Winter annual weeds are found in most turfgrasses throughout Georgia and the southeastern U.S. during the winter and early spring. Not only on golf courses, they are also found in home lawns, athletic fields, industrial sites, and other turfgrass areas. Mild winter temperatures favor germination and rapid growth of these annuals. The presence of weeds detracts from the aesthetic value and usually results in an undesirable turf. Most winter annuals initiate germination in early fall and growth continues until late spring to early summer. If not controlled, winter annual weeds will delay the spring transition of desirable warm-season turfgrasses (Johnson 1977a, 1982a).
Herbicides can be applied as preemergence (Grant, Cooper, and Webster 1990; Johnson 1975, 1977a, 1982a) or postemergence (Ebdon and Jagschitz 1982; Johnson 1977b, 1982a, 1982b, 1987; McCarty 1991) treatments for the control of winter annuals. Preemergence herbicides when used, must be applied prior to weed seed germination and emergence. Therefore, application time is important in obtaining optimum weed control (Johnson 1975). In this study, annual bluegrass (Poa annua L.) control was higher when benefin was applied in September at Lawrenceville and October at Griffin than when applied in July or August at either location. These results indicate that July and August treatments were too early at both locations and October treatment was too late at Lawrenceville for optimum annual bluegrass control.
Before selecting the preemergence herbicide, it is important to know what weed species are in the turfgrass area. When benefin at 3.0 lb ai/acre was applied in the fall, >80% control was obtained for annual bluegrass, henbit (Lamium amplexicaule L.), common chickweed [Stellaria media (L.) Cyrillo], and corn speedwell (Veronica arvensis L.), but control was <70% for hop clover (Trifolium agrarium L.), parsley-piert (Alchemillamicrocarpa Bossier Reuter), and lawn burweed [Soliva pterosperma (Juss.) Less.] (Johnson 1977a). In the same study, oxadiazon at 4.0 lb ai/acre controlled parsley-piert, hop clover, annual bluegrass, and corn speedwell, but not lawn burweed, henbit, or common chickweed. Grant, Cooper, and Webster (1990) reported that tank-mixes of preemergence herbicides applied in the fall controlled more common weed species in the southeastern United States than when a single herbicide was applied alone. Isoxaben applied alone at 0.75 lb ai/acre provided >80% control of seven to 10 different weed species, while oryzalin applied alone at 2.0 lb ai/acre controlled only annual bluegrass and henbit. When the herbicides were tank-mixed and applied as a single application, control was >70% for all ten species. They reported similar results from tank-mixes of isoxaben with pendimethalin. Therefore, the effectiveness of preemergence herbicides on winter annuals depends on time of application, weed species, and whether the herbicide was applied alone or tank-mixed with other herbicides.
Preemergence herbicides applied in the fall can delay green-up (spring transition) of common bermudagrass (Cynodon dactylon [L.] Pers.) the following March and April (Johnson 1977a). The amount of delay in green-up, however, often depends on herbicide used. Benefin applied at 3.0 lb ai/acre delayed green-up by 34% by late March and 28% by mid-April. The turfgrass recovered from the treatment, however, by mid-May. In contrast, butralin at 6.0 lb ai/acre delayed green-up of common bermudagrass only 15% by late March, while the green-up in plots treated with oxadiazon at 4.0 lb ai/acre was not affected.
The timing of postemergence herbicides can also affect bermudagrass green-up in the spring. In most instances, combinations of 2,4-D with mecoprop and dicamba, or with dicamba injured bermudagrass more when applied to semi-dormant turf in March than to dormant turf in January or February (Johnson 1980).
