There is no difference between a sales appointment with a potential customer and a job interview with a potential employer. In either case, you're trying to make a good impression so they'll give you a chance to prove your worth and earn an income.
A suit and tie can indeed be overkill; however, more times than not, overkill will garner you more respect than underkill when it comes to making a good first impression.
Sorry, nothing about a baggy flannel says, "wow, he's a real tree guy" to me. Anybody can show up with a saw and baggy flannel and call themselves a tree guy. What's going to set you apart from the rest of the guys who are all vying for the same job?
Again, I'll ask the question, "who are you marketing your services to?" Is it a 30-year old guy in a t-shirt and flannel who can proabably do the job himself or a 60-year-old banker who'd prefer to spend his day in Khakis and a polo shirt out on the golf course?