Yan Chen, Ron Strahan and Regina P. Bracy
Yellow nutsedge is one of the most troublesome and widespread perennial weeds in landscapes and gardens across the coastal plains. This fast-growing weed can be found in nearly all soil types but thrives in irrigated landscape plantings. Its upright growth habit and pale green color make it a prominent distraction in the aesthetics of high quality landscapes. Although grass-like and often referred to as nutgrass, nutsedges are not grasses but true sedges. Sedges can be identified by their triangular stems, whereas grass stems are flat or oval. Distinguishing between grasses and sedges is important to landscape managers because selective grass-killing herbicides such as fluazifop and sethoxydim will not work on sedges.
It is also important to differentiate within sedge species because the species differ in their herbicide susceptibility. The two most common sedge species found in landscape beds are yellow nutsedge and purple nutsedge. As their names imply, flower color is an easy way to identify the two species. Yellow nutsedge flowers are yellowish while those of purple nutsedge are burgundy to purple. Both species produce rhizomes and tubers, but purple nutsedge produces tubers connected by rhizomes or “chains.” Yellow nutsedge produces tubers at the end of rhizomes. When flowers are not available, leaf tip is the easiest way to differentiate the two species. Yellow nutsedge leaf tips have long and tapered points; purple nutsedge leaf tips are blunt or dagger-like in appearance.
Yellow nutsedge is difficult to manage because of its carbohydrate-storing tubers that produce numerous aerial shoots. One tuber is capable of producing as many as 1,900 shoots and up to 7,000 additional tubers each year. Tubers are viable for years and are distributed from 2 to 14 inches below the soil surface. These characteristics contribute to a long germination period that requires repeated treatment for effective control.
Although tilling and hand-pulling are common weed control methods, they are time-consuming and expensive and often contribute to the spread of the sedge in the field. Few nonselective herbicides are available that control yellow nutsedge (Table 1). Selective herbicides, including pre-emergence and postemergence products, are available, but choosing the correct product depends on proper sedge identification. For example, the herbicide Pennant provides good preemergence yellow nutsedge control but no control of purple nutsedge.
Mulching is a common practice in landscape maintenance to protect trees and shrubs from soil temperature fluctuation and to provide some weed control by suppressing seed or tuber germination. Research suggests that nutsedge tubers will emerge through bark or rock mulches in landscape plantings; however, a thick layer of mulch combined with herbicide treatment may control nutsedge at emergence or at an early growth stage.
During the past two years, LSU AgCenter scientists have been testing herbicides at the Hammond Research Station
for effective nutsedge control and their safe use around ornamental plants. A study conducted in 2006 investigated the interactions between landscape mulch types, herbicide rates and herbicide placement (above or under the mulch) and the control effects of the herbicides Sedgehammer and Eptam 5G on yellow nutsedge.
A total of 72 research plots were manually infested with yellow nutsedge tubers, and each was planted with Stella de Ore daylilies, Big Blue liriope, and Mystery gardenia. Plots were covered with one of the three organic mulches often used in our area – 4 inches of pine straw, 4 inches of pine bark nuggets and 6 inches of cypress mulch – either before or after herbicide sprays. Herbicide treatments were Eptam 5G or Sedgehammer at the label rate or twice the label rate. Sedgehammer was applied as post-emergence treatment when yellow nutsedge was at 3-to-5-leaf stage. Eptam 5G was applied as pre-emergence treatment. The numbers of yellow nutsedge plants in treated plots were compared with untreated plots. Control efficacy was based on weed density and vigor and evaluated on a scale of zero percent (no control) to 100 percent (complete control) at 4, 8, 12 and 16 weeks after treatments. Overall landscape plot quality was assessed by visual ratings on a scale of 1 to 10, where 10 represented the best quality. Visual ratings lower than 8 represented unacceptable quality. Tolerance of ornamental plants to herbicide treatments was also evaluated. Mulch effects on nutsedge control
Bare-soil plots with no mulch or herbicide averaged 250 yellow nutsedge plants per plot. Mulched plots had fewer nutsedge plants – pine bark nuggets, 75 percent less nutsedge; straw, 60 percent less; and cypress mulch, 45 percent less. These results suggest that mulching is an effective method for reducing yellow nutsedge emergence. None of these plots was aesthetically acceptable based on their overall ratings, indicating that at a high nutsedge infestation level herbicides were needed in combination with mulches to obtain acceptable controls.
When herbicides were applied over top of mulch, only Sedgehammer at the twice-label rate gave acceptable control of fewer than two nutsedge plants per 10 square feet. Eptam at the twice-label rate applied on top of mulch did not provide satisfactory control.
When herbicides were applied before pine bark nugget or pine straw mulch, Sedgehammer at both rates and Eptam 5G at the twice-label rate provided acceptable control of nutsedge. Results suggest that for heavy yellow nutsedge infestations when the herbicide is applied after mulch installation, satisfactory control of this weed is only provided by Eptam and Sedgehammer at the higher rate. These results indicate that the amount of herbicide (and associated cost) can be significantly reduced if landscapers can apply herbicides before they mulch the area. Herbicide effects on plant growth
Ornamentals planted in herbicidetreated plots showed no acute injury to overhead application of Eptam or Sedgehammer. By the end of the 12-week growing period, Eptam had little adverse effect on ornamental plant growth. However, plants exhibited different degrees of growth reduction in response to Sedgehammer. Generally, plant growth reduction was less pronounced when the herbicide was applied above the mulch than when applied under mulch.
In some instances, landscape plant response was also influenced by mulch type. Daylilies in plots covered with pine straw and treated with Sedgehammer at high rate had very short flower stalks and were aesthetically unacceptable. Daylilies in plots covered with other types of mulches with the same herbicide treatment were shorter but still acceptable. Liriope fresh weight was reduced by Sedgehammer treatments, but plants were aesthetically acceptable in terms of plant size and leaf greenness. Gardenia plants treated with Sedgehammer had similar number of flowers compared with untreated control, but plant size was smaller. Because of these potential growth reduction effects, Sedgehammer should only be applied as a directed spray around newly planted woody ornamentals or on established landscape beds. Yan Chen
, Assistant Professor, Hammond Research Station, Hammond, La.; Ron Strahan
, Assistant Professor, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.; Regina P. Bracy
, Professor, Hammond Research Station, Hammond, La.(This article was published in the summer 2007 issue of Louisiana Agriculture.)