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! Time then to whip out our packages of seeds and get to work. But how do you best set to work, to get these tiny, fragile seeds to grow without problems into small plants?
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.
The Complete Guide To Germinating Cannabis Seeds
Before you can be met with bountiful hauls of dank buds, there are several stages of cannabis growing that take precedence. Unless you can successfully germinate cannabis seeds, you won’t have a plant to harvest. Give your seeds the best possible start in life by reading our definitive guide to germination.
- The art of germinating cannabis seeds
- What to look out for in cannabis seeds
- Germination temperature plays a crucial role
- What is an expected germination time?
- Choosing your germination method
- Giving your seeds the best possible start in life
- Germination and beyond
The Art Of Germinating Cannabis Seeds
Often overlooked, it is all too easy to assume that the vegetative and flowering stages of cannabis growth are the most critical parts of the plant’s life cycle. However, with the chance of failure high unless you know what you’re doing, poor planning when it comes to germination can make or break your next grow. Giving your cannabis seeds the best possible start on their journey to bulging buds is a surefire way to encourage a healthy and robust plant.
Small, fragile, and in desperate need of a helping hand, there are several ways you can germinate your cannabis seeds. All methods have varying degrees of success, with both advantages and disadvantages. It is important to note that even with advanced growing expertise and top-of-the-line equipment, you may still end up with a few failed seeds. This is a natural part of dealing with a living organism. At Royal Queen Seeds, we provide a wide range of high-quality regular and feminized cannabis seeds. We label our genetics clearly, so you don’t have to worry about any unwanted surprises.
What To Look Out For In Cannabis Seeds
Regardless of where you get your seeds from, it is best to give them a slight (and delicate) inspection before planting. Most of the time, all seeds will germinate; however, poor-quality seeds will produce a weaker plant. Unfortunately, that is something you will not find out until well into the vegetative and flowering stages.
To avoid disappointment, seeds that have a darker colouration stand a better chance of germinating, while pale green or white seeds are likely to fail. Even if dark seeds look slightly damaged, they should be planted anyway. There is a good chance they will still germinate, even if the outer shell is somewhat crushed.
Germination Temperature Plays A Crucial Role
Before we jump straight into the germination methods, there are a couple of germination golden rules. For the best results, we recommend staying within these guidelines, no matter how you choose to germinate. That being said, of all the factors to consider, temperature is one of the most critical. Seeds will always seek out even the smallest amount of moisture, but they use temperature as a sign that they need to do so.
- The ideal temperature is between 22° and 25°C (71–77°F)
- Your growing environment should be damp/moist, but never wet
- Relative humidity range should be between 70% and 90%
- Seeds favour fluorescent lighting (Cool White code 33)
- Minimise the amount of seed handling you do
- In hydroponic/rockwool plugs, the ideal PH value is 5.8–6.2
|-15 °C||17 °C||23 °C||+25 °C|
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|-15 °C||⚠️ DANGER ZONE|
|17 °C||IDEAL NIGHT TEMPERATURE|
|23 °C||IDEAL DAY TEMPERATURE|
|+25 °C||⚠️ DANGER ZONE|
What Is An Expected Germination Time?
Three fundamental principles will trigger that first small taproot to appear: warmth, moisture, and darkness. With the promise of moisture, a single root will take shape before slowly developing into the cannabis plant we know and love. In the right conditions, seeds will begin to develop within 12–36 hours of moisture being introduced to them.
Timescales can vary, as it all depends on how ideal your germination environment is (see the golden rules above). Even the worst grower could make a seed germinate, but it may take a few weeks and, of course, increases the risk of a weaker plant.
Choosing Your Germination Method
Glass Of Water Approach
Arguably one of the least effective methods, but it is still viable. Incredibly simple to facilitate, beginner growers may opt to germinate their seeds in a glass of water. Half-fill a glass or bowl with water that is approximately 22°C (71°F).
After 3–5 days, the seeds will start to open, and you should see tiny white tips appear. Once these roots reach 2–3mm in length, use extreme care to transfer them from the water to pre-prepared soil pots.
The soil pots will need small holes (roughly 10–15mm deep) for the newly germinated seeds to be placed into. Once the seeds are secure, you will want to place a fluorescent light 13–15cm (5–6 inches) away to encourage growth. Finally, don’t risk overwatering your seeds at this early stage. Use a plant mister to make sure they stay damp but not soaking wet.
