To avoid returning to a withered, lifeless desert, plant lovers will employ plant sitters to water plants and/or tend their lawns while they're away. This could be the job for you if you're trustworthy and have a little spare time on your hands.
To advertise your service, drop leaflets around garden centres and through people's front doors. Tell friends and neighbours to spread the word, and put up some cards in shop windows. Charge a daily rate of around £5 - you could offer discounts for weekly or fortnightly sessions.
The owners will probably brief you on what needs watering and when, but if not, all the information you'll require can be found in a decent gardening manual. Normally you'll be asked to tend and water plants in your clients' homes (don't forget to ask for the key) so you'll need some form of transport.
Alternatively, you could bring potted plants back to your own nursery where you can care for them in the comfort of your own home.