The 2,4-D and herbicide mixtures that contain 2,4-D have traditionally been used to control most broadleaf weeds in turfgrasses. When these herbicides are used, however, the two- or three-way combinations will usually control weeds more consistently than 2,4-D alone (Johnson 1977b). For example, one application of 2,4-D (1.0 lb ai/acre) controlled common chickweed 43%, compared to 91% from 2,4-D plus mecoprop plus dicamba (0.66 + 0.33 + 0.08 lb ai/acre), and 100% from 2,4-D plus dicamba (1.0 + 0.5 lb ai/acre) (Johnson 1977b). A wider range of weed species can also be controlled from the two- and three-way combinations applied in multiple applications, compared with a single application. One application with 2,4-D plus mecoprop plus dicamba (1.0 + 0.5 + 0.1 lb ai/acre) failed to control henbit, parsley-piert, and corn speedwell effectively, while the control was excellent when 1.0 + 0.5 + 0.1 lb ai/acre was applied in each of two applications (Johnson 1977b, 1982b, 1987).
The consistency of postemergence herbicides in controlling winter annuals often is related to time of application. McCarty (1993) reported that postemergence herbicides applied in December effectively controlled small weeds, as opposed to the poor control when treatments were delayed until March or April. In an earlier study in Georgia (Johnson 1980), two- and three-way herbicide combinations tended to control corn speedwell, hop clover, and parsley-piert better when applied in January or February than when applied in March.
Triclopyr has good to excellent activity on broadleaf winter annuals (Jagschitz 1980; Johnson 1982b, 1987). Triclopyr, however, severely injured a mixture of red fescue (Festuca rubra L.), Kentucky bluegrass (Poa pratensis L.), and colonial bentgrass (Agrostis tenuis sibth.), and delayed spring green-up of common bermudagrass. Therefore, the successful use of herbicides depends greatly on turfgrass tolerance. The herbicide has little value if it controls weeds and causes undesirable turfgrass injury.
It has been several years since preemergence and postemergence
herbicides have been evaluated on winter annual weeds in Georgia (Johnson 1982b,
1987). Additionally, several herbicides have been introduced during this time
period. Therefore, preemergence and postemergence herbicide experiments were
conducted to determine the effects of these herbicides on weed control and on
tolerance of common bermudagrass turf.
MATERIALS AND METHODS
Winter annual weed control experiments were conducted on Kentucky bluegrass at Butternut Creek Golf Course, Blairsville, Georgia, and on dormant common bermudagrass at Beaver Lake Golf and Country Club, Gay, Georgia. Table 1 gives the nomenclature of herbicides used in all experiments. The experiments include the postemergence weed control activity from preemergence herbicides at Blairsville during 1990 and 1992 and preemergence and postemergence herbicide evaluations at Gay during 1993 and 1994. Table 2 gives treatment dates, location of experiments, and turfgrass species within each experiment.
Table 1. Nomenclature of Herbicides
|XL||1 + 1G||benefin given above;|
|oryzalin = 4-(dipropyl-||DowElanco|
|Team||1.3 + 0.7G||benefin given above;||DowElanco|
|trifluralin = 2,6-dinitro-|
|Goosegrass/Crabgrass Control||5.25 +
|bensulide = 0,0-bis(1-||O.M. Scott & Sons|
|oxadiazon = 3-[2,4-dichloro-5-|
|Flazasulfuron||ASC 67040||10WP, 25DF||1-(4,6-dimethoxypyrimidin-||ISK Biotech|
|Fluoroxypyr||XRM 5316||1.5 EC||[(4-amino-3,5-dichloro-6-||DowElanco|
|Imazaquin||Image||1.5 AS||2-[4,5-dihydro-4-methyl-||American Cyanamid|
+ 0.3 EC
|MCPA = (4-chloro-2-methyl-||Rhone Poulenc|
|mecoprop = (+)-2-(4-chloro-|
|dicamba = 3,6-dichloro-2-|
|Metsulfuron||DMC||60WD||methyl methyl 2-[[[[4-methoxy-||O.M. Scott & Sons|
