Unlike certain years where sod webworms, billbugs and stem rust are hit and miss, maybe rising to a damaging level and maybe not, grassy and broadleaf weeds are the one pest on the golf course that can be categorized as a “year in and year out” plague.
The reason lies as close as your nearest divot, tilled area or heavily aerated fairway. As soon as two weeks after soil disturbance, no matter the soil type, weeds begin to emerge and take root, beginning to cover the area with foliage. No one planted weeds there – there might not even be much soil moisture. So why is there so much weed growth?
Simply, weed growth occurs on recently disturbed soil, thin spots in fine turf and in thick vigorous stands because large numbers of weed seed are in the soil waiting for favorable conditions for growth. Some estimates put the number of seeds at 10,000 per cubic foot of soil. The very nature of the topic of postemergence weed control means that plants are growing where they are not wanted.
Make time for scouting
Scouting needs to be a dedicated practice, an actual planned activity, rather than a random act that someone “gets around to” if there is time. Monitoring is a series of scouting events, or scouting over time.
One method of scouting involves assigning a staff member to devote a certain number of hours each day to driving a golf car over the course, looking for weed infestations. As specimens are found, their exact location is marked on a map of the hole to document for the short term and long term. A determination must be made whether to implement some sort of initial control measure.
Long term, such infestations are often classified as “hot spots.” It’s very helpful to know where these are, so as to prevent their recurrence with cultural practices and preemergent herbicide applications. Elaborate software mapping products are available through various vendors, but, at least while on the seat of a golf cart, it’s wise to sketch out their location on paper so as to avoid confusion later.
Additionally, staffers can be assigned the task of looking for weeds as they go about their normal duties. For example, staff mowing fairways will be able to spot the earliest emergence of crabgrass or other weeds as part of their duties. All that is needed is initial training on weed identification and consistent follow-up by their supervisors to avoid forgetting or neglect.
This scouting method can become a well-developed habit to inspect or scout as other activities are conducted and be integrated efficiently into the normal maintenance routine. The key for success is consistent scouting and follow-up.
Prevention through healthy turf
Without a doubt, the best way to control most pests is through prevention. Growing a thick, healthy turf that shades and outcompetes weed invaders allows fine turfgrasses to gain a competitive edge, naturally becoming predominant in the stand.
Bentgrass fairways, for example, normally create a much thicker cover and can help prevent annual weeds much more effectively than can perennial ryegrass fairways. Creating such turf doesn’t come easy. However, with inputs of optimal moisture content, moderate fertility, control of other pests and adequate cultivation, desirable golf turf can cover with only a few escapes.
In terms of pest control, weeds on a golf course are an obvious target. A varying degree of importance can be placed on targets, depending on their location on the course, ease of control and potential for disruption of play. High-value areas such as tees and greens are usually more heavily scrutinized than out-of-bounds areas and tend to garner more attention.
Yellow nutsedge is a difficult-to-control weed, requiring more involved treatment efforts than say, dandelions, which usually can be eliminated with whatever is in the tank at the time.
A few target weeds to be controlled with postemergent products are:
White clover is a cool-season perennial broadleaf plant. A member of the legume family, white clover has trifoliate, rounded, dark green, waxy/shiny leaves. Closer inspection of the leaflets reveals white, crescent-shaped markings and prominent parallel veins. The leaves are arranged alternately on the stem. The flowers are (you guessed it) white, sometimes with a tint of pink.
The stems are low-growing/prostrate and covered with fine hairs.
Individual clover plants creep laterally by rooting at nodes, which allow them to persist under close mowing heights. As plants grow and enlarge in size, the stems grow together, sometimes overlapping each other. In so doing, a dense mat of closely spaced clover plants is produced. In most patches, desirable turfs are choked out. Control efforts should be directed toward elimination in the fall, when temperatures are cooler, less wax is on the leaves and stems, and greater translocation of herbicide occurs within the plants. Over the past couple of years, the authors have seen effective control with applications of herbicides containing 2,4-D, dicamba and quinclorac, respectively.
