But it’s probably the easiest and cheapest way. Another plus point would be creating habitat for smaller animals and morale boost for the city inhabitants

Well, both require making land available, and while amenity gardening is manageable, a botanical garden is often a research site that requires a lot of expertise to set up and maintain. Mind you, that's just fine, the county can and should hire people.

Looking out my window, I see trees planted on the sidewalk along a road.

It takes little space away from pedestrians, but provides a lot of shade. That seems both easier, cheaper and better that taking up whole city blocks.

We need more trees, i am shouting that for years.