What Is a Lichen? Fungus plus Alga on Bark and Rock
A lichen is a fungus plus an alga living together as one organism. University of Maryland Extension says the pair lives in a symbiotic relationship that helps both partners.
The short answer
A lichen is a partnership between a fungus plus a green alga or cyanobacteria. Penn State says the alga makes food through photosynthesis while the fungus takes up water. Lichen has no roots or leaves. It grows on bark, rock plus fences. NC State says lichens do not harm plants plus control is not needed.
How does a lichen live?
Penn State says lichens have no roots, stems, leaves or vascular system. They get water from rain plus fog plus can absorb water vapor from the air. In dry spells they cannot hold water and go dormant. Once wet, they grow again. The fungus absorbs water while the alga makes carbohydrates that the fungus uses to grow. Penn State adds that lichens need clean fresh air.
| Option | What it does | Source |
|---|---|---|
| Fungus | Takes up water | Do lichens cause harm to trees? |
| Alga | Makes food | Do lichens cause harm to trees? |
| Dry spell | Goes dormant | Do lichens cause harm to trees? |
What forms do lichens take?
Maryland Extension names three forms. Crustose lichen is flat plus crusty. Foliose lichen has raised leaf like lobes. Fruticose lichen has branched growth with fingers or threads. New Mexico State says colors include green, blue green, yellow green, brown, gray or even red. Maryland adds that lichens grow on trunks, rocks plus fences, even in Antarctica.
Crustose: flat plus crusty. Foliose: leaf like lobes. Fruticose: branched threads.
Why is lichen on older or slow trees?
Maryland says lichen rarely forms on fast growing trees, likely because they shed bark before lichen can spread. Mature or less vigorous trees often carry more. NC State says stressed plants grow slower plus shed bark less often. Canopy dieback also lets more sun reach the trunk. Both sources say lichen does not cause the decline.
Fast growth: sheds bark. Slow growth: more lichen. Thin canopy: more sun.
Troubleshooting
Gray green crust on a trunk? NC State says it is likely lichen.
Worried about the tree? New Mexico State says lichens do not harm trees.
Lichen spreading on a slow tree? Look at site conditions per NC State.
Kind Seed Co shares these plant notes for growers of all kinds. For more on site conditions see growing cannabis near trees and cannabis leaf anatomy. Read companion plants when growing marijuana for garden pairings. See mulch for cannabis for bed cover.
Sources
- Lichen, algae, and moss on trees, University of Maryland Extension
- Do lichens cause harm to trees?, Penn State Extension
- FAQ: lichens, NC State Extension Gardener
- What about the lichen on my tree? (H-167), New Mexico State University Extension
FAQ
Should lichen be removed from trees?
NC State says control is not necessary. Lichens do not harm the plant.
Is lichen a plant?
No. Maryland says lichen is a fungus plus an alga living together.
Why is lichen on dead branches?
NC State says stressed plants shed bark less plus canopy dieback lets in more sun for lichen.
Do lichens mean clean air?
Penn State says lichens need clean fresh air to survive.
What colors are lichens?
New Mexico State lists green, blue green, yellow green, brown, gray or even red.