|MON 120051||Manage||50 WP||methyl-5-[[(4,6-dimethoxy-2-||Monsanto|
|Oxadiazon||Ronstar||2G||given above||Rhone Poulenc|
|Oxadiazon + benefin||Regalstar||1+.5(38% N)||given above||Regal Chemical|
|Pendimethalin||PRE-M Turfweed grass control||60 WDG||N-(1-ethylpropyl)-3,4-dimethyl -||Lesco
O M.Scott & Sons
|Quizalofop||Assure II||0.88 EC||(±)-2-[4-[(6-chloro-2-quinoxa-||Dupont|
|Confront||2.25+.75 EC||triclopyr = [(3,5,6-trichloro-2-||Dow Elanco|
|clopyralid given above|
+ 75 DF
|2,4-D = (2,4-dichlorophenoxy)||Rhone
|acetic acid; dichlorprop =|
+ .21 EC
Table 2. Annual Weed Control Experiments, Locations, and Treatment Dates
|Postemergence from preemergence herbicides||Butternut Creek Golf Course
|Kentucky bluegrass||1990: March 5|
|1992: March 2|
|Preemergence herbicides||Beaver Lake Golf Course
|Common bermudagrass||1992: September 11|
|1993: September 13|
|Postemergence herbicides||Beaver Lake Golf Course
|Common bermudagrass||1993: January 27|
|1994: February 14|
Postemergence weed control activity from preemergence herbicides.
Preemergence herbicides were applied to Kentucky bluegrass
turf at Blairsville on March 5, 1990 and March 2, 1992 for subsequent evaluations
of crabgrass control. Early spring evaluation of these plots revealed that
some preemergence herbicides exhibited postemergence activity on corn speedwell
and chickweed (common chickweed and mouseear chickweed [C. vulgatum
L.]) (Table 3). Therefore, control ratings for these winter annual weeds
were recorded April 4, 1990, and May 4, 1992. At time of ratings, the cover
of weeds in untreated plots was 35% and 11% corn speedwell cover in 1990
and 1992, respectively, and 25% chickweed spp. cover in 1990.
Table 3. Performance of Preemergence Herbicides on Postemergence
Weed Control In Kentucky Bluegrass,
Butternut Creek Golf Course, Blairsville, Georgia.
|2 + 1G||2.0 + 1.0
3.0 + 1.5
6.0 + 3.0
|LSD @ 0.05||18||21||15|
Preemergence weed control.
Preemergence herbicides at rates given in tables 4 and
5 were applied to common bermudagrass on September 11, 1992 and September
13, 1993 at Gay, Georgia. Treatments were made to different plots each
year. Weed control ratings from 1992 treatments were made in March and
ratings from 1993 treatments were made in March and April 1994. The later
ratings in 1994 were necessary because colder temperatures in January 1994
slowed weed emergence and growth. The lowest temperature during January
1994 was 6oF and there were 15 days when the temperature was
less than 30oF. In contrast, the lowest temperature during January
1993 was 26oF, and the temperature was less than 30oF
for only five days.
Table 4. Effects of Fall-Applied Preemergence Herbicides on Winter Annuals in Dormant Common Bermudagrass, Beaver Lake Golf Course, Gay, Georgia, 1992-1993
|Annual bluegrass||Lawn burweed||Purple cudweed|
|Mar 9||Mar 30||Mar 9||Mar 30||Mar 9||Mar 30|
|2.0 + 1.0||100||100||73||62||70||66|
|2.0 + 2.0||100||100||73||62||70||66|
|2.6 + 1.4||100||100||17||0||54||40|
|2.0 + 1.0||100||93||0||0||62||62|
|LSD P = 0.05||30||24||21||22||25||22|
Table 5. Effect of Fall-Applied Preemergence Herbicides on Annual Bluegrass Control in Dormant Common Bermudagrass, Beaver Lake Golf Course, Gay, Georgia, 1993-94.
||Annual bluegrass control b|
|LSD P = 0.05||-||22|
Weeds evaluated were annual bluegrass, lawn burweed, and
purple cudweed (Graphalium purpureum L.) in 1993 and annual
bluegrass, corn speedwell, chickweed (common and mouseear), and knawel
(Scleranthus annuus L.) in 1994. When final ratings were
made, weed cover in 1993 was 26% for purple cudweed, 36% for annual bluegrass,
and 38% for lawn burweed. The cover of weeds in 1994 was 36% for annual
bluegrass, 8% for knawel, 5% for chickweed, and 2% for corn speedwell.
