Turfgrass Weed Control Research

Philip Busey and Diane L. Johnston
University of Florida - Fort Lauderdale
turf@ufl.edu
UF Turf Field Day, 16 Mar 2000

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This is a summary of current research for controlling weeds in warm-season grasses.  We are evaluating new chemical aids for turf weed management in golf, sod, and home lawn turf, including bahiagrass, bermudagrass, and St.  Augustinegrass.  We are also studying non-chemical means for managing weeds in the landscape.  While the intent is this work will be useful by turf managers in the future, this is simply a report of work in progress.  Do not interpret anything here as a recommendation or a conclusion.  In fact, some of our experimental treatments are intentionally destructive to turf or are applied because they are expected not to work.  That's part of the research.  So, read the label and use your prudent judgment in what to spray and when and where to spray it.  Also look for recommendations from the Extension Service.  Best, learn to grow healthy turf through proper mowing, watering, and fertilization, which will reduce the need for chemical controls.

Tropical Signalgrass Control

Tropical signalgrass (Urochloa subquadripara) was identified as the widespread weed commonly called "bullgrass," "crabgrass," and "small-flowered Alexandergrass" by golf course superintendents and sod producers in south Florida.  There are no immediate prospects for postemergence control of tropical signalgrass in St.  Augustinegrass turf, but preemergence herbicide research in bermudagrass turf may generate answers for sod farms.
(For more on this weed go here:
Signalgrass http://floridaturf.com/signalgrass/ )

Currently, postemergence herbicides MSMA, Illoxan® (diclofop-methyl), Drive® 75DF (quinclorac), and Sencor® (metribuzin) are being evaluated in various combinations and only MSMA has been effective in killing tropical signalgrass.  Two applications of MSMA at 1.5 Kg a.i.  /ha (1 ounce MSMA 6.6 per 1000 sq ft) at 13 and 14-day intervals reduced tropical signalgrass canopy to <10%, compared with up to 100% in untreated plots.  Tank mixing with Sencor® did not improve signalgrass control.  Addition of adjuvants did not enhance control.

Herbicide Safety to Overseeded Ryegrass

For late summer preemergence herbicide applications to fairways, herbicide labels say wait 3, 4, up to 10 months, depending on the rate, location, and number of apps.  One product label adds, "for best results.  "

For 11 weeks, replicated bermudagrass plots were treated with dithiopyr (Dimension), pendimethalin (Pendulum WDG), and prodiamine (Barricade 65 WG), both at the UF-Fort Lauderdale and Oak Tree Country Club, thanks to Bill McKee.  On 10 January 2000, all experimental plots were overseeded with Lesco Double Eagle Blend perennial ryegrass at 8.5 pounds per thousand square feet.

Barricade 65 WG® (active ingredient prodiamine) applied at 1.125 lb/A between 0 and 10 weeks prior to overseeding severely damaged perennial ryegrass emergence, suppressing visual ratings 44 to 100%.  Seedling counts validated the visual response.
(For graphic results from this project, go here:
Overseeding http://floridaturf.com/weeds/research/overseeding.html )

Dimension® (active ingredient dithiopyr) applied between 8 and 10 weeks prior to perennial ryegrass overseeding had no measurable effect on ryegrass.  Dimension® applied 5 to 7 weeks before overseeding had minimal effect on ryegrass, depending on the number of weeks and the rate of application, 1.125 vs.  1.5 oz/1000 ft2.  Dimension applied 0 to 4 weeks before overseeding was moderately damaging to perennial ryegrass, suppressing visual ratings 13 to 99%.  Pendulum WDG® (active ingredient pendimethalin) applied at 2.5 LB/A was similar in response to Dimension®.

Drive® 75DF for Torpedograss Control

Tank mixes of Drive® 75DF (quinclorac) + Illoxan (diclofop-methyl) were no more effective in torpedograss control than quinclorac alone, when evaluations were carried into a second year of observation.  Drive® 75DF mixed with eight different adjuvants including methylated seed oil (MSO) was no more effective than no adjuvant.
(For earlier results on this project go here:
Drive on torpedograss: http://floridaturf.com/drive/ )

In some dates of evaluation, Drive® 75DF more effectively (P < 0.05) controlled torpedograss (Panicum repens) in bermudagrass (Cynodon spp.  ) turf on golf courses when applied in three or four split applications than in one or two applications.  (Each set of applications totaled 2.2 Kg a.i.  /Ha  ) Despite the initially excellent (80%) reduction of torpedograss canopy using quinclorac, within 12 months the torpedograss stands had regained at least 50% of canopy lost.

Preemergence and Other Herbicides on Overseeded Ryegrass

Overseeded perennial ryegrass normally forms a tight, weed resistant stand, but chemical control of annual bluegrass would be desirable if it can be done safely.  One preemergence herbicide label says, "Do not apply to overseeded turf within 60 days after seeding or until after the second mowing, whichever is longer.  Injury .  .  .  Is likely .  .  .  before seedling secondary roots are in the second inch of soil, not thatch plus soil. " Other labels say to wait until four mowings, or three months.  The label on a postemergence product, Prograss, says to apply one to two weeks after emergence of the ryegrass.

So, how safe is to apply herbicides to young ryegrass? See for yourself in this preliminary research project.  http://floridaturf.com/weeds/research/overseed.html

Alternative Broadleaf Herbicides

Old World diamondflower (Hedyotis corymbosa) was identified as an important emerging weed in both bermudagrass turf on golf course fairways and greens approaches as well as in St.  Augustinegrass sod.  Several herbicides including combinations were evaluated for control of Old World diamondflower and other difficult broadleaf weeds such as Wedelia (Wedelia trilobata) and alligatorweed (Alternanthera philoxeroides).
(For more on hard-to-kill broadleaf weeds, go here:
Old World diamondflower http://floridaturf.com/diamondflower/
Wedelia http://floridaturf.com/wedelia/

Chemicals evaluated were 2,4-D; carfentrazone; clopyralid; dicamba; MCPA; MCPP; metsulfuron; clorpyralid; and sulfentrazone.  The 2,4-D by itself showed little injury to St.  Augustinegrass, from applications in either the summer or winter, while MCPA was very destructive by itself and in mixtures.  Carfentrazone was very highly effective in killing both Old World diamondflower and alligatorweed, and was not injurious to St.  Augustinegrass turf.  Phytotoxicity of clethodim was evaluated in St.  Augustinegrass and bahiagrass (Paspalum notatum) and it was highly injurious to both grasses.
(For the label on Manor, a newly registered formulation of metsulfuron for turf, go here:
Manor http://floridaturf.com/manor/ )

Irrigation Management of Dollarweed

"Does reduced irrigation reduce dollarweed?"  A one-half acre site was planted with 500+ sprigs of dollarweed a.k.a.  pennywort (Hydrocotyle umbellata) for study of nonherbicidal management.  A similar quantity of St.  Augustinegrass was planted as plugs.  There are 24 irrigation zones which will be programmed in replicates to provide treatments ranging from excessive irrigation to droughty conditions, along with a commercial control treatment, atrazine.  The hypothesis is that moderate irrigation will control dollarweed invasion.  However, it is not known whether a single drought is more effective than moderate irrigation year-round.
(For a photo:
Dollarweed http://floridaturf.com/weeds/weiddoll.htm

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