Greetings.
I am trying to id a plant I bought 2 years ago at Sun State Tree's nursurey. African Starburst it may have been called?

It has big wide green leaves that are purple underneath. At first it had white and purple shoot type flowers coming out of the tops.
Since the first season, I have'nt seen these flowers.
They die back every winter, but now are 8' tall (i already cut them back earlier this year)
I don't have the area irrigated, they get watered with roof runoff when it rains.
They seem not to like too much water
What can I do to get them to flower?
Bassman swears by Miracle Grow Liqui-feed, I haven't used it on them yet. Would this be acceptable?