Gadenin日本語Get the appThis is a woody shrub, not a herbaceous peony
A tree peony builds a permanent framework of woody stems that stay alive and above ground all winter. It is not a herbaceous peony that dies back to a crown each autumn, and almost every serious mistake made with one comes from treating it as though it were. The stems you see in winter, bare, gray brown and apparently dead, carry every flower bud for the coming spring. Cut them down and you have thrown the plant away for several years.
It grows slowly and it grows permanently. A young plant puts on 4 to 8 in (10 to 20 cm) of woody extension a year and reaches 4 to 5 ft (1.2 to 1.5 m) across in a decade, then keeps going for fifty years or more in the same spot. Choose the position as though you were planting a small tree, because you are. Moving an established plant sets it back three years and sometimes kills it outright.
Because the framework is permanent, the roots must be permanent too. Tree peonies resent disturbance, dislike competition at the root, and will not tolerate ground that stays saturated after rain. Get the site right on planting day and the plant asks for almost nothing afterwards.
If your plant dies back to nothing every winter and sends up soft green stems from ground level each spring, it is a herbaceous peony, not a tree peony, and the pruning and depth advice here does not apply to it.
The graft union goes four to six inches under the soil
Tree peonies are slow and awkward to raise from cuttings, so nurseries graft a woody shoot onto a fat herbaceous peony root, which acts as a nurse. That nurse root feeds the top for the first two or three years and then, in most plants, exhausts itself and dies. If the woody top has not made its own roots by then, the whole plant declines and disappears. Getting the top onto its own roots is the single most important thing you do at planting time.
Find the union. It is the swollen, often slightly crooked joint low on the stem where smooth woody bark meets the pale, fleshy storage root below. Set the plant so that joint sits 4 to 6 in (10 to 15 cm) below the finished soil surface. That means the hole is deeper than the pot the plant arrived in, and the buried part of the woody stem will look wrong to anyone used to planting shrubs at their nursery depth. Bury it anyway. Woody peony stem held in moist soil in the dark produces its own roots within two or three seasons.
Planted at nursery depth, with the union sitting proudly above the soil, the plant flowers well for two years on borrowed energy and then fades. That delay is exactly why the mistake is so common: the consequence arrives long after the cause, and by then the plant is usually blamed on the weather.
Drainage first, then the hole
Tree peonies want soil that is deep, fertile and free draining, and they are unforgiving about wet feet. Standing water around the crown for even a few days in winter produces root rot, and root rot on a grafted plant that has not yet rooted from the stem is usually fatal. If a hole you fill with water still holds water an hour later, plant on a raised mound or a raised bed rather than trying to improve the base of a sump.
Dig a hole at least 2 ft (60 cm) across and 2 ft (60 cm) deep, and loosen the base rather than glazing it smooth with the spade. Mix the excavated soil with a couple of spadefuls of well rotted compost and a handful of bonemeal or a slow release balanced feed. Do not line the hole with fresh manure; it holds water against the buried stem and burns new roots.
Backfill in layers, firming each one with your knuckles rather than stamping, so no air pocket is left against the buried union. Water in thoroughly once the hole is full, top up any settlement, then mulch with 2 in (5 cm) of compost, kept a hand's width clear of the stems themselves. Autumn is the best planting season, while the soil is still warm and the plant is going dormant; early spring while it is still leafless is acceptable second best.
| Item | Target | Why it matters |
|---|---|---|
| Graft union depth | 4 to 6 in (10 to 15 cm) below soil | Forces the woody top to root on its own before the nurse root dies |
| Hole size | 24 in (60 cm) wide and deep | Loose soil for a permanent, deep root system |
| Spacing from other shrubs | 3 to 4 ft (90 to 120 cm) | Airflow to dry the foliage and no root competition |
| Sun | Half a day of sun, shade in the hottest afternoon hours | Full sun bleaches and shortens the flowers, deep shade stops them |
| Soil pH | Neutral to slightly alkaline, 6.5 to 7.5 | Strongly acidic ground gives weak growth and poor color |
| Drainage test | A filled hole empties within an hour | Standing water in winter kills ungrafted young plants |
| Mulch | 2 in (5 cm), kept off the stems | Steady moisture without rotting the bark at the base |
Suckers from below the union, and how to tell them apart
For the first few years the buried nurse root will try to grow on its own account. What comes up is unmistakable once you know it: soft, non woody stems appearing from below the union, carrying fine, ferny, deeply cut foliage quite unlike the broad, leathery, blue green leaves of the woody top. Left alone, that herbaceous rootstock outgrows and starves the plant you actually wanted.
Remove suckers as soon as you can identify them. Scrape the soil back, follow the sucker down to where it joins the root, and pull or cut it off flush at its point of origin. Snipping it at soil level leaves dormant buds behind and it returns stronger. Firm the soil back afterwards. Expect to do this once or twice a year until the top is rooted on its own, after which it usually stops.
The reward for burying deep is that this problem eventually solves itself. Once the woody stems have made their own root system, the nurse root becomes irrelevant, and a plant on its own roots is longer lived, tougher and easier to keep than one still depending on a graft.
- Find the union before you dig Wash or rub the soil off the base of the stem and look for the swollen joint where woody bark gives way to pale fleshy root. Mark it with a loose tie so you can still see it while backfilling.
- Dig deeper than feels right Set a cane across the hole and measure down. The union should end up 4 to 6 in (10 to 15 cm) under the finished surface, which usually means the hole is 6 in (15 cm) deeper than the pot was tall.
- Backfill in firmed layers Add soil a few inches at a time and press it against the buried stem with your knuckles. Air pockets around the union stop the stem from rooting and let rot in.
- Water in, then leave it alone Soak the whole planting area once, top up any sinking, and mulch. Do not feed a newly planted tree peony for the rest of its first season.
- Mark the position A bare, leafless woody stub is easy to hit with a spade or a mower. Put in a stake or a ring of stones for the first two winters.
What to write down on planting day
Record the depth you buried the union at, measured from the finished soil surface, and the season you planted. Three years later, when you are deciding whether the plant is finally on its own roots, that number is the only way to know whether an unimpressive plant was set too shallow or simply needs more time.
Note the exact position of the plant relative to something permanent, such as a wall corner or a mature tree, and note which direction it gets its shade from. A tree peony spends winter as a set of bare sticks, and gardens change around it. A written position saves a plant from being dug through.
Keep a count of the suckers you remove and when you remove them. If the fern like herbaceous growth stops appearing two or three years after planting, that is your evidence that the nurse root has faded and the woody top is standing on its own roots. If it is still coming up every spring after five years, the union was probably not buried deep enough and the plant is worth resetting in early autumn.