Effects Of Various Maintenance Solutions On The Appearance Of Cut Rhododendrons
A.V.P. Mackay, M.D., Ph.D. and M. Kessell, O.N.D.H. Argyll, Scotland
Anyone who has exhibited rhododendron blooms at the local flower show, or more illustrious competitions, will realize the powerful influence of transport conditions on the state of their protégés at the vital moment of judging. The wilted and browning remnant of a bloom which looked so good when it left the bush can be an embarrassment not quickly forgotten. The art of preserving, or even enhancing, the specimen in transit is jealously guarded but based, by and large, on folklore and anecdote. The study to be described was a modest attempt to replace some of the art with knowledge based on scientific method.
The question was a simple one. Can tap water be improved upon as a support medium by the addition of solutes which have been claimed to improve bloom maintenance? The first, "Dettol", is known to be used by some exhibitors for this purpose. The second, "Phostrogen", is recommended by its manufacturers to prolong the vase life of cut flowers (1).
The outcome of this study was clear cut but we present it here not so much for the results themselves, which are interesting but restricted, as for a description of the methodology which exemplifies various important experimental principles.
The aim of the study was to compare the influence of four solutions on the viability, over a two week period, of rhododendron trusses. Trusses were to be as similar as possible at the start of the experiment, distributed randomly between the four solutions, and rated at intervals during the two weeks by an observer who was "blind" to the nature of the solution in which the trusses were kept. In theory, therefore, the only difference between experimental groups was the solution in which they were bathed, and the ratings of outcome were protected from any preconceived belief in the eyes of the observer.
a) Twelve similar trusses (R. cardiobasis aff.) with similar stem lengths (15 cm) cut in mid-May from the same shrub. Number of flowers per truss ranged from eight to ten.
b) Twelve vessels of similar volume (approx. 250 ml) and similar neck diameter (approx. 5 cm).
c) Experimental solutions;(i) Tap water; Ardrishaig mains supply; pH approx. 7.0.
(ii) A solution of Dettol® (chloroxylenol B.P. 4.8% w/v) to a strength of 5 ml (1 teaspoon) in 9 litres of tap water.
(iii) Phostrogen® full-strength: one level teaspoon in nine litres of tap water. Predicted pH 5.5 (1).
(iv) Phostrogen® half-strength; a 1:1 dilution of the above in tap water.
[Phostrogen composition: total nitrogen 10%, phosphorous pentoxide soluble in neutral ammonium citrate and water 10%, including phosphorous pentoxide soluble in water 10%, potassium oxide soluble in water 27%, magnesium 1.3%, iron 0.4%, manganese 200 mgm/kg.]
d) A rating scale of various aspects of bloom viability. Items rated were number of flowers per truss showing definite browning (expressed as percentage), number of flowers per truss which were wilted (expressed as percentage), number of flowers shed from each truss, and "overall appearance" - a global estimate of quality rated either "good", "mediocre", or "very poor". Leaf appearance was assessed only at the last time point, rated either "healthy" or "wilted".
Trusses were cut from twelve branches randomly distributed throughout the bush and placed in an empty bucket. Stalks were subsequently trimmed to give an approximate length of 15 cm. There was variation in stalk maturity, some were woody.
Stock solutions were made up as described above and dispensed into twelve similar glass vessels labeled simply 1-12. The key relating vase number to type of solution was kept out of sight. One truss was placed in each vase, the surface of the liquid reaching the neck of the vessel.
Three vessels contained tap water, three contained Dettol solution, three contained full-strength Phostrogen, and three contained half-strength Phostrogen; label numbers were distributed randomly among the experimental groups. The vessels containing the trusses were then placed on a white sheet on the floor of a bright, well-ventilated room with no artificial heating or lighting and not receiving direct sunlight. The containers were placed randomly in rows approximately 12 inches apart.
One of us (MK) acted as a rater, ignorant at all times of the number code key. Baseline ratings (time 0) were made immediately after the containers were laid out. Subsequent ratings were made at 2, 4, 9 and 14 days.
Originally it was planned to complete the evaluation within a week, but as it became apparent that viability in all groups was good even after a week, the experiment was continued in order to amplify differences between trusses. At the final rating point of 14 days, only leaf and overall bloom appearances were rated.
