Answer: When lichen is wet from rain or dew, it grows actively. It also likes sunshine, and will grow well in winter after the leaves have fallen from deciduous trees and no longer block the light, or on trees with badly thinned canopies.
If you have lichen growing on the trunks or branches of your trees or shrubs, don't panic. Lichen will not actually harm your trees. That's because it takes its nourishment from the air rather than from its host. Lichens are often blamed for the decline and death of shrubs and trees because they are commonly found on dead branches and limbs. In actual fact, exposed limbs on damaged plants simply give lichens access to the sun they need for growth with little competition.
Lichen will grow on all sorts of surfaces ? rocks, woody debris, soil, fence posts, rusty metal, sand and, of course, tree bark ? where they don't have to compete with other plants. They tolerate the most extreme environments, from hot dry places, to the wettest rain forests or arctic conditions.
Lichens act like sponges, taking in everything that is dissolved in the rainwater. They cannot excrete the elements they absorb from air and rainwater. For this reason, they're an excellent indicator of poor air quality. Lichens will not grow where there are toxic elements of polluted air.
Usually there is no need to do anything, as lichen is not detrimental to the health of your plants. But if you do wish to get rid of it ? if it's covering ornamental bark, for example ? there are a couple of very simple things you can do. First, you can get a scrubbing brush and water and gently rub off the lichen. Or you could cover the trunk or limb for a while with shade cloth; eventually the lichen will die from a lack of sunshine. Or, you can use a lime-sulfur spray in late winter (when temperatures are above 40F and before the buds swell) and spray the bare branches of your trees. Lime sulfur effectively halts the development of lichen.
Best wishes with your trees!
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