Successful Rhododendron Propagation Without a Greenhouse
By Warren Baldsiefen, Rochelle Park, N. J.
Fig. 15. Propagating cold frame showing the typical construction.
The comparatively recent interest, and the ever increasing demand for rhododendron hybrids in this country, and the realization that these plants grown on their own roots are better able to exhibit themselves and endure adverse conditions, are responsible for this article.
There are now many methods of rooting these fine plants, each best suited to its user for his conditions and needs, but all of them worthwhile and of merit. The system outlined below, far from being new, has been in use nearly a quarter of a century, invented by a well known botanist and horticulturist, and one of the few pioneers of rhododendron culture in this country--Guy Nearing. Devised originally for the rooting of large-leaved catawbiense hybrids (ironclads) at that time believed impossible to strike from cuttings, it has also proven itself beyond a doubt for practically all broad leaved evergreens in garden use today;--pieris, ilex in variety, leucothoe, cotoneaster, berberis, etc. And while it is true that unfavorable percentages have resulted with most of the narrow leaved genera (thuya, juniperus, tsuga, taxus, and chamaecyparis) exceptionally gratifying have been the results with many of the newer evergreen and deciduous azaleas heretofore considered impractical from cuttings.
Here then is the way of propagating, either for the commercial grower or the hobbyist. The device is inexpensive to construct, durable and lasting, and requiring no special training or skill to achieve astonishing results that are consistent. Yet not being perfect, experimentation has never ceased in an effort to better rooting percentages of some of the more difficult varieties; simplify the medium and extend its propagation uses. Positive improvements of past tests are incorporated in the article. Others not included are yet inconclusive but by all means a step forward. Modifications are sometimes slow and must be carefully considered.
The rooting shed is best described by the photo. With the exception of the bottom of the box which is tongue and groove pine flooring and the roof which can be any strong weather resistant material the remainder of the structure is cypress or redwood. Frames built with this specification will last over 20 years.
The rooting box of 1 x 12 cypress or redwood is partitioned in the middle forming two separate units each 31 inches wide and 70 inches long, (inside measurements) 12 inches deep at the outside end, and 18 inches deep on the inside (the partition). On top of each unit conveniently fits one standard hotbed sash sloping toward the outer end. To obtain this pitch a 1 x 6 board 72 inches long is nailed on the front and back of each unit beginning at the partition and sloping towards the outside ends. "A" in the photo. The bottom of tongue and groove pine flooring is nailed as tightly as is convenient without the use of clamps. The wood for the framework of the superstructure can be practically any width and thickness. 1 x 3 does the job as well as 2 x 4' s. It rises 60 inches above the box on the front end (facing north) and 12 inches above the box on the back end. Care should be taken in nailing on the roof or shade that it does not reach the ground. This space seems necessary for air circulation, preventing the accumulation of hot air during those "dog days" of summer.
Once constructed the frame should he oriented, the open end facing directly north. Unless the variation correction on the compass is understood, the north star, polaris, which is easily identified by nearly everyone, is the safest marker. In its orbit it never varies more than one degree east or west of true north, which is insignificant. In the area where the frame or frames are to be set, sight on the north star with three sticks forming a line. In tests made by Mr. Nearing with rhododendrons where the frames were intentionally offset 15 degrees east and west of true north the cuttings soon died. Whitewashing the glass may have prevented this but it would also defeat the purpose of the frame: to offer the cuttings as much reflected light as possible without any direct sunlight. Then too with the sun beating down on the frames during the middle of the day or thereabouts, watering must he increased. True during that time of the year when the sun rises north of east and sets north of west, rays do strike the glass in the early morning and late afternoon. The angle is so small most of them are mirrored off the glass and those that do enter the frames for only a short period are not very intense and are innocuous. Place your frame in as bright a location as possible. The depth it is to he set in the ground depends on the water table in your locale. Under the driest conditions it should not be sunk more than 10 inches allowing 2 inches out of the ground at the shallowest end. Earth may be mounded against this grading, away from the units, as in the foundation of a home. If sand is used weeds are kept at a minimum and during the rainy seasons prevents muddiness.
