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Identifying bud rot, mold, and root rot on marijuana plants

After growing weed for long enough, at some point you will encounter bud rot, mold, root rot, or all three. It’s important to know how to identify each of these common problems and how to fix them. These issues can stunt your plants and ruin buds, damaging your crop and reducing yields.

Diagnosing and treating bud rot and mold on marijuana plants

There’s nothing worse than going through months of time and labor to grow weed only to discover mold on it. Also called botrytis, or bud rot, mold can be hard to see on your buds. 

What is bud rot?

Bud rot is a type of mold that develops in the dense cores of cannabis buds. It starts on the stem inside of the bud and spreads outward, and it’s difficult to detect in its early stages. It typically occurs on plants in the flowering stage.

bud rot
(Aaron Rogosin for Outer Elements Photography)

After onset, bud rot breaks down the surrounding bud and then produces spores which spread to other areas of the plant or grow space. 

The rot will first appear white and wispy and then turn grey and black as the bud turns mushy and slimy.

Once a bud begins to mold, it is no longer safe for consumption and must be discarded. Growers usually cut out the infected area of the plant and to let it continue to grow. Because it is caused by moisture buildup, you may be able to save the rest of the plant. But be sure to inspect the rest of the plant for it.

However, if the infection is widespread, you may have to cut down the whole plant to protect the rest of your crop.

Mold will commonly occur in outdoor cannabis gardens after a heavy rainfall, especially toward the end of the season when buds are large and dense.

To spot mold in your cured product, keep an eye out when breaking apart buds. You should be able to clearly identify it on the inside of buds. Discard the plant material when you find it.

You’ll also be able to smell mold in buds—it will smell musty, wet, and old, and leave a sour taste in the back of your throat.

What causes bud rot in cannabis plants?

The best way to prevent mold or bud rot on your cannabis plant is to understand the conditions that allow mold to thrive:

  • High humidity
  • Mild temperatures
  • Poor ventilation
  • Dense foliage
  • Dense buds
  • Weak immune systems in plants

How to prevent bud rot and mold in cannabis plants

Molds are fungi that develop in warm, damp, and humid conditions. They develop from spores, which travel through the air, unseen to the naked eye. Mold is actually important in the ecosystem because it breaks down dead plant material, but you don’t want it on your buds.

The only way to prevent bud rot is to make sure the growing environment isn’t too damp or humid. You can invest in a dehumidifier if it’s too humid, and if it’s too hot, you may also need an AC unit.

If you see bud rot developing, you can also harvest plants early, so it doesn’t spread to the entire plant.

Once your plant has bud rot, there’s no way to get rid of it; you have to discard the infected parts of the plant.

Bud rot prevention begins with the type of marijuana strain you select to grow. Sativas, having adapted to humid equatorial regions, tend to grow light, wispy, airy buds. With improved air flow in the buds, sativas often have superior mold resistance. 

Indicas, on the other hand, adapted to the dry mountainous regions of Asia and grow denser buds that are more susceptible to mold if introduced to a humid climate. 

If humidity control is a concern, consider growing a strain that has more airy, mold-resistant buds.

Other ways to reduce the chances of mold in your garden include:

  • Pruning plants
  • Spacing plants properly
  • Watering plants appropriately

Pruning and training plants can also help prevent bud rot. Humidity is higher in dense and crowded grow spaces—think about walking through a dense jungle versus an open forest. By pruning plants effectively, airflow increases, reducing moisture collection. You can also trellis or scrog your plants to spread out branches. 

Plants with healthy immune systems are more likely to fend off mold growth. Growing organically with diverse nutrients can help increase beneficial microbes in the soil and help keep plant immune systems strong and ready to fight infections.

Most important of all, protecting plants from excessive moisture is key in the battle against bud rot. Also, watering plants in the morning can help prevent humidity in the garden when the sun goes down or when lights turn off later in the day. 

Indoor mold prevention

The best way to prevent mold growth is to ensure a consistent climate in your grow room. Fans, dehumidifiers, temperature control systems, and ventilation can keep humidity down, increase airflow, and maintain steady temperatures. This will create a stable environment and increase the overall health of your plants.

Pruning and spacing will increase airflow through your canopy, which helps reduce humidity. If you overwater plants, this will increase the humidity level, as plants will be unable to absorb water, making water evaporate into the air and stay in the room. Giving plants incorrect nutrients can weaken their immune systems and make them more prone to attack from molds.

