Bent out of shape

Weed control in bentgrass is not impossible, but choices are limited and effective management can be difficult.

While herbicide technology has led to the development of numerous products that control obscure weeds, why are so few of these products registered for use on bentgrass and bentgrass greens? It isn't a characteristic of bentgrass but our imposed cultural management practices that drive this limitation. However, with the use of certain herbicides and plant growth regulators, options for weed control aren't as limited as they might seem at first.

Environmental and cultural sensitivity of bentgrass Creeping bentgrass greens are among the most sensitive turfgrass areas maintained in the industry today. Reasons for this sensitivity include both species/environment interactions and the cultural practices that we impose. The demand for bentgrass greens has extended its use over the past 10 to 15 years into Southern regions where bentgrass is exposed to heat, drought and disease pressures uncommon in Northern climates. In some parts of the Transition Zone, visual quality is reduced and root growth may decline by 80 percent near the end of summer. However, summer declines are a result of both environmental conditions and how we mow greens.

Because bentgrass greens are often maintained at 1/8 inch or less, and rarely above 3/16 inch, cultural stress is a concern. Recent research shows that reducing mowing heights from 5/32 inch to 1/8 inch on Penncross greens results in significantly reduced quality and root biomass throughout the year. Low mowing heights and the stress that results cause bentgrass greens to be more susceptible to pesticide injury than other cool-season species or fairway bentgrass.

Although using emulsifiable (EC) formulations may cause problems under severe heat conditions, fungicides and insecticides are generally safe on bentgrass greens. However, the margin between selective weed control and turf damage is small when bentgrass is stressed. Therefore, fewer herbicides are labeled for use on bentgrass greens.

You should carefully consider turf sensitivity when applying herbicides. For example:

- Mobile materials may accidentally contact the putting area. The photo below shows how ethofumesate can injure bentgrass when it reaches a putting green. In this case, the chemical was transported to the green by excess runoff from irrigation. Also, make sure your water source has not been exposed to mobile herbicides like atrazine; they also may damage sensitive turf.

- Application accuracy is important because overdosing can occur. Drift and spray pattern overlap can result in overapplications. Be careful, use foams and marking dyes to mark spray locations and use non-drift equipment to limit spray areas.

- Also, thoroughly clean spray tanks after every use to remove chemicals that may damage bentgrass greens. The photo on page Golf 48 shows injury from pronamide that resulted from clumped solids that were left in the tank. The solids dissolved during the application of another chemical to the bentgrass green.

Bentgrasses like the A-series, G-series, L-93 and Cato/Crenshaw cultivars are gradually replacing the standby Penncross. Research has shown, though, that newer bentgrass varieties are no more or no less susceptible to herbicide injury. Thus, thankfully, these varieties do not decrease the number of usable herbicide products for putting greens. Although they are often mowed lower than Penncross, they have better shoot density and may have improved stress tolerance. Therefore, they may be less prone to weed problems.

Broadleaf weed control in bentgrass You can use numerous herbicides to control broadleaf weeds in bentgrass turf. These include the phenoxy group, (2,4-D, dichlorprop, MCPA, MCPP, etc.), the benzoic acid group (dicamba) and the pyridinecarboxylic acid group (clopyralid, triclopyr). When you apply them to established turf, these herbicides show few adverse effects. However, if you apply them to bentgrass greens, some phenoxy and benzoic acid herbicides should be mixed and applied at lower rates to avoid bentgrass discoloration. Large, established turfgrass plants with more green tissue and larger root systems can withstand some herbicide applications. Stressed plants in putting greens are more sensitive and may not withstand the same applications of herbicides. Clopyralid and triclopyr are absorbed rapidly by turfgrass foliage. Therefore, do not apply them to bentgrass greens - they are not labeled for that use.

Formulations for post-emergence broadleaf control on bentgrass greens include liquids, solubles and granules. Use granules with care. They are concentrated and can cause injury if not properly applied. Always consult the herbicide label for proper rate and frequency for post-emergent applications to bentgrass greens.

Grassy weed control in bentgrass Use pre-emergence herbicides for control of crabgrass, goosegrass and annual bluegrass. Most pre-emergence herbicides belong to the dinitroaniline (DNA) family and include pendimethalin, prodiamine, oryzalin, benefin, trifluralin and numerous combinations of these materials. They work by inhibiting both root and shoot growth of plants that encounter them in soil. They are highly effective on developing weeds, but they can be detrimental if you apply them to establishing turfgrasses or to bentgrass greens. Therefore, no DNA herbicides are labeled for use on bentgrass greens.

