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Journal American Rhododendron Society

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Volume 22, Number 2
April 1968

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Knaphill Azalea Propagation in the Northeast
Lawrence L. Carville, Middletown, Rhode Island

        Much has been written over the recent years proclaiming the merits of the group of deciduous azaleas commonly referred to as the Knaphill azaleas. Although this subject is not entirely new, I feel that the introduction of this group from England provides both the grower and connoisseur with a most stimulating challenge. These plants are dependably hardy throughout the northeast, are vigorous in growth habit, easily propagated, and provide interesting accent in every landscape plan. I was first exposed to the beauty and vigor of these azaleas in 1957 and was privileged to be associated with Dr. A. F. Serbin at Tumble Brook Nursery in Bloomfield, Conn. Over a span of almost a decade we developed several techniques in the propagation and culture of Knaphill azaleas which I feel may be of interest to other growers.
        The Knaphill azaleas are predominantly North American in origin in that six of the species used in their development are native to this country, namely, R. viscosum, R. nudiflorum, R. calendulaceum, R. speciosum, R. arborescens, and R. occidentale. Anthony Waterer began in 1860 to cross the Ghent azaleas with the Chinese molle and calendulaceum, the flame azalea. These crosses were furthered with the addition of the 'Albecan' (occidentale x molle) and arborescens, the sweet azalea. The Knaphill strain originated at Waterer's Knaphill Nursery but was further developed by Goldsworth Nursery, at Ilam Estate, Christchurch, N.Z., and beginning in 1922 at Exbury by Lionel de Rothschild. This rather extensive but interrelated breeding gives rise to four sub-groups of the Knaphill hybrids: Knaphill, Slocock, Ilam, and Exbury. Most trade publications today make little differentiation other than to list the group as Knaphill or Exbury hybrids.
        Our experience with the Knaphill hybrids indicates that as a group they are completely winter hardy and have also tolerated some extremely dry summers. Although Frederick Lee1 makes some rather definite zonal recommendations for growing these azaleas, I feel that they may be grown as far north as a line extending between Buffalo, Albany (N.Y.), Pittsfield, North Adams (Mass.), Concord (N. H.), and Augusta (Maine). There are, of course, exceptions both above and below this line, but generally speaking I would defend this as an arbitrary northern limit. More difficulty is encountered with these plants as we move south since they do not tolerate the extreme summer heat of the southern states. I agree with Lee that probably Atlanta, Georgia, is the most southerly point at which they may be grown successfully.
        Tumble Brook Nursery is located in U. S. D. A. zone 6, and the annual precipitation is recorded as 43 inches. The annual mean temperature is 39 degrees F., and the annual mean maximum temperature is 60 degrees F. However, we recorded temperatures as low as -24 degrees F. in December 1962 with no snow cover and winds at velocities of 45-50 mph. During this same winter, we experienced fourteen consecutive days of below zero temperatures with no substantial injury to Knaphill azaleas of any age.
        In our earliest years of propagation, we soon found there was a great deal to learn before we could assure ourselves that one-year-old liners would be available to fill our production schedule. So many problems became apparent from time of cutting to the first frost that each season became a new challenge, placing us in the role of the observer and student. Nature is a demanding teacher and has demonstrated that although we have much to learn, some progress has been achieved.
        Under normal growing conditions, we began taking softwood cuttings in early June and have taken them as late as August 28th. I found that the optimum time was the period between June 10-15th when cuttings are still green, semi-soft but not sticky, and before apical buds are evident. Cuttings are collected early in the day from stock plants which have been well watered at least two days previously. All cuttings are dipped briefly in a solution of captan- sevin-wiltprof2, drained, and stored in plastic bags in a cool shaded area until stock. I have found the cuttings may be stored for several days in this manner without deterioration from wilt, fungus, or bacteria.
        In preparing cuttings for the propagating bench I strip all lower foliage leaving two or three pairs of leaves; pinch the top growth, and give a light side wound where necessary. Normally, only the heavier cuttings and some of the yellow hybrids (more difficult to root) are wounded. Cuttings are dipped in number three Hormodin and stock in the benches. I prefer a medium of 100% European sphagnum peat which has been thoroughly shredded, soaked, and treated with Aquagro. The addition of Aquagro, a commercial wetting agent, hastens capillary action within the medium and insures a uniform distribution of moisture which is critical when dealing with softwood cuttings
