Herbaceous Understock Sprouting on Tree Peonies Part II: Removal

A few months ago we wrote about how to identify the herbaceous peony understock sprouting on a grafted tree peony.  This is usually the result of the grafted tree peony being planted too shallowly.  Now that the season for transplanting peonies has arrived, it’s time to dig up and remove unwanted herbaceous understock as well as replant the tree peony. It is necessary to do this because if left unchecked, the herbaceous peony can eventually overpower and choke out the tree peony. If you do like the way the herbaceous understock blooms, you can replant this and let it grow out.

If you have a tree-herbaceous chimera peony, now is the time to dig it up, remove the unwanted herbaceous peony suckers and replant the tree peony deeply enough so that it will grow its own roots on which it will thrive for many years to come.

Tree peony leaves are easily distinguished from those of herbaceous peonies. Tree peonies are dissected and three pronged, while herbaceous peony leaves are elliptical and pointed.

If you are unsure about the difference of the leaves, you can always check the stems. tree peonies are woody, while herbaceous are green.

Remove the leaves before digging up and peony. Dispose of these in a hot compost pile or outside of the garden least they spread harmful fungus throughout the garden.

The best method for digging up a large peony is to do it carefully. Being by digging around the circumference of the plant, about 18” away from the drip line. Then remove some of the soil from around the roots, lastly gently ease the plant out of the ground.

This plant had a smaller root system the anticipated.

The majority of this plants roots are herbaceous peony roots, it has very few tree peony roots. The herbaceous peony’s ‘eyes’ are the small white buds which have formed at the bottom of this year’s herbaceous stems.

Because this plant has so few tree peony roots, to remove all the herbaceous roots now would kill or weaken the tree peony. Best now to remove the herbaceous peony ‘eyes’ by trimming these pink or white buds. Replant the tree peony deeply so that it can grow its own roots over the next few years. Plant the tree peony about 4” above the graft union, or where the herbaceous peony roots begin. Take this opportunity to properly amend the soil for the tree peony. Place a shovel full or two of compost in the planting hole. As you back fill, mix in a cup of limestone dust if your soil is acidic. Mix in a few more shovel fulls of compost as you fill in the hole, mixing in a cup of Azomite to give the plant a mineral boost.

As you back fill, water in the soil/compost mix so that it settles well around the roots. Tamp down the soil and top dress with a shovel full of compost if you still have some handy. Newly planted, or replanted peonies should be mulched for their first winter in the ground to help prevent the soil around the roots freezing and thawing suddenly and heaving the roots above ground.

Tree peonies can be transplanted anytime now from now until early November in our USDA zone 6. The most important thing is to make sure that the roots do not dry out while out of the ground. Remember to plant in a sunny, well drained location, with rich soil and a pH of at least 6.5.