Care should be taken to make certain the frame is level. Do not trust the naked eye for this as the slightest tilt will cause extra work in preparing the medium and later in watering. The measures of the various substances comprising the medium are figured for the size unit described above. If a larger or smaller unit is desired the medium should be altered proportionately, since for unknown reasons this combination produces the higher rooting percentage with continuity. The relationship of each of the three layers in the medium to one another is unknown. The roots strike in the top layer and the purpose of the bottom two can only be approached in theory.
The bottom layer of three bushels of peat moss and one bushel of spent mushroom manure is mixed thoroughly and then leveled and tamped lightly. Its depth is approximately 3 1/2 inches. The use of the mushroom soil has proven far more satisfactory than any other source of nutrients yet attempted, and there is the belief that perhaps some hormone is present beneficial to rooting in the decomposed mushroom mycelium. An analysis of the spent manure will probably show results similar to these: nitrates--very high; ammonia--extra high; phosphorus--medium; potash--extra high; calcium--very high; magnesium--low; aluminum--high; manganese--medium. The lime used in mushroom farming is often in the form of gypsum (calcium sulphate), non-alkaline and responsible for the high calcium percentage, necessary to root growth. That the above elements and others are required for the well being of any garden plant is unquestionable, but their direct role in root formation is doubted since it is "'the hormones which are directly responsible for the striking of roots. Perhaps in the process of watering (later described) some nutrients are absorbed which will allow the un-rooted cutting to manufacture natural hormones for activating those cells responsible for the development of roots. However the ability of an un-rooted cutting to absorb nutrients is notoriously small. If mushroom manure is not available topsoil or dried cow manure may be substituted with worthwhile results. And if the hybrids grown on the west coast are of an easy nature where rooting is concerned a bottom layer solely of peat moss may suffice. These trials will have to be made in the west. The peat moss used, aside from being acid and having average water retentive capacities, may be of any brand. (Some peats fool us and absorb very little water. This may be tested by filling a container with half water and the remainder peat,--dry peat. Some peat absorb all the water and are not fully wet themselves while others absorb all they can and the container still has free water left in it. Since the bottom layer thru evaporation and vaporization keeps the upper two layers moist, a peat able to retain a large amount of water should be used.) The middle layer of ½ bushel sand and ½ half bushel of peat may act as a buffer between the sopping bottom layer and the sand in which the cuttings root. Cuttings that have rooted find this layer, which is about one inch thick, very welcome. This middle layer must also be leveled and tamped lightly. The top layer of washed propagating sand--three bushels per unit--6 bushels per frame--forms a layer about 3 inches in depth. This is evenly applied and leveled. Then water the unit with a light sprinkle so as not to disturb the level. Watering is continued until it rises about ¼ to ½ inch above the sand. The initial watering understandably takes longer than the rest for it is here that the peat moss takes on a large quantity of water. The sash is placed and the frame is ready for cuttings. For rhododendrons do not choose a sand that is too coarse or too fine. A sand too coarse dries rapidly and a sand too fine will not provide proper aeration. In the latter case roots will strike on the surface of the sand and down to a depth where lack of oxygen prohibits growth. Under these conditions rooting is usually poor. Where rhododendrons alone are considered a plaster sand is much too fine when used by itself. Mixed with peat it is an acceptable medium for azaleas but we have never tried it on rhododendrons. For rhododendrons, a sand that will pass 100% through a 3/16 inch screen and 80% of this through a 50 mesh screen, is very satisfactory.
As an alternate the top layer which is to receive the cuttings may comprise bushels of sand and one bushel of peat moss thoroughly mixed. Or it may be by quantity one half sand, one half peat. However with the latter admixture where the medium tends to be peaty and light we in the colder areas, during the course of a winter where there is much freezing and thawing in an open winter, are plagued by the problem of having to reset these cuttings after bad thaws. A mixture of this type is not advisable except possibly in the milder areas such as portions of Oregon and Washington where these conditions do not exist. Another fairly satisfactory medium is half peat moss and half vermiculite. It is not exceptional, but easy to prepare and handle, though it heaves the cuttings with the slightest thaw. The medium of peat and sand whatever proportions, tends to throw out a huge root mass but one that is feebly attached to the catawbiense hybrids. Whether there will be the same reaction in rooting the newer hybrids of the west coast and milder areas can only be determined by trial and error. This feebly attached root system in the sand and peat mixture can be overcome by the use of root stimulants but in my nursery these are yet in the experimental stage and until such time that conclusive results can be offered, the trials and experiences of the individual hobbyist or home gardener are as valuable a source of information as anything that might be printed here. To date all our rooting has been done without the use of auxins. Although some frown on the use of these acids as a direct aid to rooting, their reasoning is unsound and in the future of propagation these hormones or auxins will assume an important role.