Postemergence weed control.
Postemergence herbicides at rates given in tables 6 and
7 were applied for weed control in dormant common bermudagrass at Gay,
Georgia on January 27, 1993 and February 14, 1994. A nonionic surfactant
was added at 0.5% v/v with flazasulfuron, quizalofop, 2,4-D plus mecoprop
plus dicamba, MON 120051, and imazaquin. Treatments were applied to different
plots each year. Weed control ratings were made in March 1993 and in March
and April 1994. Weeds and population of weeds evaluated when final ratings
were made consisted of 46% annual bluegrass, 38% lawn burweed, 12% innocene
(Hedyotis procumbens [Walt. ex Gmel.] Fosb.), and 4% corn
speedwell in 1993. The population of weeds in 1994 was 30% annual bluegrass,
13% chickweed, 13% knawel, and 4% innocene.
Experiments were conducted on golf course fairways at Blairsville and Gay, Georgia. Kentucky bluegrass at Blairsville and common bermudagrass at Gay were managed similarly to the surrounding turf. None of the experimental sites were fertilized or irrigated. Both turfgrasses were mowed at weekly intervals to a height of 0.5 to 0.75 inch, while actively growing. Clippings were returned in all experiments. Neither grass was mowed during the winter after Kentucky bluegrass growth stopped and common bermudagrass was dormant.
Winter weed control and turfgrass density ratings were visually estimated at various times from March through May. Winter weed control ratings were based on difference in cover of weeds in treated and untreated plots, where 0 = no control and 100 = complete control. On this scale, >90 = excellent control, 80 to 89 = good control, 70 to 79 = fair control, and <79 = poor control. Crabgrass emergence ratings were made from the fall preemergence experiment on the following May 25. The ratings were based on 0 = no crabgrass present and 5 = heavy population. Turfgrass density ratings were based on 1 to 10, where 1 = no turfgrass and 10 = uniform complete dense cover.
Herbicide treatments in all experiments were arranged in a randomized complete block design with four replications. Analysis of variance procedure of SAS was used to analyze the data (SAS Institute, Inc. 1982). Treatment means were separated by LSD at P = 0.05 level. Data were analyzed and presented by year, as herbicide treatments and weed species varied between years.
RESULTS AND DISCUSSION
Postemergence control from preemergence herbicides.
Corn speedwell and chickweed spp. were controlled with several preemergence herbicides applied in early March in Kentucky bluegrass (table 3). Dithiopyr at <0.5 lb ai/acre and pendimethalin at 3.0 lb ai/acre completely controlled corn speedwell in 1992, but the control was poor (<60%) in 1990. Prodiamine at 0.75 lb ai/acre resulted in fair (78%) control for corn speedwell in 1992, but the control was poor (51%) in 1990. Dithiopyr at 0.5 lb ai/acre resulted in fair (74%) chickweed spp. control, but control was poor (<58%) in plots treated with pendimethalin or prodiamine. Oxyfluorfen plus oryzalin at 6.0 + 3.0 lb ai/acre resulted in good (88%) corn speedwell control, but the control was poor (<45%) when the herbicides were applied at lower rates. The higher corn speedwell control from the herbicide in 1992 was probably related to smaller weeds at the time of treatment in 1992. Isoxaben at 0.75 lb ai/acre was the only herbicide that controlled >78% of corn speedwell and chickweed in both years. There was little or no postemergence activity on either corn speedwell or chickweed from oxadiazon, benefin, benefin plus oryzalin, trifluralin plus benefin, metolachlor, oryzalin, oxadiazon plus benefin, or bensulide plus oxadiazon (data not given).
