More About Rhododendron Cuttings
By G. G. Nearing, Ramsey, N. J.
Delivered before the New York Chapter of The American Rhododendron Society on October 17, 1956.
There is a growing conviction among men who handle plants that a rhododendron propagated from a cutting or a layer makes a better specimen than one propagated by graftage. I have been completely convinced of this fact for so many years, and can point to so many instances where the superiority is evident, that it is hard for me to understand how any propagator can cling to a belief in graftage. Yet many do, and many others who practice and preach cutting propagation have been unwittingly strengthening the argument against cuttings.
For the cutting has a weakness, and unless this weakness is understood and provided against, the cutting-grown plant does not live up to its possibilities, may not in fact live at all, and so seems to justify the arguments against it.
E. H. Wilson was one of the first to urge propagation by cuttings, but since during his lifetime no one had devised an adequate method of rooting them, his opinion on the subject was only a theory. Now we can see the facts. When a cutting-grown rhododendron and a grafted plant of the same variety are placed side by side, and both treated as they should be, the grafted plant will make better growth than the cutting for two, three or four seasons, after which the cutting-grown plant will gain in vigor, and by the sixth or seventh year, will usually have caught up to and passed the grafted one in size and shapeliness. From that time on, there is no comparison. The cutting-grown rhododendron is superior to the graft in every respect.
However, the superiority is actually demonstrated before the sixth or seventh year. In selecting these plants of the same age, it is not fair to date them both from the moment of propagation. At that moment an important part of the graft, its understock, is already at least two years old. To grow it into that size has not only consumed time but cost money, and whether you look at it from the standpoint of the horticultural scientist or the commercial nurseryman, the plant grafted in 1956 is older than the cutting stuck in 1956. If this difference is corrected for, then the cutting will normally surpass the graft in about four years.
However, in order to prove his point, or to make money faster, or both, the owner of the cutting-grown plant is likely to pursue a practice which may cause it to die about the time when it should be passing the graft, or sometimes much sooner. To understand this failing, let us examine the weakness of the cutting and the method by which it must be corrected.
If you make a cut directly across a stem and examine the cut end with a lens, you will see concentric rings like a target. The innermost rings represent each a year's growth of that part of the stem, and are wood. Outside the wood is a very thin, soft, whitish ring, which is the growing layer, laying down new cells all the time both on the inside and on the outside. Those on the inside are more wood, those on the outside bark. The bark too has distinct layers, one of which is a secondary growing layer.
Examining the section with higher magnification, you can see within the wood rings of vertical tubes that have been cut across. Through them sap sucked from the earth is piped from the roots to the leaves, where it is manufactured into sugar and plant foods derived from sugar. In the inner layer of the bark is another set of slenderer tubes by which the cooked sap containing the food is piped back from the leaves into various parts of the plant where it is needed for growth and the maintenance of life. The crude sap goes up through the tubes in the wood and is returned through the tubes in the bark.
Now if a similar section of root is examined, the layers are seen to be arranged differently. The tubes for conducting crude sap upward are not confined to central rings, and those which bring down the cooked sap are not in the same clear layer of bark. The arrangement is much more complicated. Yet food must be brought down to the roots, or the roots could not grow, and as we know, every plant must have a large and active root system or it will surely collapse. The tubes for piping each kind of sap to the places where it is needed are right there in the root of course, but they are not arranged in the root as they are in the stem.
If you have ever done any pipefitting, you will see at once that there must be a point where the stem joins the root, at which special cells contrive the transfer of each kind of sap to its proper destination. This point is the crown of the plant, located just about at ground level, and is exceedingly important to the whole scheme of life and growth. A seedling begins to develop a crown immediately after the seed germinates, and is careful from that time on to keep it always developing and functioning.
Examine a rooted cutting. There is no crown. Thread-like rootlets spring from various points in the bark of the stem, and from confused lumps of callus where the stem has been wounded, but there are no main roots and no root system. The new plant lives only by a makeshift arrangement. Yet it must keep two kinds of sap moving to their proper destinations, and without the help of specialized crown cells. There is no crown. To convert the base of the stem into a crown, the new plant will first have to tear down the cell structure already there, then substitute for it a set of entirely new tissues. This is a major task, requiring a great deal of energy just at a time when the organism is badly handicapped in all its functions by the fact that the crown is not there.
The graft, on the other hand, has a crown-the crown of the understock, and as soon as the stem tissues of the scion form a stable union with those of the understock, it can handle the sap with reasonable efficiency. The weakness of the grafted plant is that, although it can quickly organize a fairly good arrangement for the transfer of sap, the tissues of the two parts of the stem never reach complete agreement as to how growth should be conducted. The older the plant, the more evident this disagreement at the point of union usually becomes. Only in exceptional cases do these two stems of unlike parentage settle down to an entirely harmonious growing agreement, and maintain full vigor.
