JARS v39n4 - Plant Growth and Rooting of Cuttings After Repeated Herbicide Application in Rhododendron

Plant Growth and Rooting of Cuttings After Repeated Herbicide Application in Rhododendron
G.F. Ryan and J.M. Wetherington
Washington State University
Western Washington Research and Extension Center, Puyallup, WA

Plants of rhododendron 'Mrs. G.W. Leak' ( Rhododendron X were treated with pre-emergence herbicides separately and/or in combinations twice a year from 1975 to 1982. In 1982, plants in 6 out 9 treatments were up to 12% shorter than untreated plants. With one exception these treatments were twice a year applications at rates equal to or 50% higher at each application than the maximum usually recommended for once a year treatment. The one exception was alternating fall and spring applications of Princep (simazine) at 3.2 lb ai/A (pounds of active ingredients per acre) and Casoron (dichlobenil) at 4.0 lb ai/A. Rooting of cuttings taken after 3, 4 and 6 years of herbicide application showed no significant effects from herbicide treatment. In 1982 there was a 17 to 20% reduction in rooting percentage in one sample of cuttings from each of two treatments, but no effect in the other 4 sets of cuttings taken from plants of those treatments during the same season. The treatments were Casoron at 4.0 lb ai/A twice a year plus Kerb (pronamide) at 2.0 lb ai/A once a year, and Princep at 3.2 lb ai/A plus Lasso (alachlor) at 6.0 lb ai/A, both applied twice a year.

The possibility that herbicide treatment of plants may affect the rooting of cuttings taken from those plants has been considered in several studies. In most cases there was no adverse effect on rooting except where herbicides were applied at rates much higher than recommended on the basis of the tolerance of the particular plant to the herbicide (1 - 7, 9, 10, 11). The excessive rates often caused visible symptoms of injury to the stock plants. Results have been reported with at least 10 azalea cultivars, 10 rhododendron cultivars, and about 20 other species of plants. More than 30 herbicides have been evaluated, applied separately, or sometimes in combination with other herbicides.

Occasionally there has been a case of reduced rooting of cuttings from plants treated at herbicide rates that would not be considered excessive which apparently did not visually injure the stock plants. Ticknor reported a reduction in root index value (based on a rating of the root ball) on cuttings from container grown plants of Golden Scotch Heather ( Calluna vulgaris 'Aurea') treated with Princep at 1 lb ai/A plus Lasso at 3 lb ai/A (9).

The only instances reported by Ahrens out of 15 field and container experiments over a period of about 10 years were a reduction in rooting percentage and score (quality of root system) on cuttings of rhododendron 'PJM' taken from container grown plants treated with Devrinol (napropamide) at 3 lb ai/A, and on cuttings of azalea 'Daviesi' from plants treated in the fall of two successive years with Princep at 1.5 lb ai/A plus Dacthal (DCPA) at 9 lb ai/A, or with Devrinol at 3 lb ai/A(2). A reduction in root ball size on cuttings from plants of azalea 'Rosebud' treated with Treflan (trifluralin) at 4 lb ai/A was reported by Hull et al(6).

The experiments reported here were started in 1975 to determine long-term effect on growth, flower bud formation, and rooting of cuttings from repeated application of herbicides to field-grown rhododendron plants. Effects on flower bud formation and growth during the first 5 years were reported in 1983 (8), and only effects on final plant height and on rooting of cuttings will be presented here.

Materials And Methods
One year old plants of rhododendron 'Mrs. G.W. Leak' were planted in Puyallup loam in September 1975. Plots were 4 by 20 ft, with 5 plants per plot, and 3 replications in randomized complete blocks. Rows were 6 ft apart, leaving 2 ft between plots.

Treatments were applied October 22, one month after planting, except Kerb, which was applied January 28, 1976. Treatments were repeated in June and September, 1976, and in April or May and September or October each year thereafter through 1982. Repeat applications of Kerb were in January 1977, and in November thereafter.

Weeds were removed from all plots before the June 1976 application, and they were removed periodically from the untreated plots with as little disturbance of the soil as practical.