Outdoor mold prevention

When growing outdoors, you’re more limited in protecting your cannabis garden from mold without the ability to control climate. Appropriate spacing, pruning, and feeding are essential to protecting your crop.

When marijuana plants are in their final homes—either final containers or in the ground—stake, cage, or trellis plants to keep branches spaced apart and to increase airflow. Additionally, pruning bottoms and insides of plants is especially important outdoors, to allow air to flow underneath the canopy.

Growing in a greenhouse outdoors can help protect against bud rot. After a heavy rainfall, it’s beneficial to lightly shake each plant to get the water off. 

Preventing bud rot and mold in the cannabis drying room

Efforts to prevent bud rot do not stop once buds have been harvested. Mold can also occur as cannabis buds dry and cure. Here are a few tips:

  • As you harvest, check colas for rot. If you find mold, discard the infected area while saving the rest.
  • Harvest after a dry spell. If growing outside, it’s ideal to harvest when it hasn’t rained for a few days so buds have a lower moisture content.
  • Wet trim—pull off fan leaves and trim buds before drying. Doing so will reduce the moisture in your drying room and increase airflow to drying buds.
  • Leave space between hanging branches as they dry. You can also place fans in the room to improve airflow and use humidifiers to pull moisture out of the air.
  • Create an optimal drying environment. When drying, a dark space with temperature between 60-70°F and humidity between 50-65%.
  • Adjust drying speed if necessary. Generally speaking, drying is not something you want to rush as THCA is still converting and the chlorophyll is still breaking down. 

A slow drying process is generally associated with a tastier and smoother smoke. However, if you’re finding mold in your harvested cannabis, speeding up the drying time by increasing temperature and decreasing humidity could save the rest of your crop from mold growth.

If you do find bud rot, everyone trimming needs to start looking for mold. Anything that is at all questionable should be set aside to be inspected with more scrutiny.

Mold can be detrimental, but it is also preventable with the right tactics and attention to detail. But don’t get discouraged if a little mold shows up in your garden! Many large-scale outdoor farmers have to assume a percentage of loss due to mold as it is difficult to prevent entirely. 

Remember to check your garden daily, and think about where and when mold will show up. 

Diagnosing and treating powdery mildew on marijuana

powdery mildew on cannabis
(Aaron Rogosin for Outer Elements Photography)

Powdery mildew, or PM, is a common disease that shows up as a white powder on the surface of marijuana leaves and buds while a plant is growing. It can show up on plants in the vegetative or flowering stages.

It usually starts on lower branches, which receive less sun and airflow. Once it appears, it spreads rapidly and can quickly make its way onto buds. Fortunately, because powdery mildew is so visible, it’s rare for a plant to die from it. But it will still make buds unsmokable, and you want to get rid of it as quickly as you can.

It can spread throughout all your plants quickly if nothing is done. PM loves to develop on unhealthy plants, and it will thrive without quality airflow in your grow space.

To an untrained eye, the beginnings of powdery mildew might look like young trichomes. If you suspect it, use a magnifying glass to examine your buds—you’ll see a stark difference between white mold and beautiful trichomes. 

How to prevent and treat powdery mildew on cannabis

  • Spray plants with organic fungicides
  • Prune plants to remove foliage and increase airflow
  • Spray compost tea or solutions with varying pH levels to disrupt the spread of the disease
  • Note which strains are susceptible to PM and grow a different one next time

If your cannabis garden does become infected with powdery mildew, a solution of 3% hydrogen peroxide (H2O2) and water can sterilize harvested plants and remove the mildew. This is a painstaking process, but it can save a crop.

Diagnosing and treating root rot on marijuana

Root rot is a serious issue that can take hold of a marijuana plant. Roots provide a network for collecting and transferring water, oxygen, and nutrients to the rest of the growing plant. When plants are overwatered and lack sufficient drainage, roots essentially drown, can’t take in oxygen, and start to die off. Without developed roots, a marijuana plant won’t grow properly.

Under-watering weed plants can also cause roots to die off. Healthy roots require a balance of water and time to dry out to take in oxygen.

Root rot is nearly impossible to fix and results in nutrient-deficient and stunted plants. You will most likely have to discard the plant entirely if it develops root rot.

In hydroponic systems, you can check roots—if they’re brown in color, slimy, and lack vigor, they may have root rot.

