An Amateur's Way with Rhododendron Cuttings
Dr. Dr. I. Simson Hall
Reprinted from the Scottish Rhododendron Society newsletter
Rhododendrons are a fascinating genus whose attractions may easily reach the stage of an obsession. Even before this happens, however, the urge is often felt to increase the stock of plants by propagating from cuttings and seed. When this occurs, rhododendrons have really taken hold.
To the addict the fact that cuttings of some species will take anything up to fifteen years to flower means nothing. The fascination lies in getting them going and, of course, showing off the results for the admiration of one's equally rhododendron-minded friends. To be successful at this game a start should be made in the early twenties so that there is some expectation of seeing the results of the labour. Why anyone should start an interest in rhododendrons when approaching the allotted span passes comprehension and can only be a reflection of the attractions of these beautiful shrubs.
It is easy to read one of the books dealing with the propagation of rhododendrons and to imagine rows of sturdy plants all growing happily. Unfortunately it doesn't always work that way and, in spite of having read everything readily available on the subject, success has often proved elusive.
After having made practically every mistake, and having suffered disappointment after disappointment, some principles seem to be emerging and, strangely enough, they correspond to many of the things which have been written, but with an important difference.
All these clear and easy-to-follow instructions have to be interpreted in the light of local circumstances, and that is just where the rub comes...and the mistakes.
Unlike the whole-time professional grower whose livelihood depends on the results, the amateur cannot give continuous attention to what must remain a hobby. A reasonably efficient system has been evolved which does permit the occasional game of golf, and it was thought that others might profit from some of the failures and possibly therefore be able to cut a few corners.
Most gardeners know that the two essentials for rooting any type of cutting are an even temperature and a constant humidity. It is the achievement and maintenance of these conditions which are likely to defeat the amateur.
Generally speaking the temperatures available control the speed of rooting, and the higher the temperature the more moisture is required, up to complete saturation. Many rhododendrons will not root unless they are given these conditions of high temperature and high water content and even so some of the more temperamental ones refuse to root.
The writer has gradually built up a system which provides a range of temperatures and degrees of moisture, and has experimented with various types of rhododendrons under different conditions. At one end of the scale is a wooden box with a sheet of glass over it, and at the other a propagating case, in a small greenhouse, with constant bottom heat and a mist jet. These and some other types of equipment which have been tried and could be adapted to the facilities available to any gardener, are described below.
For a few cuttings the simplest container consists of a pot enclosed in a plastic bag, the plastic being held well above the cuttings by two hoops of wire. The mouth of the bag is tied close and an elastic band just above the rim of the pot constricts the bag and causes any water which condenses inside to run back into the soil. A good diagram of this method can be seen on p. 130 of "The Small Rock Garden" by E.B. Anderson (Pan Books). It is now possible to obtain plastic pots with fitted cloches of clear plastic which serve the same purpose.
A larger number of small cuttings may be put in a wooden seed box with a close-fitting glass lid to prevent evaporation. In this case the top of the compost must be as near to the glass as is compatible with the size of the cuttings, because the smaller the volume of air, the less the leaves will transpire.
If possible, these pots or boxes should be placed in a closed frame shaded from bright sunlight in summer and protected from frost in winter and the humidity should be kept constant. The time taken for rooting will be considerable longer under these conditions than if bottom heat can be provided; nevertheless, the majority of the easier species should root without too much trouble. Amongst those rooted without have been R. mucronulatum and R. primuliflorum.
From these 'cold' methods we come to the use of heat, which not only speeds up the rooting of easy cuttings but also enables one to tackle the more difficult species. A frame with a warming cable gives a soil temperature of 55°F (13°C). At first this was watered by hand, sometimes several times a day in warm weather, but the frame is now equipped with an automatic drop watering system operated on a simple siphon principle. This provides the essential constant conditions and allows the gardener to take a holiday once in a while. The frame is covered with Netlon mesh on sunny days and with sacking in cold weather to retain the heat.
A great many small and medium-sized species have been rooted by this method, including: R. racemosum, R. lepidostylum, R. hirsutum, R. glaucophyllum var. luteiflorum, R. oreotrephes, and R. davidsonianum.
The most sophisticated method which has been used is a propagating case 30 x 20 inches on a shelf in the greenhouse, which gives a bottom heat of 65°-75°F (18 °C). The height of the case has been increased by a light wooden framework 12 inches high, surrounded by polythene, in order to insert a mist jet. The mist supply can be worked by hand according to the weather or, if it has to be left unattended for a week or two, by a time clock which gives a few seconds of mist every hour. At first a layer of peat was put over the heating wires, but this tended to encourage moss, liverwort and even ferns to appear; it has now been replaced by sharp sand on which pans of cuttings are placed and which has proved entirely satisfactory.
Species raised in this frame include: R. crassum, R. russatum, R. lindleyi, R. cinnabarinum var. roylei, R. brachysiphon and R. diaprepes. It may be of interest to know that all the equipment described above is obtainable from horticultural suppliers.
The next point to be decided is the material in which the cuttings are to be rooted. Many materials have been recommended, such as peat, sharp sand, pumice, vermiculite in various combinations: e.g., 1 part fine horticultural peat to 2 parts sharp sand; 1 part fine horticultural peat to 2 parts No. 4 pumice; 1 part peat, 1 part sharp sand, 1 part vermiculite.
Whatever is chosen should be adopted as standard. It is a great mistake to chop and change, for then nothing can be learned from failures, If the variables are kept as few as possible, the difference in behaviour of various species may be understood.
