JARS v46n2 - Influence of Nitrogen Nutrition on Flower Bud Initiation of Rhododendron

Influence of Nitrogen Nutrition on Flower Bud Initiation of Rhododendron
Robert D. Wright
Department of Horticulture
Virginia Polytechnic Institute
Blacksburg, Virginia

In contrast to other nursery crops, rhododendrons must not only be grown to a marketable size quickly, but their sales value is greatly increased if there are flower buds present on the plant. This is a problem with 2- to 3-year-old plants grown in the field or in containers, especially with some cultivars like 'Catawbiense Boursault'. Some success has been made by using growth regulators (1, 3) and phosphorus application (3, 5, 6) or a combination of the two to promote early flower bud initiation of rhododendron. There still remains much work to be done on this problem since results obtained above are often inconsistent and erratic.

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Nitrogen levels in the soil solution have a very strong influence on the growth of rhododendron and other plants. As early as 1918 (2), it has been demonstrated that vegetative and reproductive growth is strongly controlled by the N level applied to plants. If too high levels are applied, the plant will respond with vegetative growth and limited flower formation. Intermediate levels, relatively speaking, will encourage both adequate vegetative growth and flowering. We hypothesized that there was a level of N which, when the plant had reached the desired salable size, would not promote another vegetative flush, but would supply sufficient N to promote flower bud initiation and development. Growers would force plants vegetatively in the early part of the growing season, but reduce fertilizer application to prevent further vegetative development (shoot elongation) allowing enough time for flower buds to form.
Rooted cuttings of 'English Roseum' and 'Catawbiense Boursault', relatively easy and hard to flower cultivars respectively, were grown outdoors in 3-gallon plastic containers filled with quartz sand for two years. The third year nitrogen treatments of 0, 5, 10, 15, 30, 60, and 90 ppm in the irrigation water were applied with each watering. Data on flower bud number were taken in October 1984. Phosphorous and K were applied at 15 and 30 ppm respectively. The experiment was repeated in 1985, using 2-year old plants procured from a nursery. Rather than sand, the plants were grown in a ground pine bark medium and fertilized with either 30, 60, or 90 ppm N in the irrigation. Data were taken in October 1985.

Table 1. Influence of N rate and bud set of 'Catawbiense Boursault and 'English Roseum' (1984)
N Level (ppm) Bud no./plant
'C. Boursault' 'E. Roseum'
Expt. 1
0 4.9 11.1
5 4.6 6.3
10 6.6 12.2
15 9.1 13.2
30 5.8 13.4
60 7.0 24.2
90 4.6 25.8
Expt. 2
30 3.4 6.1
90 4.2 11.5
150 3.5 12.1

As the N level applied increased with 'English Roseum', the number of buds per plant increased for both experiments (Table 1). However, there was no influence on bud set with 'Catawbiense Boursault', which also had many fewer buds per plant than 'English Roseum'. With either experiment, I do not think we had high enough N levels. If the N levels had been higher, there may have been a reduction in bud set for 'English Roseum'.
During the process of taking the data on flower bud number for the first experiment (1984), it was noticed that some plants would possess a rather high number of buds, whereas some would have very few buds. This was true regardless of the N treatment. After further scrutiny, there appeared to be a relationship between the number of terminal shoots that had developed from a vegetative shoot whorl during the last growth flush of the year and the number of flower buds that developed on these new shoots (Table 2).

Table 2. Shoot characteristics of rhododendron plant with and without flower buds.
Cultivar Flower Bud? Shoot no.* Shoot diam.
Leaf area
(cm 2 )
'C. Boursault' Yes 3.9 6.7 28
No 7.0 5.3 18
'E. Roseum' Yes 4.7 5.9 26
No 5.4 4.6 17
*Average number of terminal shoots produced from a vegetative bud whorl during last growth flush.

As can be seen, if a bud is present on a shoot, then the number of shoots that had developed was lower. Further, the lower the number of shoots, the greater the diameter and leaf size of these shoots (Table 2).
These data would indicate that there may be a threshold level of mineral nutrients and/or carbohydrates in a shoot that is needed before a flower bud will form. If a limited amount of these reserves are available to elongating shoots, it stands to reason, that the greater the number of developing shoots, the greater the competition for these reserves by the elongating shoots. Consequently, the greater the number of shoots developing, the smaller will be these shoots (shoot diameter and leaf size) and thus a reduced likelihood of a flower bud developing.
These results would indicate that pruning practices in rhododendron production could influence the number of flower buds that might develop on a plant under a particular nutritional and environmental situation. The latter must be taken under consideration since such factors as light levels (shade) and nutritional status of the soil would greatly influence the nutrient and carbohydrate reserves in a plant. To verify the influence that pruning would have on the development of flower buds we conducted a preliminary experiment. We grew 'English Roseum' from rooted cuttings in 1-gallon containers for one growing season. The plants were pruned so that either 2, 3, 4, or 6 terminal shoots were allowed to develop on the last growth flush. As hoped, the greater the shoot number the fewer the number of flower buds (Table 3).

Table 3. Influence of shoot number on buds per plot of 1-gallon rhododendron ('English Roseum').
No. shoots per plant Average no. buds/plant
2 1.42
3 1.67
4 1.16
6 0.42

These data indicate the importance of canopy manipulation on bud set for rhododendron. Any factor, be it drought, shading, nutrition, or pruning, may influence bud set on a particular plant due to a direct or indirect influence on shoot size. The practice of removing spent flower trusses in early summer encourages flower buds for the next year. The new shoots that develop after trusses are removed are normally larger shoots compared to shoots that develop where trusses have not been removed, and thus more likely to be able to support flower bud development. It would also stand to reason that limiting the number of shoots allowed to develop after the old truss is removed would enhance the likelihood of bud set and also the size and quality of trusses that developed. You might want to try this.

Literature Cited
1.  Cathy, H. M. 1965. Initiation of flowering of rhododendron following regulation by light and growth retardants. Proc. Amer. Soc. Hort. Sci. 86:753-760.
2.  Kraus, E. J. and H. R. Kraybill. 1918. Vegetation and reproduction with special reference to the tomato. Oregon Agric. College Expt. Sta. Bul. 149.
3.  Meyer, S. P. and P. C. Kozel. 1971. Practical procedures for increasing flower bud initiation on rhododendron. Ohio Agric. Res. Sum. 56. Ohio Agric. Res. and Dev. Center, Wooster, Ohio.
4.  Ryan, G. F. 1979. Effects of nitrogen and phosphorus on flower bud formation in rhododendrons. College of Agric. Res. Center. Washington State Univ. Bul. 872.
5.  Ticknor, R. L. and M. H. Chapman. 1978. Effect of slow release fertilizer sources on flower formation and nutrient composition in rhododendron. Internat. Plant Propag. Soc. Proc. 28:101-105.
6.  Vanderbilt, R. 1966. System of producing budded container grown rhododendron from stem cutting to trailer, Internat. Plant Propag. Soc. Proc. 17:266-269.

The research reported here by Professor Wright was supported by a grant from the ARS Research Foundation.