JARS v64n3 - The 'April-Fool's Day Cutting Method' for Large Leaf Evergreen Rhododendron Propagation

The 'April-Fool's Day Cutting Method' for Large Leaf Evergreen Rhododendron Propagation
Willem (Bill) Morsink
Toronto, Ontario, Canada

Matthew Hilgerdenaar
Willowdale, Ontario, Canada

Starting early in April in the Toronto region, one-year shoots are prepared in a special way as described to become cuttings. These wrapped shoots are cut off in the middle to end of August regardless of whether roots are present and are then treated as cuttings, either under a mist or fogging system, or just in transparent plastic bags to keep them moist in a shade house.

Traditionally, current year growth (one-growing-season new shoots) are used in early fall to attempt to root them as cuttings. These cuttings are wounded at the basal end, treated with a rooting hormone and are stuck in a rooting medium in benches or pots inside a greenhouse. Many species and cultivars of rhododendrons do not root easily this way, if at all, and it takes two years to obtain a very disappointing number of rooted cuttings. One can also propagate rhododendrons using tissue culture techniques. This approach was taken by the Niagara Chapter for the 1998 ARS convention when 10,000 tissue culture plants were produced and then raised by the Littles, members of the Niagara Chapter, ARS District 12.

Traditional air layers are done in the growing season. Such air layers left on winter-exposed rhododendron branches do not survive in the Toronto area. Dr. Mark Konrad (2007) wrote on the topic of air layering that the optimum time for air layering is early spring for Pennsylvania. The "Fool's Day air layering method" described here by the Toronto Chapter is really for late winter, i.e., April 1, as Toronto's spring does not start until the beginning of May.

So why are we fussing around with air layering, or rather "air layer cuttings," as what we talk about here is a combination between an air layer and a cutting? At Toronto's Camellia Meeting of November 16, 2003, we heard all about camellias in a talk presented by guest-speaker Frits Morsink from the Norfolk-Chesapeake area of Virginia. This method, applied by Norfolk Botanical Garden volunteers on camellias, produces nearly 100% rooting in six (6) months when started in early April . They apply the scarification area to the three-year-old wooded part of a branch 30-50 cms (12-20 inches) long. This method differs from the traditional air layering by not partially wounding, but by total removal of the bark-phloem cylinder down to the xylem (the woody water-conducting tissues).

This approach has been successfully tried on camellias, gardenias, and the large-leaf, evergreen (Hymenanthes) rhododendrons, with camellias being the easiest to air layer this way. Bill Morsink thought that this scarification of the base of air layers may also be applicable to hard-to-root rhododendrons, so he experimented during 2004, starting on April's Fool's day.

April 2006 air layer from 
R. 'Janet Blair'
Photo 1: April 2006 air layer from R. 'Janet Blair'.
Photo by Matthew Hilgerdenaar
Four-month old air layer 
cutting, August 2006, from R. 'Janet Blair' with roots on and above scarified area Four-month 
old air layer cutting, August 2006, from R. 'Janet Blair' showing roots
Photo 2: Four-month old air layer cutting, August 2006, from R. 'Janet Blair'
with roots on and above scarified area before potting.
Photo by Matthew Hilgerdenaar
Photo 3: Close-up of four-month old air layer cutting, August 2006, from R. 'Janet Blair'
showing roots on and above scarified area before potting.
Photo by Matthew Hilgerdenaar
Repotting to a larger container in 
October 2006.
Photo 4: Repotting to a larger container in October 2006.
The plant on the right is in bloom in May 2007 in Photo 5.
Photo by Bill Morsink
Photo 5: 'Janet Blair' air
layer cutting in bloom, end of May 2007
Photo 5: 'Janet Blair' air layer cutting in bloom, end of May 2007,
14 months after start of process in early April 2006.
Photo by Matthew Hilgerdenaar

We Produce our Airlayer Cuttings in our Gardens
Matthew Hilgerdenaar's Toronto garden is a show garden for rhododendrons. His garden is enriched with a raccoon-proof pond and marsh, a circular herb garden, a gazebo with very comfortable chairs and finally a patio with retractable roof. Matthew's garden is a doable rhododendron garden because he used a simple approach of sand, peat moss and sandy loam and some acid to construct a raised bed over the horrible natural Willowdale clay. Matthew also grows other members of the Ericaceae family, including heathers ( Calluna and Erica ).

