QBARS - v28n3 Controlling Size and Flowering

Controlling Size and Flowering in Rhododendrons
R. L. Ticknor
Oregon State University
North Willamette Experiment Station

There are two means - genetic and cultural - by which plant size and flowering in rhododendrons can be controlled. Hopefully, genetic control will be thoroughly discussed at the Breeder's Symposium since this is the most reliable and least costly means of controlling plant size.
Genetic Control of Plant Size
Some examples of genetic control of size are the numerous hybrids of Rhododendron yakushimanum . These tend to be regularly - shaped, compact mounds. The use of 'Cunningham's White' and 'Boule de Neige', both R. caucasicum hybrids by Mr. A. M. Shammarello of the Great Lakes Chapter, has resulted in a series of hardy hybrids more compact than the older R. catawbiense hybrids.
Cultural Control of Plant Size
Cultural practices which can influence plant size and flowering are pruning, amount of light, fertilizers and growth retardants. Pinching or pruning of rooted cuttings and liners is the basis of developing well-branched, compact plants. Major pruning is usually the last resort in restoring an overgrown plant to useful size. Even large plants of many varieties can be cut to the ground, yet will grow again. For many years there was a flourishing business in collected plants in the mountains of the southeastern states. Hillsides would be burned, then 3 or 4 years later the catawbiense rhododendrons and mountain laurel which had sprouted from the roots would be dug and shipped throughout the eastern U. S.
Pruning to Control Size
Drastic pruning is best done in late March in most areas, allowing time for new growth to develop and mature before the following winter. An example of this and resulting growth is shown in Figures 47, 48, and 49. Some pruning (pinching) is necessary to induce branching in order to develop a compact plant if drastic pruning is done.

Old rhododendron plant before pruning. Plant after pruning.
FIG. 47. Old rhododendron plant before
Photo by Dr. Robert Ticknor
FIG. 48. Plant after pruning in
early April.
Photo by Dr. Robert Ticknor
Plant one year after pruning.
FIG.49. Plant one year after pruning.
Photo by Dr. Robert Ticknor

