Propagation by Very Soft-wood Cuttings of
Rhododendrons and Other Hard-wooded Plants
Koichiro Wada, Yokohama, Japan
Reprinted from The Plant Propagator
Most propagators know cuttings of annual plants and perennials root very quickly under mist or in a closed frame. Cuttings of some kinds of soft wooded plants root in a few days. But cuttings from very soft growths of rhododendrons, etc., generally fail to root under improperly controlled mist system or mist system with poor mist nozzles or in a closed frame and they usually rot in the rooting bed before rooting.
Try to dig out of the ground an evergreen shrub in growth with roots intact and expose the whole plant to air. Watch what will happen. It will start to get shriveled from the softest tip of growth. Before the new growth is completely shriveled, soak it in water at the root part. You will see the softest tips of growth, once shriveled, can never recover and turn brown and perish. This phenomenon is more clearly observed with a hard-wooded plant with gradual growth from spring to fall, such as Aucuba japonica.. Up to a certain point on the wood, depending on the hardness, it can recover from the shriveling but the softer wood at the tip part cannot recover. This will demonstrate to you that new soft growths of hard-wooded plants have their water absorption mechanism in each cell destroyed by the one-time shriveling and will never recover.
When such soft tips of growth are used for rooting, the soft tip cuttings must never get shriveled and must be always kept turgid from the time the cuttings are detached from the mother plants until they root in the rooting bed. This is the absolute requirement to get success from very soft tip cuttings of hard-wooded plants.
As you will see from the above explanations, the propagation houses must always have 95% to 100% humidity with a uniform mist covering all over the foliage of the cutting without missing any parts. This covering of mist must be done intermittently with a proper interval since it not only supplies the water from the foliage to the cuttings and prevents evaporation from the foliage to keep them turgid but, by evaporation, brings the air around the foliage of cuttings to 100% humidity. Reduced humidity may cause the soft cuttings to get shriveled, if changed to too low humidity. Not to have the humidity change too much, covering the foliage with fine particles of mist spray is absolutely necessary at a proper interval. The best interval of misting is automatically obtained by a properly located electric "leaf" of the electronic mist controller. Uniform coverage of mist is obtained by using nozzles which are most skillfully machined to spray to all directions an equal quantity of very fine mist particles of equal size. When the mist particles are of equal size, they get an equal quantity of negative electricity and they disperse, keeping equal distance between each particle. Thus, a most uniformly dispersed mist pattern is obtained.
The mist propagation house should have a special structure to permit chimney-type ventilation with no side ventilation and with no forced ventilation. The cutting beds must be on the ground and not on a raised bench. Of course, proper bottom heat is very important but the air surrounding the cuttings must be kept cool, say 60° to 70° F ideally and, most importantly, must be saturated in humidity.
The general practice of shortening the leaves or reducing the leaf number is very much beneficial if the cuttings are from the kind of plant which has poor water absorbing power. For such a kind of plant to keep the cuttings safely turgid this practice may be absolutely necessary. Also for such a kind of plant the cuttings must be taken with a short stem.
If you can handle your very soft wood cuttings without getting them shriveled and if you can create such an ideal environment in your propagation house, you will find a miracle in the field of cutting propagation of hard-wooded plants. They would root very quickly before callus is formed and grow out very quickly if fed properly by a much dilute liquid fertilizer. If you find your cuttings rot from the cut ends, perhaps the environment of your propagation house is bad-, the trouble probably is not from Fusarium, so you should try to improve the environment to more ideal conditions.