Also, you can apply dithiopyr, oxadiazon and bensulide for pre-emergence control in cool-season turf. Use the dithiopyr formulation, Dimension Ultra, and the oxadiazon/bensulide combination on bentgrass greens. They are labeled for this use, and they have less activity on root growth. Apply siduron on newly seeded areas; it is labeled for this use. Be careful, though; it is not registered for all bentgrass varieties. Use caution and check the label for tolerant varieties.

You can also use dithiopyr to extend your application window. It has some post-emergence activity on annual weeds and, therefore, you can apply it later in the growing season. However, do not apply the oxadiazon/bensulide combination after emergence. It is ineffective as a post-emergent on annual grasses. Apply the oxadiazon/bensulide combination from February in the southern Transition Zone to May in northern states. You may need a repeat application because bensulide and oxadiazon, although they have good residual, do not last as long as dithiopyr or prodiamine.

Annual bluegrass control on bentgrass greens Annual bluegrass is a manageable species in northern regions of the United States. However, in the Transition Zone, the same species is viewed as a weed due to its poor tolerance of summer heat. Two biotypes of annual bluegrass are commonly found in managed turfgrasses: a true annual (Poa annua spp. annua) and a weak perennial (Poa annua spp. reptans). You can control the annual biotype in fairways and roughs with common pre-emergence herbicides in late summer or early fall.

In bentgrass greens, you will find the weak perennial more often. Because of the different life cycle and herbicide restrictions on greens, control can be more difficult. However, plant growth regulators (PGRs) have recently become a promising control option for this biotype.

Use PGRs - flurprimidol and paclobutrazol - to suppress the perennial biotype of annual bluegrass. Both are Type II PGRs that inhibit gibberellin biosynthesis and, subsequently, inhibit cell elongation. Because annual bluegrass normally grows faster than bentgrass, PGR suppression of annual bluegrass gives bentgrass a competitive advantage. You will not see results immediately. A 2 to 4 year commitment (depending on the severity of the infestation) will be needed for this method to be successful. With time, the creeping nature of bentgrass will allow it to outgrow the suppressed annual bluegrass and effectively eliminate it.

For greens with less than 50 percent Poa annua coverage, apply flurprimidol in the spring after bentgrass is growing vigorously and at 3 to 6 week intervals thereafter. Make the initial application by applying 0.25 to 0.5 pounds active ingredient per acre. Subsequent treatments should not exceed 0.25 pound active ingredient per acre. For greens with greater than 50 percent coverage, apply 0.25 pound active ingredient per acre in mid-summer with repeat applications every three to six weeks through early fall. Apply paclobutrazol only in spring and late summer or fall. During the summer, nature does an adequate job of suppressing both annual bluegrass and bentgrass. Summer applications don't compromise root growth in bentgrass greens, but visual quality may be affected and the suppression of annual bluegrass is negligible at this time. Make two applications in spring, separated by four to six weeks, and one in the fall for most areas. An extra fall application may be appropriate but only in the Transition Zone where an extended fall contributes to better cool-season turf growth. To avoid any detrimental effects to bentgrass, do not exceed 0.25 pound active ingredient per acre. Also, there is a granular, paclobutrazol product that is combined with fertilizer. Make applications with this product between March 1 and April 30 and between September 1 and October 31. Make no more than three applications per year, and only one is recommended during the fall. Both chemicals enter the plant via roots, so they must be watered-in after application.

Bermudagrass suppression on bentgrass greens No herbicide products are currently registered for suppression of bermudagrass in bentgrass greens. However, apply flurprimidol, paclobutrazol and another PGR, trinexapac-ethyl, to collars and approaches to delay encroachment.

Make an 8- to 12-inch, single-nozzle application of the pre-emergence herbicide siduron around the perimeter of the green. It will also suppress the advancement of bermudagrass stolons into bentgrass greens. Use herbicides like fenoxaprop or fluazifop to selectively suppress bermudagrass in turfs such as tall fescue that may surround greens. You may have to hand remove bermudagrass to remedy existing infestations in bentgrass greens.

While weed control in bentgrass greens presents unique challenges, quality products are labeled for use in this extreme turfgrass environment. Mow greens at reasonable heights and properly implement necessary cultural practices to both minimize the stress greens experience and maximize the effectiveness of weed control programs. PGRs are most effective when used long-term and when implemented with overseeding and fertilizing programs.

Most of all, be careful with applications on and around greens. Avoid drift and other off-target movements and take care to prevent overlap and overapplication.

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