If complete beheading is not desired, it is still possible to reduce plant size by pruning. Here the important factor is to cut just above some dormant buds. These buds, produced in the axils of the leaves, remain visible for many years if one inspects the stems closely. If the cut is made where there are no dormant buds, the resulting stub apparently tends to inhibit growth of buds below it.
Light and Plant Growth
Generally the more light in which a rhododendron is grown, the more compact it will become and the more flowers it will produce. It is necessary to water heavily when plants are grown in full sun to prevent foliage burn. This limits the practice to well drained, relatively cool soils, since Phytophthora root rot becomes a major problem in wet soils, particularly if they are warm.
Flower and leaf size will be smaller on plants growing in full sun than those growing in partial shade. Also, the leaves are usually a lighter green and flowers do not last as long. Therefore, if you wish to win ribbons at your chapter show, grow your plants in light shade to produce top quality leaves while still producing flowers in adequate numbers.
Fertilizer Effects on Growth & Flowering
Use of fertilizer to increase growth is well known. Withholding fertilizer may result in smaller plants, but they will usually be off-color and not well budded. Normally, reducing fertilizer application is not used to reduce growth rate.
Use of fertilizer (primarily phosphorus) to increase flower bud formation in rhododendrons has been reported by Myhre (4) and others (2, 3, 7). Myhre reported that 180 lbs P 2 O 5 /acre incorporated pre-planting increased flowering the first 3 years when compared with annual applications of 80 lbs P 2 O 5 a/acre. After 3 years, 'Cynthia' produced as many flowers with the annual application method as with the heavy pre-planting application. Myhre also suggested that one tablespoon of treble superphosphate 0-45-0 in the planting hole of a liner-sized rhododendron was equal to the  180 lbs. per acre application.
Phosphorus Experiments
Several experiments have been and are being conducted at Oregon State University's North Willamette Experiment Station to evaluate phosphorus sources and placement in flower bud formation and plant size. Treatments used and results after 3 years of the experiment. begun in 1968. are shown in Tables 1 and 2. Ammonium sulfate supplied an equivalent amount of nitrogen to those plants not receiving 16-20-0. Ammonium nitrate brought the total nitrogen application to 180 lbs/acre the first year and 15 lbs/acre each during the second and third years.
There seems to be no truly consistent growth (height) response to the fertilizer application. The broadcast 2000 lbs./acre 0-45-0 plus 54 lbs./acre chelated iron produced more buds than without iron, suggesting that iron in the soil was tied up by this rate even though there were no visible symptoms of iron deficiency on the plants. With the exception of 'Roseum Elegans', the plus-iron plants also produced flower buds in greater quantity than those receiving the annual application of 16-20-0. The tablespoon per plant of 0-45-0 at time of planting, which is easily done in a home planting, appears equal to or better than the annual application of 16-20-0 for the production of flower buds - again with the exception of 'Roseum Elegans.'
Two treatments not shown in Tables 1 and 2 utilized 0-34-0, which contained 6 minor elements in fritted form. These treatments produced boron toxicity when applied broadcast at 1000 lbs./acre (340 lbs P20) plus 1244 lbs/acre of 0-45-0 (560 lbs/acre P 2 O 5 ), or as 4 teaspoons per plant. Visual symptoms of boron toxicity were purple spots clustered around the margins of the leaves.
1970 Trial
McGuire (3) reported that a liquid formulation of 10-34-0 banded beside the row was the most effective phosphate source for flower bud formation. We used this method, applying 240 lbs/acre P 2 O 5 pre-plant from 0-34-0 on 4 varieties, in comparison with 3 annual applications of 80 lbs P 2 O 5 per acre from 16-20-0. Again, plant heights were variable with variety but there was increased flowering in all varieties with the pre-plant application of 10-34-0, as shown in Table 3. Phosphorus content of the leaves of 'Nova Zembla' was 0.18% of dry weight on the 16-20-0 treatment and .22% on the 10-34-0 treatment.
1973 Trial
In May 1973, we initiated a comparison of granular 0-45-0 and liquid 11-37-0, both banded 4 inches deep and 6 inches from the plant on each side immediately after planting, with a pre-plant, broadcast incorporated application of 0-45-0. All treatments supplied 240 lbs P2O5. per acre. Ammonium sulfate supplied equal amounts of nitrogen to all treatments. Trial variety used was 'Cotton Candy', which is slow to bud. Unfortunately, 'Cotton Candy is also susceptible to cold injury, so we hope that another cold winter does not occur for at least the next two years.

Table 1. Average Height of Several Rhododendron Varieties (Inches)
After 3 Growing Seasons in 1968-70 Trial
North Willamette Experiment Station
Treatment Variety
Check 400 lbs 16-20-0/A annually 10.1 24.2 29.3 24.5
Cycocel 2000 ppm + 400 lbs 16-20-0/A annually 9.5 21.4 19.9 22.2
Broadcast 2000 lbs 0-45-0 pre-plant 9.6 23.3 25.9 25. 6
Broadcast 2000 lbs 0-45-0 + 54.5 chelated iron/A pre-plant 10.2 24.6 27.3 25.8
One tablespoon 0-45-0/plant at planting 9.3 23.6 24.5 26.7
Table 2. Average Number Flower Buds on Several Rhododendron Varieties
Per 3-Year-Old Plant in 1968-70 Trial
North Willamette Experiment Station
Treatment Variety
Check 450 lbs 16-20-0/A annually 9.9 5.5 25.5 6.6
Cycocel 2000 ppm + 400 lbs
16-20-0: A annually
12.1 8.9 20.7 12.6
Broadcast 2000 lbs 0-45-0 pre-plant 8.1 9.1 17.3 9.1
Broadcast 2000 lbs 0-45-0 -54.5 lbs. chelated iron/A replant 11.0 9.1 19.2 12.3
One tablespoon 0-45-0/ plant at planting 9.3 10.0 20.1 8.3

