Decorative bases do... what? (if anything)

[Dalcyone]

Hey, I'm running <abbr title="Colonial Charter">CC</abbr> 1.71 and in the Themed Sets--> Town Decorations menu, there's "Planter Bases" in several sizes/shapes. It says they're decorative bases for trees and flowers... but once I get them built, I can't place decorative trees or flowers into them (but can place decorative trees/flowers on bare soil). Am I misunderstanding what they're for? Do they grow their own trees/flowers with time?