Moss and lichen treatment can be done any time of the year. If you don't get your roof sprayed the moss and lichen can cause damage to your roof. Lichen is particularly hard on iron roofs. It eats into the paint and can permanently mark it. Sometimes if left to long rust will start to occur. Moss causes more problems on concrete tile roofs. As it grows it starts to block the channels of the tiles and opens up cracks in the mortar.
When moss and lichen is left to grow it will hold moisture and slow the roof from drying out. Also it will grow from your roof and start blocking your gutters and down pipes.