Research On Rhododendron Indicum 'Formosa'
By R. D. Dickey and R. T. Poole, University of Florida
Many of the woody ornamental plants grown in Florida are produced in containers such as metal cans, which may be located in shade (usually lath or saran) or full sun. Several experiments with Rhododendron indicum 'Formosa' and 2 or 3 other index plants grown in metal containers, have been conducted. The variables and their levels of 7 factorial experiments using 'Formosa' azalea are given in Table 1. The more important results from these experiments are discussed herein.
Potting medium of experiment 1 was ⅓ Florida peat and ⅔ fine sandy soil, while potting medium was a variable in experiment 2 (Table 1). Potting medium used in all other experiments was a mixture of ⅓ imported peat, ⅓ No. 8 perlite and ⅓ fine sandy soil, adjusted to a pH of 5.5 with dolomite.
Gallon containers used in experiments 1, 2 and 3 were No. 10 food cans (185 cu. in.), and those of experiment 4, 5 and 6 were 6" Lerio cans (145 cu. in.). Gallon and 2 gallon containers of experiment 7 were 6" and 8" Lerio cans (295 cu. in.). Shade conditions under which plants of the various experiments were grown are given in Table l.
In experiments 1, 6 and 7 the cans were not covered and when rain wet soil in the cans, the watering cycle started over. The plants were protected from rain by clear polyethylene in experiments 2 and 3 and by Mylar in experiments 4 and 5.
The fertilizer was applied in solution in all experiments. Nitrogen (N) was supplied from ammonium nitrate, phosphorus from potassium di-hydrogen phosphate, potassium (K) from potassium chloride and potassium di-hydrogen phosphate and magnesium from magnesium sulfate.
Treatment effects were expressed as visual grade and growth increment, which was growth index (height plus spread and sum/2) at end of experiment minus that at experiment's beginning. Visual grade was determined by using a grading range of 1 to 4, according to standards set forth in Florida Nursery Grades and Standards Handbook of the Division of Plant Industry. No. 1 is equivalent to a cull, No. 2 to Florida No. 2, No. 3 to Florida No. 1 and No. 4 to Florida Fancy.
Experiments 1 and 2 - 'Formosa' azalea plants watered every 4 days grew as well as those watered every 2 days, while experiment 2 plants watered every 3 and 6 days grew better than those watered every day (1). Experiment 4 - There was no difference in growth between plants watered daily and every 3 days and both produced slightly larger plants than ones watered every 6 days (4). Visual grade of 'Formosa' azalea plants in this experiment was lowered as watering frequency decreased from daily to every 3 and 6 days at both fertilization rates. Within watering frequencies, 5,000 pounds per acre of a 6-6-6-2 (N, P, K, Mg) produced a higher visual grade of 'Formosa' azalea plants than 3,000 ppa only when watered every day.
Experiment 5 - There was no difference in growth and visual grade of plants watered every 2 and 4 days and both were slightly larger and had a higher visual grade than those watered daily (5).
Experiment 6 - Growth increment data of 'Formosa' azalea show that response to watering frequency was similar at each fertilization rate and, generally growth increased as fertilization rate increased for the 4 and 8 weeks fertilization intervals. An exception to this general pattern occurred in the group of plants fertilized weekly where plants watered every 3 days and given 6,000 ppa were as large as those which had received 9,000 ppa (5).
There was no difference in visual grade of 'Formosa' azalea watered every 1 and 3 days, and plants watered every 2 days had the higher visual grade (5).
Results of these experiments and additional experience with growing 'Formosa' azalea plants in containers show that plants given the less frequent watering frequencies were as large or larger than plants watered daily. Similar results were obtained with watering frequency in companion experiments with Ligustrum japonicum and Viburnum suspensum.
Under these experimental conditions visual grade was not adversely affected if plants were watered every 2 days Watering daily did not increase visual grade or growth, only increased leaching of nutrients from the containers.
Experiment 3 - There was no difference in growth between 60, 120 and 240 ppa (pounds per acre) of K at the 60 and 120 ppa N levels, but at 240 ppa of N, 240 ppa of K produced more growth (3, 6). There was no difference in growth between plants fertilized every 2 and 8 weeks at 60 ppa of N, but at 120 and 240 ppa of N, plants fertilized every 2 weeks grew larger than those fertilized every 8 weeks. Each increase in N produced a decided increase in visual grade. The interaction differences of growth increment could not be detected visually (3, 6).
Experiment 4 - Plants which received 5,000 ppa of 6-6-6-2 grew larger than those which received 3,000 ppa (4). Visual grade data detected a watering frequency x fertilization rate interaction and these data are discussed in the section on water frequency (4).
Experiment 5 - Growth and visual grade of 'Formosa' azalea increased as fertilization rate increased from 3,000 to 6,000 to 9,000 ppa of 6-6-6-2 (5).