In the cutting all the cells are of the same parentage. Eventually they can arrive at perfect agreement and maximum efficiency, but for a long time they are so desperately busy trying to get sap enough through the makeshift arrangement which substitutes for a crown, that they have little energy left for the necessary rebuilding of tissues required before there will be a true crown to handle the interchange. Consequently the construction of this crown must proceed, as it were, only in spare time.
Now how can we give the cells of our cutting plenty of spare time to accomplish this vital task? Not, surely, by loading them with fertilizer chemicals and spurring them into hasty overgrowth. If we do that, the problem of the makeshift crown is greater than ever. It can barely take care of the sap flow for normal growth, and still have energy left over to build a good crown. If we force it to treble or quadruple the flow of sap, which rapid growth requires, how can it possibly take care of this added burden and still grow its crown? Yet that is exactly what the unthinking propagator tries to force upon it unless warned of the consequences.
To make matters worse, many cuttings are rooted under greenhouse heat. Now these make just as good plants as cuttings rooted in a cold frame, provided they are properly hardened off. But this hardening is a very slow process, much too slow for a man in a hurry to make a lot of money. Here again, senseless forcing with chemical fertilizers can only hasten disaster. The new plants are completely confused about when to expect winter frosts, which must be prepared for, at least two months in advance, by progressive hardening of the wood, another exhausting process. Harassed by the struggle with a makeshift crown and a disproportionate sappy top growth for which hasty root development cannot adequately provide, the cutting could not prepare for winter even if it had not been upset in its measuring of the seasons. (Its summer started in December or January.) Yet it cannot be coddled along in the green-house forever, and is in no condition to face the cold, even with frame protection.
The grower who pursues such practices, instead of raking in all the money his greed has dreamed about, is likely to find himself raking up a lot of dead plants, with nothing to show for his time and labor.
Let the newly rooted cutting rest and take its own time. Give it a normal soil, about one-third peat, no fertilizer, and protection from the worst extremes of weather. When, relieved from strain, it has succeeded in replacing its old stem tissues with a satisfactory crown, it will give notice by vigorous root growth that it is ready for bedding out. This takes one to three years, depending on how successful it was in putting out its first roots.
I simply carry all my rooted cuttings in 4-inch pots plunged in peat moss in a shaded, north-facing frame. When the roots begin to bind in the pot, they are ready for bedding out in spring. They receive no fertilizer at any time, because rhododendrons do not want fertilizer or manure. Fertilizers do not kill rhododendrons, but rapid growth does kill them, and nothing which forces growth should ever be applied to their roots. When they are ready, they will make all the growth that is good for them.
And now a word about growing rhododendrons in pure peat. The roots develop wonderfully in peat, but have great difficulty in jumping from the peat into normal soil. This is probably because a given plant grows different kinds of roots according to the kind of soil it is placed in, just as it grows different kinds of leaves for sun or shade. And just as the leaves refuse to change their structure when the plant is moved to a new environment, but must be dropped, and new, suitable ones grown, so the ill adapted root cannot grow a well adapted extension from its own tip. A properly adapted root must start from near the crown, but if the crown is surrounded with pure peat, then a root adapted to normal soil will not grow there. The result is familiar to anyone who has set out peat-balled plants. The roots simply refuse to leave the peat and so cannot develop an adequate system.
Peat stays frozen longer than any other component of soil. Not only is the root ball too small when the roots refuse to grow out of it, but the hard frozen condition prevents the taking up of moisture, and robs the plant of its hardiness doubly.
On the west coast, where cold winters are unexpected, except that they arrive rather regularly every four or five years, when an exceptional winter devastates the rhododendrons, as it seems to do just about that often, much of the damage is due to planting in pure peat and using fertilizers. As soon as western growers learn to expect the unexpected, which comes so often and so surely, they will perhaps abandon fertilizers and pure peat culture. But let me add that many eastern growers, who know well that the winter is going to be cold, cannot keep their itching hands off the fertilizers and the peat, and blame their losses on the Hungarian revolt or the Suez Canal.
One last thought. Many of our misconceptions about plants come from misuse of the word "feeding." We cannot feed green plants. Green plants make their own food by combining air and water to form sugar, and this is the basis of all food. Fertilizers do not feed plants, but drug them. With farm crops this does no harm. A stalk of corn will die anyway as soon as it has borne its grain. But with most ornamentals, fertilizers simply give an oversupply of certain substances necessary in small amounts, though incidental, for the process of making food. These substances are not used up unless a crop is taken away, and additions of them are not only superfluous but drug the plants into unnatural and often disastrous excess growth.