Temperatures and Medical Cannabis Growing: From Seedling to Harvest The marijuana industry created an estimated $34 billion economic impact in 2018. Legal sales of marijuana are expected to Climate is key to cannabis growing. In this post we analyse the importance of relative air humidity at every growing stage. We’ll define the ideal hygrometric level … So what is the best temperature for growing weed and what about the humidity in your grow room? Check it out!
Temperatures and Medical Cannabis Growing: From Seedling to Harvest
The marijuana industry created an estimated $34 billion economic impact in 2018. Legal sales of marijuana are expected to increase from $10 billion in 2018 to $20 billion by 2022, according to the Marijuana Business Fact Book. This means that more and more legal growers (under strict governmental regulations) will be on the American horizon seeking the best indoor growing practices. Temperature and humidity monitoring are essential for growing healthy cannabis plants throughout the entire life of the plant…seedling to harvest.
What’s the Difference Between Temperatures and Humidity Levels?
Temperature and humidity levels interact and are very similar and are closely related to one another. Humidity is the measure of how much humidity is actually being held in the air. Relative humidity (RH) which is one way to measure humidity levels, is how much water is in the air compared to the maximum amount of water that can be held in the air at that temperature. The premise is that warm air holds more water vapor than cold air so it is important to get the warmest air in the environment. It is necessary to extract the warmest air from the growing environment while allowing cool air to enter. The warm air alone has way too much vapor for ideal growing conditions.
Horticulturalists know that the key to success in any growing plant is understanding the lifecycle of the plant. This is also true with growing cannabis. Throughout every stage of growth, cannabis plants will have a constant need for water which the amounts will fluctuate with the humidity. When the humidity is high, plants use their leaves to absorb moisture from the air which results in less water drawn from their roots. On the other hand, when the humidity is low, cannabis plants will take more water through their roots.
Monitoring and having the right humidity encourages strong, healthy, nutrient-dense leafy plants with optimal growth.
Humidity levels must be monitored to prevent:
1. Bud Mold/Rot: A condition when the inside of the plant is taken over by mold. When this happens the buds are unusable.
2. White Powder Mildew: White Powder Mildew (WPM) is a rapidly reproducing fungal disease resulting in a white powder or mildew covering the plant. If caught early enough, it can be prevented, if not the plant will become unusable.
3. Nutrient Problems: If a cannabis plant takes in too many nutrients at once the leaves will turn yellow and the tips will look burnt, called nutrient burn or nute burn.
4. Slowed Growth: If humidity levels are not optimal for the growth stage, they don’t grow as fast as they could. Younger cannabis plants grow better in higher humidity than more mature plants.
So, the next question is how do you measure the ideal levels?
Cannabis Temperature Monitoring In All Stages of Growth
There are four stages or periods of cannabis growing, all requiring different temperature and humidity levels. They are seedling, vegetation, flowering, and late flowering. Experts recommend monitoring levels throughout all four stages using a digital thermometer and a digital hygrometer that include a memory feature so history can be reviewed. The four stages and what they require are:
1. Seedling Stage: Seedlings or clones, little cut-off pieces from a parent plant that needs to develop roots, require high humidity levels between 65 and 70 percent. These high humidity levels are needed because the root system is not established and humidity allows water intake through the plants’ leaves. Young growing cannabis plants prefer temperatures between 75 and 80 degrees Fahrenheit. Anything above 80 degrees Fahrenheit is not recommended.
2. Vegetation Period: This is the stage that cannabis starts developing its renowned leaf shape. It is important to note that light exposure is a very important part of this stage. The number of humidity levels should be lowered by five percent each week throughout the vegetation period. The temperature can be increased, too because roots absorb more water and evaporate through the plants’ leaves. Humidity levels should be before 40 and 70 percent. If wet spots are forming on the leaves, humidity levels are too high.
