Propagation Of Some Deciduous Azaleas From Winter Cuttings
Britt Smith and Frank Mossman
Propagators of Knaphill azaleas and similar deciduous azaleas know that the proper time to take cuttings is when the wood is "barely able to stand up," in other words, "half ripe", in June - July. After careful treatment, roots develop, and there is general agreement that these young rooted cuttings must be in a sufficient degree of health to enlarge or 'push' the growth buds slightly before going dormant in the fall season. If these growth buds fail to 'push' in the fall before the leaves are dropped, then the young rooted cuttings fail to grow in the following springtime, usually.
Cuttings of the same type can also be taken all through the winter from plants placed in a greenhouse in September to induce growth. Both systems are described in an article by David Leach in the Rhododendron and Camellia Year Book 1968.
A glance at any healthy, well-established, deciduous azalea plant growing out-of-doors in the winter reveals leafless branches, hardened wood, and well-developed, prominent growth buds. These buds have already 'pushed' very well indeed. Now, if such wood taken as cuttings could be made to develop roots, we would have another interesting technique. We have been working at this idea for the last three years. We are still far from perfection and may never attain it, but simple methods and equipment have produced promising results.
In transplanting a five year old R. prunifolium one winter, Mossman accidentally cut the plant's main trunk just above the crown. With no little exasperation, the severed stem was reinserted into the soil-sawdust mixture. Months later, this stem had leaves and the broken portion of the stem showed healthy appearing callus, without benefit of overhead plastic or even adequate watering. Later, this experience was related to Smith, who was curious about all the details, and from this episode came his idea of trying winter or leafless cuttings.
We have worked with R. occidentale and Knaphill azaleas. The cuttings have been taken in November, December, and January. Late December seems best. The cuttings are immersed briefly in a weak chlorine solution to eliminate insects and fungus spores. Other sterilants are also effective. Any remaining leaves are removed or soon drop off anyway. The cuttings are four to five inches long, for convenience. Rooting occurs more rapidly after hormone treatment. A 24 hour tip soak in a dilute solution of indolebutyric acid seems to stimulate earlier root formation. Roots emerge from side wounds, from the cut end, and also directly through the submerged stem bark. Wounding of the stem is probably unnecessary, but new roots do seem to cluster at a wound.
Various rooting media have been tried. The medium should be free of fungus spores and provide good drainage. Sand and peat in equal parts have been successful, but Perlite and peat may be even better. Some cuttings have been rooted in pure Douglas fir sawdust. Plenty of artificial light, even continuous light, is indicated, but direct sunlight is apparently too much. The cuttings are kept under plastic. but ventilated often. Temperatures up to seventy degrees are tolerated, but rooting occurs out-of-doors on a south exposure in January-February where temperatures are 35° to 50°. Rooting occurs in three to five weeks. Small tufts of new leaves often appear before roots. Humidity must remain high. Continuous light promotes much early growth at 65° and 70°. Excessive watering is to be avoided because of rot.
We have tried various types of wood, i.e., varying ages and calipers. All will form roots, but the current year's wood of 1/8 inch caliper, plus or minus, may be best.
This technique is not suggested as a substitute for other methods, but only as a supplement. Half ripe cuttings taken in June-July are rooted and 'pushing' months before winter cuttings are taken. The winter cutting provides a second opportunity. The winter cutting will tolerate air shipment that half-ripe cuttings cannot. The temperatures in the mail compartments of planes vary considerably.
We have found leafless December cuttings a useful adjunct to our R. occidentale propagation. Christmas season is our time to collect layers and small plants of this species in its native areas. No matter how many propagations of a particular clone have been made earlier in the year, more are frequently required. Of a very special clone, no wood is wasted regardless of caliper, but usually the cuttings are 1/4 inch or less in diameter. Some varieties have short yearly growth, so that a four inch long cutting may be several years old. In the field, these cuttings are placed in plastic bags with a little water, to await further treatment in the home nursery, where they are cut to four inch length, treated for insects and fungus, given the hormone treatment, and inserted about ½ their length into the growing medium (at two inch intervals) under plastic and light. The overhead plastic is removed after roots form. Transplanting is done as thinning is needed. Plants to one foot high have been produced in six months.
Leafless winter cuttings of other woody plants have been successfully rooted by plantsmen for many years. It is hoped that this discussion will stimulate some interest in this way of propagating deciduous azalea species and hybrids. Sterile media, adequate water and drainage, in combination with proper light and heat, are important. When these factors are properly combined, we feel that a fairly high production rate can be expected. So far, our percentages are low.