I purchased a bare root tree peony at the NW Garden Show this year and it was doing just fine, getting to be about 1.5 feet tall until about two weeks ago and the entire plant is now drooping, the leaves quickly sagging. It is in a semi-shady spot that receives morning sun. I water all of my plants once a day unless it has been raining. I am trying to figure out if it is dying, and what I might do.
My first thought is that your tree peony is either responding to drought, or too much water. My own tree peony which is about 12 years old has always been sensitive to excessive heat and drought. It is in a partly shady location, but it has root competition from two nearby conifers. Watering less frequently but more deeply is usually a good idea. A commercial nursery site has the following information about this plant’s water needs. Excerpt:
“Watering: This is the most common misunderstanding. Tree peonies do not have watering needs like roses or other perennials. They are woody shrubs native to northern China, which receives about 30 inches of rain per
year. Once established, tree peonies are drought tolerant plants. Excess water will suffocate the roots and is the leading cause of plant failure. Do not plant near auto-sprinkler systems that keep the soils continuously moist. Do not water until soil is dry below the surface and try not to wet leaves when watering to prevent fungus. Be observant; soil can dry out on top and still be moist 6-12″ below the surface. When you feel the soil is dry below the first 4-6″ and leaves may droop slightly, water the roots deeply. Climates of hot summer temperatures with little or no rain at all will require more attention to watering then those areas that get some rainfall. Peonies in root control bags will require more watering attention than tree peonies planted in the ground. NOTE: Droopy leaves in the first warm days of spring are caused by an imbalance of the root system and leaf production. If soil has moisture, do not water. This imbalance that will self correct as the plant settles into the growing season. You know this is the cause of the limp leaves if the plant recovers in the evening or early the next morning.”
The other possibility is a fungal disease called peony wilt. Here is more information about this problem, from the Royal Horticultural Society. Excerpt:
“Tree peonies can be vulnerable to attack by peony wilt (Botrytis paeoniae), especially during wet springs. Symptoms are wilting of the flower buds, sometimes accompanied by a fluffy grey mould and, later in the season, brown blotches on the leaves. Botrytis forms sclerotia (hardened fungal bodies) in diseased tissue, which carry the fungus over the winter, so it is important to prune out and destroy infected tissues to prevent this happening. Currently no fungicides labelled for control are available.”
If you think that the problem may be wilt, and would like confirmation, you can bring samples of the leaves to a Master Gardener Clinic for diagnosis.