Management Practices for Weed Control in Roadway Rights-of-Way

By law, Departments of Transportation are required to control noxious weeds along highway rights-of-way (ROWs). Since 2000, District 4 (D4) of Minnesota Department of Transportation (MnDOT) adopted a survey design consisting of n= 7, 3-mi segments to quantify infestations of Canada thistle (Cirsium arvense (L.)(Scop.), leafy spurge (Euphorbia esula L.), and poison ivy (Toxicodendron radicans) in chosen regions of the district. In 2004 and 2005, a second survey design was added to see if stratification by ecozone in D4, and greater numbers of 1/4-mi segments could improve precision. Comparison of matching sample statistics from the 3-mi and 1/4-mi plans in each year indicated the two plans yielded equivalent estimates of mean acres per roadway mile of each weed (a = 0.05). However, precision at the district level was much greater in all cases with the 1/4-mi plan. In addition, weed abundances varied substantially among ecozones (a < 0.05); this knowledge will allow managers to direct control efforts to problem areas. Finally, a combination of computer based mapping and resampling of the 1/4-mi segments observed in the two years suggests that additional improvements in precision and efficiency are likely to occur if segment lengths are shortened to 125' or less. Shorter segments would reduce inspection costs, increase sample sizes, improve precision, and possibly allow conversion from an area-measurement approach to one based on presence or absence of chosen weeds in selected segments. Plans are underway to compare the latter two approaches in 2007.