The most appropriate mode of transport for you trip will depend on the country you are travelling to and the locations of the gardens you intend to visit - so it is important to plan your transportation before you set off.
Fly-drive is an excellent travel mode for visiting places outside towns. The air flight usually lands you at a destination on the fringe of a city and with good transport links, without your being subjected to urban congestion. It is less cheap but one can return the vehicle to another airport and fly home.
For flights within America, see Travelocity: Airline Tickets, Hotels, Cars, Vacations, and Cruises. or Expedia Travel -- discount airfare, flights, hotels. Or, if you have time to hunt for a bargain, try ITA Software and QIXO
For cheap flights from Britain, see Cheap Flights.