Volume 22, Number 2
April 1968
Budded Container Grown Rhododendron Production Methods
Richard Vanderbilt
The Conard-Pyle Company, West Grove, Pennsylvania
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To produce a uniform crop, a stock block separate from the salable crop should be maintained. A certain amount of studied neglect is necessary to produce cuttings which root strongly and quickly. The stock plants are grown in bushel baskets where they are fertilized every other time we water until July 1.
By the middle of July the stock plants will be budding heavily. These flower buds are removed so that another growth will be produced. Our cuttings, taken in September, will be either single second growths or multiple first and second growths if the cutting originated from a branch not cut the previous year. A combination of IBA, Phygon and boric acid in talc is used to treat the cuttings, which root in twelve weeks. Formerly the cuttings were benched in peat but now they are potted into l-quart poly containers. It was found that growth was slower when the plants were cut out prior to canning.
A potting mix of peat moss-perlite, 50-50, with no limestone or superphosphate is used. These additives do not seem beneficial at this time. Dieldrin at the rate of 4 lbs. active ingredient per acre plus Geigy Chelate NaFe at the rate of 2 ounces per hundred square feet after potting are applied in water. In addition, 20-20-20 at the rate of 4 ounces to 100 gallons of water is used at this time, which will carry them until active growth begins. The potted cuttings are moved into a 40°F. night temperature greenhouse where daytime temperatures are held as cool as possible for 20 days. At that time, the temperature is then raised to 65° and supplemental cyclic lighting of 20 foot candle minimum is given six minutes out of every half-hour from 8:00 p.m. to 4:00 a.m.
When soil tests indicate a nitrate level under 7 or 8 parts per million, another feeding of 4 ounces 20-20-20 per 100 gallons of water is given. In practice, this works out to one feeding a month with low dosage. Although this is extremely safe it is surprisingly easy to build up excess nitrogen during the dark months of February, March and April. In the spring, soil tests frequently indicate feeding is necessary once every two weeks.
Shifting of the young liners to larger containers in the field starts after May 15. Canning is done in farm wagons loaded with potting mix beside the bed where the plants are to be placed. These beds are in shaded Quonset-type structures where the plants remain for the summer and following winter. Shade is provided by 4-mil white opaque polyethylene in which 5"-6" circular holes have been cut so excess heat can escape. The number of holes is increased over a 4 to 5 week period so that at the end of 5 weeks the plants are in full sun. If the variety is not sun-tolerant it is kept shaded.
A definite relationship exists between the container and the potting mix. Good drainage is always necessary but it becomes critical in a container which tends to be wetter in the lower portion of the container because it is such a limited column of soil. A mix should be so well-drained that there is not enough excess water available to allow the growth and development of rhododendron root rot fungus, Phytophthora cinnamomi. The lighter and more aerated the mix, the less root rot will occur.
A soil mix of peat moss-sand, 50-50, with the addition of 10 lbs. dolomitic limestone, 3 lbs. high-calcium limestone and 10 lbs. pulverized 0-20-0 superphosphate per cubic yard is used. If disease were not a problem, any ratio up to pure peat is satisfactory. Phytophthora becomes a problem at higher peat levels, however. These high levels of limestone and superphosphate are necessary since available phosphorus in the soil is the single most important key to early, heavy budding of rhododendrons. The chief cause of failure to bud is lack of available phosphorus.
Poor or sporadic budding of young rhododendron stock used to be almost a universal phenomenon. When we consider the availability of phosphorus in soils this is easily understood. Phosphorus tends to be most soluble when the pH of the soil approaches 6.5-7.0 and is practically insoluble at the low of pH 3.8-4.5. At the low pH, phosphorus reacts with the abundant iron and aluminum to form insoluble aluminum and iron phosphates. This means it is almost impossible to supply the phosphate necessary for a good bud set at low pH.
Normally a rhododendron growing at pH 6.5-7.0 will be very chlorotic since the iron necessary for good green color is not available to the plant. By supplying chelated iron it is possible to keep both iron and phosphorus available to the plant.
While it is possible to grow rhododendrons at a very high pH, it is not desirable to do so since the danger from Phytophthora increases with the pH. A pH of 5.5-5.8 is a good compromise level. The limestone rates mentioned earlier were calculated to raise the potting mix to this level when a very acid sphagnum peat moss of pH 4.0 is used.
The initial phosphorus level is further bolstered by the use of a high phosphate "starter-type" fertilizer with a 9-45-15 analysis in our feeding program. This program is started as soon as the plants are canned. A stock solution of 1 lb. in 1 gallon of water is proportioned out at a 1:200 dilution rate at every other irrigation. This rate is used until August 15, at which time a half-strength rate is used. The following spring the full strength rate is used as soon as the plants need water. At August 15 the second year the rate is again lowered.
Dieldrin at the rate of 4 lbs. active ingredient per acre plus Geigy Chelate NaFe at the rate of 2 ounces per 100 square feet is applied when the plants are moved to larger containers. These materials are applied through the proportioners. The chelated iron is applied again at the same rate the following spring.
A mix with roots thoroughly dispersed throughout the entire soil area will remain at a lower and more even moisture level than the same mix only 10 per cent penetrated by roots. This means that over potting can be a real danger because disease builds up in the wetter unused soil. Plastic and metal containers with bottom drain holes remain wetter than the same size container made of woven wooden strips and thus are more subject to disease problems.
We have found that a 2-gallon Lerio container is too small for two growing seasons. A plant will practically double its size if shifted from a 2-gallon to a 5-gallon Lerio can for the second growing season. Root rot problems develop if the liner is placed directly into a 5-gallon container. Shifting from 2's to 5's becomes a tremendous project when large numbers of plants are involved.
Since we have noticed that Phytophthora has not been a problem in our stock plants in bushel baskets, we plan to plant the liners directly in quarter bushel baskets. This size is officially 2 gallons, which is equivalent to the so-called 3-gallon metal pot. This should be large enough so that plants can grow two seasons without shifting. This container, with its better aeration and cooler summer temperature, should be less subject to disease than an oversized solid container.
Pinching to develop good branching starts in the greenhouse and is continued the entire first summer. All plants will be budded the second year with this system, with bud formation starting in early July.
A spray program at weekly intervals is carried out in the greenhouse. Once the plants are in the open the schedule is reduced to every three weeks using the following materials: DDT, Diazinon, Parzate and Kelthane. While some of these materials do the same job we have found the combination most effective on mites, all leaf spot fungi and most insects except leaf rollers. Thiodan used alone controls the leaf rollers.
The incidence of Phytophthora has been greatly reduced by monthly applications of Dexon even though there has been some controversy about the effectiveness of this material. We feel that it is very useful as a preventative but it is not a cure.
Covering the Quonsets in which the plants have been growing with 4-mil white polyethylene provides winter protection.
Volume 22, Number 2
April 1968
