Efficient Production of Rhododendrons From Cutting To Crop
David G. Leach
Reprinted, with the kind permission of the author and the publisher
from the American Nurseryman, Nov. 1, 1965
The rooting of rhododendron cuttings has been so perfected in recent years that production of plants is now starting to overtake the demand. A year ago an eastern nursery actually sold the familiar, standard 'Roseum Elegans' in the 18 to 24-inch size for $1.65 net, after deducting the discounts offered.
It seems unlikely that this low selling price will be repeated. Hopefully, other growers will assume that it was the result of a temporary, burdensome surplus that required emergency liquidation. They may be right, but there are signs of trouble ahead. The ubiquitous 'Roseum Elegans' is beginning to back up in some retail sales lots as customers restively search for something a little different.
End of Sellers' Market?
True, there continues to be a shortage of fine-quality plants in large specimen sizes, but most rhododendron growers must probably resign themselves to the first major market change in their professional lives. After a century of generally comfortable profits from a waiting market, there is accumulating evidence that the standard commercial clones will come under increasing price pressure as a result of the steady production increases that have followed the perfecting of propagating techniques.
It becomes increasingly important, then, that production costs be lowered and efficiency increased as rhododendron hybrids edge from the pantheon of patrician plants into the plebian company of hustlers lined up in throat cutting rivalry for the buyer's dollar. Scientific research has produced many practical aids which are scarcely used by most nurserymen.
There may be a question as to whether the demand will expand at a pace that will keep up with production. Rhododendron growers have been gleefully riding the crest of a swelling preference for their products which has been spectacularly increasing for 15 years. Without it they would unquestionably be in doleful straits today.
In the populous Middle Atlantic states, for example, the United States horticultural specialties census shows that coniferous evergreens lost a jolting 25 per cent of their share in the horticultural market place in a 10-year period. At the same time, broad-leaved evergreens almost doubled their share of the dollars spent for nursery crops, and the trend continues. So perhaps the clouds now no bigger than a man's hand are shading only scattered local maladjustments of supply and demand and are not an omen of a gathering storm.
Still, most growers will welcome any opportunity to boost their efficiency and, in turn, their profits, whether or not marketing conditions force them to it.
Standard of Effectiveness
There is a good standard by which growers can measure the effectiveness of their methods: More than 85 per cent of the 'Roseum Elegans' crop should be well-budded, premium quality 18 to 24-inch grade at the end of the second growing season from rooted cuttings. The dwarf 'Boule de Neige' will be smaller in proportion to its slower growth; several of the clones of upright habit may be a bit taller. But 'Roseum Elegans', whatever its future, has been the largest seller in the industry for nearly 100 years, and it offers a standard of measurement to which almost any grower can compare his results.
Rhododendrons root more readily and in higher percentage as the autumn progresses, but the later rooting imposes a penalty in the subsequent size of the plants at the end of the first year of growth. Since growers' main concern is production efficiency, the optimum time for the taking of cuttings will be from September 10 to 30 in the Department of Agriculture climate zone 7a of the northeastern United States.
It has been reported on several occasions and included in papers published by this author - that an 18-hour soaking of the cutting stems in hormone solution is a good deal more effective for rhododendrons than the use of a dry powder, but only a few professional propagators have accepted the additional time and inconvenience required by this method. That being the case, the hormone powder used on cuttings taken in September will include both 1 per cent and 2 per cent indolebutyric acid in talc, the stronger concentration being reserved for the clones more difficult to root. A few hybrids extraordinarily hard to propagate respond well to the addition of 0.1 to 0.25 per cent 2,4,5-TP plus 0.1 per cent naphthaleneacetic acid to the IBA.
Because cuttings taken in September are more susceptible to stem rot than those taken later, 12½ per cent of Phygon is included in the preparation of the rooting powder. Perhaps more important, the fungicide has an auxin-like effect on rooting. Boric acid also stimulates root formation, as demonstrated by British researchers. In this country Richard Vanderbilt has proven the particular effectiveness of boric acid in combination with Phygon for the rooting of rhododendrons. The addition of boric acid to the powder at the rate of 50 parts per million completes an effective formula suited to the season.