Cuttings were taken October 3, 1977; August 30, 1978; July 2 and August 17, 1979; July 1, July 23, August 12, September 3, and September 23, 1981; and June 29, July 1, July 28, August 2, and September 1, 1982. Ten cuttings were taken from each plot, except in 1977, when less than 10 were available from some plots, and an average of 8 per plot were taken. The cuttings were rooted under intermittent mist in a peat-perlite medium (1:1 ratio by volume). Before sticking they were treated by basal dip in Dip 'n Grow diluted 1 to 10, which gave the following concentration of ingredients: 1000 ppm indole-3-butyric acid, 500 ppm 1-naphthaleneacetic acid, 17.5 ppm boron, 100 ppm dichlone, and 2% dimethylsulfoxide.

After preliminary examination indicated some rooting had occurred; the cuttings were removed and rated as to size and quality of root ball. When only a small percentage of cuttings had rooted at the time of the first evaluation, the healthy but un-rooted or poorly rooted cuttings were re-treated with Dip 'n Grow, reinserted in the rooting medium, and evaluated at a later date after additional rooting had occurred. Total percent rooting recorded in the two evaluations is reported here.

Evaluation consisted of recording the number of cuttings rooted, the number still alive but un-rooted, and the number dead. The root balls of rooted cuttings were rated on a 1 to 5 scale, from which a rooting index was calculated.

Chlorotic symptoms of herbicide toxicity were apparent in 1979 in the leaves of plants treated twice a year with Casoron at 6 lb ai/A (Table 1), and fall applications were omitted from this treatment in 1979 and 1980. Toxicity symptoms appeared early in 1980 in leaves of plants treated with Casoron plus Kerb, and spring applications of Casoron were omitted in 1980 and 1981. Symptoms were not severe, and were not apparent after herbicide applications were omitted.

Six of the herbicide treatments resulted in a significant reduction of plant height at the end of the experiment in 1982 (Table 1). With one exception, these treatments were twice a year applications, and at rates in some cases 50% higher each application than the maximum usually recommended for once a year treatment. The one exception where there was a 9% reduction in height without the same herbicide being applied twice a year was the Princep and Casoron treatment, alternating fall and spring. Through 1980, the difference in height between these plants and the untreated checks was not statistically significant (8).

The plants treated with Princep plus Lasso were injured the first year with the EC formulation of Lasso, and were more than 20% shorter than the checks the first 5 years (8). Starting in 1978, the 4G formulation of Lasso was used to avoid the leaf burn from the EC formulation. By 1982 the effect on plants treated with Lasso and all other treatments was 12% or less.

The mean rooting percent for each year did not show any significant effect from the herbicide treatments (Table 1). The only significant effect on percent rooting was in the cuttings taken July 28, 1982 (Table 2). Cuttings from the Casoron plus Kerb and the Princep plus Lasso treatments on that date rooted only 77 to 80% compared with 97% for the untreated checks. On the other 4 dates of that year, cuttings from those treatments rooted essentially as well as or better than from untreated plants, and the combined means for that season were not significantly different.

Rooting index was not significantly affected by any treatment in any year or at any date during the year (data not shown).

Table 1. Effects of herbicides on height of rhododendron 'Mrs. G.W. Leak' plants, and rooting of cuttings. 1
Herbicide Rate
(lb. ai/A)
Plant height 1982 7
Percent rooting 8
1978 1979 1981 1982 Mean
None -- 51 83 53 64 87 75.8
Casoron 2 + Kerb 3 4.0 + 2.0 49 60 70 71 85 76.5
Princep + Kerb 3 3.2 + 2.0 49 67 70 73 91 80.4
Ronstar + Kerb 3 4.0 + 2.0 49 67 40 66 90 73.5
Casoron 4 6.0 45* 63 63 74 91 79.5
Princep 4.8 46* 83 53 63 89 75.7
Ronstar 6.0 47* 53 47 63 91 73.5
Princep + Lasso 5 3.2 + 6.0 45* 63 67 95 84 77.4
Princep + Devrinol 3.2 + 6.0 48* 63 57 62 89 74.0
Princep, Casoron 6 3.2, 4.0 47* 53 67 66 90 75.6
1 Planted 9-26-75. Herbicides applied 10-22-75, and each spring and fall thereafter, except as indicated in footnotes 2 to 6.
2 Casoron application omitted May 1980 and April 1981.
3 Kerb applied only once a year.
4 Casoron application omitted September 1979 and 1980.
5 Lasso 4E used through 1977; Lasso 15G used starting in 1978.
6 Princep applied only in the fall each year, alternating with spring applications of Casoron.
7 Means significantly different from untreated plants at 5% level are indicated with an asterisk (*).
8 Means for 30 cuttings per treatment in 1978 and 1979, 4 sets of 30 in 1981, and 5 sets of 30 in 1982. In the last column are means for the total of 330 cuttings per treatment.

