From late August to early October is the time to graft tree peonies in our USDA zone 5. Grafting is the most efficient means of reproducing a new clone of an existing tree peony variety. This is an ancient technique whereby the bud of the desired variety is fused onto an under-stock of either tree or herbaceous peony root. This nurse root serves as a ‘battery’ which powers the new growth for a year or two before it develops its own roots.
Cross section of a successfully grafted tree peony. This plant did not ultimately flourish because it was not planted deeply enough and the tree peony never developed its own roots.
One of the main reasons tree peonies remain expensive plants is that grafting, though a relativity labor intensive and slow process remains the most effective means of asexual propagation. Tree peonies reportedly can be rooted from summer cuttings, but success requires humidity and temperature to be carefully controlled. This is very difficult and not done in any production setting we are aware of. There have been some reports of success in cell tissue micropropagation of tree peonies, though large-scale success has thus far proved elusive. Age-old grafting techniques are the preferred method of propagation of tree peonies in Japan, China and the U.S.
Grafting will remain an essential skill for gardeners to propagate woody plants, as it has been for centuries. We present here a low tech method with little specialized equipment for those dedicated gardeners who seek to propagate their woody tree peonies.
Grafting tree peonies is similar to other kinds of grafting, but also differs in a few key ways. The following is the method we use at Cricket Hill Garden.
Preparing Tree Peony Scions and Herbaceous Peony Roots
Cuttings can be taken from tree peonies from late August through early October. In this period the buds have reached a good size and the stem have yet to become too woody. Select large terminal buds for cutting. These cuttings are known as “scions”. Each scion should contain at least one terminal bud. If you can get a secondary bud as well, all the better.
Choose big terminal buds like this for use as scions.
After cutting your scion, trim off the leaves. Leave buds lower down on the branch to develop. Realize that in cutting the terminal buds, you are cutting off next years flower buds. This may be hard for any peony grower to accept, though a new tree peony may be of some compensation.
Leave secondary buds to develop for next year.
Trimmed tree peony scions.
If you are not going to do you grafting immediately, you can store the scions wrapped in a damp paper towel in a sealed plastic bag in the refrigerator. Scions stored in this manner should be viable for several weeks.
The best herbaceous roots are 2-year-old seedling which have been ‘decapitated’ of their ‘eyes’. Seedling roots are the most vital. If you do not have seedlings, a good healthy piece of root from any herbaceous peony will work very well. To obtain roots from a well established plant, you can either dig up a whole plant and cut what you need or extract a few roots by digging around the clump. For each graft you will need a 5-7” pieces of healthy and undamaged herbaceous root. Be sure that you keep track of the terminal end of the root.
2-year-old herbaceous seedling root which has been ‘decapitated’, or had its eyes cut off.
Large herbaceous root harvested from the side of a large clump. Note that we have cut the bottom end of the herbaceous root at an angle so we can distinguish it from the top
It’s best to pick and clean your herbaceous roots the day before you intend on grafting. Store them in a cool, damp place. This will allow them to become a bit less brittle and a little more elastic when you are ready to graft. Freshly harvested herbaceous roots can also be used but are quite brittle.
Tree peony roots, usually of ‘Phoenix White’ can also be used as under-stock. These are much more difficult to work with than herbaceous roots and are not recommended.
Before using your scions and roots soak them for 10 minutes in a 10% bleach solution. Then wash them off until they no longer smell of bleach, allow to air dry before begging grafting.
A quick 10% bleach bath before grafting helps to eliminate unwanted bacteria.
There are many different kinds of cuts which can be made to secure the tree peony scion to the herbaceous root. Make sure to sterilize your blades with alcohol before doing each graft. We will discuss the two which he have had good success with at Cricket Hill Garden.
Cut scion and rootstock at complimentary angles. Make sure that your cuts are on a level plain so that there will be good contact. The aim is to have the cambium layers of the tree peony scion and herbaceous understock match up. The cambium layer consists of the undifferentiated cells of the vascular system of the plant and is where growth occurs. In a successful graft the vascular cambia of the scion and rootstock grow together.
For a side graft, make one clean and even angled cut at approximately 45° to the scion.
Make a cut at a complementary angle on the herbaceous root.
The goal is to have the cambium layer of the scion and the rootstock match up.
Align the cambia of the scion and the rootstock.
It is not necessary to have the scion and rootstock be of the same diameter. If one is larger than the other, just match the cambium layers up on one side.
Tie up with rubber band. We use specialty grafting rubber bands, though a plain old office rubber band will also do.
Cover wound completely as possible, don’t worry if there is a bit of exposed tissue at the very top.
Wedge and Cleft Graft
This is a standard set of cuts and is perhaps an easier introduction to tree peony grafting than the faster side graft. Wedge and cleft grafting required four cuts. The goal here again is to match up the vascular cambia of the scion and the understock.
First, cut your scion with an even wedge.
Make sure that the two sides are as even as possible. We like to use a very sharp grafting knife or a straight-edge razor blade for fast, clean cuts.
The wedge needs to have an even and clean bevel.
