Towards the end of July, tree peonies usually start to develop brown stems as they start becoming woody and showing their true nature. Here are a few closeups just to show how it looks. Once tree peonies darken up, they are usually set to survive as long as they are planted deeply enough to develop their own roots.
This one appears to be solid already. Like trees, their stems and branches tend to harden off completely before they ever expand in diameter.
This one is just starting to transform as the stem is getting darker towards the base.
Some of them will show a clear distinction in their transformation into woody stems instead of fading gradually. This one has darkening which starts from the base.
Even itoh peonies will darken up in the summer time, but the difference with them is that even if all of their stems turn woody, the top part of itoh peonies may die back partially.
Some tree peonies start their transformation from either dots or blotches
Some species like my kinshi tree peony can be very reluctant to transform so it is easy to see the stem become energized as it turns reddish with the flower buds for next year already forming now.