Guides, Hemp Farming

How To Identify, Prevent, and Treat Mold on Hemp Plants

Although hemp is an exceptionally hardy and resilient species, it is not impervious to disease. Mold, in particular, is a common problem that both indoor and outdoor growers face. It can quickly take hold in the garden, destroying crops before you have time to react. Therefore, it’s critical that farmers learn how to identify, treat, and prevent mold on hemp plants before it’s too late.

What Is Mold?

Mold is a fungal structure that appears where water and oxygen are present. It grows rapidly, forming spore colonies on moist surfaces within just one to two days.

Mold spreads by releasing tiny, lightweight spores into the air. These spores can stay airborne indefinitely, infecting nearby hosts, including plants, wood products, and other organic matter. Mold spores can remain viable for years; however, even when they die, they can still pose risks to humans and other animals.

There are more than 100,000 types of mold found outdoors and indoors. Some molds are harmless to humans and animals, but others can pose serious health risks. Some of the allergenic, pathogenic, and toxigenic effects of molds include:

  • Congestion
  • Throat irritation
  • Skin irritation
  • Dizziness
  • Lung infections

Types of Mold on Hemp Plants

Gray Mold

Gray mold on hemp budGray mold, also known as botrytis or bud rot, is one of the most common molds found on hemp and cannabis. It tends to infect plants during the late flowering period but can also spread to drying buds after harvest. Gray mold will quickly destroy hemp plants, making the flowers unsafe for consumption.

This type of necrotrophic fungus is hard to spot, as it usually infects the inside of hemp flowers, spreading outward over the course of several days. It often appears as fuzzy, brown, or grey spots on the flowers of hemp plants. Other signs include fragile, brown stems, dying leaves, and stunted growth. The easiest way to identify gray mold is to break apart the buds and look for discoloration or fuzzy growth.

Powdery Mildew

Powdery mildew on hemp leafPowdery mildew, also known as white mold, is another common pathogen that infects indoor and outdoor hemp plants. This parasitic fungus thrives on living plant tissue, obtaining nutrients from the host via intracellular feeding structures. Because of its parasitic nature, this type of mold can kill a healthy hemp plant quickly, attacking the leaves, stems, and buds.

Powdery mildew is characterized by dusty, white, flour-like patches on the tops or undersides of leaves. It may also cause abnormal leaf development and yellow or brown stems. New leaves are likely to be infected, but older growth can also become colonized.


Fusarium wilt in hemp plantFusarium is one of the most threatening pathogens to hemp, as it is challenging to identify and can swiftly destroy plants. This soil-borne mold can live underground for years, attacking plants when the soil temperature rises in the spring and summer.

Fusarium infects the roots of hemp plants, diminishing their ability to uptake water and nutrients and causing wilting. As the infection spreads, it damages the stems and leaves, eventually killing the plant. By the time symptoms become noticeable, the mold has likely already caused irreparable damage to the plants.

What Causes Mold Growth on Hemp?

Mold growth is entirely dependent on the environmental conditions of the area. Most types of molds favor these conditions:

  • Moderate temperatures
  • High humidity
  • Lack of airflow
  • Dense foliage
  • Weak or young plants

How To Prevent Mold on Hemp Plants

Once mold has infected a plant, stopping the outbreak from spreading is very difficult. Therefore, preventing mold infections before they take hold in the garden is essential. Fortunately, there are many steps farmers can take to deter fungal growth in outdoor and indoor gardens.

Outdoor Mold Prevention

  • Grow fast-flowering, mold-resistant hemp strains that will reach maturity before autumn rainstorms.
  • Plant in direct sunlight to stop spores from germinating on leaves and flowers.
  • Give your plants plenty of room to encourage airflow and slow the spread of infections.
  • Prevent weeds from growing near the plants, as they can spread disease.
  • Use well-draining soil and allow it to dry out before each watering.
  • Opt for a drip irrigation system rather than a pivot irrigation system, which will prevent the leaves and buds from becoming wet.
  • If heavy rain is expected, cover the crops with a tarp to keep the flowers dry.
  • Use outdoor fans to dry the plants after a rainstorm.
  • Prop up sagging branches to keep them from touching the ground.
  • Sterilize farming equipment to prevent fungal infections.
  • Inspect the plants daily to spot signs of mold outbreaks.

Indoor Mold Prevention

  • Grow mold-resistant hemp strains that can thrive in humid climates.
  • Calculate the vapor pressure deficit (VPD) and monitor the temperature and humidity of your grow room.
  • Use an air conditioner or heater to keep temperatures between 68 and 77 degrees Fahrenheit.
  • Use a dehumidifier to lower the humidity level in the garden, especially during the late flowering period.
  • Space the plants at least six inches apart and set up fans to blow gently across the entire canopy. Airflow will prevent mold from attaching to the plants.
  • Use high-efficiency particulate air (HEPA) filters to trap airborne fungal spores.
  • Lightly defoliate the plants during the vegetative growth period to encourage airflow and light penetration.
  • Use a powerful grow light that covers the entire canopy.
  • Sterilize shears, trowels, and other tools to prevent infections.
  • Examine the plants every day to spot infections.

How To Treat Mold on Hemp Plants

Once you have identified a fungal outbreak in the garden, you must act quickly to stop it from spreading. Because some types of fungal infections are systemic, treating them can be very difficult. What type of treatment you use depends on what kind of mold has infected your plants.

In most cases, the best way to get rid of mold is to cut off any infected parts of the plant or remove the infected plant from the garden. You’ll have to either burn the infected plant matter or dispose of it far away from your garden to prevent it from spreading. Also, don’t forget to wash your hands and clothing after handling infected plants.

If your plants are still in the vegetative growth period, you can spray organic fungicides such as neem oil or baking soda solutions on the leaves and stems of the plants. These will help slow mold growth but may not eliminate it entirely. Just be sure to avoid spraying anything onto the plants during the flowering period, as the buds can become contaminated.

How To Identify, Prevent, and Treat Mold on Hemp Plants: Conclusion

Mold is a very common plant pathogen that most hemp farmers will encounter at some point. Even the most experienced growers lose plants to fungal outbreaks from time to time. While there is no effective way to stop mold from entering your garden, there are plenty of steps you can take to minimize the risk of it spreading. By following the tips outlined in this article, you can identify, treat, and prevent mold infections before they ruin your harvest.

What are your tips for treating mold on hemp plants? Please share your advice in the comment section below. While you’re here, shop our collection of premium feminized hemp seeds.

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