Growth Retardant Experiments
The last method of size and flowering control involves growth retardants, which provide an effective method of reducing growth and often increasing production of flower buds.
1965 Trial
Several experiments using growth retardant chemicals have been conducted at the North Willamette Experiment Station since 1965 (5, 6). Effects on height and flower bud formation of 'Roseum Elegans' from applications of B-9 (succinic acid - 2, 2 dimethylhydrazide), Cycocel (2-chloroethyltrimethylammonium chloride) 2000 ppm, Cycocel at 4,000 ppm and an untreated check are shown in Tables 1 and 2. These treatments were applied 4 times, twice to the first growth flush and twice to the second during 1965 and again in 1966. B-9 at this rate had no effect on height nor flowering. Cycocel caused statistically significant decreases in height at both rates. The only statistically significant increase in flower bud formation resulted from the Cycocel 2000 ppm application.
1968 Trial
Cycocel at 2000 ppm was included as a comparison to the phosphate treatments on 'Elizabeth Hobbie', 'Pink Pearl'. 'Roseum Elegans' and 'White Pearl'. Statistically there were no significant increases in flower bud formation, although variation due to treatment did occur as shown in Table 3. Maximum flower bud production in 'Elizabeth Hobbie' and 'White Pearl' resulted from the Cycocel treatment.
Cycocel reduced plant height of 'Pink Pearl'. 'Roseum Elegans' and 'White Pearl'. With the dwarf 'Elizabeth Hobhie' the effect on height was not noticeable.
1969 Trial
A screening trial of growth retardants on 'Roseum Elegans' was started in 1969 and continued in the field for three growing seasons. Plants used had grown one year in gallon cans, thus were larger than the usual liners planted into the field. Retardants were applied twice at 8 to 10 day intervals only to the first flush of growth. Many products tested were numbered compounds which have not become commercial products. Off-Shoot-O (methyl esters of fatty acids C6-C12), normally used as a chemical pruning agent, was included with several of the retardants at a rate of 5000 ppm or which is lower than the pruning rates of 1 to 4%, to try to increase absorption of the retardants.
Height reduction, as shown in Table 5, was the major effect in this trial; amount of flowering was not significantly increased. Cycocel continued as an effective height retardant, while B-9 also became effective when the application rate was increased to 7500 ppm.
Off-Shoot-O increased effectiveness of B-9 and some other growth retardants. Later work in Alar (which contains the same active ingredient as B-9) also had increased response when 5000 ppm of Off-Shoot-O was included in the spray.
1970 Trial
An experiment combining phosphate application methods and growth retardants was initiated in 1970, using the varieties 'Cosmopolitan', 'Holden', 'Nova Zembla' and 'Pink Pearl'. Statistically there were differences between 16-20-0, supplying 80 lbs. Of P 2 O 5 annually for 3 years, and 10-34-0, banded pre-planting to supply 240 lbs of P 2 O 5 , with the amount of nitrogen applied annually held equal as shown in Tables 3 and 4. More flowers and taller plants were produced where the 10-34-0 was used pre-plant.
Only 'Nova Zembla' responded with a statistically significant difference between the check and the growth regulator-treated plants receiving the same fertilizer treatment. This occurred following application of Cycocel at 2000 ppm to plants which had received 10-34-0.
1972 Trial
To determine whether Cycocel at 2000 ppm applied in the third year in the nursery would increase flowering with a minimum of plant size reduction, a trial was begun June 5 in two nurseries utilizing 20 varieties. A second application to the first growth flush was made June 12. Ten plants of each variety were tagged and sprayed: then during October the treated plants and 10 closeby untreated plants were measured.
Response to Cycocel varied with variety. No increase in flower shoot production was observed in the treated plants of varieties 'A. Bedford', 'Anna Rose Whitney', 'Carol Jean', 'Clementine le Maire', 'Lamplighter', 'Lord Roberts' nor 'Vulcan'. Flowering shoots constituted 75-95% of total shoots on the check plants of 'Anna Rose Whitney', 'Carol Jean', 'Lord Roberts' and 'Vulcan'.
A number of varieties shown in Table 6 responded to the Cycocel spray by producing 10% or more increase in the ratio of flowering shoots to total shoots. Part of this increase is accounted for by reduction in number of vegetative shoots on treated plants and part by increase in flower buds. Still another group of varieties had less than 10% increase in ratio of flowering to total shoots: 'Antoon van Welie', 'Cynthia', 'Mrs. Charles Pearson’, 'Pink Pearl' and 'Queen Mary'.
Since many varieties did not respond to an application of Cycocel at 2000 ppm. It should not be a general practice to apply this chemical to all varieties. Research should be continued to identify varieties which will benefit from Cycocel in enhancing flowering during the second or third year in the nursery.

