Cannabis Buds with Mold or Powdery Mildew: Causes, Signs & Prevention Tips

In this article, I will explain everything you need to know about mould and powdery mildew, including what causes them, signs to look out for to identify mould, and the optimal temperature, humidity, and environmental settings to prevent mould. Also, what do you do if you find mould spreading during the flowering stage? My top tips and how to avoid it in the future!

What Causes Mold and Powdery Mildew on Cannabis Buds?

Mould is a fungus that can quickly spread and attack organic plant matter. It's an airborne fungus that releases spores on damp surfaces, particularly in areas with poor ventilation. You can get mould on walls at home, out-of-date food in the fridge and worst of all, on the inner and outer parts of cannabis buds during the later stage of flowering. The primary causes of mould are high moisture levels, high humidity, and inadequate ventilation. 

The same applies to powdery mildew, which is far easier to spot, appearing as a chalky white powder that forms on the surface of the leaves. It is essential not to confuse resin production with powdery mildew spores, especially during the final week of the 12/12 cycle. 

MSNL advises that most types of fungus love warm, damp environments, and Powdery Mildew is no exception. Indoor cannabis grows check off both of those boxes, making it an ideal breeding ground for this pesky fungus.

How to Identify Moldy Cannabis Buds

• There will be a cotton-wool or spider-web-like characteristic to your buds.
• Your buds will develop brown, grey, or black spots on them.
• The colour of the buds can become yellow or brown.
• Buds will have a mushy and soft texture when squeezed or torn.
• There will be a fungal, earthy odour when smelling the buds.

Signs of Powdery Mildew on Cannabis

• The leaves will have a white powdery substance on the surface.
• The surface of the buds will have a white chalky appearance.
• Leaves, buds and growing tips can become deformed.

The Optimal Temperature and Humidity for the Cannabis Flowering Stage

The main culprit for a flowering room that has become infected with moulding buds will be a lack of fresh air circulating through the buds. When indoor growers operate with high temperatures and humidity levels above 60%, mould can infect the buds in a short amount of time, and very quickly spread through the colas. 

• The temperature should stay between 20 and 28 degrees Celsius.
• Relative humidity levels should not exceed 60% and stay close to 40%.
• There should be a constant flow of cool air blowing around the buds.
• Use a digital hygrometer to check the temperature and humidity.
• Oscillating fans will produce consistent air currents throughout the grow room.

How to Deal With a Moulding Flower in your Grow Room

As an indoor grower, checking on your frosty and beautiful-looking buds is an essential part of quality control. However, there may come a time when you unzip your tent and notice that some of the buds are slightly off colour and have taken on a candy floss fuzzy appearance. 

Then again, you may notice that inside your biggest cola, the twigs have become brown, and when you break them apart, the twigs snap and have a soft and mushy consistency. Depending on the severity of the mould problem, there are ways to deal with the situation and allow you to save what non-contaminated buds remain.

• Wear latex gloves that can be thrown away after removing the infected parts.
• Use a clean and sharp pair of scissors to cut away the contaminated buds carefully.
• Increase the amount of air that is blowing at the cola or lower buds.
• Throw away the moulding buds and leaves into a bin outside the grow room.
• Wash your scissors afterwards with alcohol and sterilise them.

How to Prevent Powdery Mildew from Forming in Your Grow Room

1. Provide more space between the plants, which will improve air flow.
2. Maintain consistent temperature and humidity levels.
3. Add more fans to the grow room at different heights.
4. Prune and throw away contaminated leaves.
5. Use 3% hydrogen peroxide to try to kill the fungus.
6. Spray the tops and undersides of the contaminated white leaves.
7. Use commercial or organic fungicides, but avoid spraying buds.
8. Beneficial bacteria can attack the fungi and fight off the infection.

5 Top Tips for Avoiding Mould and Powdery Mildew During 12/12

Prevention is the cure when it comes to growing indoor cannabis. Below are five top tips to consider to ensure that your plants have the best chance of growing without any signs of mould or powdery mildew.

Tip #1: Air flow is key!

Damp and humid environments will attract mould and fungus straight to your flowering plants, so the best way to keep your room fresh is to use multiple fans. Using a floor fan, combined with 2-3 small oscillating fans inside the grow room / grow tent, will provide plenty of fresh air currents and keep your growing environment well ventilated and fresh.

Tip #2: Avoid high humidity during 12/12

Unless you are using carbon dioxide, there is no reason that you need to exceed 28 degrees Celsius during the final 4 weeks of the flowering stage. Maintain a room temperature between 22 °C and 24 °C, and keep the humidity level at 40-50%.

Tip #3: Provide adequate space between plants

When plants are placed too close together, there can be a lack of fresh air circulating around the lowest, middle and highest parts of your canopy. Plants growing too close together, with the leaves touching each other, is one of the easiest ways for Powdery Mildew to spread. Give your plants space and room to grow, and ensure air currents are blowing the leaves.

Tip #4: Use a digital hygrometer

A digital hygrometer is a small, battery-operated device that measures the exact temperature and humidity level. Place the hygrometer inside the tent and hang the sensor close to the top of the canopy. Suppose you notice that the relative humidity is above 50%. In that case, this is a sign to act quickly and do your best to lower the humidity. If your temperatures are too high, try increasing the distance between the grow lights and the top of the plants.

Tip #5: Clean the tent after every harvest

It's a good idea to establish a habit of giving your grow room a thorough clean after every harvest. Using bleach and hydrogen peroxide to clean the floor, walls, fans, pots, and any other equipment will help create a sterile and pathogen-free environment. Even if you do not have any signs of mould or mildew, it is still best to give the room a thorough cleaning.

My Conclusion

A severe case of mould or powdery mildew can become a devastating loss on harvest day. Not only should you discard the contaminated buds and leaves, but you will also need to sterilise your grow room. Under no circumstances should you ever dry and smoke mouldy buds, even if the damage does not look bad. 

Never make hash or extracts with leaves that have powdery mildew spores; cut your losses and start the grow from scratch. Setting up your indoor grow room properly will reduce the chances of infection and keep the temperature and humidity optimal.