Wet Kitchen Towel Method
Probably one of the most common methods of germination. The kitchen towel method comes in several iterations. Some growers use cotton wool pads or absorbent pieces of paper. For this guide, we will be using kitchen towel as it is readily available and holds moisture relatively well.
Place one sheet of damp kitchen towel on a flat surface. Space your seeds a few centimetres apart before placing the second piece of kitchen towel over the top. You need to ensure both pieces are damp, not wet. Once again, when the white root tips reach 2–3mm, move the seeds (carefully) to soil pots. Use the same guidance found above for planting techniques.
Planting Directly Into Soil
Planting directly into your growing medium prevents having to move seeds when they are at their most fragile. That first root tip is covered with microscopic filaments that are easily damaged. Given that both a cup full of water and moist paper towels are more prone to temperature fluctuations from their environment, planting in soil is a much safer option.
Start by filling pots with a premium-quality soil that has been soaked in water. Many growers also choose to lace the water with a root stimulator. Make a hole roughly 10–15mm deep. This will be your seed’s new home. Remove the seeds from their packet and place them into the pre-dug holes. Loosely cover the seeds, but be careful not to compress the soil above the seed too much. The root will struggle to penetrate solid soil, slowing plant growth. Lightly spray the top of where you placed the seed so that your growing medium stays moist.
If you don’t like the idea of pre-soaking your soil, you can use a spray to moisten the holes before you plant each seed. With enough moisture surrounding your seeds, you can still encourage a root to develop.
Your growing pots will need to be placed in a damp climate that is within the temperature range listed under our golden rules. After 4–10 days, you should see a young seedling sprout, while the roots will have begun to develop underneath the soil. The entire plant and its soil can now be transferred to a larger pot, where normal growing routines should start.
Using Stone Wool Blocks
Maintaining the ideal temperature (between 22–25°C/71–77°F) and moisture for germination is tricky. Leaving seeds out in the open environment or on a windowsill is far from ideal; a DIY climate-controlled cupboard would do a much better service. A warming mat is perfect for maintaining a constant temperature, but it doesn’t tackle the issue of moisture.
You will need to invest in a few pieces of unique equipment, but by using stone wool blocks, you can create a perfect utopia for germinating cannabis seeds. Soak the stone wool blocks in the same way you would a soil medium. The wool will retain the moisture and prevent the need to water during the early stages of germination. After the wool blocks are soaked, stick them in a plastic tray that also has a lid. Large cake tubs are ideal.
The dome of the plastic container will create your seeds’ own mini tropical climate. If you then place all the components in a temperature-controlled cupboard, you will have created a self-perpetuating supply of moisture—no need to touch the seeds again until they are ready to be transferred to your final growing medium as a young seedling. Using the stone wool block method, your seeds should germinate in one to two days.
Two or three weeks after germination, your young seedlings should be ready for their new home. At this point you have two options; transplanting them into soil pots, or taking on the challenge of hydroponics. You’ll know when the seedlings are ready to be moved because the root system should start to poke out of the bottom of the wool blocks. As long as the roots haven’t begun to engulf the bottom half of the wool block, they will seek out water and nutrients in their new surroundings and continue to grow downwards.
RQS Soil Plugs
RQS Soil Plugs are another great option when it comes to germinating seeds. They are easy to use and contain a carefully crafted formula of ingredients designed to help seedlings thrive. Composed of the highest-quality peat and coir, they provide a superb balance of aeration and water retention. This ensures seeds don’t dry out, while also preventing fungal diseases that cause the dreaded damping off.
RQS Soil Plugs contain key micronutrients that seedlings require for optimal growth and development. The addition of active biologicals also helps to establish the root microbiome with beneficial organisms that fend off pathogens and assist in nutrient uptake and organic matter breakdown. Furthermore, the pH-balanced nature of these plugs means cannabis roots can easily absorb nutrients without the risk of lockout.
They’re super easy to handle and take any hassle out of transplanting. Once roots begin to appear, you can transplant seedlings directly into soil or hydroponic systems.
To use RQS Soil Plugs, place a single seed of your choice into each plug. Place your plugs into a propagation tray and mist them with water—just enough to get them moist. Use a clear-plastic lid to maintain humidity, and aim for the temperature and humidity conditions discussed above. Use a small hygrometer to keep track of these important figures.