Table 3. Average Number of Flowers in 1972 per 3-Year-Old Plant in 1970-72 Trial
North Willamette Experiment Station
Treatment Variety
No. Growth Regulator Fertilizer & Rate Cosmopolitan Holden Nova Zembla Pink Pearl
1 Cycocel 2000 ppm 16-20-0 2.5 11.5 8.1 1.2
(80 lbs P2O5 annually)
2 Cycocel 2000 ppm 10-34-0 5.9* 18.0 17.6** 2.2
(240 lbs P2O5 preplanting)
3 Alar 7500 ppm 16-20-0 1.3 8.9 5.5 1.6
(80 lbs P 2 O5 annually)
4 Alar 7500 ppm 10-34-0 4.1* 12.8 12.4 2.2
(240 lbs P20-, preplanting)
5 Check 16-20-0 2.1 12.5 5.4 1.0
(80 lbs P 2 O5 annually)
6 Check 10-34-0 4.5 14.1 6.9 1.8
(240 lbs P20, preplanting)
*LSD 5% 2.5 N.S. 5.7 N.S.
3.7 N.S. 8.5 N.S.
Table 4. Average height in Inches in 1972 of 3-Year-Old Plants in 1970-72 Trial
North Willamette Experiment Station
Treatment Variety
No. Growth Regulator Ferlilizer & Rate Cosmopolitan Holden Nova
Pink Pearl
1 Cycocel 2000 ppm 16-20-0 16.0 13.1 13.9 15.1
(80 lbs P.O.. annually)
2 Cycocel 2000 ppm 10-34-0 17.4 14.9 17.0 5.8
(240 Ibs P 2 O5 preplanting)
3 Alar 7500 ppm 16-20-0 17.0 15.1 15.4 15. 2
(80 lbs P2O5 annually)
4 Alar 7.5110 ppm 10-34-0 17.3 16.8 16.9 15.3
(240 Ibs P2O5 preplanting)
5 Check 16-20-0 19.0 19.4 18.6 15.5
(80 lbs P 2 0., annually)
6 Check 10-34-0 19.4 18.6 19.0 16.5
(240 lbs P205 preplanting)
Table 6. Average Plant Size, Shoot Length and Bud Counts of 10 - 3-Year-Old
Field Grown Plants of Several Varieties of Rhododendrons Treated
With Cycocel 2000 PPM on June 5 and June 12, 1972
North Willamette Experiment Station
Veg. Shoot Flower
Flower to
Variety Treatment Height Width Length Length Buds Buds Total Buds
Anah Kruschke Check 3.6 32.6 5.5 8.0 27.8 26.8 49.1
Cycocel 22.3 28.0 4.8 7.4 19.7 31.6 61.6
Alice Check 19.2 27.7 5.1 6.2 51.1 3.6 6.6
Cycocel 20.2 26.3 4.5 5.8 40.9 11.3 21.6
Cotton Candy Check 19.8 26.0 5.0 6.2 12.4 15.6 55.7
Cycocel 17.0 24.3 8.1 5.5 8.2 17.9 68.6
Mother of Pearl Check 16.9 26.8 4.4 6.4 38.2 4.3 10.1
Cycocel 19.2 25.9 5.1 6.6 21.4 5.9 21.6
Parson's Gloriosum Check 19.6 23.0 3.8 6.6 10.3 18.1 63.7
Cycocel 20.4 26.0 4.5 7.3 3.5 27.2 88.6
Hugo de Vries Check 19.6 28.0 6.6 6.9 29.7 4.6 13.4
Cycocel 20.7 25.3 6.6 6.9 16.3 7.0 30.0
Roseum Elegans Check 24.5 36.7 4.6 7.5 7.9 52.5 68.6
Cycocel 21.5 34.0 4.4 7.9 3.8 51.4 93.1
Unknown Warrior Check 19.4 26.8 5.0 7.4 15.2 12.6 45.3
Cycocel 18.3 26.2 3.9 7.7 9.1 17.4 65.7
Results of flower-inducing experiments.
FIG.50.  Drench treatments have been most consistent in
inducing flowering. Results of the two most effective
materials Cycocel left and Phosphon, right, are shown
Photo by Dr. Robert Ticknor

