Why did it have to be something on a roundabout.
For instance Liverpool has the Lamb bananas, Canaries - different kinds of dogs as does Vancouver - I know these are not sculptures but draw a lot of interest.
Why not sculptures ( trees for growth if we must ) of different sizes winding their way around parts of the town each one a little larger than the last one. It would encourage visitors to follow the pathway of trees and also maybe spend some money in the shops/cafes as they go.
Too late!