Tree peonies generally respond quite well to pruning, which is best under taken in late winter or early spring, just as the buds begin to swell - so now is the ideal time!
If your tree peony was planted in the last two years then you should really only take out the occasional dead stems while the plant is still establishing. Cut stems back to a live bud or to just above ground level.
However if the plant is already well established but somewhat misshapen, then you can prune each stem back hard to about 15cm (6") from ground level to encourage new stems to be produced. However, it is important that hard pruning should be spread over a number of years, pruning back just one main stem per year. After several years your tree peony should have developed a better shaped framework of stems. Of course, this process will risk the loss of some flowers but the end result will be worth the sacrifice!
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