Mold growth on a Tree Trunk: How to Prevent and Treat?

There are many things that can affect a tree during its lifetime. One of these things is mold growth.

Although mold is caused by fungi there are many different types of fungi that can cause these problems. You may see black mold, green mold, white mold, or even red mold growing on the trunk of your tree.

Depending on the type of tree, surrounding habitat, and other conditions like climate, water, and sunlight, the trees may experience one or more of these problems.

mold lichen

In most cases, mold is not harmful to the tree to be concerned about. Most mold on trees is just a symbiotic relationship between the tree and the fungi. The tree gets nutrients from the fungi and the fungi get a place to live.

To give an example, the redwood tree and its partner, the humus fungus, are an example of a symbiotic relationship between a tree and mold.

Although we don’t exactly know why this symbiotic relationship exists, scientists believe it’s because the humus fungus assists the redwood tree in obtaining nitrogen.

Because trees can’t move around to find new sources of nitrogen, the humus fungus helps to provide them with what they need.

However, sometimes the mold can be harmful to the tree. Not all molds represent a symbiotic relationship.

There are some molds, like the black mold Stachybotrys chartarum, that can be harmful to both humans and trees.

Also, sometimes something good for the tree can turn bad. For example, a mold that is helpful to the tree in one stage of its life may become harmful in another stage.

Similarly, if the relationship gets out of balance, it can harm the tree.

We know mold is typically caused by fungi and they are opportunistic organisms. This means that they will take over an area if the conditions are right and there is nothing stopping them.

Also, in some cases what you observe as mold may not be fungi but a lichen.

Lichens are a sophisticated life form that is the result of a symbiotic relationship between two different creatures, a fungus, and an alga. Therefore, they aren’t exactly mold, but they can look like it.

Before we delve any further if you want to get rid of the mold on your tree, there will be no “one size fits all” solution that will work for every tree and every type of mold.

Every tree is different and what you may observe as “green mold” on your neighbor’s tree may in fact be something entirely different.

Therefore, before you go out and begin treating your tree with some sort of fungicide, it is important to have a basic understanding of what you’re dealing with.

If unsure, contact a certified arborist or tree care professional in your area. They can examine the tree to see what type of organism developing on the trunk of your tree. and tell whether it is harmful.

Now that we’ve got that out of the way, let’s take a look at how to identify, prevent and treat mold on a tree trunk.

Why do mold and mildew grow on trees?

Molds are fungi that grow in moist, humid environments. They can grow on any organic material, including leaves, wood, paper, and even clothing, in addition to trees.

They are an essential ecosystem organism that decomposes organic materials. They exchange nutrients between trees and aid in the breakdown of deadwood, releasing nutrients back into the soil.

They use the tree trunk as hosts because they are attracted to the moisture and nutrients in the bark. They do this by sending out filaments called hyphae, which penetrate the tree’s bark.

Once the hyphae have penetrated the bark, they begin to grow and reproduce. As they grow, they spread out and begin to cover the trunk of the tree.

The reason you see mold as a “mossy” growth on the tree is that the hyphae are producing spores.

These spores are released into the air and can be transported to other trees or surfaces, where they will grow if the conditions are right.

What does mold need to grow?

Mold needs four things to grow:

  • Moisture
  • Organic matter
  • Oxygen
  • Proper temperature

Moisture: Mold and mildew need moisture to grow. They get this moisture from the air, rain, or dew. In fact, most molds will only grow if the humidity is above 60%.

Organic matter: Mold needs organic matter to grow. This can be leaves, wood, bark, or any other type of organic material.

Oxygen: Although mold typically grows in damp, humid environments, it still needs oxygen to grow. Because they are fungi, they need some oxygen to break down the organic matter that they are feeding on.

Proper temperature: Mold needs the proper temperature to grow. The optimum temperature for most mold growth is between 77 and 86 degrees Fahrenheit.

Mold thrives in warm, humid environments where there is plenty of organic matter for it to feed on.

Once the environment has all of the necessary conditions for mold growth, it reproduces by releasing spores into the air.