Indolebutyric acid may be bought from Distillation Products Industries Division, Eastman Kodak Co., Rochester, N. Y., or from Fisher Scientific Co., Fairview, N. J., in 5-gram lots at about $1.15 per gram. Most nurserymen will find it convenient to ask their druggists to prepare the hormone powder for use. Unfortunately, there are no ready-made powders tailored to the special needs of the rhododendron propagator now on the market.
The best cuttings, those that root more readily, come from stock plants grown in 30 to 40 per cent shade. The stems are wounded on both sides, and the leaves are reduced in length, so that they will not decay from overlapping with a spacing of 2½ x 2½ inches in the propagating bench. After treatment with the hormone powder they are inserted in a mixture of 50 per cent European sphagnum peat and 50 per cent coarse sand, with a bottom heat of 72 to 75 degrees under intermittent mist applied six to eight hours per day in a range of one minute out of four to one minute in nine, depending on the weather and the season. The rooting medium can be used but once, due to the progressive loss of aeration from compaction.
After the cuttings are rooted they are transplanted with a spacing of 3 x 3 inches into benches of pure sphagnum peat, where they are chilled for 20 days at about 40 degrees to fulfill their dormancy requirement. The night air temperature is then raised to a minimum of 65 degrees, and the rooted cuttings are given flash lighting from 200-watt clear bulbs spaced 12 feet apart, which provide the 20 foot candles at bench level required for the response.
Two controls and two contactors are required for a typical installation of 20 bulbs. First in the electric supply line is an automatic switch, which turns the system on at 8 p.m. and off at 6 a.m. A 1,000-watt Tork model 919 (T) switch is available from Allied Radio Corp., 100 North Western avenue, Chicago, Ill.
This master switch is followed by another, which turns the lights on for one and a half seconds of each minute. An Aemco 1-minute timer, model 601, and a metal housing cabinet, purchased separately, are available for a total of $17.20 from Herbach & Rademan, 1204 Arch Street, Philadelphia, Pa. The timer then triggers the contactors, which do the actual on-off switching of the heavy load imposed by the 20 bulbs. The wattage is multiplied by 13 to compute the initial load, so 20 200-watt bulbs produce an initial load of 52,000 watts.
With the night-lighting supplement and a minimum temperature of 65 degrees, the cuttings will start growing in about 30 days. The first fertilizing of the rooted cuttings as growth begins in the peat benches is with Peters' 21-7-7, or a similar formula, applied in a solution containing not more than 10 ppm of nitrogen. Any further fertilizing of the peat invites trouble. Instead, the plants are given weekly fertilizer foliage sprays containing 30 ppm of nitrogen for as long as the plants remain under glass.
Every two weeks the watering of the peat includes an addition of Morsodren at a concentration of 1 part to 4,500 parts of water, which effectively forestalls attacks of root rot (Phytophthora cinnamomi). Sequestrene, NaFe (Geigy) should be applied once, shortly after growth begins, at the rate of two ounces to 100 square feet of bench area.
Under both night light and heat, a second flush of growth will follow and, with many clones, a third, so that by the time the young plants are ready to be transplanted outdoors into ground beds they are considerably larger than is usual in commercial practice. All growths but the first are pinched to induce branching.
When the danger of frost is over, the plants are given a final watering, which includes a wetting agent, and then are transferred from the greenhouse into outdoor beds under lath shades with a spacing of 15x15 inches. The shading remains in place for one year. The wetting agent plus the cutting of the root masses into 3-inch cubes when the plants are lifted from the bench insures the extension of the new roots from the peat moss into the surrounding soil outdoors. This has been a formidable problem in the past for some growers, both in the east and on the west coast.
