I have some very old peonies growing in my yard. For the last few years, the blooms have not been as large or as many. What can I do?
First, if the environment has changed, such as trees growing up and obstructing the sunlight, this could be the problem. If possible, limb up (prune away lower branches) the trees to permit the sunlight to reach the peonies,
Secondly, is it possible that soil and mulch have been piling up over the peony roots? Peonies like to be very shallowly rooted.
Finally, if the peonies have been in that bed for many years, they may be suffering from lack of nutrients. You might want to add some good aged cow or sheep manure in a band around each plant about 20-30 cm away from the centre. You can also use a low nitrogen commercial fertilizer (5-10-10 or 5-10-5) to focus on blooms over foliage.