Influence of application timing on the impact of glyphosate on giant reed (Arundo donax L.)

Experiments were conducted near Davis and Fresno, California, to determine the impact of glyphosate timing on giant reed control. At Davis, giant reed (Arundo donax L.) in pots received an application of 1.5% glyphosate solution in September 2006, October 2006, November 2006, April 2007, June 2007, or August 2007. Leaf greenness, number of living and dead stems, and number of newly emerging stems were measured monthly until September 2008. By 1 year after treatment none of the treated container-grown plants at Davis had living stems, regardless of treatment month. For naturally growing giant reed near Fresno, leaf greenness values decline following treatment but recovered in the spring following treatment. Plants treated in September and October had the lowest proportion of living stems m(-2) one year after treatment. The lowest number of new stems produced for the Fresno study was observed for plants treated in September and October. The combination of greater stem kill and reduced production of new stems indicates that a fall treatment with 1.5% glyphosate resulted in greater kill of giant reed. Differences between the field- and container-grown plants were likely due to smaller plants in the containers, which may be less able to recover from damage due to the reduced rhizome mass.