Is A Steamy 90-Degree Room Too Hot For Healthy Cannabis Growth?

Marijuana plants can grow and survive in almost any climate, but they will struggle and likely not produce desirable effects if the temperature is too hot. A 90-degree room may be too hot for growing pot, as the maximum temperature for marijuana grow tent success is 85°F.

At high temperatures, marijuana plants are susceptible to issues such as nutrient burn, white powdery mildew, wilting, and reduced bud potency and smell. If the temperature is too high, the plants may even die.

To prevent heat problems when growing marijuana, it is important to select strains that do well in your particular climate and create an ideal environment using grow rooms or greenhouses. Additionally, growers should pay close attention to the temperature and watch their plants carefully for signs of heat stress, such as curling leaves, wilting, and browning leaf tips.

The maximum temperature for successful growth

Marijuana plants are resilient and can grow and survive in almost any climate. However, they will struggle and may not produce the desired effects if the temperature is too hot. The maximum temperature for successful growth will depend on the growth stage of the plant.

Seedling stage

The ideal temperature range for seedlings is between 70-78°F (21-25°C). If the temperature rises above 78°F, you may need to take steps to cool the environment.

Vegetative stage

During the vegetative stage, marijuana plants thrive in temperatures between 70-85°F (20-30°C). This is also the maximum temperature for marijuana grow tent use.

Flowering stage

During the flowering stage, the temperature should be kept between 65-80°F (18-26°C). The second half of the flowering stage is when the temperature should be at its coolest to encourage flower colour, trichome production, density, and smell.

General guidelines

Maintaining the right temperature for growing marijuana plants is crucial for a successful harvest. In general, temperatures between 68-86°F (20-30°C) are safe for indoor cultivation, but a range between 68-83°F (20-28°C) is considered optimal. It is important to avoid large temperature fluctuations as extreme changes can stress the plants and affect their growth, health, and yield.

Signs of heat stress

Even in a grow tent, marijuana plants will start to have problems if the temperature is too high. Signs of heat stress include leaf edges that curl upwards, wilting and drooping leaves, burnt leaves with brown tips, and the development of foxtails.

Preventing heat stress

To prevent heat problems, select strains that do well in your particular climate and use grow rooms or greenhouses to create ideal environments. Ensure proper ventilation and use air conditioning or fans to maintain the correct temperature. Choose low-heat lighting options and keep lights at least three feet away from plants.

High temperatures and bud quality

High temperatures during the flowering stage will slow down bud growth and may cause buds to become airy and loose. The heat can also reduce the potency and smell of the buds, burning away terpenes and cannabinoids.

Signs of heat stress

A 90-degree room may be too hot for growing cannabis, as the optimal temperature range for proper plant development is around 19°C at night and 25°C during the day. However, most cannabis strains can tolerate higher temperatures without issues. When the temperature surpasses 30°C, symptoms of heat stress may start to appear.

• Curling leaves: Leaves with edges curled upwards, appearing dry and withered.
• Brown spots: Irregular brown spots on the leaves, especially along the edges of the fan leaves.
• Leaf discolouration: Yellow patches on the leaves, along with the brown spots.
• Foxtailing: During the flowering stage, additional shoots may form on existing buds, resulting in elongated structures protruding from typically rounded buds.
• Increased white pistils: During the final weeks before harvest, more white pistils may appear instead of the expected coppery colour.
• Buds lacking smell: Terpenes burn away at high temperatures, resulting in buds with reduced potency and smell.
• Stretching: The plants may exhibit increased stretching as a response to the heat.
• Nutrient burn: Heat stress can cause increased water transpiration, leading to nutrient burn.
• White powdery mildew: High temperatures and humidity can create favourable conditions for mildew growth.
• Wilting: Heat stress can cause a lack of oxygen, leading to wilting of leaves and the entire plant.

It is important to note that heat stress can have negative impacts on the growth, yield, and quality of cannabis plants. Therefore, monitoring temperature and taking steps to mitigate heat stress are crucial for successful cultivation.

Controlling temperature with lighting

Lighting is a critical component of growing cannabis, but it can also be a source of unwanted heat. Here are some ways to control temperature through lighting:

Choose the Right Lights

The type of grow lights you use can have a significant impact on the temperature of your grow room. LED grow lights, for example, emit less heat than traditional HID lights. When choosing your grow lights, consider the power, spectrum, and intensity of the lights, as these factors will influence both the temperature and the growth of your plants. Higher wattage lights will generate more heat, so it's important to select the appropriate wattage for your growing area. Additionally, the spectrum of light can affect the growth and development of your plants. For instance, blue light is important for the vegetative stage, while red light is crucial for the flowering stage.

