North America Native Plant

Thelotrema Eximium

Botanical name: Thelotrema eximium

USDA symbol: THEX

Habit: lichen

Native status: Native to North America  

Thelotrema eximium: The Mysterious Lichen Living in Your Backyard Have you ever noticed pale, crusty patches on the bark of trees in your yard and wondered what they were? You might be looking at Thelotrema eximium, a fascinating lichen that’s quietly going about its business right under your nose. While ...

Thelotrema eximium: The Mysterious Lichen Living in Your Backyard

Have you ever noticed pale, crusty patches on the bark of trees in your yard and wondered what they were? You might be looking at Thelotrema eximium, a fascinating lichen that’s quietly going about its business right under your nose. While you can’t plant this curious organism like you would a flower or shrub, understanding what it is and why it matters can help you appreciate the hidden biodiversity thriving in your garden.

What Exactly Is Thelotrema eximium?

Let’s clear up any confusion right away: Thelotrema eximium isn’t a plant in the traditional sense. It’s a lichen – a remarkable partnership between a fungus and algae that work together to create something entirely unique. This particular lichen is what scientists call crustose, meaning it forms a thin, crusty layer that’s tightly attached to whatever surface it’s growing on.

Think of lichens as nature’s ultimate roommates – the fungus provides structure and protection, while the algae photosynthesizes to create food for both partners. It’s a win-win arrangement that’s been working for millions of years!

Where You’ll Find This Native Lichen

Thelotrema eximium is native to North America and particularly fond of the eastern regions of the continent. You’ll typically spot it in deciduous and mixed forests, where it makes itself at home on the bark of various tree species. If you have mature trees in your yard – especially in shadier, more humid areas – there’s a good chance this lichen is already part of your local ecosystem.

How to Identify Thelotrema eximium

Spotting this lichen takes a bit of detective work, but once you know what to look for, you’ll start seeing it everywhere:

  • Look for thin, pale gray to whitish patches on tree bark
  • The surface appears crusty or granular, almost like dried paint
  • It forms irregular patches that can vary in size
  • You’ll typically find it on the shaded sides of trees
  • It prefers smooth-barked trees over deeply furrowed bark

Is It Beneficial for Your Garden?

Absolutely! While Thelotrema eximium might not add the flashy color of a flowering plant, it’s working hard behind the scenes to benefit your garden ecosystem:

  • Air quality indicator: Lichens are sensitive to air pollution, so their presence indicates relatively clean air around your property
  • Biodiversity support: Many tiny insects and other creatures use lichens for food and shelter
  • Soil enrichment: As lichens slowly break down, they contribute organic matter to the soil
  • Natural beauty: They add subtle texture and visual interest to tree bark

Can You Grow Thelotrema eximium?

Here’s where things get interesting – you can’t actually plant or cultivate this lichen like you would a traditional garden plant. Lichens are incredibly slow-growing and have very specific environmental requirements that are nearly impossible to replicate artificially. The good news? If you have the right conditions, Thelotrema eximium will likely find its way to your trees naturally.

How to Encourage Lichens in Your Garden

While you can’t plant lichens directly, you can create conditions that welcome them:

  • Maintain mature trees with relatively smooth bark
  • Avoid using harsh chemicals or pesticides near trees
  • Keep some areas of your garden naturally shaded and humid
  • Resist the urge to clean lichen off your trees – they’re not harmful!
  • Plant native trees that lichens naturally prefer

Living in Harmony with Garden Lichens

The beauty of Thelotrema eximium and other lichens is that they’re the ultimate low-maintenance garden residents. They don’t need watering, fertilizing, or pruning. They simply exist, quietly contributing to the health and biodiversity of your outdoor space.

Next time you’re walking through your garden, take a moment to appreciate these remarkable organisms. They’re living proof that some of nature’s most fascinating partnerships are happening right in your backyard, no planting required!

Thelotrema Eximium

Classification

Group

Lichen

Kingdom

Fungi - Fungi

Subkingdom
Superdivision
Division

Ascomycota - Sac fungi

Subdivision
Class

Ascomycetes

Subclass
Order

Graphidales

Family

Thelotremataceae Stizenb.

Genus

Thelotrema Ach. - barnacle lichen

Species

Thelotrema eximium R.C. Harris

Plant data source: USDA, NRCS 2025. The PLANTS Database. https://plants.usda.gov,. 2/25/2025. National Plant Data Team, Greensboro, NC USA