Growth Retardants on Container-Grown Plants
Possibly —the most promising use of growth retardants in rhododendrons is with plants grown in containers. Size is not so important. since they are often sold by size of container rather than size of plant. Thus size reduction that accompanies increase in flower bud formation is not a problem.
Cathey (1) reported in the Quarterly Bulletin on techniques for the production of budded plants from cuttings in one growing season. Since this procedure required several months in a 65° F greenhouse with night lighting. It was decided to determine whether similar results could be achieved with less expensive facilities.
Following are procedures used at the North Willamette Experiment Station to produce budded plants in containers in one growing season. Cuttings are taken in July and potted in November into 4-inch pots filled with ground fir bark. These pots are placed in a plastic house with heat regulated to prevent temperature dropping below 35° F. During February, the plants are shifted from 4-inch to 6-inch pots filled with a fertilizer potting mixture, the contents of which are shown in Table 6. Minimum temperature is raised to 45° F at this time.
Minimum temperature in the plastic house is raised to 55° F and fluorescent lights are turned on from 10:00 p.m. to 2:00 a.m. on March 1. Lights and heat are turned off May 1. In early June after all danger of frost the plants are moved out-of-doors into full sun. Ideally, if there were room for spacing the plants in the plastic house, then only the polyethylene cover of the house would have to be removed to control heat without moving the plants. In late fall. the plants are again placed under plastic for cold protection. An unheated plastic shelter provided protection from successive low temperatures of 5°, 5°, 9° and 5° F in early December 1972. There was no loss of flower buds as there was on stock plants growing in the open field.
Although a few varieties such as 'Purple Splendour' and 'Mrs. Betty Robertson' will bud well without growth regulator treatment, it is necessary to treat most varieties to obtain flower buds consistently. We have tried many experimental as well as commercially available compounds, both as drenches and foliar sprays. Although more costly because of quantity of solution applied and time necessary to apply the solution, drench treatments have been most consistent in inducing flowering. The two most effective materials, applied in 200 milliliters of solution per 6-inch pot, have been Cycocel at 5900 ppm (a 1-in-20 dilution of the 11.8% product) and 0.4 grams of Phosphon (a 1-in-50 dilution of Phosphon DL).
For most normally growing varieties, drenches are applied as the first flush of growth matures, usually in April. With dwarf varieties, treatment is delayed until after two flushes of growth are produced. Otherwise, treated plants are too small and out-of-scale with the container.
Because many pot plants are marketed during holidays, we have attempted to force bloom for Valentine's Day, one of the earlier holidays. In mid December we bring the plants into a 65° F glasshouse for the forcing period. Time necessary to open flowers varies from 44 days for 'Yellow Bells' to 85 days for 'Kluis Sensation'. Blooming rhododendrons for holidays later than Valentine's Day would be simpler to produce. Other than potential florist markets. there should be outlets in garden stores for small plants with flower buds. One advantage of this market is that no forcing would be necessary.
Some varieties most promising for production of budded gallon-can rhododendrons are 'Blue Ensign', 'Carolyn Grace', 'Catawbiense Album', 'Gomer Waterer', 'Holden', 'Kluis Sensation', 'Madame Masson', 'Nova Zembla', 'Purple Splendour' and 'Roseum Elegans'. We are continuing to screen varieties for adaptability to this method of production. It is planned that after the 1973 crop is forced, another article will be written listing successes and failures for all varieties tried so far.

