Is Lichen on Trees Bad? Treating Trees with Lichen

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If you’ve noticed a frilly, greenish-gray growth on your tree’s bark, it may have one of the many types of lichens. 

Many people blame this organism for a tree’s failing health, but lichen is not a parasite and merely affects your tree’s appearance. You can treat trees with lichen manually and chemically.

lichen on tree trunkPin

Lichens are a complex life form that is a symbiotic partnership of two separate organisms, a fungus and an alga. 

Lichens of North America and all over the world are found. They are found in a vast diversity of habitats and climates, from the Sonoran desert in the Coronado National Forest, 

There are at least 13,000 species of lichens living throughout the world. Lichen species are so numerous and diverse that there are individual exceptions to most general statements about them. 



What Is Lichen on Trees?

Lichens may look like moss, but they differ in that they have a mycobiont and a photobiont living in symbiosis. Mycobionts include fungi, while photobionts can be green alga or cyanobacteria. 

Photobionts photosynthesize to obtain nutrients, meaning that lichens do not affect a tree’s health. You can find them growing on nutrient-free rocks as well.

Lichens are found worldwide and occur in a variety of environmental conditions. A diverse group of organisms, they can colonize a wide range of surfaces and are frequently found on tree bark, exposed rock, and as a part of biological soil crust.

Wind spreads lichens by blowing their reproductive structures around. These lightweight organisms can settle on any still object, like a brick wall, tree, or rock. They prefer slow-growing trees like oaks because their surfaces take a long time to change.

People associate lichen with the failing health of trees because it often appears on stressed plants. 

These trees may become stressed from insufficient moisture, sunlight, soil, planting depth, or insect infestation. Lichens can appear on trees of any health level, but they become more apparent on ones that look sick.

They are also necessary for the survival of the ecosystem around them, such as partnering with plants and trees for nutrients and survival. 

Lichen Body Type

There are three main lichen body types: crustose, fruticose, and foliose. 

Lichens that form a crustlike covering that is thin and tightly bound to the substrate are called crustose. 

Squamulose lichens are small and leafy with loose attachments to the substrate and are usually considered to be a special type of crustose lichen. 

Foliose lichens are large and leafy, reaching diameters of several feet in some species, and are usually attached to the substrate by their large platelike thalli at the center.  

Fruticose lichens can be hanging or upright and may be hairlike, cuplike, or shrubby in appearance. 

In addition to their morphological forms, lichen thalli are also classified by the ratio of phycobiont cells (i.e., cells of the photosynthetic partner) to mycobiont cells (i.e., cells of the fungus).

What Damage Does Lichen on Trees Cause?

Lichens do little damage to trees alone; however, they can spread to the tree’s leaves and interfere with its photosynthesis.

If the lichens grow too thick, they can weigh down a tree’s branches or stems and cause them to break. Nevertheless, lichen is not a parasite, so it is completely safe and even beneficial in small amounts.

A lichen is not a single organism. Rather, it is a symbiosis between different organisms – a fungus and an alga or cyanobacterium. Cyanobacteria are sometimes still referred to as ‘blue-green algae,’ though they are quite distinct from the algae. 

Although cyanobacteria are called blue-green algae, they are actually bacteria and are part of the bacteria kingdom, Monera.

The “blue” in the common name refers to the fact that they need to live in water, and “green algae” refers to their photosynthetic abilities, like green algae.

Fungi do not contain chlorophyll or any other means of producing their own food, so they rely on other organisms for nutrition. Fungi are widely known for their role in the decomposition of organic matter. 

Lichen on trees can house beneficial lacewing larvae. These critters eat dangerous pests and camouflage themselves in lichens. Some small animals and birds use lichen for their nests. Squirrels use it as a food source.

About 20 species in the tropical and temperate rainforests are Basidiomycetes, the “mushrooms.” About 40 genera of algae and cyanobacteria are found in lichen partnerships. 

Different lichen species can grow on many types of surfaces, including tree bark, dead wood, bare rock, cleared soil, rusty metal, animal bones, glass, plastic, and cloth. 

Some lichens can grow on many types of surfaces, while others are confined to specific types of trees or rocks. 

How to Control Lichen on Trees

You can remove lichen manually with potassium spray, baking soda, or copper sulfate. The chemical methods may harm the tree, so try to remove them by hand first.

Manual Extraction

Manual extraction is the safest way to remove lichen. You will need a wooden or plastic scraper that isn’t so hard that it will harm the tree but can still scrape off the lichens. Try to scrape the tree bark gently to remove the lichens.

Lichen reproductive parts containing both algal and fungal cells may occur asexually for dispersal. In vegetative reproduction, any fragment or shred of lichen containing both the algal and fungal components that break off the original can form a new lichen body.

You will likely see healthy tree bark under the lichen once you have removed the excess vegetation. The best time of year to manually extract lichens is in spring or when the tree is dormant, as they will separate easily, and the trunk can heal faster.

The best type of lichens to remove manually hang from the branches and have a hair- or moss-like look. If they appear loose enough, you can lift them off by hand. Make sure to do it gently without jerking the tree to prevent damage.

The algal partner can usually live outside of the lichen in streams, ponds, or wet soil. The fungal partner, however, has become dependent on the algae for its food and cannot live off of decaying organisms like other fungi. 

Some scientists think the relationship between lichenized fungi and algae is actually a controlled form of parasitism. 

Potassium Spray

Another relatively safe way to remove lichens from trees involves spraying them with potassium. This element is a contact killer that will eliminate the lichens soon after spraying.

All of the sprayed lichens should die within a few days, and some trees will clear within a few hours. Since potassium is a root fertilizer, it will not harm your tree. Any excess may fall to the roots and benefit your plant.

Keep in mind that some commercial potassium sprays may damage your tree as the manufacturers designed them for patio use. Make sure to check the usage and try to get the purest product possible to avoid pollution.

Baking Soda

The safest chemical solution to treat trees with lichen is baking soda. This treatment is high in salt, so you should not use it on trees with new growth. Baking soda is another powerful contact killer. Before spraying your plants with baking soda, try to remove as much lichen as possible manually and use it sparingly.

Copper Sulfate

The final lichen treatment is the most dangerous. Copper is an antifungal treatment employed in many dry fertilizers.

This systemic spray takes the most time but should kill most lichen. While effective, copper sprays can damage tree growth. By spraying your trees late in their sprouting season or before the next one begins, you can prevent them from damage.

Check that the spray you buy works for plants. Most copper sulfate sprays will kill other bacterial and fungal diseases. When used correctly, they make an excellent addition to your plant care regimen.

Prevention

You can prevent lichen from growing on your tree by keeping it healthy. Make sure to provide your plant with enough sunlight, water, and fertilizer.

Try to grow it in its native soil type and prune it to maintain its appearance. While you cannot prevent the wind from blowing lichen onto your tree, you can minimize its appearance by keeping your tree healthy.

Lichen management for aesthetic purposes can be accomplished, but if lichens are on a living plant, their removal could damage the plant and create wound sites where plant pathogens could enter and cause disease symptoms to develop.

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