QBARS - v33n2 Add Some Color to Spring Indoors with Rhododendrons

Add Some Color To Spring Indoors With Rhododendrons
R. L. Ticknor and J. W. Fox

Azaleas have long been accepted as seasonal pot plants while their near relations - the rhododendrons - are known as landscape shrubs. A wider color range is available in the rhododendron group than in the azaleas. Bold foliage, spectacular trusses and in some cases fragrance are other desirable characteristics of the rhododendron.
The potential market for flowering or budded rhododendrons grown as pot plants is great, particularly if identification with a major holiday could be established. Mother's Day falls within the normal blooming period of rhododendrons and does not have a plant such as the Easter lily identified with it. Very little costly heated forcing time would be necessary for this date but cultivars normally blooming later than Mother's Day will be necessary since plants receiving winter protection bloom earlier than normal. The plants, if the cultivars are matched with the climate, can be set out of doors following flowering to become a permanent feature in the landscape.
Valentine's Day which would require early blooming cultivars is another possible gift holiday for rhododendrons. Unfortunately, there is a lack of early blooming red cultivars. Of course, any time between these gift days would be a possible sales time for flowering potted rhododendrons.
Considerable research (1, 3) has been done on the cultural techniques necessary to produce flower buds on small rhododendrons. Most of this work has utilized relatively high night temperatures with extended day length, plus growth regulators. Using these techniques plus refrigerated cooling, it has been possible to produce flowering plants throughout the year from a rooted cutting within one growing season.
For rhododendrons to compete successfully with other pot plants, reliable production of budded plants at the lowest production cost is necessary. Since growing facilities and fuel cost are major expenses, our research has attempted to keep these costs at a minimum.
Growing Conditions
All growing is done in polyethylene covered houses having 1/2" pipe framework. Rooted cuttings are held at a minimum temperature of 35°F till February 1st when the thermostat is advanced to 45°F. On March 1st the minimum temperature is advanced to 55°F and lights are turned on to provide a 24 hour day which produces maximum growth according to Doorenbos (2). Delaying higher temperatures till the outside temperature rises, saves fuel and is compatible with a production program aimed at the Valentine's Day to Mother's Day period.
Extending the day length accelerates the growth of the rooted cuttings so that all will have three and some will have four flushes of growth by the end of the growing season. The lights are turned off in mid May. The plastic is removed between mid May and early June depending on the outside temperatures. The plants are covered in late November to prevent freeze damage before marketing.
Plants are grown pot to pot on raised benches till the plastic is removed. They are then spaced 6" apart each way. Plants can be consolidated during the winter protection phase if necessary.
Propagation Procedures
Production starts with taking cuttings between July and September. Cuttings with flower buds are preferred since the axillary buds are better developed than on vegetative cuttings. Getting three or more breaks from a rooted cutting is the key to a quality pot plant.
Cuttings are double wounded and treated with rooting hormone before sticking into 2¼" square by 3½" deep rose pots filled with peat-perlite 1:1 by volume. As roots show through the drain holes, the pots are removed from the mist bench which is maintained at 72°F. The cuttings remain in the rose pots till transplanting.
Fertilizer Practices
After rooting, 30-10-10 at 1½ teaspoons/gallon is applied at monthly intervals till potting into 1 gallon cans during February. The growing media is 1/4" minus Douglas fir bark to which the following fertilizers are added per cubic yard of mix: 11.1 lbs. Osmocote 18-5-11, 3.76 lbs. treble phosphate 0-45-0, 2 lbs. calcium and 1 lb. of dolomitic limestone, and 1.5 lbs. gypsum or 5.25 lbs. ureaform, 5 lbs. treble phosphate, 1 .5 lbs. potassium sulfate, 2 lbs. calcium and 1 lb. dolomitic limestone and 1.5 lbs gypsum. Some trace elements should be supplied either as post planting liquid applications of Peter's S.T.E.M. or other multiple element product or in solid form in the potting mix. Esmigran at 4 lbs. per cubic yard or FTE 503 at 4 oz. can be used. The plants are fertilized with 30-10-10 at 1½ teaspoons per gallon rate every 10-14 days during February and March and with 10-50-10 during April and May. Osmocote 18-6-12 at 1 teaspoon per gallon has been applied to the plants before they are spaced in the container yard. A trial of slow release fertilizers for top dressing in 1978 indicated that fertilizers with more nitrogen in relation to potassium such as 10-6-4 or 31-5-3, or 25-10-10 may produce more flower buds.
