Just plant more trees.

就是多种树。

We should plant more trees and grasses.

我们应该种更多的树和草。

We should plant more trees around our neighbourhood.

我们应该在周边多种点树。

We can plant more trees.

我们可以种更多的树。

If we really have to, plant more trees.

如果不得不这样做，那就种植更多的树木。