How to prevent root rot in marijuana plants

marijuana plant root rot
Root rot. (NokHoOkNoi/AdobeStock)

You can lessen the risk of root rot with some preventative measures:

  • Have healthy soil with beneficial microbe and bacteria populations. These populations help keep the fungus responsible for root rot under control.
  • Water your plants correctly. This means measuring the amount of water given each plant and observing day to day how plants respond. It’s better to see a plant begin to wilt than to overwater plants.
  • Have breathable soil. Growing in smart pots and adding perlite to soil are two ways to help facilitate oxygen flow and allow soil to drain properly.

How to improve the health of your marijuana plant’s roots

Without a functioning root system, the basic functions of a cannabis plant will fail and make the plant weak and sick. When a plant is young, many gardeners choose to focus on the development of its roots rather than the actual plant to give the plant a strong foundation.

Here are some tips to keep the roots of cannabis plants healthy.

Oxygen

Roots uptake oxygen during cellular respiration, a process in which ATP energy is created. Without oxygen, the plant won’t be able to create energy needed for it and roots to keep growing.

To keep a healthy flow of oxygen into the root system:

  • Let soil dry in between waterings. When soil is saturated, it can’t breathe effectively; as it dries up, roots pull in oxygen. Dry periods between waterings are essential for oxygen absorption.
  • Use a smart pot. These pots are made of a porous material that allows oxygen in through the sides. Plants buried in the ground or in hard pots are limited to intaking oxygen from the surface.
  • Add perlite, vermiculite, and peat moss. These materials help fluff up soil, giving more space for oxygen to flow through and to keep soil from compacting.
  • Use complete soils. These promote life for fungus, insects, and microbes that help keep soil loose, allowing for the development of root networks throughout soil.

In hydroponic systems, make sure your roots are always submerged in water, and measure dissolved oxygen with a meter to make sure plants are receiving enough.

When growing outdoors, if tilling soil, make sure to keep off the soil as much as possible to prevent it from compacting, which prevents oxygen flow.

Warmth

Roots are active at night and happiest around 75°F (24°C). With an indoor setup, a good tip is to set up air intake below the canopy and exhaust above the canopy to keep a fresh, healthy cycle of air.

If growing outdoors, growing a cover crop—like clover—with cannabis can keep soil temperature more consistent while also protecting the topsoil layer.

Spreading mulch or hay around the base of plants can also help regulate temperature, but be sure to keep a 6-inch radius around the stalk of each plant free, as excessive moisture in mulch can lead to rot if it touches the stalk.

Proper watering

Roots are constantly on the hunt for water as they grow and move farther away from the main taproot. As a plant gets bigger, so should the watering radius—the area around the stalk of the plant that you water. Doing this will help guide roots to the edges of the pot as they seek available nutrients in soil.

But watering too far away from where roots currently are can create standing water, which can lead to root rot, mold, and pest issues.

Topsoil should be sufficiently dry before watering the plant again. To see if a plant is ready for more water, feel for dryness by putting your finger 1-2 inches down.

Choose the right container

Roots need plenty of space as they grow out and explore for water, oxygen, and nutrients. When cramped, a plant’s root system can strangle itself, becoming rootbound. Roots can begin to die off and root rot can set in, possibly killing the plant entirely.

Be sure to put a plant in a big enough pot, anticipating how big it’ll get by harvest time. If a plant gets too big for its original pot, transplant it to a bigger one, adding more soil. If planting in the ground, it’s even more important to give it plenty of space, as you won’t be able to move it.

To see if a pot is too small, observe the roots from the drainage holes—if roots are crossing over the holes, it’s time to transplant to a bigger pot.

Be sure to check roots before flowering, as the plant shouldn’t be moved or repotted during that time.

Give your roots some friends

Plant roots interact directly with mycelium, the substance mushrooms sprout from. Mycelium helps make nutrients available to plant roots and helps roots find water. In exchange, a plant sends down carbon, helping mycelium grow and expand its own network.

This symbiotic relationship—called mycorrhiza—is important in keeping roots healthy so they can access all available nutrients.

Complete soils contain mycelium and are a good way to start using it. There are also mycorrhizal powders that can be added to soil when potting plants.

Compost teas will also help keep soil full of beneficial insects, microbes, and bacteria that in turn will help keep nutrients available for roots.

Next time you repot plants, take a moment to observe its roots and see if they need attention. A healthy root system is critical to growing a healthy cannabis plant, and buds you’ll be proud to harvest.


Trevor Hennings contributed to this article.

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