New fruit trees need extra care during their first spring and summer, because their roots will not have had time to establish themselves. New pot-grown trees are particularly prone to drying out so pay attention to these. Regular watering is perhaps the single most important and useful thing you can do to help your new fruit tree get established.
Usually a bucket of water once a week will be sufficient, but if the weather is hot and there is no rain it may be necessary to water every 2-3 days. Apply the water slowly so that it soaks right into the soil.
Whether you have a sophisticated automatic irrigation system, or are just using a watering can or bucket, it is better to water periodically and allow the soil to dry out a bit between waterings. Fruit trees do not expect to be growing in perpetually wet soil. Over-watering can be just as harmful as lack of watering.
As mentioned in our fruit tree planting guide, applying a thick mulch around the base of the tree is one of the most useful things you can do. The mulch will help retain moisture, as well as preventing other plants competing with the tree.
After the first year you don't need to be quite as vigilant as the tree will have established its root system. As a rule of thumb the period when fruit trees need most water is just before, during, and just after the blossom period (about 6 weeks in total) and in the period leading up to the harvest.