These results indicate that most preemergence herbicides
applied for crabgrass control in late winter have little or no postemergence
activity on winter annuals. Therefore, if postemergence activity is desired
on emerged weeds at the time preemergence crabgrass herbicides are applied,
it would be desirable to tank-mix a postemergence herbicide with preemergence
herbicide and apply as a single application (Johnson 1983).
Preemergence weed control.
All preemergence herbicides applied on September 11, 1992 controlled at least 80% annual bluegrass the following March, with the exceptions of isoxaben, metolachlor, pendimethalin, and simazine (table 4). The control with simazine at 2.0 lb ai/acre was poor (52%) when rated on March 9. Annual bluegrass control was excellent (>90%) with simazine in earlier studies (Johnson 1977b, 1982a). The poor control in the present study was probably related to 11.6 inches of rainfall in November 1992. This was 7.5 inches above normal. Pendimethalin resulted in fair (74%) annual bluegrass control by late March.
Isoxaben (0.75 lb ai/acre) was the only preemergence herbicide applied in September 1992 that effectively controlled lawn burweed (>98%) when ratings were made the following March (table 4). Oxyfluorfen plus oryzalin (2.0 + 1.0 lb ai/acre) provided fair (73%) control when rated March 9, but the control was reduced to an unacceptable level (62%) by March 30.
Of the 14 herbicide treatments applied September 1992, only isoxaben (74%) and simazine (100%) controlled purple cudweed when ratings were made on March 30 (table 4). Purple cudweed control with oxyfluorfen plus oryzalin was 70% in early March, but reduced to 66% by late March. Lawn burweed control with oxyfluorfen plus oryzalin was similar to that observed for purple cudweed.
All preemergence herbicides applied on September 13, 1993 resulted in good to excellent (>81%) annual bluegrass control the following April (table 5). Poor control with isoxaben and metolachlor during 1993 (table 4) and 1994 indicates that these herbicides have minimal activity on annual bluegrass.
Corn speedwell, knawel, and chickweed were present in the test area. The populations were variable, however, and prevented reliable control ratings. The 1994 test area was selected the previous year because it contained a uniform stand of several winter weeds. There was evidence that lateral movement occurred with some herbicides across plots during 1994, which may account for variation in weed population. Rainfall was 1.8 inches the six weeks following the September treatments in 1993. The highest rainfall at any date during this period was <0.5 inch. A total of 4.3 inches, however, occurred within a seven-day period at six to seven weeks after herbicide treatments. The lateral movement of the herbicides probably occurred during this period. Most preemergence herbicides applied in the fall effectively controlled annual bluegrass, but not lawn burweed or purple cudweed. In 1993, isoxaben controlled both broadleaf weeds, while simazine controlled only purple cudweed.
It was observed that dithiopyr applied at 1.0 lb ai/acre
in the fall was the only treatment where crabgrass had not emerged by late
May during 1993 and 1994 (data not given). Some of the other herbicides
controlled crabgrass by this date during one year, but not in the other
years. These results suggest that preemergence herbicides applied in the
fall have some activity on crabgrass the following spring. Additional information
is needed, however, to determine how much activity is present from the
Postemergence weed control.
Flazasulfuron at rates of >0.04 lb ai/acre was
the only herbicide applied in January 1993 that provided good to excellent
(>87%) annual bluegrass control (table 6). The control with flazasulfuron
at 0.02 lb ai/acre was fair (72%) when rated on March 9, but the control
was reduced to 45% by March 30. Control was effective throughout March
when the rate of flazasulfuron was increased to 0.04 lb ai/acre. Imazaquin
at 0.5 lb ai/acre provided fair control of annual bluegrass (<79%)
in early March, but reduced to an unacceptable level (<57%) by
Table 6.Effects of Postemergence Herbicides on Winter Annuals in Dormant Common Bermudagrass, Beaver Lake Golf Course, Gay, Georgia, 1993.