Table 5. Average height of 'Roseum Elegans' After Four Growing Seasons
and Average Flower Bud Formation Following Growth Retardant
Treatment in Three Successive Years
North Willamette Experiment Station
B-9 7500 ppm 25.5* 2.5 4.5 12.9
B-9 7500 ppm + Off-Shoot-O 5000 ppm 22.6* 1.3 8.4 17.6
Cycocel 2000 ppm 23.6** 1.3 11.5 12.0
Cycocel 2000 ppm + Off-Shoot-O 5000 ppm 25.2 1.6 9.6 20.9
Check 29.1 1.9 8.1 14.9
*L. S. D. 5% 3.1
**L. S. D. 1% 4.8
Table 7. Potting Mix for Container-Grown Rhododendrons
North Willamette Experiment Station
Material Amount / 4 cu. ft. Rate / cu. yd.
Ground fir bark 3.75 cu. ft. 25.3 cu. ft.
Ground lignite .25 cu. ft. 1.7 cu. ft.
Magamp 335.9 grams 5.0 lbs.
Ammonium sulfate 100.8 grams 1.5 lbs.
Potassium sulfate 100.8 grams 1.5 lbs.
Gypsum 100.8 grams 1.5 lbs.
Dolomite 201.6 grams 3.0 lbs.

Size and flowering in rhododendrons can be controlled by selecting proper varieties, by pruning, by fertilizer treatment and by growth retardant chemicals. Selection of appropriate variety for the location is most important. Pruning can reduce size of overgrown plants and develop good habit in young plants. Fertilizers, particularly phosphorus, may increase flower bud formation. Growth retardants can reduce growth of rhododendrons and in some cases increase flowering. Production of budded plants in 6-inch pots in one growing season is possible by varietal selection and growth retardant application.

Literature Cited

  1. Cathey, H. M., and R. L. Taylor, 1965, Guidelines for regulating flowering of rhododendrons - light and growth retardants, Quar. Bul. Amer. Rhode. Soc., 19(1):26-35.
  2. Criley, R. A., 1969, Controlling rhododendron flower bud initiation, Florists' Review, 143:3715:3637, 93-95.
  3. McGuire, J. J., 1969, Record outing for Rhode Island group, R. I. Nurs. Newsletter, 39:1, 3 & 6.
  4. Myhre, A. S., and W. P. Mortensen, 1964, The effect of phosphorus on rhododendron flower bud formation, Quar. Bul. Amer. Rhodo. Soc., 18(2): 66-71.
  5. Ticknor, R. L., and C. A. Nance, 1968, Chemical control of rhododendron growth and flowering, Quar. Bul. Amer. Rhodo. Soc., 22(2): 90-95.
  6. Ticknor, R. L., 1968, Influence of fertilizers and growth regulators on flower bud production of field-grown rhododendrons, Proc. Int. Plant Prop. Soc., 19:305-309.
  7. Vanderbilt, R., 1967, System of producing budded container-grown rhododendrons from cutting to trailer, Proc. Int. Plant Prop. Soc., 17:266-269.