Lucky is the caretaker of a garden that hosts lichens. At least this is how I feel about lichens now, having cast off earlier frustration with ornamental bark hidden under a coat of crusty gray. Now I spend winter days exploring granite surfaces exposed by the sun’s warmth, constantly amazed at the diversity of lichens growing there: crusty lichens that cover sections of the rock in intricate patterns of gray, green and orange; colonies of brown, leafy lichens that trail down the sides of the rock; pillowlike masses of silver-gray lichens that branch like miniature shrubs.

Lichens also grow on the trunks and branches of trees. I am often asked if lichens harm the tree, if they are parasitic or if they mark a diseased or dying tree, misconceptions that arise from the abundance of lichens on dead branches or declining trees. In fact, lichens increase their growth rate in full sun and are thus more abundant where lack of foliage admits more light.

The presence of some lichens in the garden, whether growing on trees or rocks, is an indicator of pollution-free, moist air. The shrubby and leafy lichens are the “canaries in the coal mine” – they will not grow where the air is polluted – while the crusty lichens, the ones that cover the surface of rocks and pavement, are more tolerant of sulfur dioxide and other pollutants. Tree branches draped with the lichen called Old Man’s Beard or tree moss are reliable indicators of a pollution-free garden.

What are lichens? In the strictest sense, they are not plants at all, but two organisms – a fungus and algae – living intimately together in a symbiotic relationship. As one of my students eloquently described this arrangement, “The photosynthetic algae provides the food, the fungus provides a place to hang out.” Perfect!

Lichens hang out on rocks or the old headstones in the cemetery. And they hang from the branches of trees. They are remarkably tough, enduring exposure to freezing temperatures in winter and extreme desiccation during summer droughts.

The crusty lichens that cover the surface of granite boulders are pioneers in soil genesis, secreting organic acids that weather the surface of the granite, eventually forming a pocket of mineral soil deep enough for a grass plant to grow. All the while, moisture trapped by the lichen at the surface of the rock cycles through freezes and thaws, slowly expanding tiny cracks in the rock, making way for the roots of a tree or shrub seedling. It is part of the education of a gardener to sit atop a granite outcrop and survey these various stages of soil formation.

The gardener cannot cultivate lichens, only make room for them. They are an important part of the rock garden and xeriscaping because they are not dependent on a constant supply of moisture. Their ability to survive alternating periods of drought and abundant moisture give them an advantage in colonizing stressful environments.

Wherever lichens appear in the garden, on rocks, garden benches, or in trees, they should be embraced as signs of a healthy environment, of a garden in tune with nature.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to reesermanley@shead.org. Include name, address and telephone number.