Vegetative Growth of Rooted Cuttings of
Rhododendron X P.J.M. During Winter
John J. McGuire and Valerie J. Bunce
University of Rhode Island, Kingston, R. I.
Rhode Island Agricultural Experiment Station Contribution Number 1333
The relatively new rhododendron hybrid 'P.J.M.', named after Peter J. Mezzit, has become extremely popular in New England because it blooms early in spring at the time of forsythia and it blooms profusely. It is also desirable for its hardiness and compact habit. In several years of observation in various exposures in Rhode Island, it has not exhibited any foliage burn or bud blasting.
The hybrid is not difficult to propagate from cuttings that may be rooted throughout midsummer or fall, but commercial propagators in Rhode Island have experienced some difficulty in getting young plants to initiate shoot growth after rooting. This is particularly true of softwood cuttings taken in summer. Losses of 40% have been reported the following spring when cuttings propagated in August were held in heated plastic or glass houses until spring. When cuttings were taken in late fall there appears to be no problem in getting the plants to initiate shoots, but subsequent growth of shoots is slow until days lengthen in March. The purpose of this study was to determine whether growth might be stimulated by exposure to long days and/or low temperature.
A total of 350 rooted cuttings was obtained from a local nursery in mid November, 1968. Twenty cuttings were potted immediately and placed in the green house. Half of these were exposed to long day (LD) photoperiod by illuminating them with daylight fluorescent lamps between the hours of 11 p.m. and 2 a.m. Lamps were extended 4 feet above the bench to provide approximately 100 f.c. The other ten cuttings were placed at the same temperatures, a minimum of 60° F but at normal short day lengths (SD). The remaining 320 cuttings were placed in two groups at minimum temperatures of 40 and 32° F. One group of twenty cuttings was removed from each temperature each week, potted, and placed in the greenhouse. Half of each group were exposed to LD and half to SD photoperiods. Records were made of number of plants showing shoot initiation each week and photographs were made at the termination of the experiment.
Plants given the LD exposure developed new shoots regardless of the length of time in cold storage (table 1). Shoots developed rapidly and had to be pinched several times. Plants removed from storage after exposure from one through four weeks initiated shoots slowly in both LD and SD treatments. Plants exposed to five or more weeks of low temperature and then placed under SD initiated shoots quickly but shoots did not develop until natural day lengths lengthened. Plants receiving similar cold treatment and LD developed shoots rapidly.
There was no difference in plant response when plants were stored at 32° or 40° F. The effect of the two cold treatments was the same and statistical analysis of the resultant growth data indicated no significant differences in shoot initiation or development. There was a significant difference in number of plants with newly initiated shoots under LD as compared to those under SD after 4, 6, and 8 weeks in the greenhouse. There was no difference after only two weeks. This indicated that photoperiod was at least one governing influence, and if low temperature exposure was influential the effect was masked by day length.
Since only one light intensity was used, nothing can be said about effects of light intensity, although Cathey (1) reported a minimum of 25 f.c. was necessary to shoot initiation on the rhododendron he was testing. Low light intensity is probably not a critical factor in commercial operations. It is possible that if minimum temperatures were not maintained at a high enough level in these operations, unfavorable results would have been obtained. It has been reported in many cases that artificial lighting must be accompanied by a minimum temperature of 60°F to 65°F (1) (2) (3).
Exposure to low temperatures (below 40°F) for at least four weeks may hasten shoot initiation, but unless days are naturally long it will be necessary to provide an extended day of at least 12 hours or provide a light break in the middle of the night period.
As can be seen in Fig, 1, total growth of plants exposed to LD would be at least twice that of plants which have occupied the same space but received no supplementary lighting. This was similar to results of Weisner (3). Two possibilities appear feasible: 1) take softwood cuttings in summer and keep them under LD with a light break throughout the fall and winter, whereby a substantial plant will be developed and at least two more crops of cuttings will be provided from the necessary clippings, 2) propagate hardwood cuttings in October or November and put them under LD until March 30th. A smaller plant will be obtained but less time will be spent obtaining it. Present commercial practices of holding extensive greenhouse areas with plants in an inactive state are inefficient, since no plant growth results.
- Cathey, H. M., 1965, Initiation and Flowering of Rhododendron Following Regulation by Light and Growth Retardants, Proc. Amer. Soc. Hort., 86:753-760.
- Skinner, H. T., 1939, Factors Affecting Shoot Growth and Flowering Bud Formation in Rhododendrons and Azaleas, Proc. Amer. Soc. Hort. Sci., 37:1007-1011.
- Weisner, C. J. & L. T. Blaney, 1963, Rooting and Night Lighting Trials with Deciduous Azaleas and Dwarf Rhododendrons, Amer. Hort. Magazine, 42:95-100.
Fig. 15. Effect of day length on shoot growth of rooted
cuttings of Rhododendron 'P.J.M.' (Nil=No
storage, 7 wks=7 wks at 40°F, L=Light Break,
O=Natural Day Lengths)
Table 1. Average number of plants initiating shoots after exposure to low temperatures and/or long day lengths.
Weeks After Potting 2 4 6 8 Total LD SD LD SD LD SD LD SD LD SD
0 5 0 8 0 9 7 9 8 31 21 1 0 0 7 2 8.5 3.5 9.5 4.5 25 10 2 0 0 0 0 9.5 .5 9.5 1.5 25 2 3 1 0 3.5 0 7.5 2 10 4.5 22 6.5 4 1 0 8.5 3.5 9 4.5 9 7 27.5 15 5 2.5 2.5 6.5 6.5 7.5 8.5 8 8.5 24.5 26 6 4 5.5 6.5 9 7.5 9 7.5 9 25.5 32.5 7 5 4 8 8.5 10 9.5 10 10 33 32
18.5 12 54 35.5 68.5 44.5 72.5 53