Use High Fertility Level
The fertility level in the beds should be high, from an April application of a fertilizer formulated to fit the local soil need. However, there are two important considerations to keep in mind: The nitrogen must be ammonium nitrogen and not nitrate nitrogen. and calcium from gypsum should be supplied in most soils.
The nitrogen payout is much slower from ammonium sulphate, for example, than from a nitrate. And frequent light applications foster the continuing presence of the desirable ammonium radical in the soil to off-set the conversion to nitrate nitrogen which takes place as a continuing natural process. Light applications, frequently given, also constitute the most economical way to maintain soil acidity in regions where the soil tends to become alkaline.
The nitrogen level must be allowed to decline, of course, in late summer to allow the plants to harden off for winter. But Colgrove and Roberts in experiments at the Oregon agricultural experiment station demonstrated the congenial character of ammonium nitrogen for rhododendrons a few years ago, and subsequent experience in the field has fully supported their findings. It is an important consideration for maximum growth at whatever rate and frequency the nurseryman finds practical for his situation.
Since lime is not a requirement of rhododendrons, there is a widespread belief that calcium need be of no concern, whereas the exact opposite is actually the case. I think there is little doubt that growers are losing valuable size increments in many fields due to lack of calcium. Spectrographic analyses of leaves from plants in good growth at my own plantations have invariably shown, for many years, a much larger content of calcium than of the major elements, potassium and phosphorus! I have repeatedly observed the beneficial result of recommended calcium applications on rhododendrons which gave no sign of deficiency in the fields of professional nurserymen. If the soil is excessively acidic, the calcium can come from limestone, preferably dolomitic. If no upward change in soil pH is desired, calcium should be supplied by gypsum (calcium sulphate).
An exceptionally efficient grower in New Jersey makes the following April application with gratifying results for the soil constitution at his site:
Table I 80 pounds actual Nitrogen and 230 pounds actual Phosphoric Acid-as di-ammonium phosphate. 100 pounds actual Potassium-as potassium sulphate. 200 pounds actual Magnesium-as magnesium sulphate. 2,000 pounds Gypsum--as calcium sulphate. 30 pounds NaFe (Geigy)-as Sequestrene.
For the greatest economy and effectiveness, fertilizers must be tailored to the local soil conditions, but the above table should provide valuable clues.
Growers in the south, however, must irrigate carefully, keeping in mind the hazard of salt accumulations, to which rhododendrons are particularly sensitive. Many failures with rhododendrons by nurserymen who have made their first tentative experiments with them south of Mason and Dixon's line have probably been due to this factor. Dr. Harry Kohl has shown that tolerance to fertilizer salt residues depends on day temperature. A concentration which showed no injury at 50 degrees produced obvious damage at 70 degrees and killed the plants in 18 days at 78, 86 and 100 degrees. This probably accounts for the chlorosis which was so often reported by growers in the north in the course of the dry, hot summer of 1961.
Rhododendrons are acid-soil plants, but it is far from true that "the more acid, the better." The soil acidity which produces maximum rhododendron growth is about pH 5.5. Below that point the decay-producing bacteria and molds which reduce organic matter to nitrogen compounds and other plant stimulating essentials are inactivated progressively as the acidity increases.
Altering Soil pH
Specialists receive many inquiries about the best chemicals for altering soil pH and how much of them to use. My recommendations are as follows:
Table II To Change
to pH 5.5
Lbs. of Ferrous
Per 100 Sq. Ft.
7.0 16.5 6.5 11.8 6.0 7.1 Lbs. of Ground
Per 100 Sq. Ft.
4.0 6.8 4.5 4.8 5.0 2.9
Ferrous sulphate, often called copperas, is a by-product of steel mills. It is the fastest and safest acidifier, effective almost immediately upon application. Finely ground sulphur-the finer the better-is cheaper but so slow that it may require six months to complete its effect. To convert for the amounts of sulphur required for the same pH change, multiply the copperas quantities by 121.3 per cent. Aluminum sulphate should never be used. Aluminum ions are toxic to rhododendrons in rather low concentration, and the more acid the soil, the greater their availability in rapid progression.
