What? No. Plants colonize new areas in complex succession sequences that require hundreds of years to play out. It might start with grasses, then shrubs, then dense stands of trees that die off and are replaced by stands of other trees... maybe there's a fire that needs to happen in between... and let me remind you that a "city park" is not exactly a natural formation anyway.

If you want a park in 30-50 years, you need to plant trees and design it with specific public-use objectives in mind.