Tree peonies have some of the most sumptuous flowers of any plant. Once rare in gardens, they are now becoming more available.
Tree peonies are very hardy, but they come into growth early and may be caught by frost. In the garden, choose a reasonably open position, but one sheltered from strong winds and early morning sunshine. Do not plant in deep shade or near the roots of large trees.
Planting
Plant when dormant from late autumn until late winter. Existing plants can also be transplanted, with care, during the winter months. They do best in fertile, well-drained soil that is not too acidic or alkaline. Avoid waterlogged situations and dry soils. Prepare the ground by thorough digging and incorporating well-rotted organic matter. This is particularly important on very well-drained soils, where generous amounts of organic matter will improve moisture retention.
Ensure the roots are thoroughly teased out before planting. It may well be worthwhile carefully washing off all the compost and treating them as bare rooted. True species will be grown on their own roots but most cultivars will be grafted. These should be planted deeply with the graft union about 10-15cm (4-6in) below the surface of the soil. This allows for the rooting of the graft and encourages development of growth buds at the base. Should the original stems then prove poor or straggly they can be cut hard back to within 10cm (4in) of the ground in September.
After planting water thoroughly once a week in the first summer. It helps establishment to remove the flower buds for the first two years - if you can bear to do it!
Tree peonies need little further attention. Apply a general-purpose fertiliser at the end of February and again in the autumn and mulch with organic matter. Deadhead after flowering.
No regular pruning is needed apart from removing dead or weak stems in late February. Many cultivars naturally have an upright form, others will be more branched and spreading. Where growth becomes crowded, older shoots can be cut out after flowering. Cut off any suckers that might develop from the herbaceous rootstock; these are easily recognised as the foliage is very different.
Container growing
Tree peonies can be grown quite successfully in containers for a few years. Use pots at least 30cm (12in) in diameter and fill with loam-based John Innes No 3 compost. Water regularly during the growing season, and in winter ensure the roots do not freeze. Tree peonies are vigorous plants and quickly use up the nutrients in the compost, so feed each spring with a small amount of slow-release fertiliser and regular liquid feeds. When plants start to decline, transfer them to a border and cultivate them as you would any other tree peony.
Propagation
True species can be propagated from seed. Sow in pots of John Innes Seed Compost and place in a sheltered spot outdoors or a cold frame. It takes two years for a shoot to emerge and a few more years for flowering. Cultivars will not come true from seed.
Vigorous plants with numerous stems may be suitable for division but layering may be a less risky procedure. Plants can be layered after flowering in May or June by cutting a notch at the junction between the current year’s growth and the previous year’s wood.
Commercially, most tree peonies are grafted onto herbaceous peony rootstock, but this is not a method most home gardeners would undertake.
Problems
Tony Dickerson