As Dave recommends, ask for current examples of their work. Lately I've reviewed a couple of sites for people on here that were built using 10 year old code and antiquated SEO methods. One landscaper-focused web design company's entire portfolio I just reviewed is 9 years or so old and looks like it.
If you can budget to work one-on-one with a developer, and not a mass-producing hosting company, I highly recommend doing so. The biggest difference is SEO (search engine optimization) that is specifically tailored to your needs and target market. This is something you tend not to find with any of the large web mills.
If you have any specific companies/firms you're looking at, feel free to post them here, or pm, and we'll gladly give a deeper review of the actual under-the-hood workings of their product.
a guy who knows both the lawn business and web strategy