3. Flowering Plants: The humidity levels of cannabis plants at the beginning of the flowering stage should be lowered to 40 to 50 percent. At this stage, the plants have created a robust root system to meet their water needs. They still absorb water through the leaves. A lower humidity level is required to prevent mold and mildew from forming. Note that mold and mildew infect the plant from the inside the buds, so it isn’t visible at first.
4. Late Flowering Period: This period lasts anywhere between six and 12 weeks and when plants mature. During this time careful temperature and humidity monitoring are necessary. It is recommended that humidity levels be lowered between 30 and 40 percent to improve the yield, flavor, and appearance of the plant. Temperatures can range between 70 and 80 degrees Fahrenheit.
Tips To Adjust Temperature and Humidity for Cannabis Growing
It can be difficult to keep humidity and temperature levels at the right balance throughout the plants’ life cycle. Keep in mind that warm air retains more water than cold air and that relative humidity and temperature interact to create the perfect balance. The following are hands-on tips for adjusting these levels but note that there is technology available in the SaaS Cloud that will wirelessly do the work for you.
Tips to Lower Humidity and Temperature:
- Increase fresh, cool air in the growing area
- Water plants right when lights are turned on in the growing area
- Add a dehumidifier to regulate humidity in the growing area
Tips to Increase Humidity and Temperature
- Keep the soil surface moist
- Surround the room with moist towels and open containers of water
- Use a humidifier to constantly add moisture to the room
It is difficult to be present for the entire life cycle of a cannabis plant. There are more innovative and technological methods for monitoring temperature and humidity levels. Contact SensoScientific for a free trial and to learn more about the most advanced wireless monitoring solution available for cannabis temperature monitoring.
What is the optimum humidity level for each stage of cannabis growing?
In order to start growing in the best possible conditions, it is adamant to understand the meaning of this technical term. Relative air humidity levels give us information on the concentration of water vapour in the air.
It is important to realise that air humidity is closely connected to air temperature, as hot air is more humid than cold air.
That’s why an air conditioning unit dehumidifies and cools the air at the same time.
When air humidity levels come close to 100%, the air is not capable of retaining the excess moisture, which leads to condensation in the shape of small droplets of fog, morning dew, or rain.
For instance, if the temperature of your grow space is 30ºC, with a relative humidity of around 33-35%, and the temperature suddenly plummets to 10-12ºC, the humidity level can quickly reach 100%.
This can become a critical situation at the end of the flowering period if the ‘dew point‘ is reached. This is a parameter that reflects the cannabis flowers’ humidity level. Bad management of the relative humidity levels of your grow space, caused by abrupt temperature variations, could lead to the appearance of mould in your buds.
Air circulation for cannabis growing
So what is the most favourable humidity level for each growing stage?
Here we show you the ideal humidity levels for every growing phase of your cannabis grow.
Germination / beginning of the cuttings’ rooting period
It is really important to maintain a relative humidity of 80-90% in your mini greenhouse during this phase. This applies to seeds that have just sprouted and also to young seedlings (0-10 days old). These still don’t have a strong radicular system and are therefore pretty fragile.
They require high moisture levels so they can concentrate all their resources on the development of a strong and vigorous root mass. The correct day temperature should be 25-26ºC, and 21-22ºC at night.
Young seedlings and 10-day-old clones
At this point, the required hygrometric conditions should range between 70 and 80%, with similar temperatures to those in the initial stage.
The moisture level for this phase needs to be quite high as the root mass is not very vigorous and actually non-existant in the case of the young seedlings. That’s why it’s necessary to point all technical parameters towards this objective.
It is essential to maintain the correct relative humidity as cuttings will start to develop their first roots between 12 and 20 days after having been planted in the growing medium. This is a crucial point for the appropriate evolution of these young plants.
At this stage the young seedlings, derived from seeds or cuttings, will have developed a radicular mass that is significant enough for them to feed through and therefore reach their maximum potential before being transplanted to a new pot for further development.
