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I have a tree with a fairly heavy growth across its trunk and branches; I suspect that it's a form of lichen but I'm not sure.

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Is this something that will negatively affect the growth or health of this tree? Should I spray it down or leave it be?

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up vote 16 down vote accepted

These look like the tree lichens that are growing on my own trees. A lichen is a composite organism where by a fungus lives together with an algae in a symbiotic relationship. They don't hurt the tree, and are more commonly seen on the bark of older trees where the bark is not constantly being shed and regrown. So, it may also be an indicator of poor health if a younger tree is not growing as well as it should.

Although more common in damp areas, they can require very little moisture to grow, and do better in clean air.

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Thanks, Graham. The tree isn't too old nor very young (probably around 15-20 years old), but seems to be otherwise healthy. I'm in the midwest, so hot & humid summers and cold and dry winters. – Mahmoud Al-Qudsi Mar 14 at 17:30
Lichens, moss ok, shelf mushrooms, weeping chancre, bad. Basically, the growth must not penetrate the tree's bark. – Fiasco Labs Mar 14 at 19:27

As Graham says they are lichen and moss. A tree will commonly live in a symbiotic relationship with millions of macro and micro-scopic organisms. In tropical and temperate rain forests overgrowth on the the bark is very common.

I cannot find any research which indicates that they harm the tree in any way at any age as these publications indicate.

That they are growing so well indicates good things about the environment!

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Thank you; I have accepted Graham's answer but I appreciate your links to the resources. I'm in the midwest (Chicago) and it's in my front yard, not a heavily-forested area... but the tree does seem to be doing fine. – Mahmoud Al-Qudsi Mar 14 at 17:29

Lichens are a marker for air purity. They vary from algal lichens to the mossy shrubby lichens you will see hanging from the branches of pristine forests. The more complex organisms they are the purer the air. They're very good to see close by.

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Rosie, as a point of interest, the connection between 'clean air' and lichen growth,although its a belief I've subscribed to myself, is something I'm beginning to question - I see lichen growth on many trees here in West London, and the air is, frankly, anything but clean - a couple of trees right next to the busy A40 have lichen growth... very mysterious.... – Bamboo Mar 19 at 12:27
Me too with your findings Bamboo. Although I've only seen the shrubby lichens in pristine forests of the highlands, Wales etc. – Rosie Mar 19 at 12:30
Hmm, when I've got a bit of time, I'll do some research, cos I'm beginning to think its not actually true... – Bamboo Mar 19 at 12:31
I'd be interested to know your findings. Maybe it's something that is partially true, or that lichens are evolving to cope with pollutants. Reindeer feed off lichen and their feeding grounds are only In harsh conditions in northern climates and in the artic circle. – Rosie Mar 19 at 12:34
I'm guessing the truth is, as always, different varieties of lichen grow in different conditions, and maybe some have adapted, but I will spend some time on it at some point... – Bamboo Mar 19 at 12:35

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