At my last home, I had a neighbor that used to plant her impatiens in such a way to create these beautiful balls, perfectly round. Any idea on how she did that??? She didn't use any kind of planter, just the ground.
Here's a link of a picture showing what I am talking about. However, my neighbor didn't have the 'balls' touching each other.
Here is a link that might be useful:
Water, fertilize and have the perfect amount of light.
That's not a matter of pruning just good management.