I planted this tree peony 20 years ago without knowing much about them. Over the years it has started blooming regularly but as you can see is pretty tortured looking. Can I prune it back hard without killing it? Any suggestions are welcome!

I would advise against proceeding with the pruning actions. Here's my explanation and alternative recommendations:

The peony you have appears to resemble a tree peony that might not have been planted deep enough to establish its own root system. It is possible that the peony gets all its nutrition from the understock of the herbaceous peony, which is not sufficient. This is likely why it is growing in an upward manner. While there are some side shoots present, there are no visible new shoots emerging from the ground as there should be. As a result, the peony is unable to develop into a proper bush and continues to grow primarily upwards. It is worth noting that the plant does not seem to lack sunlight or light, which further suggests an issue with root development.

Instead, I would suggest the following steps:

Right now, add additional soil around the base of the peony (e.g., three or four inches).

Wait until September to carefully dig up the peony and examine its roots. That is, peony roots start growing sometime by late August and grow through the fall. By inspecting the root system of your peony, you will have a better understanding of the root structure and any potential issues that need to be addressed. You would be able to see if the peony has any of its own new roots. There are some good pictures of tree peony roots here in this post below. If you scroll down, you will see what the tree peony roots are like and you will be able to tell them apart from the herbaceous peony understock.

https://www.treepeony.com/blog...

If the own root system (made of whiteish roots) appears shallow or underdeveloped, dig a much deeper hole when replanting the peony. I would dig a hole deep enough to accommodate planting the main branches of the peony at least five to six inches into the ground. Alternatively, for optimal results, consider digging a hole deep enough to bury the lowest side branches, approximately five to six inches below the surface.

In this way, you can provide the peony with a better chance to establish a robust root system, allowing for healthier growth and the development of a proper bush-like form.

I would not prune anything at this time. Next spring I would eventually prune only dead branches. The peony should start growing from the ground and side branches, and hopefully, new shoots will start growing from the ground, so those leggy branches will not be the problem anymore. With a little bit of luck it will be a beautiful bush in a couple of years

Please do not hesitate to ask additional questions. We are all happy to help

Great advice, Jasmin.
One thing to add; is the peony getting at least 6 hours of direct sun? It may be stretching for more sun. If it is not getting at least 6 hours of sun, then when you follow Jasmin's advice, plant it in a sunny, well drained spot.

Jasmin said: I would advise against proceeding with the pruning actions. Here's my explanation and alternative recommendations:

The peony you have appears to resemble a tree peony that might not have been planted deep enough to establish its own root system. It is possible that the peony gets all its nutrition from the understock of the herbaceous peony, which is not sufficient. This is likely why it is growing in an upward manner. While there are some side shoots present, there are no visible new shoots emerging from the ground as there should be. As a result, the peony is unable to develop into a proper bush and continues to grow primarily upwards. It is worth noting that the plant does not seem to lack sunlight or light, which further suggests an issue with root development.

Instead, I would suggest the following steps:

Right now, add additional soil around the base of the peony (e.g., three or four inches).

Wait until September to carefully dig up the peony and examine its roots. That is, peony roots start growing sometime by late August and grow through the fall. By inspecting the root system of your peony, you will have a better understanding of the root structure and any potential issues that need to be addressed. You would be able to see if the peony has any of its own new roots. There are some good pictures of tree peony roots here in this post below. If you scroll down, you will see what the tree peony roots are like and you will be able to tell them apart from the herbaceous peony understock.

https://www.treepeony.com/blog...

If the own root system (made of whiteish roots) appears shallow or underdeveloped, dig a much deeper hole when replanting the peony. I would dig a hole deep enough to accommodate planting the main branches of the peony at least five to six inches into the ground. Alternatively, for optimal results, consider digging a hole deep enough to bury the lowest side branches, approximately five to six inches below the surface.

In this way, you can provide the peony with a better chance to establish a robust root system, allowing for healthier growth and the development of a proper bush-like form.

I would not prune anything at this time. Next spring I would eventually prune only dead branches. The peony should start growing from the ground and side branches, and hopefully, new shoots will start growing from the ground, so those leggy branches will not be the problem anymore. With a little bit of luck it will be a beautiful bush in a couple of years

Please do not hesitate to ask additional questions. We are all happy to help

wow Jasmin, thank you for this excellent information and advice! I will definitely do that come September or so. Not sure how deep I can replant it as it's about 4 feet tall, so to bury that much of the main stem/branches could be a challenge, but definitely worth a try.

I think it would be sufficient if you cover these lower branches with soil. Please see the red arrows in the picture below. That could be approximately six to eight inches. Right?

I also agree that the peony should receive a lot of sunlight. Good luck