Experiments With A Nearing Propagating Frame
Walter Kern, Woodlyn, PA
Since 1958, all my asexual propagation of rhododendrons and deciduous azaleas has been done in a Nearing frame(*), using for the rooting medium the simplified composition and arrangement of layers perfected by Warren Baldsiefen and for the cuttings the hormone treatment devised by David Leach.
The results obtained bore out the claims made for this method of propagation: consistently successful rooting, saving in space and labor and the production of rooted cuttings more vigorous than those produced with methods resorting to bottom heat.
I soon found out, however, that both the deciduous azalea cuttings taken in May/June and the rhododendron cuttings taken in October were fully rooted only by June/July of the following year. This meant that by the time I could lift the rooted cuttings it was too late in the season to permit emptying, disinfecting and refilling of the frame in time for sticking a new set of deciduous azalea cuttings, which at the time were my principal interest.
My first attempt to shorten the period required for rooting ended in failure. While a soil heating cable laid at the bottom of the rooting medium unquestionably speeded up rooting, it also magnified the disease problem, with the consequence that many more cuttings were lost than with the cold method. So I dropped the use of bottom heat. I was back where I had started.
At this point I remembered that ever since I first heard about the Nearing Propagating Frame I had been puzzled by two of its features: first, why was direct sunlight excluded and only diffused daylight used when it is known that plant growth is essentially a function of photosynthesis and, presumably, up to a certain point, proportionate with light intensity; second, why was the extreme wetness of the rooting medium necessary when it is known that both azaleas and rhododendrons require sharp drainage and grow best in an airy, loose medium? I reasoned that if I could find ways of increasing light intensity and increasing drainage of the rooting medium while maintaining the humidity of the air above the cuttings, I ought to be able to speed up the rooting of the cuttings sufficiently for my needs. So, although the instructions for the use of the frame warn that changes are likely to be expensive in terms of failure, I decided to take the chance and find out for myself.
When a windstorm in 1964 damaged part of the roof of my Nearing frame, I took the opportunity of inserting a 3' wide strip of translucent, corrugated green fiberglass in the top of the roof. Subsequently I replaced also the sides, and a year later the entire roof with the same material. Concurrently with these changes, I increased the drainage in the bins by drilling about a dozen one-inch diameter holes in the floor of each bin. As I had noticed that during heavy winds, especially those from the north and northwest, I was unable to maintain the desired high humidity in the bins, I applied weather-stripping to get a tighter fit between sash and frame. To the same end, I also used shading of the roof during the hottest part of bright, sunny days. Furthermore, I used plywood sheets to cover the sashes overnight whenever a sharp drop in temperature was expected. In addition, instead of the weekly flooding of the rooting medium, I adopted the method of sprinkling the foliage of the cuttings whenever the absence of condensation on the underside of the sash glass indicated an excessive drop in the humidity.
Most changes were made gradually, over a period of years, and with extreme caution. Most were fully adopted for the entire frame only after they had been tested in one of the two bins, as at no time did I dare to risk complete failure.
While the care required for the operation of the Nearing frame with these changes is considerably greater than that needed for the successful operation of the frame as originally conceived, or for the operation of an electronic mist system, they have unquestionably solved the problem which gave rise to them in the first place. As a result I have been getting two crops of rooted cuttings from each of the two bins for the last four years.
The first crop consists of deciduous azalea cuttings stuck during the last days of May or the first days of June. They are lifted late in September and transplanted to a cold frame, where they are shaded till late fall, then covered for the winter. The second crop consists of large-leaved rhododendron cuttings stuck in October, lifted in May, transplanted into a cold frame in the field, where they are shaded, to grow until the following year.
Those desiring further information about the Nearing frame, the selection and preparation of cuttings, and the propagation of azaleas and rhododendrons in general, are referred to Rhododendrons of the World, by David Leach.
(*) For those not familiar with the Nearing Propagating Frame, an over-simplified description would be: a box consisting of two bins arranged lengthwise containing the rooting medium, each covered by a 3' x 6' sash, with a superstructure open to the north only, with opaque sides and an opaque roof slanting to the south, so as to exclude virtually all direct sunlight.