Table 2 was computed to alter the pH of average medium loam. If the soil to be modified is sandy, somewhat less of the chemicals will be needed, perhaps only two-thirds as much; a heavy clay loam, which would contain many more of the soil's natural buffers, may require up to one-third more of the chemicals to produce the desired change in pH.
With the fertility and acidity levels as described, rhododendrons transplanted from the greenhouse in the spring after having made two or more flushes of growth under glass will continue to progress rapidly. By the end of their first growing season in U.S. D.A. zone 7a (southeastern Pennsylvania, central and southern New Jersey, Long Island, southwestern Connecticut, southeastern Rhode Island, Cape Cod) the plants will have made a total of four to five flushes of growth, which is much beyond that obtained in usual commercial practice. Each flush made in outdoor beds the first season is pinched to induce the continued branching that is essential for handsome, well-foliaged plants a year later.
Obtaining the maximum possible size the first year is critical in producing the finished stock of premium value at the end of the year following. Each successive growth is smaller than the growth preceding because of the progressively greater branching that has occurred. In the second year, little can be done beyond maintaining the appropriate soil fertility to encourage added size; so it is from the springboard of the first year's impetus that the quality of the final crop is determined.
The heavy budding which contributes so much to the value of the crop is also produced by management in the first year. A massive application of triple super phosphate supplying 500 pounds of actual phosphoric acid per acre is given the beds of 1-year plants in the autumn. To produce the budding it is essential that the soil contain between 21 and 30 pounds of available phosphoric acid per acre during the following growing season.
Because soils vary so greatly regarding their chemical constitutions and consequently in the amounts of phosphorus actually available from an application of the phosphate, it may be necessary to supply additional quantities to reach the critical level, but, however much may be required, it is vital to have a range of 21 to 30 pounds per acre of available phosphoric acid present in the soil early during the second growing season to induce the budding response. In some soil types it is not an easy level to reach, but persistence in testing and making applications until the level is achieved pays off handsomely in both sales and extra dollars of profits.
The budding response of rhododendrons to high levels of available phosphorus was discovered by A. S. Myrhe and W. P. Mortensen at the Western Washington experiment station and demonstrated to be commercially practical in the east by Richard Vanderbilt, then with the Koster Nursery, Seabrook, N. J.
The nitrogen must be maintained during the second year of growth at about the same status as recommended for the first year if the phosphorus is to produce the maximum budding. At the high phosphorus level, more buds will be set by the end of the second year of growth with adequate nitrogen in the active growing season than without it. The total effect on budding is phenomenal, an increase of about 600 per cent, and at a fraction of the cost of such growth retardants as BNine, Cycocel and Phosfon, which are inconsistent in effect under field conditions of rhododendron cultivation in any case.
With conscientious pinching of the terminal buds after each growth the first season, the plants will have formed such a fine, multiple-branched framework that no pinching is required in the second year, and none is desirable for the end result. A single exception in the east is the notoriously lanky 'America,' and, no doubt, 'Princess Elizabeth' would be representative of several such exceptions on the west coast.
Two strong, vigorous growth flushes will be made with the high concentration of available phosphoric acid in the beds. Without the elevated phosphorus in the soil, three growths could be obtained, but the value of the consistently heavy budding, regardless of weather conditions, is much greater than the value of the added size, for the plants are in the upper range of the 18 to 24-inch grade in any case, are of premium quality and can command premium prices.
I am indebted to Richard Vanderbilt and to the Koster Nursery for the opportunity to observe the successful application of much of this research to the large-scale commercial production of rhododendrons. In addition, Mr. Vanderbilt solved experimentally several previously troublesome problems inherent in the program or adapted findings with other plants.
These procedures represent the increments of knowledge from many researchers intelligently applied to practical use. If they are accepted and put into practice by nurserymen, they will bring to the production of rhododendrons in the industry a standard of efficiency far beyond the best that has been possible in recent years.