"The April Fool's Day Air Layer Cutting Method"
This modified method of propagation of air layer scarification must be carried out before shoot growth starts in May, and was initiated by Bill in 2004. It resulted in eight out of 16 rooted air layer cuttings. Matthew joined the trial in 2005, with results presented at the September 2005 meeting. The preliminary conclusion was that it would be desirable to have many members joining the trial in 2006, with hopefully a hundred or more rooted air layer cuttings from a large range of cultivars.

The Experiment Started Very Early in April
At this time, sometimes with snow on the ground, only "fools" attempt to break tradition. Last year's "one growing season" shoots having a cluster of five or so leaves were selected; select the longest possible shoots with the larger shoot diameters. Scarify each one cm (half an inch) in length at the point where the air layer is to be made towards the base of the shoot. Do not apply rooting hormone or fungicide, but apply a clump of sphagnum moss around the scarified section and securely wrap it with a piece of aluminum foil. New shoots will form from the terminal buds of these last year's shoots, starting about the end of May. So in July you will have a new shoot on top of last year's shoot that will later form the air layer cutting.

Cut the air layer during August or September when convenient, just below the scarified area and pot them as if they were simple cuttings. Again only "fools" would break the tradition and cut the airlayer at this time when hardly any or no roots have formed. Our rooting results in May 2006 (13 successfully rooted from 22 air layer cuttings) of the 2005 air layer cuttings are shown in Table 1. Note that unprepared cuttings with new June shoots taken in July 2005 all died one month later.

Steps in Making Air Layer Cuttings of Large Leaf Evergreen Rhododendrons (example date is for Ontario, Canada)
1. Start very early in April, very important (not in May)!

Last year's "one–growing season" shoots with a cluster of five or so leaves are selected; select the longest possible shoots with the greatest shoot diameters. Scarify a one cm (half an inch) length at the point where the air layer is to be made at the base of the shoot, see Fig. 1.

Do not wound the two-three cm (one inch) scarification area with an Exacto-knife or propagating knife because you will injure the water conducting outer xylem layers of the cutting and likely increase the chance of infection and death of cutting. The idea is to carefully wrench–off the outer bark-phloem layer without injuring the woody-xylem part of the cutting.

Do not apply rooting hormone, or fungicide , but instead apply a clump of sphagnum moss around the scarified section and two-three cm (one inch) of stem above the scarified area. Securely wrap the moss with a piece of aluminum foil. New shoots will form in the middle of May from the terminal buds of the last year's shoots, see Fig. 2. Thus, you will have as in our example last year's (2005) and this year's (2006) shoots forming the future air layer cutting. Such prepared stalks should be shaded from direct sun if so exposed.

Time will tell whether you prepared the air layer properly, because if not, the air layer, which is wrapped with sphagnum moss and foil wrap and still attached to the mother plant, will otherwise die within one month around June. If the air layer stalk is still green and good looking before you cut it in August, then you know so far so good (Figs. 1 and 2).

Fig. 1: How to prepare the cutting 
in early spring at the start of the air layering process
Fig. 1: How to prepare the cutting in early spring at the start of the air layering process
Fig. 2: Air layer cutting preparation.
Fig. 2: Air layer cutting preparation. During the winter, the lower leaves will usually
fall off from the lower part of the cutting (last year's growth) and roots may form
from this area, emerging from vertical cracks in the bark above the scarified area. Roots
may also form from the callus area; this happens when the one cm scarified zone on
the air layer cutting is incompletely debarked (scarified). Roots developing along
the vertical cracks of the lower end of the cutting produce better root systems
as compared to roots forming from only the callus growth.