Maintain Optimal Distance from Plants

The distance between your grow lights and your plants is crucial. Lights that are too close can cause heat stress, while lights that are too far away may not provide sufficient light intensity for optimal growth. As a general rule, the closer the lights are to the plants, the higher the light intensity, and vice versa. However, it's important to note that some lights, such as LED grow lights, can be too intense for young plants, so you may need to adjust the distance as your plants grow.

Implement Proper Ventilation and Exhaust Systems

Ventilation is essential to managing the temperature in your grow room. Ensure you have a sound exhaust system in place to pull out hot air and increase air circulation for your plants. If using HID lights, look for models that come pre-built to accommodate cooling via an exhaust system. Additionally, consider using fans to improve air circulation, but avoid pointing them directly at your plants, as this can cause damage.

Adjust Your Light Schedule

If you're experiencing temperature extremes at certain times of the day, consider adjusting your light schedule. Change the timer so that your grow lights are on at night and off during the hottest parts of the day. This can help equalize day and night temperatures and reduce heat stress on your plants.

Utilize Shading Techniques

Excessive heat from lighting can be mitigated by using shading techniques such as shade cloths or curtains. These can help regulate light intensity and reduce temperature fluctuations caused by heat-generating lights. Just be sure to strike a balance between light and temperature to create an optimal growing environment.

Nutrient concentration

Seedlings

Cannabis seedlings obtain their nutrients from the seed and absorb water through their leaves as their root system develops. Therefore, it is crucial to maintain a warm and humid environment for the first three to four weeks of their growth.

Vegetative Stage

During the vegetative stage, a 2:1:2 or 4:2:3 fertiliser ratio can be used to introduce seedlings to nutrients and prevent nutrient burn. As the plants progress to the mid-vegetative phase, a more aggressive approach with a 10:5:7 fertiliser ratio is recommended to promote lush foliage and bud site development. In the final week of this stage, it is advisable to lower the nitrogen levels and prepare the plants for the flowering stage with a 7:7:7 fertiliser.

Flowering Stage

During the flowering stage, cannabis plants require less nitrogen and more potassium to encourage the growth of large, resinous flowers. A 5:7:10 fertiliser ratio is typically used during the first two weeks, gradually increasing to 6:10:15 by mid-flowering. In the last weeks of flowering, the nutrient levels are reduced, and a milder 4:7:10 fertiliser is applied to smooth the transition to the pre-harvest flush.

Micronutrients

In addition to the three primary nutrients, cannabis plants also require micronutrients such as calcium, magnesium, and sulfur, which play vital roles in their growth. Calcium aids in cell wall development, improves water penetration, and helps reduce soil salinity. Magnesium is crucial for photosynthesis and carbohydrate metabolism, while sulfur is necessary for chlorophyll formation and protein production.

Organic vs. Chemical Fertilisers

Organic nutrients are released more slowly into the soil, reducing the risk of nutrient burn and benefiting the soil ecosystem. They are particularly suitable for outdoor growers as organic soil improves over time. On the other hand, chemical fertilisers are absorbed faster, providing more control over nutrient ratios and making them ideal for treating deficiencies.

The role of humidity

Humidity is an important factor in growing cannabis. Relative humidity is a measure of how much water vapour is present in the environment relative to the temperature. Relative humidity is important because warm air can naturally hold more vapour than cold air, so the same humidity reading at different temperatures will have different effects on the plants.

Humidity plays a necessary role in influencing how much evaporation occurs in cannabis plants. Your plant will take in more nutrition and water when the humidity is low. If the evaporation strain gets to be too much, marijuana plants will guard themselves against scarcity by closing their stomata, which stunts growth due to a lack of water absorption.

The ideal humidity levels for cannabis plants depend on the stage of development. Seedlings, for example, like warm and humid conditions, while flowering plants usually prefer cooler nighttime conditions and less humidity.

Seedlings and clones do well in temperatures of 20–25°C and relative humidity of 65–70%. These conditions promote rooting and allow seedlings/clones to take up water via the leaves until they develop roots.

Vegetative plants like warmer, less humid conditions. A temperature range of 22–28°C and relative humidity of roughly 60% RH is recommended.

Flowering cannabis plants like cooler temperatures and low humidity. Keep temperatures between 20–26°C and relative humidity at 50% during the lights-on period. If possible, drop temperatures by roughly 5–8°C during lights off. During the final 2 weeks of flowering, it is recommended to drop the temperature to 20–23°C during lights on and 18–20°C during lights off, with relative humidity at 30%.

The easiest way to control humidity is to use humidifiers and dehumidifiers, which add or extract water from the air. If you don't want to invest in a humidifier or dehumidifier, you can try spraying water to raise humidity or using extractor fans to lower humidity.

Frequently asked questions