Thank you for sharing your question.
Here is what you asked:
"How do I prune these to encourage vertical growth"?
Vertical shrubs pruning - Hi Cara, from your question and the picture you sent I assume the original purpose for planting these plants was to cover the large brick wall at the front of the house.
You mentioned that the shrubs have remained at the same height for the last 3 - 4 years and you want to prune them to encourage vertical growth.... I hope all that is right and I have understood correctly...
OK then, lets prune and help these slow growers develop into towers :-)))
See how quickly they grow when you get up close......? all you have to do is snap a close-up with your camera and the problem's solved ;) OK, lets get serious. These shrubs (I can't recall their names, they might be part of the "Buxus" family) are very slow growers and there is little that you can do about it. But there is something you can do to help nature chug along a little faster....
So, what can you do to help them grow? First, in each one of these "towers" choose 2 - 3 mid-height stems at the back of each plant - and prune them. This will seriously impact the following three things, simultaneously:
Keep in mind: Your vertical shrubs pruning challenge is not a 'must prune' situation at all. It's just nice to have. And it won't result in your shrubs shooting up to the sky within a week. This one has a very specific shrub anatomy so don't change anything in terms of the water regime, fertilizer etc... your plants look fabulous - strong, green, and rich with leave. Perfect!
If you have any other Q please feel free to ask - I'm always happy to chat about pruning :)
I hope that was helpful!
Providing general guidelines on 'how to prune' any kind of plant is important, but there are plants, and there is YOUR plant. So despite there being loads of information on this site, I also provide specific and personalized support. If something wasn't clear, or you're not sure of your plant's name, simply send me a picture with your question. Use the form below and I'll get back to you. In the mean time, see what others are asking.