You can buy plastic or earthen pots and make small holes and close it with cotton or coir in such a way that water just drops from the pot. Then bury this pot in 3 places around the trees 5 to 6 feet away from the tree and fill water in the pot. The pot should be closed so that water should not evaporate. By this you can save water and can provide enough water for the trees. Using this method people are planting trees in Rajasthan. They are saying watering in the pot once in a month is sufficient. You can also try this.