|Annual bluegrass||Lawn burweed||Innocene||Corn
|Mar 9||Mar 30||Mar 9||Mar 30||Mar 30||Mar 30|
|Triclopyr + clopyralid + isoxaben||0.75+0.25+0.75||0||0||89||97||100||100|
|2,4-D + mecoprop + dicamba||1.0+0.5+0.1||0||0||83||99||100||75|
|MCPA + mecoprop + dicamba||1.42+0.68+0.16||0||0||77||100||100||42|
|2,4-D + dichlorprop||0.94+0.91||0||0||28||100||100||67|
|LSD P = 0.05||14||19||14||18||8||18|
All postemergence herbicides applied in January 1993 effectively controlled (>78%) lawn burweed, except fenoxaprop, diclofop, and quizalofop (table 6). The control, however, by late March was not as good with clopyralid (<80%) as when treated with flazasulfuron (100%), metsulfuron (100%), fluroxypyr (100%), MCPA or 2,4-D plus mecoprop plus dicamba (100%), and 2,4-D plus dichlorprop (100%).
All postemergence herbicides controlled innocene >75%, except diclofop, quizalofop, and MON 120051 (table 6). In contrast, the combination of triclopyr plus clopyralid, alone or with isoxaben, and 2,4-D plus mecoprop plus dicamba were the only herbicides that controlled >75% corn speedwell at the same date. In earlier studies (Johnson 1977b, 1987), corn speedwell control was generally improved when a second application was made at a two-week interval. Since most of the postemergence herbicides included in the present study have not been previously evaluated in Georgia, it is not known whether a second application would improve weed control.
Flazasulfuron was the only herbicide that controlled >80%
annual bluegrass during both years (tables 6 and 7). To obtain this level
of weed control, flazasulfuron must be applied at rates >0.04 lb
ai/acre. The results with flazasulfuron in 1994 were similar to 1993. Annual
bluegrass control with imazaquin at 0.5 lb ai/acre, however, was higher
in 1994 than in the previous year.
Table 7. Effect of Postemergence Herbicides on Winter Annuals in Dormant Common Bermudagrass, Beaver Lake Golf Course, Gay, Georgia, 1994.
||Annual bluegrass||Chickweed||Knawel||Innocene||Corn speedwell|
|Herbicide||Rate||April 4||April 4||April 4||March 21||April|
|Triclopyr + clopyralid|
|2,4-D + mecoprop|
|MCPA + mecoprop|
|2,4-D + dichlorprop||0.94+0.91||0||86||94||77||48|
|LSD P = 0.05||21||18||10||7||20|
Flazasulfuron at 0.04 lb ai/acre was the only postemergence herbicide that controlled >70% of all five weed species (annual bluegrass, chickweed, knawel, innocene, and corn speedwell included in the test area) (table 7). Imazaquin at 0.5 lb ai/acre resulted in good to excellent control of all species except corn speedwell. The control was similar among the combinations of triclopyr plus clopyralid alone (1.5 + 0.5 lb ai/acre), or with isoxaben (0.75 lb ai/acre), 2,4-D plus mecoprop plus dicamba (1.0 + 0.5 + 0.1 lb ai/acre), metsulfuron (0.0376 lb ai/acre) and fluoroxpyr (0.5 lb ai/acre) for all weed species except annual bluegrass. Corn speedwell control was poor (<70%) when the rate for triclopyr plus clopyralid was reduced to 0.75 + 0.25 lb ai/acre, fluoroxypr reduced to 0.25 lb ai/acre, and metsulfuron reduced to <0.018 lb ai/acre. Combinations of MCPA plus mecoprop plus dicamba and 2,4-D plus dichloroprop controlled chickweed, knawel, and innocene, but not annual bluegrass or corn speedwell.