These summer annuals are very similar in appearance. The stems radiate from a taproot, forming a dense mat. Spotted spurge has smooth stems, while prostrate spurge has hairy stems. Leaves are dark green and oblong and may have a reddish splash in the center. The leaves of spotted spurge are somewhat toothed, while prostrate spurge has smooth leaves with some hairs on the underside. All plant parts contain a white, milky sap. Compacted, thin turf is an open invitation for spurge invasion. Postemergent control is best when first observed. Sequential applications of various three-way herbicides has been an effective control method.
Often confused with white clover, oxalis has pale green leaves consisting of three leaflets, which are distinctly heart-shaped and appear folded along the midrib. Both annual and perennial types are found. The stems are thin, sparsely hairy and able to root at the nodes. The yellow flowers are small and funnel-shaped. Oxalis has an interesting seed dispersal mechanism, with the capacity to project seed a considerable distance from the mother plant. With this unique property, a small infestation of oxalis can become problematic if not addressed quickly.’
A low-growing annual, prostrate knotweed is especially well adapted to compacted soils. Small, slender, dark-green leaves are born on thin, wiry stems that radiate from a central growing point. Prostrate knotweed germinates in cool soils in early spring and tolerates close mowing.
While relatively easy to control when young, it is very difficult to control when aggressively growing later in the season. Knotweed is a significant problem on lower-maintenance facilities with excessive traffic and resource limitations relative to appropriate aerification equipment. Early-season applications of combination broadleaf products are most effective in the seedling stage. Control drops dramatically as the weeds mature. Additionally, isoxaben-containing products provide good residual control of knotweed within landscape beds or in turfgrass areas.
Yellow and purple nutsedge
Many sedge species can invade turf, but purple (Cyperus rotundus) and yellow (Cyperus esculentus) nutsedge are the most common. From a distance, yellow nutsedge resembles a grass, but quite different characteristics are evident upon close inspection. The leaf blades are mostly basal, light green, V-shaped with a prominent midrib. The stem is erect and triangular in cross section. The root system is extensive, consisting of rhizomes and tubers. If unmowed, yellow nutsedge produces a seedhead with (you guessed it) yellow flowers, which are surrounded by several leaves.
Purple is commonly found in the South and resembles yellow nutsedge in several ways, yet some differences are visible after careful observation. The tubers of yellow nutsedge occur only at the end of the rhizomes. Tubers of purple nutsedge plants are covered with red or reddish-brown scales and are formed in chains with several tubers on a single rhizome. Also, in general, the leaves of yellow are slightly wider than purple. Yellow nutsedge appears as a slightly more stout or husky plant than purple nutsedge.
The most vulnerable time for these species is upon first emergence. Nutsedge control or suppression has been effective using sequential applications of halosulfuron-methyl, sulfentrazone and mesotrione.
Smooth and large crabgrass are warm-season annuals that are light- to medium-green and tiller profusely. Developing quarter-inch leaf blades and stems that grow close to the ground and root where their nodes contact the soil, individual plants often reach the size of 6 to 8 inches in diameter.
Crabgrass begins germination when soil temperatures warm to the 55-degree range, which can be accelerated through close mowing and exposure of bare soil that warms quicker in spring than taller-cut turf. Though controlled most commonly via preemergent herbicides, escapes of crabgrass are common in thin and pest-ridden turf. A number of products are used for control: quiclorac and mesotrione appear to be two of the more effective active ingredients, especially at the earlier stages of growth.
Field bindweed is a warm-season perennial that grows low to the ground and will tolerate close mowing. Its slender stems are wiry and can grow several feet long. The leaves are generally spade-shaped, an inch and a half in size and have pointed basal lobes. The root system is extensive. White to pink flowers are conspicuous in summer.
Bindweed is primarily a problem in thin turf subject to excessive wear. It is a prolific seed producer, and seed can remain dormant in the soil for up to 20 years. The long-term seed survival and bountiful seed production make short-term control strategies less than desirable. Fall treatment is preferred.
Because of its aggressive stoloniferous nature, avoid pulling or tilling. Excellent control has been reported with quinclorac. Methylated seed oil (MSO) is strongly recommended when using quinclorac to treat field bindweed.
Postemergent control of weeds can be challenging, but with proper cultural practices, monitoring, identification and herbicide applications, weed pressures and visibility can be minimized.