Landscape gardening requires some form of construction, this might require heavy lifting equipment and a certain amount of design and planning. You might want a pond, path, fence or irrigation system installed. You may want a completely new garden, designed and installed from scratch. If so, you probably need a landscaper.
A landscape designer or gardener does not require any formal qualifications but would normally have at least a City and Guilds qualification and a professional association.
A landscape architect has a degree in Landscape architecture and a professional qualification.
Architects can work on the design and planning of small garden project through to the design of large private and public developments.
Designers would primarly work on smaller projects including residential gardens and landscape gardeners usually implement the work of the architect or designer.
A landscape gardener or contractor will also more than likely complete maintenance contracts as a contract general gardener.
If you want to hire a landscaper, it would be a good idea to check there professsional association photos of previous work and references from other customers.