These spores are invisible to the naked eye and can travel long distances before landing on a suitable host. Once they land on a host, they begin to grow and reproduce.

Do mold and mildew cause any harm to trees?

Mold spores are typically aren’t harmful to trees. But if there are people and animals in the vicinity that are allergic to mold, it can cause respiratory problems.

That said, what is observed as “mold” on a tree can be other organisms that are harmful.

Also, if mold growth overtakes a large portion of the tree’s trunk, it can cause the tree to become structurally unsound. This is because the hyphae that mold uses to penetrate the bark can also damage the tree’s vascular system.

A tree with a large amount of mold growth may eventually die if the mold continues to spread and the tree is unable to get the nutrients it needs.

Here are a few potential problems that can be caused by tree trunk mold:

Weakened tree defense

Some types of mold can break down the tree’s natural defenses. The trunk with the fungi growing on it is more susceptible to outside elements like harsh weather, pests, and diseases.

Because mold creates small openings in the bark, it allows water and insects easy access to the tree’s inner layers.

Increased susceptibility to pests and diseases

Mold and mildew can also make trees more susceptible to pests and diseases.

The fungi growing on the tree trunk can provide entry points for pests and diseases. Pests and diseases can then spread throughout the tree, causing further damage.

Interference with the tree’s ability to photosynthesize

If a tree is covered in the mold it can affect the tree’s ability to photosynthesize.

This is because the mold progress from the trunk of the tree and covers the leaves. When the leaves are covered, they are not able to effectively get the sunlight they need to photosynthesize.

If the tree cannot photosynthesize, it will not be able to produce the food it needs to survive. This can lead to reduced growth and even death in extreme cases.

Increased fruit or flower production

In some cases, mold cover on a tree can actually increase fruit or flower production. The increased production is due to the fact that the mold and mildew are providing extra nutrients for the tree.

However, increased production is not always a good thing. In fact, it damages trees by taxing their energy and resources.

Poor root growth

The mold on the tree trunk can also spread to the roots, which can cause poor root growth.

The roots are responsible for anchoring the tree in the ground and taking in water and nutrients. If they are not able to do their job properly, the tree can become unstable and may eventually fall over.

Reduced wood quality

Mold and mildew can also reduce the quality of the wood. This is because the fungi break down the cellulose and lignin in the wood, which makes it weaker. The infested wood is more likely to break and can be less valuable.

Increased frost damage

Mold growth can increase frost damage. Because the fungi growing on the tree trunk can absorb water from the air, which leads to an increased risk of frost damage.

Reduces gas exchange

Mold growth can also reduce gas exchange. The fungi growing on the tree trunk can block the pores, which limits the amount of carbon dioxide and oxygen that can be exchanged.

This can lead to problems with photosynthesis and respiration.

Allergies

If you are allergic to mold, you may experience symptoms such as a runny nose, watery eyes, sneezing, and coughing when you are near the trees.

This can be a concern especially if you need to closely work around the tree, such as trimming branches or removing leaves.

Also, if you have other animals, such as dogs or cats, they may also be allergic to the mold and could experience similar symptoms.

As you can see, there are many potential problems that can be caused by the mold that grows on tree trunks.

What are the most common types of mold and mildew found on trees?

There are many different types of mold but some of the most common ones found on trees are:

Sooty Mold (Black Mold)

Sooty mold is a type of fungi that grows on the honeydew secreted by insects.

Honeydew is a sugary substance that insects secrete. Sooty mold gets its name from the fact that it looks like soot or coal. It can cause the tree bark to turn completely black if not properly controlled.

The problem with sooty mold isn’t necessarily the mold itself, but the insects that secrete the honeydew. These sap-sucking insects will have a negative impact on your trees if they are not treated.

Therefore, it’s important to act proactively if you see sooty mold on your trees.

Although black discoloration is the primary symptom of sooty mold, other symptoms include stunted plant growth, early leaf drop, honeydew residue, and tree bugs sucking up sap.

