During a recent home visit to inspect some declining shrubs, a homeowner showed me some frilly, pale-green growths on the trunk and limbs of a dwarf Japanese maple. “Is this some kind of fungus I should spray,” the homeowner asked. The answer to that question was easy, as was the identification of the pretty growths. No, they should not be sprayed; they are not fungi, but unusual organisms called lichens.

I find lichens beautiful, the intricate whorls, the crispy edges, the soft blue-gray-green color of many species, but what are lichens? Are they parasites? Will they kill trees and shrubs? Do they have a purpose or a message for us?

Lichens are complex structures. Each lichen is composed of two living organisms, a fungus and a green alga or cyanobacterium sometimes referred to as blue-green alga. The two organisms survive in a symbiotic relationship in which each one contributes to and derives benefits from existing with the other. The fungus provides the physical structure of ...