The mixture adopted here is 1 part peat to 1 part sharp sand. This is varied slightly as required. A softer type of growth might like a little more peat, a harder one a little more sand, but this will be a matter of experience and it is better not to be in a hurry to make changes.
The modern plastic pots save a lot of time but are more difficult to manage water-wise than clays. More drainage is required and they do not need as much watering since there is no evaporation through the pot.
On the other hand, for long-term use the clay pots, in the writer's experience, have the advantage that if they are sunk in a well moistened peat in a glass-covered box or frame they will survive a surprising amount of neglect because they will soak up moisture from the damp peat and this has the effect of keeping the humidity constant.
Taking The Cuttings
So far we have not touched the most important point of all for successful propagation - the time to take the cuttings. Unfortunately this is a most difficult thing to decide, for there are few reliable guides. Some say that the smaller the leaf of the rhododendron, the later in the year the cutting should be taken, but this can be no more than a generalization. The most important thing is to take the new wood just when it is ripe enough, which takes years of experience and many failures to learn.
For a beginner who has some plant he is specially anxious to propagate, the best way is to take one or two cuttings each fortnight of each month over a wide period until he finds the time at which they root the best. He can then note the results for future reference.
Some rhododendron seem to root better without heat and without too much moisture; amongst these the types with thin, small leaves appear to fall. The larger-leaved and softer types seem to prefer close conditions. One is constantly having surprises and disappointments, as when two apparently similar cuttings are given identical treatment and one thrives while the other fails. Possibly the two shoots had not ripened at the same time; even cuttings from different aspects of the same parent bush might not be in comparable condition. This illustrates the complexity and difficulty of standardizing propagating techniques.
Now for the cutting itself. How long and how big should it be? There are different opinions as to what constitutes a good cutting, but on the whole thinner, longer cuttings seem to take more easily than thick, short and sturdy ones. On the other hand, they must not be spindly and weak; very often the most suitable shoots will be found away from the vigorous front of the bush.
The length of the cutting varies according to the type. In some of the dwarf rhododendrons it may be difficult to find a cutting an inch long, whereas in well matured bushes of large types it is easy to take them up to six inches long.
When removed from the parent plant the cutting should be placed immediately into polythene bags. It is a great mistake to walk around the garden with a bunch of cuttings in a hot hand and it is essential to keep them shaded until they are inserted in the potting medium. If there is to be a considerable time lapse before they can be potted, it is better to stand them in water to keep them turgid.
Immediately before potting the cutting should be re-cut with a very sharp knife or razor blade and shortened if necessary. The top growth of larger species should be reduced to two or three leaves. A 'heel' is not necessary. In fact it is probably a disadvantage since it prevents proper 'wounding' of the stem of the cutting. This means the removal of a vertical strip, about half to one inch in length, of the outer layer of the stem to expose the cambium layer from the cells of which the new growth eventually comes. The wound should not be deep enough to penetrate the woody centre.
Finally, the stem of the cutting is moistened and dipped in hormone rooting powder when it is ready for insertion in the compost. It must be pushed into the compost and pressed firmly into position to ensure that the stem is in close contact with the mixture, leaving no air gaps to cause the cutting to rot.
Once the cutting has formed roots there are further problems to be faced. Where high temperature and humidity have been used, the roots have been absorbing and have to become used to taking in nutrient solutions before the plant will grow. The plant has to be toughened by gradual weaning from the soft conditions in which it has been living. The professional with elaborate equipment does this part of the operation by use of an automatic 'weaning unit' which reduces temperature and moisture so that plant gradually become accustomed to normal conditions. The do-it-yourself amateur must imitate this process with what ingenuity he can command. One effective way is to remove the pan from the propagating unit and put it in a polythene bag on the greenhouse bench or in a frame, making sure it is shaded. The bag is opened for lengthening periods each day and in this way the plants are gradually acclimatized to ordinary conditions. Naturally, where conditions of rooting are nearer normal, with lower temperature and humidity, less weaning is needed. Where the cuttings are in a box the glass lid should be raised a little each day until the cutting become accustomed to the fresh air.
To prevent any interruption in the growth of the young plants they may now require feeding. Sand and peat do not contain any great store of food and if it is inconvenient or inadvisable to pot on the rooted cuttings, a little weak fertilizer, preferable one of the seaweed derivatives, may be given.
It is at this stage, in the writer's experience, that many rhododendrons turn up their toes and fade out. That famous propagator in the Royal Botanic Garden, Edinburgh, L.B. Stewart, used to say that it was easy enough to put roots on anything, the trouble came in persuading the plants to grow.
The potting mixture is of considerable importance; it should have plenty of drainage material and yet hold moisture without becoming waterlogged, since these young plants dislike soggy conditions. The mixture must be fairly acid, having a pH of 5.5 or even 4.5, and the following proportions have been found satisfactory:
2 parts turf loam, lime free
1 part course sand
2 parts granulated peat
1 part John Innes compound No. 2 or 3
The figures refer to parts by volume; handfuls, trowelfuls or pailfuls, not parts by weight.
Following the work of Dr. Henry Tod reported in the S.R.G.C. Journal of April 1968, the writer has made comparisons between the above mixture and the same with the addition of chopped sphagnum, which have shown interesting results. Where sphagnum has been included the plants seem to be superior in health and growth. Whether this is due, as Dr. Tod suggests, to something in the sphagnum, or to the physical properties imparted to the mixture, remains to be discovered, but it looks as if sphagnum does do something for rhododendron cuttings.
Throughout this stage it is important that the cutting are never allowed to dry out, and equally that the soil never becomes sodden. Protection will be required during the first winter, in a frame or cold house, as cold drying winds spell disaster to young plants. The following spring the plants may be transplanted to the open ground.