Leaf analysis of field grown rhododendrons (5, 7) and of our container grown plants has provided guidance for changes in our fertilizer program. The container grown plants have been high in phosphorus, potassium, and magnesium compared with good field grown plants. Also nitrogen has been low in fall leaf samples. Trials to evaluate possible changes in our container fertilizer are continuing.
Growth Regulator Test
Initially, growth regulators seemed to be the key to production of flowering plants in one growing season and many different candidate chemicals were tried. Quite consistent production of budded rhododendrons of many cultivars was possible using drenches of Cycocel at 5900 ppm or Phosphors at 0.4 grams per plant. In both cases 200 mil of solution was applied per plant in a one gallon can.
The disadvantages of using growth regulators were cost, size reduction, and persistence. Cost of the chemicals was 13 cents for Cycocel and 18 cents for Phosphors per 6" pot plus the labor of applying 200 ml of solution to each plant. Internodes became very short producing, in many cases plants not in scale with the pot. Short internodes persisted for 3 years after Phosphors treated plants were set into the field (6). Phosphors is no longer produced.
Suitable Cultivars for Pot Plants
Over 140 cultivars of rhododendrons have been evaluated for propagation, growth, plant habit, flower bud formation, and forcing characteristics. Many cultivars which we have tried have not rooted in high enough percentages. In other cases plant growth was too slow to develop a large enough plant in one growing season. Even with pinching some cultivars did not form well branched plants. Many cultivars have not formed sufficient flower buds the first growing season. Long pedicels which cause the flower to appear droopy or flower buds which abort on forcing have disqualified other cultivars. A list of the most promising cultivars is shown in Table I. Plants marked with ** are the best found so far. Table II shows some of the unsuitable cultivars.
Forcing Trials
Time from moving potted rhododendrons into a heated greenhouse till the first open flower is an important factor in determining the economics of potted rhododendrons. Most of our forcing trials started in mid December when Poinsettias would be moved out of the greenhouse with Valentine's Day as a target date and are shown in Table III. Forcing temperature has been 55°F at bench height. This 57 day period has proved to be too short for most cultivars tested, only 'Cheer', 'Christmas Cheer', 'Hello Dolly' and 'Rosa Mundi' have been successful. These cultivars will be in bloom for Valentine's Day when started January 4th. The number of days to the first open flower does decrease (4) the later forcing is started as shown in Tables III and IV.
To reach flowering size in one growing season, growth continues later than is normal in field production. Flower buds form late and must mature and have a chilling period before forcing. Starting growth earlier by light and higher temperatures would correct this problem but at increased costs. March and April dates could be covered with many cultivars. Dates in May such as Mother's Day will take late blooming cultivars or a change in handling techniques. Plants held in a plastic house without heat come into bloom too early. Possibly a lath house would give the protection from winter injury needed without accelerating growth in the spring.
One gallon 6"x7" deep pots have been used for most of our trials. Generally we have found the larger the pot in which the rhododendron is grown the larger the resulting plant. Of course the larger the pot the higher the shipping cost. We have found that the roots of several rhododendron cultivars can be trimmed to fit a 6"x5" or an 8"x6" pot without affecting bloom quality or time to bloom.
Conclusions
Rhododendrons with flower buds can be grown from rooted cuttings in one growing season without expensive facilities. Only selected cultivars will set flower buds on a well shaped plant that forces well. Forcing time depends on the cultivar and the time forcing is initiated.

Literature Cited
1. Cathey, H. M. and R. L. Taylor, 1965, Guidelines for Regulating Flowering of Rhododendrons - Light and Growth Retardants, Quar. Bul. Amer. Rhodo. Soc. 19(1),26-35.
2. Doorenbos, J. 1955, Shortening the Breeding Cycle of Rhododendrons, Euphytica, 4:141-146, 3. Myers, S. P. and P. C. Kozel, 1972, Practical procedures for increasing flower bud formation on rhododendrons, Amer. Nurs., CXXXV 3 :13 114.
4. Stockton, David, 1974, Unpublished Senior Project, Horticulture Dept. Oregon State Univ.
5. Ticknor, R. L. and M. H. Chaplin, 1979, Effect of slow release fertilizer sources on Rhododendron Flower Formation and Nutrient Composition, Proc Int. PI. Prop. Soc. Vol. 28. In Press.
6. Ticknor, R. L. and J. W. Fox, 1976, Residual effects of growth retardants on rhododendrons, Quar. Bul. Amer. Rhodo. Soc. 30(4):242-244.
7. Ticknor, R. L. and J. L. Long, 1978, Mineral Content of Rhododendron Foliage, Quar. Bul. Amer. Rhodo. Soc., 32:150-158.