        Overhead intermittent mist is controlled with a time clock to apply a misting spray over all leaf surfaces for sixty seconds every six-and-one-half minutes. The time of the mist cycle is most important and must be adjusted to each local situation. Foliage most not become dry during the first several weeks when daytime temperatures within the propagating house may rise to 90-95 degrees F. The mist interval is less frequent on cloudy days and is gradually discontinued after six weeks when rooting is apparent. A bed temperature of 70 degrees F. is maintained and I have found this to be very beneficial since without bottom heat bed temperatures have sometimes dropped as low as 55 degrees F. in mid-July. Extreme temperature fluctuation has a delaying and often inhibiting effect on root initiation.
        Most cuttings are lifted from the propagating bench after eight or ten weeks and flatted for growing on. Growth is continued under lights in the propagating house until Sept. 15th at which time the cuttings are placed in cold sash houses for the winter. I have found that unless rooted cuttings make some growth after flatting and prior to wintering, a large percentage of these plants will not break into growth the following spring.
        A recent innovation which I utilized to eliminate the need for chilling and fall lighting was to plant rooted cuttings from the propagating bench directly into prepared beds in the sash houses. The mixture in these beds consisted of sterilized soil, peat, and perl lome (1-2-1 by volume.) These sash houses were heated with Reznor LP gas heaters to maintain a minimum night temperature of 50 degrees F. Bedded plants continue to make root development throughout the winter until February 1st when the minimum temperature is increased to 60 degrees to promote top growth. Lights are turned on February 18th until May 1st at which time the plants are bushy, 8"/10" in size, and have been pinched twice.
        Once cuttings have wintered their first winter and begin making new growth in early April (in unheated sash houses) they have passed the most critical period of their development. I believe our most substantial losses occur in the period from rooting to outdoor bed planting.
        Cuttings are bedded in open beds which are prepared in late April. Fresh shredded peat moss is thoroughly tilled into a sandy loam planting mix which is then fertilized with a castor pomace-cottonseed meal and triple superphospate material at a rate of 5 pounds per 100 sq. feet. Plants are spaced six by six" in the beds, mulched with 1½" of Stazdry, and shaded with snow fence.
        Weeds have not been a serious problem in the mulched beds but I have used Vapam as a soil sterilant on an experimental basis and have had excellent weed control without a mulch. The Vapam can be applied in late fall or in early spring at least fifteen days prior to planting. Soil temperatures must be above 50 degrees for an application of Vapam so I prefer to apply this material in late September or early October.
        Plants are pinched again at time of planting to induce continued branching. If new growth is soft at planting time, a spray of Wiltpruf is applied to prevent wilting. From time of transplanting until late fall, the plants receive no additional feeding and very little cultural care other than irrigation when necessary. Snow fence shade is removed in mid-August to allow the plants a gradual hardening process. When the ground is frozen, we apply d marsh bay mulch over the bedded liners. This mulch is left on the plants until mid-March and is removed before buds begin to break. Larger plants are grown in fully exposed areas, receiving no additional mulch in the late fall and have withstood the rigors of winters in the northeast without damage.
        To discourage damage from rabbits during the winter months, all plants are sprayed with a solution of Arasan 42-S3 plus a spreader sticker. This spray is applied in late fall and remains effective throughout the winter. Temperature should be above 40 degrees F. when application is made to insure that spray material has an opportunity to dry without freezing. Since we began using this repellant several years ago, I have encountered no damage from rabbits on any of our plants.
        I feel that the Knaphill azalea hybrids deserve a place in the landscape field, the wholesale nursery, and in the amateur's garden. Properly used these hybrids offer outstanding selections of color when in bloom, are free of disease pests, and many varieties combine to provide vivid fall foliage displays. No opportunity to speak or write about these plants is complete for me without passing along my personal selections of the outstanding varieties as I have observed them over the years:

Color Name Hybrid   Color Name Hybrid
red 'Krakatoa' Knaphill    yellow 'Annabella' Exbury
  'Gibraltar' Exbury     'Old Gold' Exbury
orange 'Ginger' Exbury     'Golden Oriole' Knaphill
pink 'Cecile' Exbury   white 'Oxydol' Exbury
  'Sylphides' Knaphill     'Toucan' Knaphill
        bi-color 'Pavane' Knaphill
          'Marionette' Knaphill

(1) Lee, Frederick P. The Azalea Book, 1965.
(2) Cutting dip: Wiltpruf - 1 part to 20 parts H2O
     Add to above solution:
          2 TB/gal Sevin 50% WP
          2 TB/gal Captan 50% WP
(3) Arasan 42-S - 1 gal to 3 gal water, 1 pint sticker to 4 gal of Arasan solution


Volume 22, Number 2
April 1968

DLA Ejournal Home | QBARS Home | Table of Contents for this issue | Search JARS and other ejournals