JARS v37n4 - Herbicide Effects on Growth and Flower Bud Formation in Rhododendron

Herbicide Effects on Growth and Flower Bud Formation in Rhododendron
G.F. Ryan and J.M. Wetherington
Washington State University Western Washington Research and Extension Center
Puyallup, WA

The decision to recommend use of a particular herbicide in a crop is based on evidence of weed control without apparent injury to the crop plants. Numerous herbicides have been evaluated for use in rhododendrons, and effects on appearance and growth of rhododendron plants during one to three years of treatment have been considered in several studies (1, 2, 3, 5, 6, 7, 8, 9). Longer term effects from repeated herbicide use have not been reported, nor does there appear to be any report of effects of herbicides on flower bud formation.
An experiment was started in 1975 to determine the long-term effects of repeated application of herbicides in rhododendrons on growth, flower bud formation, and rooting of cuttings. Some treatments were at excessive rates of application for comparison with recommended rates.
Only effects on growth and flower bud formation during the first five years of the experiment are reported at this time. Any effects on rooting of cuttings will be reported after the experiment has continued for an additional one or two years.

Materials And Methods
One year old plants of rhododendron 'Mrs. G. W. Leak ( Rhododendron X) were planted in Puyallup loam in September 1975. Plots were 4 by 20 ft. with 2 ft. between plots. There were 5 plants per plot and 3 replications.
Treatments were applied October 22, one month after planting, except Kerb (pronamide), which was first applied January 28, 1976. Treatments were repeated in June and September, 1976, and in April or May and September or October each year thereafter. Repeat applications of Kerb were in January 1977 and November 1977, 1978 and 1979.
Kerb, Princep (simazine), and Devrinol (napropamide) were applied as wettable powder formulations. Ronstar (oxadiazon) was applied as a flowable formulation and Lasso (alachlor) as an emulsifiable formulation through 1977, and both were applied in granular formulation thereafter. Granular Casoron (dichlobenil) was used throughout the experiment.
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.
Plant height was measured each year and the shoots with and without flower buds were counted.

Perennial ryegrass ( Lolium perenne L.) was not controlled in some plots before the first re-treatment in June 1976, and common chickweed was not controlled by Ronstar (Table 1). After the June re-treatment, weed control was 90% or higher in all treated plots except that Princep failed to control common groundsel ( Senecio vulgaris L.). The simazine-resistant form of common groundsel is widespread in western Washington.

TABLE 1. Weed control and phytotoxicity on rhododendrons from October 1975 and June 1976 herbicide applications.
Weed control (%) rating,
Rate 5-25-76 8-12-76 8-12-76
Herbicide 1 (lb ai/A) Grass Broadleaf Grass Broadleaf (0-10) 2
None -- -- -- -- -- 0.3 a
Casoron 4.0
+ Kerb 3 2.0 100 a 5 100 100 a 100 a 0.3 a
Princep 3.2
+ Kerb 3 2.0 100 a 98 100 a 90 ab 1.3 ab
Ronstar 4.0
+ Kerb 3 2.0 100 a 100 100 a 99 a 1.3 ab
Casoron 6.0 99 a 100 100 a 100 a 0.1 a
Princep 4.8 95 a 96 100 a 57 b 3.1 c
Ronstar 6.0 69 b 88 99 a 100 a 1.7 b
Princep 3.2
+ Lasso 6.0 62 b 100 100 a 100 a 5.1 d
Princep 3.2
+ Devrinol 6.0 99 a 99 99 a 99 a 2.8 c
Princep,  Casoron 4 3.2,  4.0 97 a 99 100 a 95 a 0.3 a
1 Herbicides were applied October 22, 1975 and June 9, 1976, except as indicated in footnotes 3 and 4.
2 Visual rating of plants. 0 = no injury, 10 = dead.
3 Kerb was applied January 28, 1976.
4 Princep was applied in October, and Casoron in June.
5 Means followed by the same letter are not significantly different at the 5% level as determined by Duncan's multiple range test.

Plants treated with the Princep + Lasso combination showed severe foliage burn and distortion in 1976, as a result of using the emulsifiable formulation of Lasso (Table 1). These plants were 25% shorter than untreated plants when measured in September 1977, and remained approximately 25% shorter through 1980 (Table 2). Plants in five other treatments were 18 to 24% shorter than untreated ones in 1978. These were Casoron + Kerb, Casoron alone, Princep + Kerb, Princep + Devrinol, and Princep alone.

TABLE 2. Effects of herbicides on height of rhododendron 'Mrs. G.W. Leak'.
Rate Plant height (inches) 6
Herbicide 1 (lb ai/A) 1977 1978 1979 1980
None -- 15 26 32 40
Casoron 2 + Kerb 3 4.0 + 2.0 13 21* 27* 34*
Princep + Kerb 3 3.2 + 2.0 13 21* 28 35*
Ronstar + Kerb 3 4.0 + 2.0 16 25 32 37
Casoron 4 6.0 13 20* 26* 30*
Princep 4.8 13 21* 27* 36
Ronstar 6.0 15 24 30 35*
Princep + Lasso 3.2 + 6.0 11* 19* 25* 32*
Princep + Devrinol 3.2 + 6.0 13 21* 28 35*
Princep, Casoron 5 3.2, 4.0 14 23 29 36
1 Herbicides were applied 10-25-75; 6-9-76; 9-13-76; and in April or May and September or October 1977, 1978, 1979 and 1980, except as indicated in footnotes 2 to 7.
2 Casoron application omitted May 1980.
3 Kerb was applied only on 1-28-76, 1-14-77, and in November 1977, 1978, 1979 and 1980.
4 Casoron application omitted September 1979 and 1980.
5 Princep was applied only in October or September each year, alternating with June 1976 and April or May 1977, 1978, 1979 and 1980 applications of Casoron.
6 Means significantly different from untreated plants at the 5% level are indicated with an asterisk (*).

