Best Grow Room Conditions For Maximum Yield

Start the grow with temperature at 20°C or 68°F and increase to 26°C or 80°F in flowering. Maintain humidity around 50% to 60% for the duration. Ventilate continually with fresh air to supply CO₂ to the plants. Grow light wattage should be 30 to 50 per sq. ft. (350 to 500 watts per sq.m.) for a high performance light intensity.

Providing the best grow room conditions for temperature, CO₂ and light intensity will ensure your plants will grow fast and healthy. Plants grow by Photosynthesis

Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide. The plant then releases the oxygen back into the air, and stores energy in the form of glucose which the plant used to fuel growth.

Optimising each of these inputs will maximise the photosynthesis rate which will determine how fast your plants grow and how big they get by harvest time.

The best light intensity for plant growth during flowering

The light plants receive is the energy source the plants use for photosynthesis and growth. The measurement of light used for photosynthesis is called Photosynthetically Active Radiation (PAR).

Photosynthetically Active Radiation (PAR) is a crucial concept for indoor growers to understand as it refers to the portion of the electromagnetic spectrum that is essential for photosynthesis in plants.

PAR is the range of electromagnetic radiation wavelengths (measured in the 400 to 700 nanometer range) that plants use for photosynthesis. This is the same as the visual range i.e. from blue to green to red.

Recommended PAR intensity for flowering plants

Cannabis plants can utilise high PAR intensity and growth rate increases at a high rate up until about 1,400 µmols/m²/second. This is a very high performance level which requires a lot of skill to support this level of plant growth with environment control and feeding systems. This level is most suitable for commercial and expert home growers.

Most home growers will get a very good return from an average PAR intensity of about 900 µmols/m²/second. The plants will not be under a lot of stress and managing the environment and feeding systems can be less precise. This is why most grow light manufacturers design grow lights to deliver an average PAR at around this level.

Above 1,400 µmols/m²/second the rate of growth reduces and the system is less efficient as plants have limitations on the amount of light they can absorb in one light cycle. 

The best light intensity for plant growth during vegging

The PAR intensity levels required during vegetative growth are about half of the requirements for flowering. We recommend a PAR level of 400 - 500 µmols/m²/sec for your grow room.

You can achieve these levels by reducing the power consumption of your grow light to 60% using a dimmer. If this is not available you can halve the number of lights (if possible) or raise the grow light to about two times the optimum hanging height.

The best light intensity for Autoflower plants

The photoperiod or amount of time grow lights are on for autoflowers is generally much longer than for photoperiod plants, usually about 20 hours. Plants are limited to the amount of light they can absorb in one day so we recommend a reduced average PAR intensity for Autoflowers.  The PAR intensity levels required for Autoflowers are 500 - 600 µmols/m²/sec.

What should the temperature be in my grow room?

Controlling temperature is the most effective way of increasing harvest quality and potency and is known as crop steering.

Temperature Control for Early Vegetative Growth

The vegetative stage is critical for establishing a strong, healthy plant structure that can support prolific flowering. During this stage, medicinal herbs benefit from a warmer, stable temperature range. Maintaining temperatures between 20-28°C (68-82°F) during the vegetative stage promotes vigorous growth. Warmer temperatures accelerate metabolic processes, including photosynthesis and nutrient uptake, leading to faster growth rates and stronger plants.

Transitioning to the flowering stage

The flowering stage is where medicinal herbs focus energy on producing buds rich in cannabinoids like THC. Temperature adjustments during this stage are crucial for maximizing THC content. High bud temperatures will cause the volatile cannabinoid compounds to evaporate and reduce the concentration in the flowers.

Reducing the temperature in the last four weeks of flowering will reduce growth rate a little but is very critical to delivering high potency buds at harvest.

Night time temperatures must be maintained above 50 Degrees Fahrenheit or 12 Degrees Celsius to prevent cold shock which will stunt growth and may result in stunted growth or even plants dying.

Initial Flowering Phase: In the early flowering stage, maintaining slightly warmer temperatures around 24-26°C (75-79°F) during the day and 18-22°C (64-72°F) at night helps sustain vigorous growth and initiate bud formation.

Late Flowering Phase: As the plants progress into the late flowering phase, reducing temperatures further can enhance THC production. Lowering nighttime temperatures to 15-20°C (59-68°F) while keeping daytime temperatures between 20-24°C (68-75°F) creates a mild stress condition known as "cold shock." This controlled stress response stimulates the plant to increase resin production, which is rich in THC.