Establishing new climate conditions at this stage is vital as the aim is to achieve a reasonably high humidity level, but slightly lower than at the beginning. A good balance should range between 60 and 70%.
Temperatures must be kept between 22-28ºC during the day and 18-22ºC at night. These conditions guarantee the plants’ well-being and facilitate good metabolic function, as well as overall optimum development.
At this point, the radicular system is strong enough for the plant to absorb a higher amount of nutrients and develop a much more resistant immune system.
At the start of the flowering period, when the ‘stretching’ or the final growth boost occurs, plants need specific climate conditions for this transitional phase.
As this is still a transition period in which plants progressively end the veg phase, the humidity level needs to be set at 50-60%. It is also essential that the temperature remains between 20-26ºC both day and night.
This prevents thermal shock and guarantees that plants will be ready to produce those delicious flowers. It is vital that their well-being is taken care of so they can concentrate all their resources on their final objective: quality and quantity. These measures are valid for the first 3 to 4 weeks of flowering, depending on the strain.
In the second part of the flowering phase the humidity level must be reduced to 40-50%. In the two to three final weeks it’s preferable for this figure to be closer to 40%.
This is absolutely essential, particularly if you’re growing strains that are sensitive to mould or that produce dense flowers.
If you follow our guidelines, chances of botrytis hitting your flowers will be slim. As far as temperature goes, we advise you to stick to 18-24ºC during the day and 17-20ºC at night.
This way you’ll be able to control your flowers’ dew point, which is crucial in order to prevent fungi and avoid ruining all your hard work.
Brian Worms After completing my studies in International Trade and Marketing in France, I’m more interested in the world of communication today. Blogger, grower and real cannabis toker for more than 20 years, always looking into the rarest and finest cannabis genetics. My passion for cannabis is so deep that it’s become something of a lifestyle now… My work is like a dream come true.
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Best Temperature and Humidity for Growing Weed
Getting the temperature right for your cannabis plants is an essential part of each stage of the growing process. If the temperature in your grow room or outdoors is too hot or too cold, your young weed plants will not be able to photosynthesise properly, and this will inevitably affect their growth and development. However, it is not as simple as saying ‘this is the correct temperature; the temperature needs of the plants vary according to their stage of development, so you must have some knowledge of their requirements. To help you, we have written this guide which will explain the best temperature and humidity for growing weed for each phase of your young weed plants.
Now, we understand that establishing the ideal temperature for weed plants can be tricky – some say that it is as much instinct as it is science. However, it is certainly possible to work things out for the growing conditions of your plants, and this article will tell you everything you need to know.
What you need to know about temperature and humidity
The first thing you need to understand is the close relationship between temperature and humidity levels. Let’s look at science. Humidity refers to the amount of water vapour present in the air. At higher temperatures, molecules move at top speeds; thus the probability of condensation of water vapour due to the collision of molecules is less. This makes the air hold more water vapour at high temperatures. In other words, as the temperature increases, the humidity of air also increases.
Most plants including cannabis plants, need humid air to thrive. This is because the pores through which they breathe lose most of their moisture when the surrounding air is dry, a loss that the plant can’t always replace through the water absorbed by its roots. The thinner the leaf – and cannabis plants have quite small, narrow leaves, the higher its need for humidity.
When deciding the correct temperature for your plants, therefore, it is also sensible to assess the humidity levels as the two go hand-in-hand. To do this, you should invest in a good-quality thermometer and a hygrometer, ideally digital models with the ability to store historical maximum and minimum levels.
When the temperature is 68°F (20°C), air can absorb 7.2 ml of water maximum. When this occurs, the air has a relative humidity of 100 percent. There will be so much water vapour that the air will be as dense as a cloud and will affect your ability to see anything. Air with a temperature of 32°F (0°C) is only able to absorb about 5 ml of water. This is the reason why the air is a lot drier in the winter than in the summer.
