I have a patio on north side of house. It is made of patio brick. Moss forms in the cracks. it is very unsightly. I have to scape it out by hand. I have used weed killer and moss begone, but it doesn't seem to work. Any suggestions? The patio is approximately 12 by 20 feet. It is a hard tedious job, digging the moss out by hand about every 3-4 months.