They could be encouraged by saying the importance of trees to them, and how the plants and trees, give out Oxygen by just taking the Carbon-dioxide from the air, Sunlight and water. Also,
they must be made to know about,
1.Global Warming,
2.Excessive CO
 emission in the air,
3.Mixing of more and more chemicals in the air; and,
4. about Acid Rain.
By making them to know about these, if we say planting more and more trees is the only remedy to control the rate of losing the naturality of the air.

Planting trees will surely give a better result in the mixture of air, by increasing the percentage of Oxygen, around the particular area.
planting trees more and more will make the earth's average temperature to go normal, and the alarming rate of global warming would be gone.
Due to excessive Deforestation, we had lost many trees; and, 
most of our forest covers.
is the only remedy for all these.

Thus, we can make them to know about the importance of planting trees, and they can be encouraged to participate. :)

Thank you.
Hope this will help you.

I would like to make then aware about the tress and how they are useful for them. I would even tell them the importance of the tress. I would take the help of my friend to tell them the importance and what loss they would get when the neglect the tress and when they don't care about them. I would first request them to help and figure out the help our classmates about taking care of the plants outside which are helpful and needed. I would even try to help my classmates by motivating them and even helping them from my side.

hope so my answer is useful and mark as best if possible..............^_^
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