Then, cut a complementary cleft out of the rootstock.
Keep in mind that there is some give to the herbaceous root so the wedge should be narrower than the cleft.
The wedged scion should fit snugly into the cleft in the herbaceous rootstock.
Again, don’t worry if the scion and rootstock are of different diameters, just make sure that the cambium layers match up on one side.
Tie up the graft. I like to tuck the tail in at the end and forgo tying a knot.
Tightly tied up.
Don’t futz around trying to get the scion and rootstock to fit just right. The Chinese say the excessive rubbing of the cut surfaces decreases the change of the graft taking. Better to practice making sharp, clean cuts before attempting grafts for the first time.
Even high volume commercial operations do not achieve 100% success in tree peony grafting. Our first year we had about 70% of our grafts come up in the spring, last year was lower, but that is because we began too early in the summer. Hopefully this year we will achieve 80-90% success.
Most tutorials on tree peony grafting have you cover the rubber band and wound area with grafting tape or polyfilm before setting aside to heal. We do not feel that this step is necessary because we heal the grafts for a short time at a high heat as opposed to a relatively longer period at room temperature.
Healing the Grafts
After you have tied up your grafts, they need to be healed. We have found the best way to do this is for a short time at a high heat. Place grafts in a 4” deep tray with clean, slightly damp sand at the bottom. The sand must not be too wet or the grafts will fail. Cover grafts with at least 1” of damp sand.
Be careful not to jostle the scions too much in fitting the grafts into a try of sand.
Keep the sand covered grafts at 90° F for six or seven days. We use a heat mat to do this. Ensure that the sand remains damp by occasionally misting the top and covering with a piece of plastic. If the medium is too wet or too dry the grafts may fail.
We allow the grafts to heal for six or seven days.
After six days, the grafts will have ‘taken’ and the healing will have begun. We find that if the tree peony buds are still firm there is a good chance that the graft was successful.
Select a planting location which is has deep, well drained soil (pH at least 6.5) and receives about 6 hours of sun.
We use an iron breaker bar to make the holes in the garden bed in which the grafts are planted.
Plant grafts so that top bud is 2-3” beneath the soil, space grafts 8” apart in 1′ rows.
Make sure that the graft is snugly planted with 2-3” of soil covering the top bud.
Mulch with a light layer of straw or leaves to help retain moisture. Right before or after the ground has frozen in the late fall cover your graft bed with a a layer of clear plastic and add additional mulch on top of this. We do this in November.
Mulch graft beds after planting to retain moisture. Cover with clear plastic and more mulch in the late fall, when the ground begins to freeze. Covering with plastic too early will invite mice to nest in your graft bed and ruin your results. This has happened to us, so we warn you of this problem. Covered grafts are dormant all winter and begin to grow in the spring.
In the Spring…
Uncover your grafts by removing all mulch in mid to late March or when the crocuses bloom. The young shoots should be poking through the soil at this time.
Emergent tree peony graft in the very early spring.
Expect first year grafts to grow to 8-16”. Some may even form viable flower buds, though it is best to pick these off at an early stage so that the plant does not waste energy on a small flower. Begin to fertilize in the later spring after the leaves have turned fully green.
The amount of first year growth is contingent on a number of factors; ranging from scion and understock size to planting conditions and the relative vigor of a given variety.
This comparison illustrates the importance of using large and vigorous herbaceous roots as understock. These are both 1st year grafts of the same variety, the only difference was the size of the herbaceous root used as understock. Bottom line: the bigger the root the greater the strength of the ‘battery’.
In the fall of the first year, you may want to further space out your grafts. Space to 2′. Remove rubber bands. If the graft has not developed any tree peony roots by this point, replant it deeper, at least 4” above the graft union.
1st year graft which has thus far failed to develop any tree peony roots. These are merely roots from the herbaceous understock. It should be replanted above the first two buds.
Grafted tree peonies which are not planted deeply enough often never
develop tree peony roots and will fail to thrive.
2nd year graft with two vigorous stems. Most varieties will begin to flower in the 3rd year.
Basic tree peony grafting supplies.
Sharp grafting knife or straight-edge razors
sharp gardening snips
isopropyl alcohol (to sterilize cutting instruments)
rubber bands or hemp twine (specialty grafting rubber bands are available from nursery supply companies, but plain old rubber bands will also do)
herbaceous peony roots
tree peony scions (bud cuttings)
tray with damp, clean sand
heat mat (not strictly required, if there are other means of keeping grafts at 90 degrees for the healing period)
‘Itoh’ or intersectional hybrid tree peonies can also be reproduced by grafting using the technique described here.
Tree peonies can also be propagated by means of root layering and division. We will be posting on these two techniques later in the fall.
In one of my first posts, I discussed experimental Spring grafting. Unfortunately all but one of these failed, though this could have been for a number of reasons. The scions were picked from winter damage and could have been dessicated, also the root stock was very small. Bottom line is that grafting is best done in the fall.
Some other links of interest on tree peony grafting
A good tutorial prepared by the Mid-west peony society.
An article by English peony grower Jo Bennison on tree peony grafting.
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