By making them understand the importance of it
do campaigning
make it a competition
best class who plant trees and water them daily will get a prize
You should tell them the importance of plants and trees and in what ways they help us. Also tell them that how they help the environment and the future generation by planting more and more trees.You can assign them duties by giving responsibility of taking care of plants to all students.Like let one of them water the saplings, and the other one clean all the weeds and another one take care of the saplings and put mesh on them so that they couldn't be eaten by animals. By organizing a competition between the students on tree plantation. By this they start taking keen interest on planting and nurturing trees. And  meanwhile you can also prepare a presentation for the students on the benefits of planting trees.