Cloning American Chestnut Trees
Before You Begin.
Last updated 1 May 2003 by Charlie A Allen
Before you can successfully clone American Chestnut trees, you will need to understand the survival traits of the species and how you can use them to your advantage. The American Chestnut is an opportunistic canopy tree of the eastern hardwood forests. It achieves dominance over the oaks and hickories that share its habitat by lurking patiently as a bushy shrub in the shady understory until some accident opens a hole in the forest canopy. Then the chestnut races upward straight for the light and blocks the sunlight from its rivals. To be successful, the tree must concentrate all possible energy on the top-most, the most vigorous and the straightest shoot, and severely limit the energy to all other competing shoots. If that most-favored shoot is damaged, the tree will quickly send up a new crown shoot to replace it.
Under the best of circumstances, our grafts will appear as severely damaged shoots, and the tree will make every effort to shift its energy to a healthy non-grafted shoot. If our graft is less than ideal, or if the tree is stressed, or if we fail to remove healthy competing shoots, our graft will fail. However, once our successful graft has grown for a year or two, the tree recognize it as the most favored shoot, and will pump all its available energy into supporting the grafted stem.
Successful Whip Graft
Shirley Allen displays a three-year old American Chestnut seedling that has been successfully whip grafted with a B2-F2 American-Chinese scion. The graft is three months old in this photo. This tree is now a regular source on nurse-nut grafting scions.
Whip Grafts on
American Chestnut Trees
Last updated by Charlie A Allen
on 1 May 2003
There are many ways for a chestnut graft to fail, but only one way for it to succeed. You will need to do everything almost perfectly to get a good graft. Some elements are not under our control, but we can improve our chances by doing correctly the things we can control.
See our technical article on Graft Compatibility, titled "What I Did With My Children's Inheritance" on the Projects Page before proceeding. Then come back and read the rest of this section.
Only healthy, vigorous rootstock of pure American or mostly-American stock will yield a healthy graft to an American chestnut scion. Only a blight-resistant Oriental or partially-Oriental rootstock will graft to a blight-resistant hybrid or Oriental scion. A fast-growing central shoot works best. The diameter of the scion must match almost exactly with the stem on the rootstock.
Only healthy, vigorous scions will succeed, so discard any weak, damaged or diseased scions.
The scion must be dormant and the rootstock starting to break bud when the graft occurs, to ensure that the root sends energy to the scion and issues a "wake-up call". If the scion breaks bud first, the scion will use up its available energy and liquid, and then wither and die before reinforcements arrive from the roots. Cut scions in mid-winter during maximum dormancy and store in the refrigerator in a damp zip-lock bag (clearly labeled) until spring.
If you have never grafted before, practice on some more forgiving plant such as a plum or apple to develop your technique. Read carefully the additional web pages on cloning below for details
Keep your tree stress-free until the graft is well established.
Nurse Nut Grafts
Last updated by Charlie A Allen
on 12 April 2001
Chestnuts ripen around Halloween; earlier if the weather turns prematurely cold and rainy. If you are gathering fallen nuts off the ground, collect them promptly before they become waterlogged or mouldy and before they freeze. If you are collecting off the tree, gather them early, while the burrs are still slightly green and before the squirrels collect them for you. Allow the nuts to air-dry at room temperature for several days before storing them in a cool place.
Chestnuts are similar to potatoes and onions in their storage requirements. Too dry or too cold and the living germ cells die; too wet and they mould and rot. All American chestnuts and most European and Oriental chestnuts require a short period of "winter chill" at about 40 degrees F. before they will sprout properly.
The objective of nut storage is to keep the nuts alive but dormant for as long as possible, so that the outside air temperature will be above 60 degrees F when the grafted plants are ready to be transplanted outdoors. In Northern California, that is late-April; in Pennsylvania it may be mid-May. As soon as the nuts are harvested and briefly air-dried, store them in a labeled zip-lock bag with several 'air holes' and a handful of damp* peatmoss in the veggie compartment of the refrigerator; above freezing but below 40 degrees F. Cooler is better, but freezing is deadly, so exercise care.
* To test the peatmoss for the proper water content, pick up a handful and squeeze as tight as possible. If your hand and fingers do not become wet,the peatmoss is too dry; if a few drops of water squeeze out between your fingers, it is too wet.
Three weeks before you plan to make grafts, remove the nuts from the refrigerator and sort for root length. Roots over 1/2 inch; return to cold storage for the final three weeks. Roots less than 1/2 inch, store at room temperature for the final three weeks, to force the roots to develop more rapidly.