All of these treatments involved application twice a year at the maximum rate recommended for once a year application, or in the case of Casoron and Simazine alone, the rates of application were 50% higher than maximum recommended rates. Thus the total annual application were two or three times the recommended amounts.
Flower buds were produced in 1977 on 17% of the shoots of plants treated with Casoron + Kerb, compared with 0% for plants with no herbicide (Table 3). Plants treated with Casoron or Ronstar also produced significantly more flower buds than untreated plants, although both had flower buds on less than 10% of the shoots.

TABLE 3. Effects of herbicides on flower bud formation, rhododendron 'Mrs. G.W. Leak'.
Rate Percent of shoots with flower buds 2
Herbicide 1 (lb ai/A) 1977 1978 1979 1980
None -- 0 d 16 c 56 ab 80 ab
Casoron + Kerb 4.0 + 2.0 17 a 24 c 47 abc 52 cd
Princep + Kerb 3.2 + 2.0 1 cd 26 c 40 bc 66 abc
Ronstar + Kerb 4.0 + 2.0 4 bcd 79 a 74 a 52 cd
Casoron 6.0 9 b 16 c 24 c 40 d
Princep 4.8 1 cd 8 c 47 abc 73 ab
Ronstar 6.0 5 bc 41 b 68 ab 62 bc
Princep + Lasso 3.2 + 6.0 0 d 24 c 43 bc 62 bc
Princep + Devrinol 3.2 + 6.0 0 d 16 c 42 bc 84 a
Princep, Casoron 3.2,  4.0 3 cd 19 c 61 ab 73 ab
1 See footnotes on Table 2.
2 Means followed by the same letter are not significantly different at the 5% level, as determined by Duncan's multiple range test.

In 1978, plants treated with Ronstar + Kerb or Ronstar alone were the only ones that had significantly more flower buds than untreated plants (Table 3). No treatments significantly reduced the number of flower buds.
In the summer of 1979, plants treated with Casoron showed chlorotic and necrotic spotting on much of the previous year's foliage, and these plants produced less than half as many flower buds as untreated plants. Casoron application was omitted from these plots in September 1979 and 1980, and phytotoxic symptoms disappeared. No other treatments significantly affected flower bud formation in 1979. Plants treated with Casoron + Kerb showed a small amount of the same kind of spotting in the summer of 1979, but flower bud formation was not affected. Casoron application was omitted from these plots in May 1980.
By 1980, flower bud production was only half as much in the plants treated with Casoron alone as in the untreated plants, and it also was significantly reduced in plants treated with Casoron + Kerb.
Ronstar + Kerb was the only other treatment that significantly reduced percent of shoots with flower buds in 1980. The drop in flower bud production on those plants might have been a reaction to the two previous years of unusually high flower bud production. The average flower bud production for those plants in 1979 and 1980 was about the same as the average for other treatments excluding the two Casoron treatments discussed above.
Flower bud formation in rhododendrons, as well as in other kinds of plants, is generally thought to be favored by mild stress due to environmental factors such as sun exposure, drought and cold (4). It also may be increased by the stress resulting from temporary interference with normal translocation from the roots as a result of stem girdling, or from damage to the root system caused by digging and moving a plant.
The early flower bud formation on plants treated with an excessive amount of Casoron may have resulted from slight stress on the plants through an effect on the roots during the first two years of treatment, before the effect became so severe as to cause leaf symptoms and the reduction in growth and flower bud formation that was apparent by 1979 and 1980.
Where Casoron was applied only in the spring, alternating with a fall application of Princep, plant height and flower bud formation were not adversely affected as they were with twice a year application of Casoron.
The flowable formulation of Ronstar was available to us for experimental use before the granular formulation was available commercially. Spraying that formulation on rhododendron leaves resulted in light green spots, but not the severe burning that would occur on plants with less resistant foliage such as azaleas. There was distortion of foliage in these treatments in 1977. Possibly this mild spotting and leaf distortion had just enough of a stressful effect on the plant to influence flower bud formation in 1977 and 1978 without significantly affecting plant height.
The results reported here indicate that plant growth and flower bud formation in rhododendron 'Mrs. G.W. Leak' were not adversely affected by repeated use of herbicide treatments that do not produce phytotoxic symptoms or growth effects during one or two years of application. Excessive use of these same herbicides or use of a different formulation of the same chemical may result in phytotoxic symptoms, reduced growth, and/or reduced flower bud formation.

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3.  Myhre, Arthur, S. 1965. Chemical pre-emergence weed control in western Washington. Proc. Internat. Plant Prop. Soc. 15:306-309.
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7.  Ryan, G.F. and C.C. Doughty. 1978. Control of quackgrass ( Agropyron repens ) in highbush blueberries ( Vaccinium corymbosum ) and hybrid rhododendron ( Rhododendron x). Weed Sci. 26:516-520.
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We wish to acknowledge financial support from the American Rhododendron Society during the first year of this study.