Here are is the best way to get rid of it:

The best way to get rid of sooty mold is to remove the insects that are secreting the honeydew. This can be done with a variety of methods, including:

  • Insecticidal soap
  • Horticultural oil
  • Neem oil
  • Diatomaceous earth

After you’ve removed the insects, you can then focus on removing the sooty mold. This can be done with a pressure washer applying high-pressure water to the affected area.

Once you apply pressure water don’t point the waterjet in a way that swipes the sooty mold off the tree and onto other areas of your tree as this will just spread the problem.

Instead, move the waterjet in a way that lifts the sooty mold off the tree.

You can also use a garden hose with a nozzle attachment to remove the sooty mold. Simply point the nozzle at the sooty mold and turn it on to a high setting. Move the hose in a way that lifts the sooty mold off the tree.

In some cases, you may need to apply some elbow grease and scrub the sooty mold off.

Never touch or breathe in the sooty mold as it can be harmful to your health. Wear gloves, a mask, and protective clothing when handling sooty mold.

Once you’ve removed the insects and the sooty mold, you can then use a preventative measure to keep it from coming back.

This can be done by applying above listed products at least once a week for a month. The mold may look like it’s gone but it can still be lurking in the tree. Therefore, doing multiple treatments will ensure that the mold is gone for good.

After that, although you can stop using the preventative measure, it’s always a good idea to keep an eye out for any new insects or mold growth.

Lichens (Green Mold)

Lichens are one of the most common and diverse types of organisms that consist of a fungus and algae living together.

Lichens on tree

Lichens can be found in all types of climates and ecosystems and come in many different shapes, sizes, and colors.

Most lichens are harmless and can actually be beneficial to your trees as they help with nitrogen fixation. If you see lichen growing on your trees, there’s no need to worry as it’s not causing any harm.

However, sometimes lichens can become a problem if they start to cover a large area of the tree.

This can cause the tree to become stressed and affect its growth. If you see lichens starting to cover a large area of your tree, you can remove them with a garden hose or a pressure washer.

You typically don’t need to use any chemicals to remove lichens as they are not harmful to your trees. Simply washing them off with water will suffice.

If you don’t want to periodically remove lichens, you can apply a preventative measure such as zinc sulfate or copper sulfate.

These products will help to prevent lichens from growing on your trees as they create an unfavorable environment for the lichens.

Lichens are extremely sensitive to dry conditions and will eventually die off if the area around your tree becomes too dry.

The lichen problem your tree may have can be related to low sun exposure caused by other trees or buildings that are shading your tree.

If this is the case, you can try to increase the amount of sunlight your tree gets by trimming back other trees or removing obstacles that are blocking the sun.

There are a number of ways to remove lichens from trees. You can prune them, powerwash them, or treat them with an herbicide. However, a lot of times you should just leave them be as they are not causing any harm other than being an eyesore.

There is no permanent solution to prevent lichens from growing on your trees.

Even if you remove them, they will eventually come back. The best you can do is to periodically remove them and create an unfavorable environment for them to grow.

Most anti-moss chemicals or lichenicides work by killing the algae in the lichen.

This will cause the lichen to turn brown and eventually die off. These chemicals are available on the market in both liquid and granular forms. They are typically applied at the base of the tree.

Some people also use home remedies, such as vinegar or saltwater, to kill lichens. However, these home remedies are not as effective as the chemicals and can actually damage your tree if used in large amounts.

White Mold

The white mold you see on the tree trunk is typically another type of lichen and is very similar to the green mold we have just mentioned. Lichens can be in many different colors, not just green.

white lichen on trunk

While white mold is not harmful to your tree, it can be unsightly. If you want to remove white mold from your tree, you can use the same methods as you would remove green mold.

As per all types of mold issues, identifying the type of mold is the first and most important step. Because the problem could be not white lichen, but powdery mildew or some other type of tree fungus.

While not all molds are dangerous, some types of mold can be harmful to humans and animals. If you have concerns about white spots on your tree, it is best to have a professional tree service come and take a look.