SCION COLLECTION & STORAGE
Scions should be partially dormant
( start of swelling of the bud, but no green color showing ). The nut hormones will cause the scion to wake up and start to grow. If the scion is awake before the nut hormones arrive, the bud will use up the little bit of energy stored in the scion and then shrivel and die. Partially dormant scions can be stored for up to two months in the veggie section of the refrigerator, using a zip-lock bag and a damp paper towel. Select only vigorous, healthy scions; a dead scion wastes everybody's time. Scions smaller than 3/16 inch in diameter work best for nurse-nut grafts; larger scions stress the nut opening. Harvest the scions about a month before the normal time for bud-break. ( early March in California, mid-April in Pennsylvania )
Rinse off the nut in tap water to remove peatmoss, and cut the root shoot to a small one-eithth inch stub. With a single-edged razor blade or equivalent, slice off about 1/8 to 3/16 of an inch from the sprouting end of the nut, leaving a clean flat elliptical surface 1/4 inch long by 3/16 inch wide, centered, more or less, on the root shoot. Using a square-ended Exacto blade 1/4 inch wide, make an incision 5/8 inch deep, aligned with the long dimension of the oval and centered on the circular residue of the root shoot. Do not cut all the way through to the base of the nut, but deeper is better.
Inspect each bud, and use only healthy-looking buds. Terminal buds are preferred, but side buds are good also. Starting at the terminal bud, cut scions 2 to 2 1/2 inches long, leaving 1/2 inch of scion stub above the next side bud. Proceed down the scion; some cuttings will contain only a single bud, others will have two or more, depending on internodal spacing. Stop when scion diameter exceeds 3/16 inch. Carefully with a sharp knife remove all excess buds from the multiple-bud scions, leaving only the last bud. Now cut a wedge 1/2 inch long on the base end of each scion section. My preferred method is to remove a sliver about 1/4 of the scion diameter from opposite sides to form a very blunt wedge, then trim the ends with a second set of cuts about 3/16 inch long.
Grasp the scion firmly close to the wedge, and insert it into the cut nut, being careful not to damage the wedge section or the bud. The final 1/16 inch of the wedge should show above the nut when the scion is fully seated. Discard any damaged scions or nuts. Record data of nut type, root length, scion type and size, date and name, etc. Place each grafted nut with the scion upright, along with a few handfuls of damp peatmoss, in a ventilated zip-lock bag. Nut and scion should be completely enclosed in loose peatmoss. Transfer graft data to zip-lock bag using Sharpie Fine Point Permanent Black marker pen. Store bags in warm (70 - 75 degrees F ), dark place for four weeks or more to allow nut regrowth to scar over the exposed wedge end on the scions. Grafted nuts are easy to store at this stage, so let them grow in this state as long as possible.
After four weeks storage at room temperature, inspect grafted nuts for signs of growth. Select those showing vigorous root growth, and pot into 2-quart milk cartons with drainage holes or 4X4 inch citrus pots, using a loose pearlite-peatmoss mixture. Cover nuts and scion to within 1/4 inch of the scion bud. Transfer important data to the potting container. Return less vigorous nuts to room-temperature storage for an additional week or two.
Place potted grafts in warm humid environment sheltered from wind and direct sun to develop leaves. Water sparingly, being careful to keep liquid water away from the graft area. After two or three large green leaves develop, carefully transplant to their final destination for further growth, disturbing the roots as little as possible. Again, protect the sensitive leaves and stem from wind and direct sun until they have 'hardened'.
REPORTING YOUR RESULTS
As the old cartoon used to say, the job isn't over till the paperwork is done. Share your results; the good, the bad, and the ugly.
1) Initial scion sections were too short, leaving the root stem too little room to come out the top of the nut and turn around and re-enter the soil. We now make the scions 2 1/2 inches long or longer.
2)Upright position during first four weeks: roots were not reversing direction completely if nuts were in random orientation. We now place new nut grafts into individual 12-ounce paper cups to maintain upright position during early growth.
3) Dormancy: Fully dormant scions take several weeks to two months longer to respond to elevated temperatures than partially dormant scions. After this year's results are analized, we will recalibrate our time line to correct for this difference. Next year, we will use only partially dormant scions and see if the grafts respond better.
More Cloning Information
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Identification Clues for
American Chestnut Scions
Note the thin scion and small buds
which hug the stem
Look for long inter-bud spacing
Some American scions have a