How and when do you prune a tree peony

Tree peonies don't need to be pruned/trimmed. You can deadhead the blooms as they finish, but only cut just below the bloom, removing excess stem can cause less productive growth the next year. Removing the spent blooms will allow for more root growth instead of putting the plants energy into seed production.

The only time you would prune is to shape a plant, but that will remove bloom potential for the next year, as they set bloom for the following year, right after blooming.

I agree with Lynn. Tree Peonies in my experience grow rather slowly. One thing to keep an eye open for is growth coming from the graft/soil line. Tree Peonies are often grafted and given a chance the root stock will try to take over. If you see it coming from the base of the plant rub it out or trim if off just as it starts and continue to watch for it.

How tall can I expect mine to get (Pacific Northwest, where everything seems to get bigger than the stated height). Hoping I have not planted it where it will become a screen. It's about 5.5 feet tall now, I think 3rd year, has not yet bloomed.

Wow, that is awesome Deb. I saw one that was just under 6 feet tall. It was about 15 years old, and so beautiful.
But I think if would depend on the variety and growing conditions. It can take up to 10 years to reach maturity.

It IS an impressive plant but I'm hoping I have not misplaced it -- if it gets much taller it will start blocking my view from the kitchen sink. I suppose I can trim it if need be, but it so far has really nice form and I don't want to mutilate it. I'm rather hoping it will bulk out rather than get any taller. And really hoping for some blooms next year, no idea what color it might be, it was a gift from my neighbor who started it from seed. She 'thinks' it may be yellow but couldn't remember herself. Thinking back, this may be its 4th year, I don't exactly remember. It kind of did nothing for a couple years then shot up. Kitchen window is the one on right, and looks over our firepit and pond area.

Bev, how did you site it? How much sun, etc?
I have three to plant this week. Everything I read says morning sun or filtered sun.

Mine is pretty much in full afternoon sun. This corner faces south and west and is sheltered from the prevailing wind. I don't get such burning hot afternoon sun as others, but it is definitely not in either morning or filtered sun. So far it's done well, but I have not yet had any blooms.

It just says that the blooms will fade faster in full/or afternoon sun.
Yours is looking wonderful.

Many of the things we grow have short bloom seasons but they are so beautiful that they are worth it. Here is Ezra Pound in a friends garden. It receives morning sun. The blossoms only last a few days but it is so spectacular it is still worth. This plant is 10+ years old. Oops.....I'm going to have to look for the picture......

Can't wait to see the photo Paul.

I'm having a problem finding it but look at Ezra Pound on the data base.

Oh my, that really is a beauty Paul.

They do grow very slowly and they wait several years to start blooming, but they're virtually eternal. There's a Chinese proverb that a tree peony is most beautiful in its second century.

How about giving it some bloom fertilizer next summer? It won't effect next years bloom but may help the following year.