Plant Growth Regulators To Increase Purple Nutsedge Control With Herbicides

Mike Edenfield, Barry Brecke, and Grady Miller

Purple nutsedge has been identified as one of the most common and troublesome weeds in turf throughout the southeastern US. It is a low-growing perennial that can be found throughout warm climates, with only low temperature and moisture limiting its distribution. In addition to being an aggressive competitor for moisture and nutrients, researchers have also identified an allelopathic potential associated with the decaying tissues of both vegetative and reproductive tissues of purple nutsedge.

Purple nutsedge is also known to the world as nutgrass due to its extensive underground system of bulbs, rhizomes and tubers. Tubers are the source of most new plants since only a fraction of the seeds produced are viable. Though the production of tubers by purple nutsedge vary according to environmental conditions, the infestation potential is great considering that reproductive tubers may be formed within 10 to 20 days after seedling emergence. Subsequently, as a result of natural growth and development a single tuber may produce 146 bulbs and tubers in 14 months.

Purple nutsedge has been identified as one of the world’s worst weeds because it is difficult to control. Conventional weed control in turf generally includes mowing and/or herbicides. Applications of 2,4-D and/or the organic arsenicals such as MSMA have been historically used for postemergence chemical control of purple nusedge. However, this herbicide combination is only effective with multiple applications. More recently, chemical control of purple nutsedge has been observed with sulfonylurea and imidazolinone herbicides such as Manage and Image, respectively.

Chemical control is often less than adequate because many tubers remain dormant on the tuber chain of actively growing plants thus reducing translocation of many herbicides. A greenhouse study was conducted at the University of Florida Turfgrass Envirotron to evaluate the use of plant growth promotors to alter the morphological characteristics of purple nutsedge during the spring of 1998. Additionally, based on the results of the greenhouse study a field study was conducted at the University of Florida West Florida Research and Education Center near Jay, FL to investigate pre-conditioning purple nutsedge with plant growth promotors to facilitate its control with herbicides.

Promalin (a proprietary formulation) was evaluated as a POST treatment in an effort to alter the growth and development of purple nutsedge shoots and tubers. PromalinÔ was applied at 0, 25, 50, 100, 200 and 300 ppm of each growth hormone to purple nutsedge 1, 3, or 6 weeks after emergence (WAE) with a second application 1 week after the initial treatment. Promalin ³ 100 ppm increased shoot number four weeks after treatment by at least 50 and 20% when applied at 1 and 3WAE, respectively. There was no increase in shoot number when applied 6WAE. Promalin had no effect on tuberization when applied 1WAE, however, tuberization was reduced 68 and 74% when applied at 3 and 6WAE, respectively.

In the field study treatments were applied to an area naturally infested with purple nutsedge. Treatments included herbicide applied 3WAE, herbicide 3WAE followed by followed by herbicide 6WAE, and PromalinTM 3WAE followed by herbicide 6WAE. Herbicide treatments included paraquat at 0.75 lb/A, MSMA at 2.0 lb/A and glyphosate at 1.0 lb/A. Two applications of either glyphosate or MSMA were most effective for purple nutsedge control and reducing re-infestation. There was no advantage to pre-conditioning purple nutsedge with Promalin in reducing plant population using either paraquat or MSMA. However, PromalinTM followed by glyphosate was more effective than a single application of glyphosate and was as effective as two applications of glyphosate in reducing purple nutsedge re-infestation.

In summary, it appears that pre-conditioning purple nutsedge with growth promotors facilitates weed control with systemic herbicides. Additional research is currently being conducted at the University of Florida Turfgrass Envirotron to further investigate the use of growth hormones as a means to alter the morphological characteristics of purple nutsedge. Future research will include pre-conditioning purple nutsedge with Promalin prior to application with glyphosate and systemic turfgrass herbicides such as Manage, Image and sulfentrazone. Additionally, studies will be conducted to evaluate the use of growth hormone tank-mixes to increase translocation and thus control with systemic herbicides.