What is Rim Lichen and Should You Be Concerned About It in Your Garden?
If you’ve noticed grayish-green crusty patches spreading across tree bark in your yard, you might be looking at rim lichen (Lecanora conizaeoides). Don’t panic – this isn’t a disease harming your trees! Rim lichen is actually a fascinating composite organism that’s part fungus, part algae, living together in perfect harmony. But before you get too excited about this natural wonder, there are a few things every gardener should know about this particular species.
The Not-So-Native Newcomer
Here’s where things get interesting: rim lichen isn’t actually native to North America. This European hitchhiker has made itself quite at home across several states and provinces, including British Columbia, New Brunswick, Nova Scotia, Illinois, Massachusetts, Oregon, Washington, and Newfoundland. While it reproduces and spreads on its own without any help from humans, it’s definitely not an original resident of our ecosystems.
What Does Rim Lichen Look Like?
Identifying rim lichen is pretty straightforward once you know what to look for:
- Gray-green to yellowish crusty patches that form circular or irregular shapes
- Grows directly on tree bark, rocks, or other hard surfaces
- Has a somewhat powdery or granular texture
- Often appears in areas with some air pollution (it’s surprisingly tolerant of urban conditions)
- Forms thin, flat crusts that can spread several inches across
The Air Quality Detective
One of the most intriguing things about rim lichen is its relationship with air pollution. Unlike many lichens that flee at the first whiff of smog, Lecanora conizaeoides actually thrives in slightly polluted environments. In fact, scientists sometimes use it as an indicator species – if you see lots of rim lichen around, it might suggest your local air quality isn’t pristine. It’s like having a natural air quality monitor growing on your trees!
Is Rim Lichen Good or Bad for Your Garden?
The short answer? It’s mostly neutral. Rim lichen won’t hurt your trees or plants – lichens don’t parasitize their hosts, they just use them as a place to set up shop. However, since it’s not native, it’s not providing the same ecological benefits that our native lichen species would offer to local wildlife and ecosystems.
From a garden perspective, rim lichen:
- Doesn’t damage trees or structures it grows on
- Can indicate environmental conditions in your area
- Adds a bit of natural texture and interest to bark and surfaces
- Requires absolutely zero maintenance (because you can’t really manage it anyway!)
Can You Remove or Control Rim Lichen?
While you technically can scrape lichens off surfaces, it’s generally not necessary or recommended. They’re not harming anything, and they’ll likely return anyway if conditions are right. Plus, trying to remove all lichens from your trees would be a never-ending battle that’s probably not the best use of your gardening energy.
If you’re concerned about the abundance of non-native species in your landscape, focus your efforts on supporting native plants and creating habitat for native wildlife instead. These actions will have a much bigger positive impact on your local ecosystem.
The Bottom Line
Rim lichen is one of those garden inhabitants that simply exists alongside everything else. It’s not going to make or break your landscape design, and it’s not something you need to actively encourage or discourage. Think of it as part of the complex web of organisms that make up any outdoor space – some native, some not, but all finding their own way to coexist.
If you’re fascinated by the natural world happening right in your backyard, rim lichen offers a perfect opportunity to observe how non-flowering organisms colonize and spread in urban and suburban environments. Just remember that while it’s interesting to observe, supporting native species in other areas of your garden will provide the most benefit to local wildlife and ecosystems.
