I have two tree peonies, a Renkaku and a much older and larger Shima Daijin.
Both have started to sag under the weight of their blooms, to the point their limbs could break (and indeed did), so this past Winter I took the drastic decision of pruning both.
I simply trimmed all branches on the Renkaku by 3", or to the closest gem. This was the result:
I can consider this a success, especially given flower size wasn't affected by pruning.
The Shima Daijin is a much harder case because it's just so much larger, so I took the middle road of completely cutting just some of the worst sagging branches while dormant. As with all middle roads, I wasn't exactly satisfied:
The front branches I didn't cut sagged once again all the way to the ground: I ought have done a much through "blade job" as there's a lot of new solid growth at the back that will bloom next year.
Another thing I deeply suspect is Shima Daijin is extremely susceptible to fertilization. I usually fertilize all my peonies with standard agri NPK fertilizer (12-12-17), but I think that excess of nitrogen is at least in part to blame for the sagging growth. So next year another experiment besides more pruning: nitrogen-free fertilizer.