2. Cut the air layer some time convenient to you late August-September
Cut the hopefully still green and healthy looking air layer plus new shoot to make a normal air layer cutting, but do so at a non-traditional time, i.e., the late-summer period. Cuttings prepared as air layers a month or so later than April Fool's Day and taken in mid summer almost always die within six weeks.

Removal Steps

1. Cut the air layer at the top of the 2-3 cm area scarified in April (Fig. 2). If callus tissue developed at the higher position of the scarified area, make the cut below the callus so as to include the callus. Practically no roots, but maybe some callus, are formed if you leave detachment of the air layer cutting until late September-October for many cultivars. If you do so, you will have lost the best two rooting months, i.e., September and October. Airlayer cuttings of Rhododendron 'Janet Blair' produced numerous roots (Figs. 3,4), in contrast to many other cultivars.

2. Pot the air layer cutting as you would any cutting, i.e., in prepared sterilized acidic (pH 5-6) growing medium or just in a peat moss medium.

3. Cover with a transparent plastic bag and place in a shaded spot ; roots likely will form between August-December. The air layer cuttings need the September to end of November period to form roots; root growth in the rooting medium ceases when the temperature drops below freezing.

If the potted air layer cutting is still green and good looking in the pot after six weeks, i.e., around the end of October, then again you know so far so good. Roots may still develop in late fall or not until the next spring.

4. At the end of October, take the plastic-bagged pots with air layer cuttings to an over-wintering place. Keep them cool during winter, below 5° Celsius (40° F) either in a refrigerator, protected within miniature poly-houses (bags) in a protected spot of your garden or in airtight cold frames. Matthew Hilgerdenaar uses miniature, cool poly-houses just large enough for individual rooting plants.

If you over winter them indoors at warmer mild temperatures, fungi may kill them and such cuttings miss out on the necessary natural winter cold-chilling requirement. Unchilled cuttings will delay new shoot growth to middle or late summer, which is undesirable for new shoot maturation.

5. Do not check for new roots on potted air layer cuttings in the fall period. Wait till May–June the following year to avoid damaging tender rootlets. Live air layer cuttings in the process of rooting should be healthy and green looking; any brown, shriveled ones are failures and can be eliminated in the fall. Results after overwintering and some time in June are shown in Photos 4 and 5.

Overwintering of the October-cut Green Air Layer Cuttings
Comparing the two year's results, Matthew over-wintered his air layer cuttings in a cool window in a glassed insulated porch; temperatures did not fall below 0º C (32º F), but air temperature on sunny days may have risen to 10º C (?50º F). Bill overwintered his air layer cuttings in a dark outdoor cold frame protected by plastic sheets and covers to exclude sun and wind. Robert over-wintered his cuttings in a basement. The best results seemed to come from holding the air layer cuttings at even temperatures just above freezing up to 5º C (41?º F) in a refrigerator, or an equivalent overwintering situation. Overwintering in cold climates can likely be improved for potential better results.

