Herbaceous Understock Sprouted on Tree Peonies: ID and Removal


Do you have a chimera peony in your garden, at tree peony which also blooms as a herbaceous peony? In fact, this is not some rare mutation, but a rather common circumstance in which the the tree peony’s herbaceous peony understock, or rootstock has sprouted.

A ‘Souvenir du Maxime Cornu’ tree peony in our garden on which the grafted herbaceous understock has sprouted and blooming. In this case the herbaceous understock is blooming simultaneously with the tree peony.

Tree peonies are most commonly reproduced by grafting. Briefly, this is a method of propagation in which a tree peony scion, or cutting, is fused to the root of herbaceous peony. This acts as a ‘battery’ to power the tree peony in its first few years of growth. I detailed this process in a post last year.

An unhealed tree peony graft, with the tree peony scion held to the herbaceous peony understock with a rubber band.

Ideally, by the time tree peonies leave the nursery, they will have grown their own roots. This is achieved by planting young tree peonies deeply to entourage the grafted scion to sprout its own roots. At Cricket Hill Garden, we strive to only send out grafted tree peonies which have also sprouted some of their own roots and also mark the correct planting depth on each tree peony we ship. Many of our tree peonies are propagated by division and are on their own root. Unfortunately, many retail garden centers and non-specialist nurseries do not plant young grafted tree peonies deeply enough, nor do they give customers proper instructions for planting.

This  tree peony is beginning to send out its own roots. The bulbous portion of the root is that of the herbaceous undertstock. For this plant to thrive, it needs to be planted very deeply so that the tree peony continues to spout its own roots.

This is a young grafted tree peony which has been planted correctly in the nursery and has established more its “own roots.” The tree peony roots are the more slender, lighter color ones located above the darker herbaceous understock. We like to mark the depth to plant, as all tree peonies are individiual. On this tree peony,  you are planting to a depth up the stem, right below the first branching. This is typical of the grafted tree peonies that we grow and sell at Cricket Hill Garden.

If a tree peony is planted too shallowly, the plant is unable to form its own roots. This can lead to both sprouted understock as well as overall plant weakness.

This tree peony was planted too shallowly, in this case below the graft union. In this case it will be impossible for the tree peony to ever form its own roots. In an instance like this, more soil should be mounded up around the plant.

Herbaceous understock sprouting next to the tree peony in the early spring.

If left unattended, sprouted herbaceous peony understock on a tree peony will gradually sap the strength from the tree peony, to the point where it will eventually be overcome and killed out by the herbaceous peony. To avoid this the herbaceous rootstock must either be cut away below the soil line in spring or separated from the tree peony at the root in the fall. The tree peony is more rare and valuable and therefore is the plant to save. The simplest thing to do is cut back the herbaceous peony foliage BELOW the soil line and then mound up 6” of soil around the base of the tree peony. This can be done at anytime, and will discourage the herbaceous peony from re-sprouting as well as encourage the tree peony to form its own roots.

If you do want to save the herbaceous peony and plant it out in another part of your garden, wait until the fall to do this. In a few months, when it’s time to dig and divide peonies I will do a follow up post which will illustrate dividing a herbaceous understock from a tree peony.

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.

 

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