Trusses in tap water showed no change over 9 days, one truss had three browned flowers at the outset but no further browning occurred. Two out of three trusses in Dettol developed browning; one within 2 days, the other by 9 days, and to the extent of a maximum of 33% of flowers. Two out of three trusses in half-strength Phostrogen developed discolouration by 9 days, the other showed 33% browning at 4 and 9 days. Full strength Phostrogen produced discolouration in only one truss; 10% at 4 and 9 days.
b) Flower wilting
No wilting occurred in any truss kept in tap water. The Dettol group showed 100% wilting by 9 days but none before. Half-strength Phostrogen was associated with wilting in two out of three trusses; 50% and 30%, both at the 9 day stage. Full-strength Phostrogen was associated with wilting of all trusses at the 9 day stage, to the extent of 40%, 50% and 70%.
c) Dropped flowers
No truss in any group shed even one flower.
d) Overall appearance
All groups appeared good until the 9 day stage. At the 14 day stage, appearances had changed as follows; in the tap water group one truss remained good, one had changed to mediocre/good and one to mediocre. Dettol was associated with uniformly poor appearance. Half-strength Phostrogen was associated with deterioration in two out of three trusses - one to poor the other to mediocre. Full strength Phostrogen was associated with deterioration in all trusses mediocre/good, poor/mediocre, and poor.
e) Appearance at 2 weeks
The results are shown in Table 1. By two weeks there were marked differences between groups, with obvious relative preservation of the tap water group.
Table 1 Leaf Appearance Overall
Treatment Healthy Wilted 1 W x Poor* 2 P ++ V. Poor 3 D +++ V. Poor 4 HP + V. Poor 5 HP + V. Poor 6 P +++ V. Poor 7 W x Good 8 D +++ V. Poor 9 W x Good 10 HP x V. Poor 11 D +++ V. Poor 12 P ++ V. Poor * Woody stem.
W = tap water.
D = Dettol solution.
HP = Half-strength Phostrogen solution.
P = Full-strength Phostrogen solution.
The experiment was successful in that clear differences emerged between the viability of trusses maintained in four different solutions. Tap water was the most successful medium and Dettol the least successful. Phostrogen solutions were inferior to tap water but generally rather better then Dettol. No attempt has been made to subject the results to statistical analysis, the group size being too small, but we believe that qualitative between-group differences were sufficiently clear to obviate the need.
Browning of the flowers was the earliest sign of deterioration and it is interesting that no change occurred in this respect in any of the trusses maintained in tap water for a period in excess of nine days. It is also of interest that the overall or "global" appearance of the trusses remained generally good in all groups for at least four days.
The fact that times well in excess of the usual delay between cutting a truss and displaying it were required to demonstrate noticeable deterioration in any group raises a question about the relevance of the findings to the problem of preserving specimens for competition purposes. However, the process of getting cut material to a show entails a large number of variables, of which storage solution is only one. Temperature and the physical trauma of handling and transportation are other obvious examples. Furthermore, it is highly likely that significant interspecies differences exist in the in vitro susceptibilities of rhododendrons. It seems reasonable to propose that these various stresses and susceptibilities can act collectively to produce more rapid deterioration than was seen here, but variables have to be isolated and evaluated separately. All other things being equal, we suggest that tap water is associated with slower deterioration than Dettol or Phostrogen. These results have perhaps more direct relevance to the gardener who merely wants to display his rhododendron blooms around the house and who is interested in vase life.
The chemical or physico-chemical reasons for the inferiority of the Dettol and Phostrogen solutions cannot be elucidated by the results of this small trial. The experiment should only be considered as a pilot study and, strictly speaking, the results can only be taken reliably to apply to trusses of R. cardiobasis aff. cut in mid-May.
Cut rhododendron trusses survive well in a cool and undisturbed environment for as long as two weeks. As a maintenance fluid tap water at neutral pH cannot be improved upon through the addition of either Dettol or Phostrogen.
1. Phostrogen Ltd., (1986): Leaflet No. 10 (1048), "Chemiculture for general garden use"; Phostrogen Ltd., Corwen, Clwyd LL21 OEE, UK.
We gratefully acknowledge the help of Mrs. Sarah Goldsworthy in the preparation of this manuscript.
Mr. Kessell is the Research Director of the Scottish Rhododendron Society.