Looks like you have several, not just one. Looking at where this plant is, you are going to have trouble unless you move the plant away from the fence. You might want to start all over again in a different spot. But I am no expert, maybe someone else will have a better idea. good luck.

In a rare situation, I've seen them freeze to the ground and vigorously regrow. I doubt anything you do will impact it too much.

I think it will be hard (well, basically impossible) to get rid of those after they've spread so much.

You can prune them as drastically as you like and they will probably grow right back vigorously.

Even in order to control their spread you'll have to keep pulling off sprouts repeatedly.

If you think these are too much just wait until you plant an elder there! Maybe instead you should just cut the Hibiscus down short every year, at the end of winter. Since this species flowers on new wood you can maintain it as a small shrub in this manner without losing the bloom display.

Ha! Well, I'm not sure I'm going to be able to get the spirea out of the way in any case - it looks pretty well entrenched too! I was only thinking of putting the elder there because I'm running out of easily accessible (for fruit picking), sunny areas in my yard. We'll see what happens...some of the berry plants may have to stay in containers for the time being!

Good luck raising elderberry for fruit, though I am a bit curious as to why you would want to. They have always seemed a bit tasteless to me. :>) In my garden the fruit gets eaten by the birds as soon as it starts to ripen. You will have to net them if you want fruit or create a tall cage of PVC or metal pipe, electrical conduit, wood, etc that you can cover with either wire or plastic mesh. IME the cage works better than just a net draped over the plant.

Spirea can also be cut down to the ground and it will resprout into a much nicer looking plant than previously, denser and without the dead twiggy matter that tends to form over time. For both the RofS or the spirea, if you don't want them they can be removed by cutting to ground level and painting the surface of the cuts where the bark meets the inner wood with undiluted glyphosate.

If it's sunny, the base of the slope where you will be planting the vinca might be a good spot for berry bushes.

I actually haven't had elderberries in years, but I have such fond memories of my grandmother's elderberry pie that I wanted to give them a go! I love the flowers as well. We'll see how it goes - I'm willing to net if it means recreating grandma's pie!

I like both the ROS and spirea, but since I am running out of sunny spots I may have to sacrifice one of them for things I want more. The base of my slope does get sun but I'm planning to use it for herbs. I'll post a photo on the other thread so you can see what I'm dealing with - our backyard is awkward for sure!