The 2009 ASHS Annual Conference

Weed control is an essential aspect of managing high quality turf.  Controlling weeds improves turf uniformity and density, but also reduces plant competition for light, water, and nutrients.  Monosodium methylarsonate (MSMA) is a widely used post-emergent herbicide for summer weed control in bermudagrass turf because of the low cost and broad spectrum of weeds controlled.  The United States Environmental Protection Agency has limited the use of MSMA in turf until December 31, 2013, when all use will be prohibited.  Turf managers will then have to rely on other products to control summer weeds or implement different weed control strategies.  One option for summer weed control is using a pre-emergent (PRE) herbicide.  Pre-emergent herbicides for summer weed control are applied in early spring, prior to weed seeds germinating, to form a protective chemical barrier near the surface of the soil.  Several PRE herbicides work by affecting cell division and thus causing death in susceptible plants.  Turf plants emerging from dormancy produce new root initiates that must penetrate this chemical barrier.  Four PRE herbicides (Dithiopyr, Oxadiazon, Pendimethalin, and Quinclorac) were applied to dormant ‘Tifway’ bermudagrass in mid-March and their effects on root architecture (surface area, length, and mass) were evaluated and compared to an un-treated control.  Root samples were harvested using a 17.8 x 1.9cm soil slab sampler and washed clean of foreign material.  Samples were divided into top and bottom profiles at a depth of 7.6cm.  Root surface area and average root length were determined using the Win-Rhizo^tm system and root mass determined gravimetrically.  The roots were evaluated 2, 4, 8, 12, and 16 weeks after treatment (WAT) and all sampling intervals revealed significantly reduced average root length and surface area.  The greatest decrease in root length occurred 8 WAT by Pendimethalin (55%), followed by Quinclorac (40%), Dithiopyr (25%), and Oxadiazon (25%).  The greatest decrease in surface area occurred 8 WAT by Pendimethalin (60%) followed by Quinclorac (40%), Dithiopyr (35%), and Oxadiazon (35%).  Twelve WAT root length was not as severely affected.  The data showed a decrease in length by Dithiopyr (40%) followed by Pendimethalin (20%), Oxadiazon (5%), and Quinclorac (0%).  Twelve WAT root surface area was not as severely affected.  However, data showed that Dithiopyr had the greatest reduction (40%) followed by Pendimethalin (35%), Oxadiazon (3%), and Quinclorac (0%).  This study shows that the PRE’s tested have a negative influence on hybrid bermudagrass root parameters well into the growing season.