Growing Cannabis Indoors and Signs of Heat Stress

Growing Cannabis Indoors and Signs of Heat Stress


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Growing Cannabis is easy enough to do, and most times a good strain will produce great results even for a first time grower. However, dealing with stress caused by a grow room which is too hot can cause problems for a beginner. In this article, we break down the different signs of plant stress that may occur inside a hot grow room, as well as how to fix your environment and our final thoughts.

Heat stress

Cannabis plants rely on water as a way to uptake nutrients from the roots, as well as transpire moisture through their leaf tissue. When the grow room environment is not favourable for a vegetative or flowering plant, then this can start a chain reaction of events causing heat stress.

What can cause a hot grow room?

● Growing indoors during the hottest part of the year.

● Poor extraction and not removing air fast enough.

● Wasted heat energy from hanging grow lights.

● A high volume of hot ballasts sat on the floor.

● Too many fans blowing hot air around.

7 Signs of heat stress to look out for

1) Leaf curl

The curling of fan leaves, and sometimes small leaves, can be a clear indicator that the air is too hot and dry for the leaf tissue. The reason why the leaves begin to curl inwards like a taco, is to significantly reduce the amount of intense lighting, as a way to cope with the stress.

When you notice the sides of the leaves curling inwards, you should check the temperature and humidity levels inside your grow room. Often times, leaf curl can be due to too much air being blown directly at the plants.

2) Dry and crispy foliage
Watching your lush green plants slowly but surely start to suffer and lose vigor, can be heartbreaking and once the leaves become dry and crispy, you can only assume the worst. A total confidence breaker for a beginner grower, however, this is another indicator that the air is too warm, combined with hot air being circulated by oscillating fans.

Once the leaves become dry and lifeless, showing clear signs of heat stress, you should do your best to reduce the temperature and humidity until the plants can bounce back with healthy green growth.

3) Stunted growth
One of the worst effects of heat stressed Cannabis plants, responding to hot and dry conditions, can be stunted growth. This means that the plants will slow down considerably, producing new foliage during 18/6 or flower production when the timer is on 12/12.

Heat stress is often times responsible for low yielding plants, with low quality buds when it comes to harvest time. Stunted growth is just one of the many downsides associated with a hot indoor garden.

4) Small yields
Unless you are really familiar with how productive a strain can be, you may not even know that the reason you only hit 25 grams dry per plant on your last harvest, due to heat stress. During the pinnacle weeks of flower production, your plants may seem to not be going crazy like previously.

There will be a high level of resin, however the buds are not as dense, swollen and chunky as they should be, resulting in low yields. What is worse, is the dried buds from heat stressed plants will not be the best quality, to add insult to injury.

5) Moulding buds
Mould can be a serious crop killer if not taken care of as soon as possible. It may not always be easy to spot moulding buds, however in a hot and humid grow room, mould can spread quickly and begin to grow from the inside outwards. Always check your buds, especially during the final 3 weeks, as they begin to really swell up and pack on the pounds.

6) Dry growing medium
Cannabis plants depend on moisture in the pot as well as moisture in the air. A hot grow room can quickly dry out the air, and it is not long before the water around the roots is used during leaf transpiration.

As the growing medium dries out faster, the grower is expected to water more frequently to keep up with the amount of water being used by the plants. Over time, there is a negative chain of events occurring which can lead to nutrient deficiency or toxicity for a beginner grower. Beneficial bacteria and fungi will also be affected by the drastic shift from wet to dry.

7) Crispy buds
Cannabis plants that were allowed to grow too close to the grow lights, will produce bleached and crispy buds. Luckily, this may only be the case with the tallest buds, however plants flowering in a consistently hot grow room may suffer severely.

The buds will have a yellow or light brown colouration and will be bone dry even harvesting. You will also notice the smaller leaves covered with resin, normally used for hash making, will also be bone dry and lifeless.

Spider mite infestation
If you have never had to deal with a spider mite infestation, then count yourself lucky. Spider mites seem to love two things, which are hot and humid environments and flowering Cannabis plants.

Once these little critters are in your grow room, they can rapidly reproduce and take over the garden, causing a path of destruction. The best way to treat a spider mite problem is to use a high ratio of predators and let them fight it out.

How to fix your indoor growing environment

1. The best way to know the exact temperature and humidity level of your indoor grow room is to use a digital hygrometer. It is essential you are able to know the highest and lowest temperature and humidity levels over a 24-hour period when producing high grade quality Cannabis.

2. During the vegetative stage, Cannabis plants like a warm and wet environment. Making sure that the environment remains consistent with the lights on and aiming for a relative humidity level of 70-80% and temperature of 24 degrees Celsius will be ideal. The easiest way to increase the humidity is with a small humidifier kept inside the room.
3. Investing a good quality extractor fan, capable of blasting out air, will play a massive role in how well the air inside the grow room can be recycled and blown out. A low poweredextractor connected to a large carbon filter can lose efficacy over time, resulting in a hot room with stagnant air.

4. A dehumidifier is a real lifesaver during the flowering period, and will allow you to remove the excessive amounts of moisture that are present in the air. There can be many reasons for high humidity, and the main culprit being the weather or time of year. Investing in a dehumidifier can help eliminate the threat of mould and powdery mildew, and won’t be too expensive to buy.

Our Conclusion

As a beginner grower, there will be many things which can knock your confidence and have you scratching your head, wondering what went wrong. Making sure you have a hygrometer, dehumidifier and a proper extraction kit will pay off massively, especially as you approach the end of the year, and will face cold, wet and harsh weather.

Foliar spraying with a mild solution of seaweed is well recommended to help keep your plants healthy and vibrant during the vegetative stage. LED grow lights are an excellent solution to hot grow lights and can be a great way to save on electricity bills also! Good luck mastering your grow room environment and producing the ultimate crop.

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