How to reduce temperatures in the grow room

• Draw more air into the grow room from outside by increasing extract fan speed or size or opening ventilation flaps in your grow tent. A temperature controlled fan is a great way to manage temperatures at optimum levels.
• Use an air conditioner if the outside temperatures are too high
• reduce heat generation in the grow room by moving heat generating equipment outside or dimming the lights
• Run the lights at night time when the outside temperature is cooler

How to Increase temperatures in the grow room

• Draw less air into the grow room from outside by reducing extract fan speed or size or closing/restricting ventilation flaps in your grow tent
• Use a heater and thermostat
• Insulate your grow space to keep the heat generated by your lights in the grow room

Humidity levels in the grow room

Humidity refers to the amount of water vapour that can be measured in the air. Reduced humidity percentage in the air will cause water loss from your plant’s tissues. High humidity levels can cause microbial and fungal pathogens to develop resulting in mould and bud rot. 

Relative humidity, often expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature.

Humidity level for seedlings and clones

During the seedling or cloning process, humidity should be on the higher side at about 80% relative humidity. Clones will be healthier and root quicker in a warm and humid environment. Use a clear plastic dome or lid to increase moisture in your cloning trays.

Humidity level for the vegetative phase

At this stage the plants have developed a good root system and can uptake water and nutrients at a good rate. This means we can reduce humidity as the plants can take up water as required.

At this stage we recommend between 60 and 70% relative humidity.

Humidity level for the flowering stage

At this stage the plant canopy is dense and the plants are growing at a high rate. To reduce the risk of mould or mildew reduce the relative humidity to about 50% at this stage. Ensure there is good air flow around the plants by trimming leaves in the lower canopy and using circulating fans to move the air around to prevent moisture build up in parts of the canopy.

Humidity level for drying Flowers

To ensure buds dry but not too quickly maintain a relative humidity level of about 40% in the area you are hanging your flowers to dry.

Controlling humidity in the grow room

Giving plants the correct amount of humidity, allows their pores to open up inside the leaves, respire more CO2, and grow faster. However, exposing them to too much humidity can cause diseases such as bud rot due to water build-up encouraging bacteria and fungi to overrule the foliage. 

How to Increase humidity in the grow room

• Reduce the power of your exhaust fan – this keeps moist air inside the growing area
• Increase the temperature in the room 
• Gently spray/mist your plants and soil to keep them moist – avoid flowering plants as this can cause the buds to rot
• Surround your plants with water (in open containers)
• Use a humidifier to deliver moisture into the growing area

How to Reduce Humidity in the grow room

• Increase the power of your exhaust fan – this will draw less humid air from outside the room
• Remove any open water source or cover them and cover the open surface the plant pots.
• Use a dehumidifier or air conditioner if the outside humidity is too high
• Water the plants at the start of their light period/when lights are turned on
• Cool the air in the growing area

How much does added CO₂ increase growth rate?

The normal atmospheric CO₂ level is about 400 parts per million (PPM). The best way to achieve this level in your grow room is to draw fresh air from atmosphere into your grow room at a high rate.

It is recommended you change the air in your grow room frequently with the use of an extract fan and ducting drawing fresh air into the grow room. At these rates you will maintain very close to atmospheric levels of CO₂ of 400 PPM and achieve good growth.

With this setup it is not practical to achieve any increase in CO₂ in the grow room. This is because any CO₂ added to the grow room will be removed quickly with the constant air flow out of the grow room.

If you want to increase CO₂ levels in the grow room you will have to seal the room so the CO₂ does not escape and use a CO₂ generator or CO₂ cylinder with a CO₂ controller and regulator.

Sealing the grow room will mean you will have to manage the temperature and humidity with air conditioning. This is relatively expensive and complex and requires a significant investment in equipment and time to setup but can increase growth rate and yield by up to 30%.

Using the naturally higher CO₂ levels in the house to increase growth rate

You can also use your own CO₂. Humans expel CO₂ when breathing out so the level within our homes is often twice the concentration outside. This is most common in winter months when the air circulation in homes is minimal. It's common for CO₂ levels in the home to range from 700 -1,000 PPM. In a bedroom at night CO₂ levels may reach as high as 2,000 PPM. Check out an article here on the CO₂ levels in the typical day of a person at home, work etc. https://www.co2meter.com/blogs/news/co2-levels-at-home