Precipitation and mowing effects on highway rights-of-way vegetation height and safety

Managing highway rights-of-way (HROW) for safety depends upon the annual precipitation received and vegetation mowing practices. This study was conducted along a precipitation gradient in Texas, USA, to evaluate how precipitation and mowing treatments influence vegetation heights. The experiment was initiated in spring 1999 and completed in autumn 2002. Each site had ten treatments with four replications and consisted of forty 15 m × 6 m subplots. Commercial highway mowing equipment was used to mow the subplots at 5-, 10-, or 20-cm heights with an annual mowing frequency of one, two, or three times per year and a non-mowed control. A vegetation height of >30 cm is generally considered a safety hazard. Control subplot vegetation heights increased along the annual precipitation gradient. Most of the subplot vegetation heights at the more arid Andrews site (35 cm precipitation year^?1) were <30 cm. Subplot vegetation heights at the Tahoka (50 cm precipitation year^?1), Brady (66 cm precipitation year^?1), and Lufkin (109 cm precipitation year^?1) locations exceeded 30 cm, regardless of mowing frequency or height. Mowing treatments for HROW safety must be based on site-specific conditions. In areas that receive <35 cm precipitation year^?1, mowing treatments have little effect on vegetation height, and management practices do not necessarily affect visual safety. In areas that receive >35 cm precipitation year^?1, mowing three or less times per year, regardless of mowing height, makes it unfeasible to maintain the vegetation heights of <30 cm needed to maintain the HROW visual safety criterion.