JARS v57n2 - Rooting Desiccated Evergreen Rhododendron Cuttings: A Preliminary Report

Rooting Desiccated Evergreen Rhododendron Cuttings: A Preliminary Report
Joe Bruso
Hopkinton, Massachusetts

Ever had a tree limb fall on one of your choicest rhododendrons, breaking off a large branch? Ever had a stem borer destroy most, if not all of a small, choice plant or have the whole plant inexplicably die? Murphy's Law seems to cause such calamities to happen most often to plants you would least prefer them happen to. This has been my experience, anyway. Usually I don't notice until several to many days or weeks after the event. By this time, especially in the summer, the foliage looks completely desiccated, with the leaves curled up and seemingly dried out, although still green. In an attempt to rescue something from affected plants, I've been experimenting with rooting cuttings from these dried out branches.

How Cuttings are Supposed to be Treated

I have read, and been told more than once, that one of the key components for successfully rooting rhododendron cuttings is to take them in the early morning, or when they are otherwise turgid (fully hydrated). Furthermore, I've been told, one should put them immediately into a high humidity/cool environment (such as a plastic bag with moisture in a cooler), until stuck.

An Alternative

While there may be some merit to this traditional approach, and it does lead to success for most rhododendrons, the results of my informal experimentation indicate that this treatment is not necessarily a requirement, and may not even be desirable! In the table below are the results of attempts to root very desiccated cuttings over the past 1½ years. The cuttings were treated in the commonly accepted manner, with one exception: After taking cuttings of the current year's terminals, they were immersed in a closed container filled with water, where they remained for one to several days, until the leaves looked fully re-hydrated. After this treatment, most looked as good as if taken from a normal, turgid plant.

Results have so far greatly exceeded expectations. If the remaining stuck cuttings root, results will exceed my typical results using non-stressed cuttings.

Preliminary Results as -- of February 15, 2003.
Rhododendron Date Stuck # Stuck # Rooted % Rooted Notes
'Scintillation' x (ssp. yak x R. macabeanum ) August 13, 2001 3 3 100 A borer broke off the main stem of this two year-old seedling.
R. degronianum ssp. yakushimanum Bovee's dwarf form October 15, 2002 5 5 100 Potted plant died. Leaves were  hanging limply when discovered.
'Jericho' (lepidote) October 15, 2002 11 10 91 The whole plant (potted) died for unknown reasons.
R. bureavii 'Ovate-Twist'** August 15, 2002 15 4 27 The plant's center died for unknown reasons. These branches were in
very bad shape. Re-hydration took several days. Six rooted, two subsequently died.
R. degronianum ssp. yakushimanum September 12, 2002 15 6 40 * A borer caused a large branch to  break off. The unrooted cuttings are
R. bureavii Lem's form November 23, 2002 15 6 40 * A tree limb broke off a large branch. Unrooted cuttings are callused.
R. minus Carolinianum Group Gable's compact white form October 12, 2002 11 4 36 * Discovered plant in "dead" condition. No response to watering. This, along with R. bureavii 'Ovate-Twist', was in the worst condition of those tested.
* Additional cutting(s) are still stuck and at least some look likely to root.
** Name is not registered.

Others' Experience

In response to my inquiry on the Yahoo! Rhododendron E-Group, several people have shared their similar experiences. Below are excerpts from some of the responses.

From Don Hyatt: "I too have had great success rooting cuttings from stressed or dying plants. I have a basement full of cuttings salvaged from branches that were being lost to botryosphaeria dieback. I have been very successful re-hydrating and then rooting branches that were broken during the winter. I have been amazed at how dried up some branches can appear after being on the ground for many weeks, and yet will come back to life. If the branch rehydrates, the cuttings will usually root." Don's method of re-hydration is essentially the same as mine.

Kathy Van Veen of Van Veen Nursery provided the following information: "I received some cuttings from Pennsylvania that were absolutely dry - almost crispy. I told the sender that they were probably hopeless but would try anyway. I soaked the cuttings in water until the umbrella opened again. They rooted.

Martin Wappler, who had a nursery in Portland, Oregon, used to swear that desiccated cuttings rooted best. In fact, he would leave them out to wrinkle up before he stuck them."

