Mapping the distribution of torpedograss and evaluating the effectiveness of torpedograss management activities in Lake Okeechobee, Florida

Thousands of hectares of native plants and shallow open water habitat have been displaced in Lake Okeechobee's marsh by the invasive exotic species torpedograss (Panicum repens L.). The rate of torpedograss expansion, it's areal distribution and the efficacy of herbicide treatments used to control torpedograss in the lake's marsh were quantified using aerial color infra red (IR) photography. During a 5-year period, torpedograss coverage increased by 21 %, from 89 to 107 ha, inside three 1-km(2) study sites. In 1996, torpedograss covered more than 6,700 ha of the lake's 40,000 ha marsh. A torpedograss management program was implemented in 2000 to control the spread of torpedograss and allow reestablishment of native vegetation. Between 2000 and 2002 nearly 4,300 ha of torpedograss were treated with the herbicide imazapyr (2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-pyridinecarboxylic acid) or a combination of the herbicides imazapyr and glyphosate (isopropylamine salt of N-(phosphonomethyl) glycine). In some areas, torpedograss control exceeded 95% for greater than one year. Overall, torpedograss control averaged 83% and 60% in the central and northwest marsh, respectively. We estimate that the herbicide treatments resulted in the control of more than 3,000 ha of mostly monoculture torpedograss. Following a reduction in torpedograss coverage, native plants including spikerush (Eleocharis cellulosa Torr.) and fragrant water lily (Nymphaea odorata Sol.) became established in many of the previously impacted areas.