A:

Mosses and lichens grow naturally on any substrate that has enough moisture and sunlight to support their growth. If you want to remove the moss from the trunk of a maple tree you can either remove it by hand or use a pesticide to kill the moss/lichen.

It sounds like you are leaning towards the pesticide. I can't say if the spray will hurt the maple tree or not but the definitive way to tell is to read the label found on the chemical. It will say what precautions need to be taken to protect you and and the environment (including other plants like the maple). If you can't tell form the label you can follow up with me with specific information about the pesticide and we can see what we can find.

Also, it's important to note that usually moss and lichens don't harm trees that they grow on an they are usually good for birds which use them for nesting material and insect foraging. So unless the moss is too unsightly, you might consider leaving rather than fighting it.

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