Table 1 . Rooting results of "April air layered - August cuttings" 2005: vs. unprepared August cuttings 2005.
April-air layered - August cuttings 2005 Unprepared August cuttings 2005
Grower Species/cultivar name No. of scarified shoots
Rooted Air layer
Potted green
Rooted Airlayer
Unprepared Cutting
Dead Cutting
Matthew 'Janet Blair' 2 1 1 4
'Purpureum Elegans' 4 0 4 0
Arnold 'Blue Ensign' 2 0 1 1
Lynne unknown 1 0 1 1
Willem 'America' 1 1
'Calsap' 1 0 1 1 1 1
'Chionoides' 1 1
'Cunningham's White' 1 1
'Hachmann's Charmant' 1 1
'Helsinki University' 1 0 1 1 1 1
'Ponticum Roseum' 2 2 2 1 1
'Saint Michel'
(syn. Mikkeli)
1 0 1 1 1 1
'Nova Zembla' 1 0 1 1 1 1
'P.C.Bosley'* 2 0 1 1 1 1
'Wyandanch Pink' 1 1
R. maximum 1 0 0 0 2 2
R. smirnowii 2 0 1 0 1 1
PJM Group (lepidote) 2 2 0 2 0 0
All Total 22 5 13 13/22 (65%) 14 14
Notes: Column 3 = number of air layered shoots at the beginning of April, and then severed in August as cuttings and potted with plastic bag and placed in a shady spot; column 5 = green leaved air layer cuttings in the process of rooting; column 6 = over wintered and rooted cuttings.
Growers: Matthew Hilgerdenaar; Arnold Werver from Holland; Lynne Melnyk; Willem Morsink.
Cuttings by Lynne were harvested in late October and were over-wintered in a refrigerator.

Table 2 . Rooting results of "April air layered - August cuttings" 2006-2007.
Propagator Species/cultivar name Scarified Shoot
Potted green
Rooted Airlayer
Matthew 'Janet Blair' 4 3 3+ shoots
'Lee's Dark Purple' 2 0 0
'Purpureum Elegans' 2 2 0
'Nova Zembla' 3 0 0
'Catawbiense Boursault' 2 2 0
Robert 'Gloxineum' 9 5 4
Willem 'America' 1 1 Green?
'Calsap' 1 1 1+ shoot
'Chionoides' 1 1 0
'Gustav Melquist' 1 1 Green?
'Haaga' 1 1 1
'Hachmann's Charmant' 1 1 1
'Looye's White'* 1 1 Green?
'Looye's Rose Pink'* 1 1 1+ shoot
'Saint Michel'
(syn. 'Mikkeli')
1 1 0
'Nova Zembla' 1 1 0
'P.C. Bosley'* 1 1 Green?
'Ponticum Roseum' 2 2 1
R. smirnowii 1 1 1
'Wyandanch Pink' 1 1 0
All Total 39 29 13-17
* Unregistered, abandoned no- name plants from Jack Looye, Niagara-On-The-Lake
Column 3 = number of leaved air-layered cuttings severed in August as (green) cuttings and potted inside a plastic bag and placed in shady spot
Column 5 = successfully over-wintered rooted cuttings, June 2007 – green means not yet rooted, but not dead either, so rooting may still potentially occur.
Matthew Hilgerdenaar; Robert Ramik; Willem Morsink.

Rooting Results
The best rooting was obtained from 'Janet Blair'. This air layer cutting method may be useful for those hybridizers wanting to produce some offspring of desirable crosses from this cultivar (see MacMullen 2006). Many other well known cultivars, as shown in the tables will root going into the following spring. Many attempts to root R. maximum failed.

In summary, last year shoots must start to be prepared from about April 1 as outlined in this paper in climates like Toronto's, when snow may be on the still frozen ground. This date likely differs in Pacific Northwest America, where hundreds of rhododendron cultivars are already in flower by early April. An equivalent dormant shoot condition may be present in January there, and the appropriate date must be determined there and in other mild climate places by their residents.

* = unregistered cultivar name.

Matthew Hilgerdenaar
Matthew Hilgerdenaar is a past president and newsletter editor for the Toronto group of ARS District 12, the Rhododendron Society of Canada (ARS District 12).

Willem Morsink
Willem Morsink also served as past president of this group and contributed several articles for the ARS Journal. Mr. Morsink passed away on March 23, 2010, at the age of 74 after a short battle with bone marrow cancer.

Konrad, M. 2007. Tips for Beginners: Propagation by Air Layering, J. Amer. Rhododendron Soc . 61(2): 105-106.
MacMullen, D. 2006. 'Janet Blair' and her children, J. Amer. Rhododendron Soc . 60: 135-136.