Factors to consider when hiring a landscape designer include education, experience, and compatibility. Ask what level of formal training the designer has achieved. Ask to see samples of their work. This may be in the form of photographs or plans from a portfolio or website, or the designer may refer you to an installed design.
Equally important is finding a designer whose style and personality are a good fit for your project. Landscape design is a collaborative process, so trust and confidence are key to a successful designer/homeowner partnership.