If you've ever tried to clean your drive with a cheap jetwash machine, don't be disheartened if the results are not so good. The lichen, mosses and moulds clinging to your paving have been around for a while. The oldest lichen in the Arctic is 8,500 Million years of age. The mosses that clog up your drive, your drains and your drainpipes are relative newcomers at only 470 Million years old. Aspergillus, the black mould that spreads on your drive beneath trees and digs into the cracks of your paving is a youngster in comparison. The lichens are proven to live in outer space; the moulds and mosses will happily survive a nuclear war. No wonder they are difficult to get rid of.