Q. I have a Mexican petunia (Ruellia) that is about 4 years old. In 2013 the freeze caused all of the stems to die back. I cut all of them off at ground level and the plant grew back nicely and flowered beautifully. Apparently, this winter the plant has very little dieback. Should I cut all of the stems off at ground level or just the ones that have frost damage? Will it flower from the existing stems or does it only flower from “new wood”?
A. Mexican Petunia is considered an herbaceous perennial which means it freezes to the ground if it gets too cold and grows back from the base. It is summer flowering so the flowers develop on new growth. Flowers will develop on new growth from older wood as well as from new growth at the bottom.
You have two choices. You can prune it to the ground again just as if it froze back. It will grow from the base and flower just like it had in 2014. That’s the easy way.
Your second option is to keep it at this height and remove anything dead or weak to the ground. Next, you would cut the remaining stems back to a height where you want it to branch. Cutting it back or shearing it will cause it to grow more densely above where it is sheared and flower.
The second method will give you a taller plant if you want one. If you need to keep it small, then use the first method.
Remember to fertilize it now with an all-purpose fertilizer for promoting flowers such as a rose fertilizer, fruit tree fertilizer, tomato fertilizer or something similar.