Temperature and humidity for the germination phase
The first moments of your cannabis plant’s life are crucial. Germinating seeds indoors will allow you to have more control over the temperature, but it is also possible to do so outdoors if the climate conditions allow. Having a high temperature (from 22 to 26°C) is an important factor for your seeds to decide to emerge into the world.
However, the temperature must not fall to less than 20 degrees Celsius nor rise to more than 30. If you are using a greenhouse for the germination phase, be warned that it can get sweltering so you must regularly monitor the temperature. Humidity must also be continuously monitored. If the seed trays dry up, germination will not take place or recently sprouted shoots will wither and die. You must not forget to add water when necessary – a spray dispenser is gentle for this, but you should not give them too much water. The seeds should only be soaking wet during the first two days.
Once the first seedlings emerge with two cotyledons (the first two round leaves that develop), they should be carefully transplanted. If the seeds are autoflowering, you should transplant them to a large pot. If they are feminised seeds, however, you can transplant them to a smaller container and move them to the large bowl when they are a bit more established.
Best temperatures and humidity for the seedling phase
The optimal conditions for cannabis seedlings are a sunny, warm (20-25ºC) and humid environment with well-circulated air. Their underdeveloped root systems mean that seedlings need to be exposed to lots of light and heat. Problems arise as their immaturity means that they cannot withstand high temperatures. Therefore, we recommend that growers maintain 20ºC to 25ºC indoor environments when the lights are on.
Humidity levels also need to be high. Levels between 65 percent and 70 percent are sufficient for seedlings to absorb enough water through their leaves.
Optimal temperatures for the vegetative phase
You can start to relax a bit during the vegetative phase. As cannabis plants mature, they grow stronger and better able to cope with extremes of heat. They start to be more resistant to heat and cold. You can raise the temperature to as much as 30ºC.
As the roots have sufficiently developed to absorb plenty of water, you may wish to reduce humidity levels by 5 percent each week. The relative humidity should ideally fall between 40 percent and 70 percent.
The best humidity and temperatures for the flowering phase
The last phase of the cannabis growth cycle is the flowering phase. Growers should aim to maintain warm temperatures that are pretty much the same as when the plants were at the seedling stage. Ensure that the temperature stays at a nice warm 20ºC to 25C to prevent buds from burning their terpenes. This will allow your plants to produce deliciously potent flowers.
It is also a good idea to lower humidity levels to about 40 percent to 50 percent, especially during the late flowering phase. This encourages the production of highly-prized resin and a higher terpene content leading to a more aromatic and flavourful crop.
Cultivators should be advised that typical temperature variance when the lights are switched off is 5ºC, and this applies to all the phases of development.
Temperatures that are too cold
Temperatures that are too cold will slow down a cannabis plant’s metabolism and its roots system will become increasingly ineffective. If temperatures drop below 12 degrees, the plant is even at risk of death.
The obvious solution to boosting the temperature of the grow room is to get an electric heater and plug it in. This has several disadvantages, however, mainly due to its high consumption of electricity but also because of its drying nature which can affect levels of humidity, and the direct distribution of the heat which makes it hard for all the plants to enjoy the uneven heat. An oil-filled portable radiator is another option if you can get one which isn’t too large to fit into the grow space. We recommend small tubular electric heaters which can be placed conveniently on the floor and only use a few watts of electricity. Radiating cables for indoor use are another excellent alternative, especially if you need to heat trays of seedlings.
Temperatures that are too hot
Reducing heat levels in a grow tent can be much harder to control than the reverse. We recommend using electric fans to circulate the air as much as possible to disperse the heat and thus cool the young cannabis plants.
Placing an exhaust fan in the ceiling vent of your grow tent is also highly effective at reducing heat. After all, heat always rises to the top.
You could also use a cooling humidifier which reduces temperatures while boosting humidity levels. Just take care that the humidity does not increase too much for the growth phase. Remember, seedlings need higher levels of humidity than vegetative marijuana plants, for which 40 percent is optimal.