Experiment 6 - Treatments duplicated those of Experiment 5 except it was placed in full sun. The watering frequency x fertilization interval x fertilization rate interaction growth data are discussed in watering frequency section (5). Interaction differences of growth increment were not detected visually. Visual grade of 'Formosa' azalea increased with each increase in fertilization rate (5).
Growth and visual grade of 'Formosa' azalea increased as fertilization rate (6-6-6-2) increased in experiments 4, 5 and 6. In experiment 3 growth and visual grade increased as N levels increased. Growth response to K, in experiment 3, was obtained only at the high levels of N and K, and to fertilization interval at medium and high N levels.
Experiment 3 - The fertilization interval x nitrogen level interaction on growth of 'Formosa' azalea is discussed in fertilization rate section (3, 6). Fertilization intervals of 2 and 8 weeks had no effect on visual grade (3).
Experiment 4 - Fertilization intervals of 2 and 4 weeks had no effect on growth or visual grade of 'Formosa' azalea (4).
Experiment 5 - Fertilization intervals of 1, 4 and 8 weeks had no effect on growth or visual grade of 'Formosa' azalea (5).
Experiment 6 - Growth increment data gave a significant watering frequency x fertilization interval x fertilization rate interaction and these data are discussed in watering frequency section (5). Fertilization intervals of 1, 4 and 8 weeks had no effect on visual grade.
Fertilization interval levels of these experiments generally had very little effect on growth of 'Formosa' azalea. In experiments 3 and 6, where growth increment measurements detected growth differences, these interaction differences could not be detected visually.
Many growers believe that fertilization of woody ornamental plants should stop in central and northern Florida by about August 15, until danger of cold is past next spring. Fertilization much later than this, it is thought, will increase susceptibility of such plants to cold injury.
Cold weather during 1960-61 winter and the severe freeze of December 1115, 1962, supplied data on effect of watering frequency and fertilization rate and interval on cold injury to flower buds (flowering) and foliage of 'Formosa' azalea.
Experiment 4 - Plants watered every 3 and 6 days had less cold injury to flower buds than ones watered daily, and plants receiving 5,000 ppa of 6-6-6-2 had less flower bud injury than those given 3,000 ppa. Fertilization intervals of every 2 and 4 weeks had no effect on cold injury to flower buds (4).
Experiment 5 - As watering frequency decreased from daily to every 2 and 4 days and fertilization rate increased from 3,000 to 6,000 to 9,000 ppa of 6-6-6-2, cold injury to 'Formosa' azalea flower buds decreased. Fertilization intervals of 1, 4 and 8 weeks had no effect on cold injury to flower buds (5).
Experiment 6 - 'Formosa' azalea plants in this experiment (full sun) evidenced cold injury to both flower buds and foliage. Flowering-Plants given the 6,000 and 9,000 ppa rates of 6-6-6-2 and watered every 3 days, and those fertilized weekly at the 9,000 ppa rate had less cold injury to their flower buds than other treatment combinations (5). Foliage-As fertilization rate increased cold injury to foliage decreased at each fertilization interval. The greatest leaf injury was to plants fertilized every 1 and 4 weeks at rate of 3,000 ppa of 6-6-6-2 (5).
Recent work on cold injury has shown that plants optimally supplied with all essential elements at all times will survive lower temperatures and recover faster from injury than plants not so supplied. Our data presented on cold injury agrees with general trend indicated in the literature (4,5).
A tip burn of 'Formosa' azalea leaves appeared at shoot terminals during second season in experiments 5 and 6. There was a high positive correlation between tip burn rating and soluble salts concentration of substratum. As watering frequency decreased and fertilization rate increased tip burn and soluble salts increased. Fertilization interval did not affect soluble salts content of substratum but did increase tip burn of plants receiving 6,000 and 9,000 ppa of 6-6-6-2 that were fertilized every 4 and 8 weeks.
Plants watered daily had no tip burn at any fertilization rate, but plants watered every 2 days showed slight and 4 days severe tip burn at 6,000 and 9,000 ppa rates of 6-6-6-2 (5). The increase in soluble salts concentration caused by this reduction in soil moisture content could account, in part, for increased tip burn of plants in these treatments.
The degree of tip burn was less for plants in full sun, possibly because of leaching of salts from the foliage by rainfall.
Chemical Analysis-Foliage and Soil
Levels of N, P, K, Ca and Mg in the foliage of 'Formosa' azalea was generally lower than that reported in research from other parts of the country for several container grown woody ornamental plants (3, 4, 5). Tissue Mg was generally higher than expected, probably because dolomite was used to adjust pH and 2 percent Mg was applied to all plants.