Table 2. Rooting of cuttings taken in 1982 from herbicide treated rhododendron 'Mrs. G.W. Leak' plants. 1
Herbicide Rate
(lb. ai/A)
Percent rooting
6-29 7-1 7-28 7 8-2 9-1 Mean
Check -- 80 80 97 97 80 86.8
Casoron 2 + Kerb 3 4.0 + 2.0 83 90 77* 97 80 85.4
Princep + Kerb 3 3.2 + 2.0 97 87 93 97 83 91.4
Ronstar + Kerb 3 4.0 + 2.0 83 90 90 100 87 90.0
Casoron 4 6.0 80 100 90 93 87 90.6
Princep 4.8 83 90 97 90 87 89.4
Ronstar 6 97 90 97 93 80 91.4
Princep + Lasso 5 3.2 + 6.0 80 87 80* 93 80 84.0
Princep + Devrinol 3.2 + 6.0 87 80 93 100 87 89.4
Princep, Casoron 6 3.2, 4.0 80 90 90 97 93 90.0
1-6 See footnotes 1 to 7 on Table 1.

These data confirm results generally observed in one or two-year experiments, as reviewed in the Introduction. Herbicide treatments that are not visibly or measurably injurious to stock plants usually do not affect the rooting of cuttings taken from those plants.

Some herbicide applications in this experiment were purposely at rates 50% higher than the maximum normally recommended for once a year treatment, and with twice a year application the total amount was excessive by 200%. Some of these treatments resulted in up to 12% reduction in plant height. No other symptoms were apparent, except from Casoron, and those symptoms disappeared when applications were reduced to once instead of twice a year.

The high rates were used in an effort to determine whether even excessive herbicide application in the upper range of tolerance by stock plants would adversely affect rooting, and the results showed it did not. With careful application as close to the recommended rates as practical, there should be no significant effects on rooting from long term use of the herbicides used in this study.

Literature Cited
1.  Ahrens, J.F. 1972. Rooting of rhododendron cuttings from container-grown plants treated with trifluralin and simazine. The Plant Propagator 18(3):12-15.

2.  _____ 1972. Rooting cuttings from plants treated with herbicides. Proc. Intern. Plant Prop. Soc. 22:374-389.

3.  _____ 1979. Cuttings from herbicide treated nursery stock - what can we expect? Proc. Intern. Plant Prop. Soc. 29:348-357.

4.  Cohen, Michael A. 1979. Effect of several herbicides on weed control and propagation of container-grown ornamentals. The Plant Propagator 25(1):8-10.

5.  Fretz, T.A. 1973. Evaluation of several herbicides on subsequent rooting response of two container-grown azalea cultivars. The Plant Propagator 19(4):23-26.

6.  Hull, R.J., J.J. McGuire, and R.D. Wyman. 1977. Rooting capacity of herbicide treated woody ornamental plants. Weed Sci. 25:452-455.

7.  McGuire, J.J. and J.L. Pearson. 1972. Rooting of softwood cuttings taken from container-grown plants treated with simazine and diphenamid. Proc. Northeast. Weed Sci. Soc. 26:62-66.

8  Ryan, G.F. and J.M. Wetherington. 1983. Herbicide effects on growth and flower bud formation in rhododendron. Jour. Amer. Rhod. Soc. 37:222-224.

9.  Ticknor, R.L. 1965. The effect of herbicides on the rooting of juniper cuttings. The Plant Propagator 11(1):8.

10. ______ 1972. Effects of several herbicides on propagation of four ornamentals. Proc. Inter. Plant Prop. Soc. 22:129-131.

11. Whitcomb, C.E. 1976. Effects of herbicides on growth of container nursery stock. SNA Nursery Research Jour. 3(2):1-12.