Success has been spotty with rooting rhododendrons using this system where the cuttings have been taken in early July and potted up two months later. It can be done but as a constant regular thing it has not been successful. There may be the great value of hormones. For azaleas, both deciduous and evergreen, the picture is more favorable. Cuttings set in late June or July or early August are ready for transplanting to flats or beds the same autumn.
We take our rhododendron cuttings in the autumn, in New Jersey any time after the first week in September through the middle of October but this will vary with the area under consideration. On stool plants grown only for cuttings, the wood of the second growth is removed after it has had time to harden off. Although a few of our varieties put out only one growth a season, the majority throw out two. To explain the feel of a "ripe" cutting presents a problem and finding one variety ripe is no indication that all are ready since inherent qualities characteristic of the individual variety govern this factor. In the hybrids it may all be blamed on their progenitors. Fortunately being so exact is not important or at least has never proved to be with this method where over a period of years cuttings of the same varieties set at dates varying from late August to late October rooted almost identically. Choosing almost the exact time is only important in those types which are extremely difficult to strike. Suffice it to say that cuttings are ready after the stickiness has gone, when the leaves have fully obtained their natural color, and they are rigid enough not to wilt should a hot day occur. Delay making the cuttings rather than take them too soon. Some hybrids yield more cuttings than others without damaging the stock plant. A safe rule to follow is not to remove more than from 5 to 33 percent of the terminal shoots from any plant. As high as 100 per cent have successfully been removed as a regular thing from some varieties. On others when too many are taken, usually as a result of being too eager to build a supply, the following year practically none was available. More than once a bush was destroyed.
The cuttings or scions should not be over 2½ inches in length and those as small as 1¼ inches develop into equally fine plants. If cuttings are longer than suggested above they will enter or rest upon the middle layer. Should this occur, although it has no effect on the percentage of rooting, the root system form on the bottom of the cutting only developing a root mass that does not anchor the plant firmly, much the same as though the entire top layer of the rooting medium were sand and peat. As such a plant matures wind can ruin or so weaken a plant of this character as to make it almost useless. Keep the scions on the short side. The belief that a short scion makes a small plant is drivel.
Along the shank of the cutting inflict a wound with a knife or other sharp instrument about an inch long and deep enough to cut away a small sliver of the xylem. Now placed in the sand, this cutting will root not only along the bottom but on the side forming a well balanced root system--the main requisite of a mature specimen. (Incidentally wounding an azalea cutting is not only unnecessary but seems to delay rooting since before rooting it is the nature of the plant to develop a callus over the wound, then root. And since azaleas readily throw out roots along the stem this wounding is only wasted time.) Apparently roots race out through the sand in all directions in search of water and mineral elements. Many rooted cuttings develop so large a root mass that a 1 inch pot is inadequate. In such cases the roots are trimmed to fit the pot and to allow ample space for the potting mixture in which the permanent root system is started. A cutting potted with a root ball the size of a shelled black walnut, will have filled a pot by spring.
Before placing the cuttings in the frame the number of leaves must be reduced on each to three. A scion usually has from 7 to 9 leaves per rosette. Pull or cut off all but three spaced equidistantly around the stem. On our ironclads, grown in the sun, the length of the leaves of most varieties is about ¾ inches. Longer leaves are cut in half or that portion may be removed which twill allow the cuttings to conveniently fit in the frame without too much overlapping. Or they may be spaced at greater intervals and left entire. Where a flower bud exists on a cutting it is wise to remove it before placing the cutting in the frame. If it is not removed, when it opens in the spring, it draws too much water and energy from the un-rooted cutting, leaving it in an unhealthy state. Added to this the inconvenience of removing the expanding buds in the frame is reason enough to do the de-budding prior to setting the cuttings. If the cuttings are set 2½ inches apart in either direction a unit will hold approximately 300, or 600 per frame. A unit will hold 1200 azaleas spaced 1¼ inches in either direction--2400 per frame.