In some instances there was a positive correlation between plant response as measured by growth increment or visual grade and the foliage and substratum content of an element, but in other cases there was no correlation (4, 5). The data suggest that much more work is needed before predictions as to expected plant response can be made from soil analyses for container grown woody ornamental plants in Florida.
Lath Shade vs. Full Sun
Duplicate experiments (experiments 5, 6, Table 1) were set up to obtain information on effect of lath shade vs. full sun on growth of 'Formosa' azalea because more and more nurserymen in Florida and elsewhere are growing container grown plants in full sun. 'Formosa' azalea plants grown under one-third lath shade were considerably larger than plants grown in full sun and had a higher visual grade (5).
Table 1. Treatment variables and their Levels of Seven Factorial Experiments
with rhododendron indicum 'Formosa'.*
Fertilization Interval 1 1956-57** 2 Days (½ Lath Shade) 4 Days 2 1958-59*** Daily (½ Lath Shade) 3 & 6 Days 3 1959-60 N - 60, 120, 240 ppa/yr 2 Weeks (½ Lath Shade) — K - 60, 120, 240 ppa/yr 8 Weeks 4 1960-61 Daily 3,000 ppa/yr - 6-6-6-2 2 Weeks (½ Lath Shade) 3 & 6 Days 5,000 ppa/yr - 6-6-6-2 4 Weeks 5 1962-63 Daily 3,000 ppa/yr - 6-6-6-2 Weekly (⅓ Lath Shade) 2 Days 6,000 ppa/yr - 6-6-6-2 4 Weeks 4 Days 9,000 ppa/yr - 6-6-6-2 8 Weeks 6 1962-63 Daily 3,000 ppa/yr - 6-6-6-2 Weekly (Full Sun) 2 Days 6,000 ppa/yr - 6-6-6-2 4 Weeks 3 Days 9,000 ppa/yr - 6-6-6-2 8 Weeks Container Size Time in Container Transplanting Methods 7 1960-64 Quart 8 Months No root pruning. (50% Saran) Gallon 14 Months Pot-bound roots removed 2 Gallon 20 Months Inch deep vertical cuts on periphery of ball.
* All experiments initiated in spring of one year and terminated in fall of the following year.
** Three other variables of this experiment were: (1) holes in cans-side or bottom; (2) substratum on which cans rest on soil or sawdust; (3) drainage material in bottoms of cans-none or 1 inch of gravel.
*** Two other variables of this experiment were: (1) 2 plant species-'Formosa' azalea and Ligustrum japonicum; (2) 8 kinds of potting media.
Container Size - Time in Container - Transplanting Method
Treatments of experiment 7 are given in Table 1. All plants were watered and fertilized alike. Container size had no effect on growth of 'Formosa' azalea after 8 months in containers (2). After 14 months in containers, plants increased in size with each increase in container size and the greater increase was made by plants in 2 gallon containers. Size of container greatly influenced size of plants grown in them for 20 months. Compared with quart cans, the growth index of 'Formosa' azalea plants was 21 and 54 percent larger for the gallon and 2 gallon containers.
At termination of time in container periods the 'Formosa' azalea plants were given 3 transplanting method treatments (Table 1) and planted in the field. There was no effect on subsequent growth in the field resulting from size of container in which the plants had been grown or time in containers, nor was there any difference in growth caused by transplanting methods (2).
- Dickey, R. D., 1960, Some factors affecting growth of three ornamental plant apecies in containers, Fla. Sta. Hort Soc., 73:358-361.
- Dickey, R. D., Effects of container size, time in container and transplanting method on growth of Poducarpus inacrophylla maki, Rhododendron indicum and Ligustrum japonicum, 1962, Proc. Fla. Sta. Hort Soc., 75:491-494.
- Dickey, R. D., R. T. Poole and J. N. Joiner, 1961, Effects of nitrogen and potassium levels and two application intervals on growth and chemical composition of Rhododendron indicum'Formosa' and Viburnum suspensum, Proc. Fla. Sta. Hort. Soc., 74:436-440.
- Dickey, R. D., R. T. Poole and J. N. Joiner, 1963, Effect of watering frequencies, time and rate of fertilization on growth and chemical composition of Rhododendron indicum 'Formosa' and Viburnum suspension, Proc. Fla. Sta. Hort. Soc., 76: 431-436.
- Dickey, R. D., R. T. Poole and J. N. Joiner, 1964, Effect of certain cultural factors on growth and chemical composition of Rhododendron indicum 'Formosa', 1964, Proc. Fla. Sta. Hort. Soc., 77
- Poole, R. T. and R. D. Dickey, 1960, Effects of levels and time of application of nitrogen and potassium on the growth of container grown Viburnum suspensum and Rhododendron indicum 'Formosa', Proc. Fla. Sta. Hort. Soc., 73:394-397.