Before You Go:
In An Established Garden:
Mature plants will tend to be more resilient and can usually cope with a week or two without water if it does not rain while you are away. It is, however, still a good idea to do what you can to help plants hang on to what water there is and to make sure that you do not come back to an over-grown jungle or a garden that is past its best.
Mulch around vegetables and amongst flowers and shrubs in beds and borders with good organic mulches to fertilise and hold moisture in the soil.
In A Small-Scale/Container Garden:
Containers will dry out far more quickly than the soil in garden beds and borders, so it is even more important to take steps to reduce moisture loss while you are away.
Soak each container well, ideally by placing each one in a larger container of water and allowing it to soak up water from the base.
Whatever kind of garden you have, if you are lucky enough to have someone reliable to come and check on your garden now and then while you are away, water and maybe even do a few jobs then of course this will be preferable to leaving everything to its own devices. But not all of us are lucky enough to have someone to help us out in this way. Follow the above advice and while you may have one or two casualties along the way, your garden should still look great when you return from your much needed summer holiday.