I have been power washing since 1984. In 1988 we started doing some landscape contracting after being asked over and over by a couple of nurseries to help them. In 1995 we bought 2 acres near the downtown and built a nursery that we operated for 10 years before selling it. I am opposed to any licensing requirement, at least in Ark. Let me explain>
You are required to be licensed by the State of Arkansas to sell or plant any plant material as a business. You are also required to pass a test and be licensed to apply pesticides. After complaining for years and paying $550.00 a year in license fees, in 2004 I did not renew my licenses and did not attend any recertification classes as required to maintain my license requirements. The state was suddenly going to assess fines and issue stop work orders - until my attorney presented all my documention for the previous 6 years of complaints I filed that were never investigated. I've not bought a license since and the state boys don't come anywhere near me. Did all the license fees make me a better landscape contractor and nurseryman? Nope!!! I still carry my insurance as I always have. Most others don't do that either as far as I know.
Try practicing law or medicine without a license. You are going to jail.
If the government will get out of my way and just leave me alone, I'll do just fine. I can't think of a single valid reason to be licensed to power wash.