Red / Pink Mold

The red/pink mold growth on the tree is frequently lichen, which is a mix of algae and fungi as with the green and white mold. The red/pink color of algae is due to a high concentration of carotenoids in the algae.

red lichen on trunk

This type of lichen is not harmful to your tree and can actually be beneficial as it helps the tree to retain moisture.

If you don’t like the look of red/pink mold on your tree, you can remove it using the same methods as you would remove green or white mold.

What parts of the trunk are most susceptible to mold?

The lower trunk is the most susceptible to mold and mildew because it is closer to the ground and often has more moisture. The upper trunk is less likely to be affected because it is further from the ground and has better airflow.

Also, the side of the trunk that is facing the ground is more likely to be affected than the side that is facing up. This is because the side facing the ground is less likely to receive direct sunlight, which can help to prevent mold growth.

The north side of the trunk is also more susceptible to mold because it is usually cooler and has less sunlight.

Obviously, this is the only case for the trees in the northern hemisphere. For trees in the southern hemisphere, the south-looking side of the tree will have less sunlight and be more susceptible to mold.

How can you tell if your tree has mold?

There are a few ways to tell if your tree has mold or mildew. One way is to look for a powdery substance on the bark of the tree.

This substance is called mycelium, and it is the part of the mold or mildew that is responsible for sending out hyphae.

Another way to tell if your tree has mold is to look for black, green, or brown spots on the bark of the tree. These spots are called fruiting bodies, and they are where the mold produces spores.

Spores are like seeds, and they are how mold reproduces. When spores land on a suitable host, they germinate and begin to grow.

How do you prevent mold and mildew growth on trees?

As we have mentioned earlier, mold growth is typically not a health concern for trees. However, some people don’t like the way mold looks on their trees. If you want to prevent mold growth, there are a few things you can do.

Give tree enough space

Trees planted too close together are more susceptible to mold because they cannot get enough sunlight. When trees are close together, they also create a microclimate that is more conducive to mold growth.

To prevent mold growth, make sure to plant trees at least 20 feet apart. This will give them enough room to grow and receive the sunlight they need.

Prune trees regularly

Pruning trees regularly will also help to prevent mold growth. This is because pruning removes dead and dying leaves and branches, which can be hosts for mold spores.

In addition, pruning also increases the sun exposure together with air circulation to the tree canopy, both of which will help to prevent mold growth.

Remove weed around trees

Weeds compete with trees for water, nutrients, and sunlight. When weeds are present, they can create a microclimate that is more conducive to mold growth. Removing weeds will help to prevent mold growth on trees.

Increase sun exposure

Another thing you can do is to make sure that the tree has good sun exposure. Sunlight helps to keep the tree’s bark dry, which prevents mold and mildew from growing.

If you have a tree that is in shade most of the time, you may want to consider moving it to a location that gets more sun.

If you cannot move the tree, you can try pruning some of the branches to allow more sunlight to reach the bark. A lot of times spread-out branches block sunlight from getting to the trunk of the tree.

Proper watering

Another important thing you can do to help prevent mold and mildew growth on your tree is to water it properly.

Overwatering can actually be just as bad as not watering at all because it can create the damp, humid conditions that mold and mildew need to grow.

The best way to water your tree will depend on the type of tree you have. If you are not sure how often and how much you should water your tree, you can consult a professional arborist.

Paint the trunk

Painting the trunk of the trees can also help to prevent mold and mildew growth. This is because paint creates a barrier between the tree’s bark and the air, which can help to keep the bark dry.

That said you cannot use any type of paint on your tree. You will need to use special tree paint that is non-toxic and won’t harm the tree. The paint is also expected to last for several years before it needs to be reapplied.

The best types of paint to use are white latex paint or tree tar. You can find these at most hardware stores. If you do paint the trunk of your tree, be sure to repaint it every few years to ensure that the paint barrier is still effective.

Remove dead leaves and branches

Another way to help prevent mold and mildew growth on your tree is to remove dead leaves and branches.

Dead leaves and branches can create the damp, humid conditions that mold and mildew need to grow, so it is important to remove them from the tree. You can remove dead leaves and branches by hand or with a pruning tool.

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