Dick Gustafson's experience indicates that desiccation may actually improve rooting success rates: "I have had the same experience with 'dead' rhododendrons on many occasions. By dead I mean the plant has curled hanging leaves that will not respond to watering of the root system. They may appear dead for a week or more. Take cuttings and submerge them in a container of water. The next morning the cuttings are usually turgid and may be stuck in the usual way. The first time I tried this was 12 years ago when one of my favorite hybrids died. Thirteen out of thirteen rooted. This is one of the few times that I had 100% rooting [author's emphasis]. This past spring [2002] I did the same with a broken branch from 'Kristen Marie', unnoticed for several days, and got a very high incidence of takes."

Ken Cox expressed the following opinion which supports Dick's experience: "We have had good results from cuttings sent from abroad which are usually a bit on the dry side. So I would say that for lepidotes there does seem to be some advantage of having some desiccation."

What's Going On?

How can cuttings that appear to be in such terrible shape recover and root so readily?

A Theory

Don Hyatt relayed a theory held by him and others in his chapter that since there is no flow of sap out of the cutting on dying branches, there is an increased sugar buildup in the tissues and that this aids root formation. One member of his group soaks her cuttings in a dilute sugar solution overnight to improve rooting, and gets excellent results on many difficult varieties.

Determining why desiccated cuttings root so well will require research. A clue as to why they recover and root at all may be provided by an article from The Quarterly Bulletin of the American Rhododendron Society , January 1969 (23: 1), titled "Winter Desiccation Injury of Rhododendron" by John R. Havis of the University of Massachusetts. In the course of his investigation into desiccation injury to lepidote and elepidote rhododendrons, he found that leaves and stems can withstand various levels of water deficit (up to 70% in this study) and still fully recover. This is apparently an adaptation to winter conditions when the ground is frozen and water can't be transported to the stems and leaves. During sunny, warm days, these parts lose water, becoming dehydrated. Perhaps the same adaptation that allows for recovery from winter desiccation also allows desiccated branches to re-hydrate and root successfully. Whether the success rate is higher for desiccated cuttings than cuttings treated the traditional way could be determined by controlled experiments. My results so far plus anecdotal information from others support this proposal.

Research Possibilities

Assuming these preliminary results are confirmed (and this should be done with more formal, controlled experiments), they raise several questions that might be answered with more research. For example:

• Is desiccation an effective way to treat cuttings of difficult to root types?

• What is the optimal dehydration level that maximizes rooting success?

• What is the desiccation point of no return beyond which various types cannot be successfully rooted?

• Is the point of no return related to winter hardiness? Havis partially rejected this idea. However, of the plants he tested, Rhododendron PJM Group, known for its winter hardiness, tolerated the greatest degree of water deficit (70%) with full recovery.

Practical Implications

Many times I have read of important plants being lost by their owners, hybridizers and otherwise. Contained in the notes of Joseph Gable as published in Hybrids and Hybridizers: Rhododendrons and Azaleas for Eastern North America are examples of the best plant from a cross or seed lot dying due to disease, mechanical breakage, etc. Some examples are listed here:

• 'Atrosanguineum' x R. thomsonii (page 52): "1953: Have lost this 'best' seedling...apparently died of blight...This was a real loss as I know of no other hardy clear deep red with such handsome flowers and foliage so early in the season."

R. catawbiense x R. fortunei x R. campylocarpum (page 58): "1954: Another seedling of this turned up when it flowered and looks very, very good but it was broken back to one little side twig."

R. davidii (page 62): "This plant was picked out by a borer, and now (fall 1950) seems to be doubtfully alive." (Last surviving seedling from the seed lot.)

R. wardii x R. discolor (page 76): "1958: Old plant died but not from cold. Have no clonal progeny."

I would guess that like me, by the time the owner of such plants realizes the situation, they are shriveled up and look dead. The results of this study point to a way to save this genetic material. Another pertinent point made by Havis is that cuttings of some desiccated plants, when placed in water, are capable of recovery beyond the point where the whole plant can recover. Rhododendron carolinianum (now R. minus ) cuttings were cited as being capable of recovery from a 70% water deficit, whereas the whole plant could not do so beyond a 50-60% deficit. The reason for this difference may be the inability of the roots to recover and take up water. When the plant itself is beyond hope, cuttings can still be salvaged, however.

For the ordinary gardener, loath to take cuttings from a small or prized plant yet desirous of more, these results show that small disasters resulting when Murphy's Law strikes can be turned into opportunities. Instead of throwing out broken branches and "dead" plants, try re-hydrating and rooting them.

Joe Bruso is a member of the Massachusetts Chapter.