The activity of fenoxaprop, diclofop, clopyralid, quizalofop, and MON 120051 was not as good for a wide range of weed species as was flazasulfuron, metsulfuron, fluoroxypyr, triclopyr plus clopyralid, 2,4-D plus mecoprop plus dicamba, imazaquin, MCPA plus mecoprop plus dicamba, and 2,4-D plus dichlorprop (table 7). MON 120051 controlled only two species (chickweed and knawel), while clopyralid (innocene), diclofop (chickweed), and fenoxaprop (knawel) controlled only one species. Quizalofop did not control any of the weed species (<50%).
These results indicate that flazasulfuron has fair to excellent activity on annual bluegrass and most broadleaf weeds included in this study. The broadleaf weeds include lawn burweed, chickweed, innocene, and knawel. Corn speedwell control with flazasulfuron at 0.04 lb ai/acre was >70% control in one of two years. Metsulfuron controlled all broadleaf weeds effectively, except for corn speedwell. Generally, 2,4-D plus mecoprop plus dicamba at 1.0 + 0.5 + 0.1 lb ai/acre over the two-year period was as effective as any other herbicide for winter annual broadleaf weed control.
The density of common bermudagrass was significantly lower
in April 1993 in plots treated with metolachlor at 4.0 lb ai/acre in September
1992, when compared with untreated turfgrass (table 8). The delay in green-up
with metolachlor from September treatment agrees with that reported from
a late February treatment in an earlier study (Johnson and Murphy 1993).
There were no other observed differences from preemergence herbicides at
the April 20 ratings.
Table 8.Effects of Fall-Applied Preemergence Herbicide on Common Bermudagrass the Following Spring, Beaver Lake Golf Course, Gay, Georgia, 1993-94
|Herbicide||Rate||April 20||May 25||April 4||May 23|
|lb ai/acre||1 to 10|
|+ oryzalin||2.0 + 1.0||3.9||5.2||3.9||4.0|
|+ oryzalin||2.0 + 2.0||3.3||4.8||4.0||3.9|
|+ trifluralin||2.6 + 1.4||3.6||4.3||4.6||4.0|
|+ benefin||2.0 + 1.0||4.2||5.9||5.1||4.8|
|LSD P = 0.05||0.9||0.9||0.9||0.7|
When common bermudagrass density ratings were made May 25, 1993, there were several preemergence herbicides applied in September 1992 that delayed turfgrass growth (table 8). Oryzalin at 1.5 lb ai/acre did not affect the spring growth of common bermudagrass at any time during the spring of 1993. With oryzalin at 2.0 lb ai/acre, however, the growth was less than in plots not treated or treated at 1.5 lb ai/acre. In an earlier study (Johnson 1994), oryzalin applied at 3.0 lb ai/acre in late February delayed growth of `Tifway' bermudagrass (C. transvaalensis Burtt-Davy x C. dactylon [L.] Pers.) during April and May. Common bermudagrass in the present study was delayed when treated in September 1992 with dithiopyr, benefin, metolachlor, pendimethalin, and benefin plus trifluralin. During this period, the growth was not delayed when treated with oxadiazon, butralin, isoxaben, prodiamine, simazine, oxyfluorfen plus oryzalin, benefin plus oryzalin, and oxadiazon plus benefin.
None of the preemergence herbicides applied in September 1993 delayed turfgrass growth when ratings were made in April or May 1994 (table 8). These results indicate that preemergence herbicides may affect turfgrass green-up in the spring differently across years. Therefore, selection should be made not only for weed control, but also for turfgrass tolerance.
