ways to increase yields, size, and overall health of our plants – temperature & relative humidity. This blog summarizes ways to keep both of these factors within an optimum range, and provides specific information what conditions should be maintained to achieve best results.


It’s important to know that humidity levels and temperatures are closely related to one another. When we talk about humidity, we usually mean relative humidity (RH), which is the ratio of partial pressure of water vapor to the maximum vapor pressure of water at the same temperature. You get the whole idea when knowing the basic principle that warm air holds more water vapor than cold air. This is one of the reasons why it’s necessary to extract a lot of warm air from our grow room, and ideally allow cool air to enter – warm air simply holds too much water vapor in it.


We need to define what humidity and temperature control actually means when growing cannabis. It makes sense to divide the life of cannabis plants into 4 different stages in which humidity levels, and temperatures, should be adjusted to ensure healthy growth. Don’t think that humidity and temperature control is complicated and not worth it! It’s generally very easy, and more about keeping parameters within a certain range, and as constant as possible.

The first thing you need to do is to buy a hygrometer and thermometer, preferably a digital one with memory function, also showing maximum and minimum values of the past. Some hygrometers aren’t the most accurate, so don’t bother having several devices in your grow room to compare values. Now that we’re able to closely monitor our conditions, we can get to the essence of humidity and temperature control – the actual humidity levels and temperatures we aim for.

1. Seedling Stage

  • Seedlings and clones like high humidity levels of 65-70%
  • Reason: The root system is not established
  • High humidity levels allow water intake through leaves
  • Temperatures with lights on: 20-25 C° (lights off: 4-5 C° lower)

2. Vegetation Period

  • Humidity levels can be lowered by 5% each week (acceptable range: 40-70%)
  • Temperatures can be increased a little bit (no obligation)
  • Reason: Roots absorb more water; evaporation through leaves cools plant(s)
  • Temperatures with lights on: 22-28 C° (lights off: 4-5 C° lower)

3. Flowering Period

  • Humidity levels need to be lowered to 40-50% (extremely important)
  • You can get away with 55% (anything over 60% is real bad)
  • It’s best to slightly lower temperatures in flowering
  • Temperatures with lights on: 20-26 C° (avoid high temperatures)

4. Late flowering (1-2 weeks before harvest)

  • The following steps are no necessity, but can improve yield, flavour and appearance
  • Bring down humidity levels as much as you can: 30-40%
  • Lower daytime temperatures, and also increase the temperature difference (day/night)
  • Temperatures with lights on: 18-24 °C (lights off: minus 5-10 C°)


We’ve got a pretty good idea on humidity levels and temperatures we aim for. Now it’s time to get to the practical part, and to find ways to bring things back in balance when they’re not. Most growers will struggle to keep both relative humidity and temperatures down, which is of primary importance in the flowering period – we got that. In some colder regions, and depending on the lighting solution, the opposite scenario might be the case, and temperatures or humidity levels must be raised.

Remember the basic principle that warm air holds more water than cold air? Keep this in mind, and be aware of the fact that relative humidity and temperatures interact with one another.

Finding ways to control humidity and temperatures is crucial when growing cannabis indoors. This blog shows practical steps for best results.