Once set, the cuttings should be watered. This is done with a light spray. Watering is continued until it rises above the sand to a depth of ¼ inch or more. It is here that the importance of leveling the frame and layers of rooting medium is realized if all the cuttings are to receive an equal amount of water. Watering depends on the type of weather-how hot and dry and whether the beds are frozen. Never water when the beds are frozen; only occasionally during cold weather. Water more frequently during the hot weather and then when the cuttings are rooted, only about once each week until all are potted. Remember at each watering the bed is flooded. If there is sufficient water pressure in your locality a contraption can be rigged of ½ inch copper tubing and a series of elbows and tees with as many as five sprinklers, thus reducing the watering time in proportion to the number of heads. A watering schedule varies with the seasons. Listed here is one for an average year with a cold winter.
Cuttings were set from Oct. 1 to Oct. 13. Those set on the 1st were then watered, and again a week later and so on with all the beds until the last were inserted on the 13th.
October 15. 22
November 6, 25
March 20, 31
Aril 5, 10, 16, 20, 24
May 2, 6, 10, 17, 21, 23, 27
June 1, 7, 10, 14, 17, 21, 21, 28
July 1, 6, 12, 19, 27
Plants were potted in August and an occasional sprinkle was given the un-potted cuttings as time permitted.
Fig. 16. Rooted rhododendron cuttings spaced 2½ inches
in the cold frame July 15, 952
This of course is not a schedule to he followed. Its purpose is only to cite the great differences in times of watering according to the season and condition of the cuttings. If peat is used with the sand in the top layer watering is still further reduced but since it only requires about a minute to water a unit there is actually not much tune involved.
If all "clean" cuttings are placed in the rooting frames spraying is reduced to a minimum. As soon as all are set a preventive spray of Bordeaux 2-2-50 is applied. In spring as the buds expand in the units the same spray is applied and once more in June. Arsenate of lead can be incorporated with this against worms and crickets chewing the leaves but their damage is so slight that this seems unnecessary. If aphids, lice, lace hugs or other sucking insects should infest the frames, they are easily controlled by placing a good sized teaspoonful of Nico-Fume on a piece of paper in a small receptacle in the unit. Light the paper and close the glass for about an hour. Air the unit for a minute or so and repeat the same remedy in about 10 days if a later hatch develops. Remove all dead leaves and cut away any diseased portions. Aside from these practices the frames are never ventilates.
The rooted cutting; may be potted, flatted. or bedded out in a good friable soil with half shade or maybe just a bit more. Potting facilitates transplanting come spring. The first winter the plants are covered with glass in our section of the country. This may not he necessary in the milder climates. If potted in July an occasional sprinkling keeps them in good health until autumn rains come. After September the glass is set over them. Water as they need it in October. In placing the glass be certain to allow ample air and circulation until winter closes in. Then before the first serious freeze, water thoroughly, close the sash tightly and replace on the lath. It is important for constant good results to make certain there is at least a foot of space above the bedded rhododendrons to the sash. This prevents winter damage through overheating, and shows itself again in the spring with normal sturdy growth.
After the rooted cuttings are removed from the units in late summer or early autumn the old medium is taken out and replaced with new and the process is again repeated year after year.
This method followed reasonably close, will give results that are more than satisfactory, especially since many have been led to believe that the propagation of rhododendrons is an art attempted only by experts. The parade is under way, gaining newcomers at every turn of the road, and the day is not far distant, when the propagation of these garden gems by grafting will be nothing but a memory.
This paper, covering all the necessary aspects of so vast a subject as propagation of rhododendrons is, admittedly very brief. If there is a sincere desire for any further information on any portion of the subject, every effort will be made to offer as complete and thorough a reply as possible.