Postemergence herbicides applied in January 1993 and February
1994 generally did not delay spring green-up and reduced density of common
bermudagrass very little when ratings were made in April and May (table
9). Two exceptions were observed in 1993, as the density was lower in plots
treated with triclopyr plus clopyralid alone or with isoxaben. Triclopyr
plus clopyralid applied at 0.75 + 0.25 lb ai/acre did not delay bermudagrass
growth, but the growth was delayed when the rate was increased to 1.5 +
0.5 lb ai/acre. Common bermudagrass growth on April 20 was delayed when
isoxaben (0.75 lb ai/acre) was tank-mixed with the lower triclopyr plus
clopyralid (0.75 + 0.25 lb ai/acre) rate, but the turfgrass fully recovered
by May 25. Three exceptions were observed in 1994, as the density was lower
in plots treated with fluoroxypyr at 0.5 lb ai/acre, triclopyr plus clopyralid
at 1.5 + 0.5 lb ai/acre, and 2,4-D plus mecoprop plus dicamba at 2.0 +
1.0 + 0.2 lb ai/acre than in untreated plots when ratings were made in
April. None of these herbicides, however, delayed turfgrass growth in April
when applied at normal rates (fluoroxypyr at 0.25 lb ai/acre, triclopyr
plus clopyralid at 0.75 + 0.25 lb ai/acre, and 2,4-D plus mecoprop plus
dicamba at 1.0 + 0.5 + 0.1 lb ai/acre). Turfgrass growth affected by herbicides
in April completely recovered by late May. Of the postemergence herbicides,
only triclopyr plus clopyralid applied at 1.5 + 0.5 lb ai/acre reduced
turfgrass growth both years. Therefore, the use of most postemergence herbicides
for winter weed control did not affect early spring growth of common bermudagrass.
Table 9.Effects of Mid-Winter-Applied Postemergence Herbicides on Common Bermudagrass the Following Spring, Beaver Lake Golf Course, Gay, Gergia, 1993-94.
|Herbicide||Rate||April 23||May 25||April 4||April 20||May 23|
|lb ai/acre||1 to 10|
|+ clopyralid||0.75 + 0.25||2.3||3.9||1.6||2.7||2.9|
|1.5 = 0.5||1.3||3.1||1.2||1.9||3.0|
|Triclopyr + clopyralid|
|+ isoxaben||0.75 + 0.25 + 0.75||1.7||3.9||1.5||2.7||3.3|
|2,4-D + mecoprop|
|+ dicamba||1.0 + 0.5 + 0.1||2.9||4.2||2.0||2.9||3.0|
|2.0 + 1.0 + 0.2||2.5||4.1||1.2||1.7||2.1|
|MCPA + mecoprop|
|+ dicamba||1.42 + 0.68 + 0.16||3.1||4.4||2.8||3.9||3.4|
|2,4-D + dichlorprop||0.94 + 0.91||3.0||4.4||2.6||3.4||3.3|
|LSD P = 0.05||0.9||0.6||1.1||0.8||1.1|
SUMMARY AND CONCLUSIONS
Herbicides were applied for evaluation of preemergence and postemergence control of winter annuals in turfgrasses in Georgia. The major findings are as follows:
1.Although exceptions may occur, preemergence herbicides applied in late winter for crabgrass and goosegrass control, generally will not control emerged winter annuals.
2.Annual bluegrass control ranged from fair to excellent when oxadiazon, butralin, oryzalin, dithiopyr, benefin, prodiamine, pendimethalin, oxyfluorfen plus oryzalin, benefin plus oryzalin, benefin plus trifluralin, and oxadiazon plus benefin were applied in September. The control from simazine was poor in one of two years. Neither isoxaben nor metolachlor provided annual bluegrass control.
3.By late March 1993, isoxaben was the only preemergence herbicide that controlled lawn burweed (98%), while simazine (100%) and isoxaben (74%) were the only herbicides that controlled purple cudweed.
4.Flazasulfuron was the only postemergence herbicide that controlled annual bluegrass consistently during 1993 and 1994.
5.Combinations of 2,4-D plus mecoprop plus dicamba at
1.0 + 0.5 + 0.1 lb ai/acre controlled all broadleaf weeds as effectively
as any other postemergence herbicide.
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The authors thank Louis Henderson, Butternut Creek Golf Course, Blairsville, and J. C. Patel, Beaver Lake Golf and Country Club, Gay, Georgia for their cooperation. The authors also gratefully acknowledge the technical assistance of W. Olson, M. Gilmer, and T. Dinkins.