Michael L. Agnew, PhD , is Senior Field Technical Manager for Syngenta Professional Products, Greensboro, North Carolina. He is coauthor of Mathematics of Turfgrass Maintenance, Fourth Edition (Wiley). Nancy H. Agnew, PhD , is Student Programs Instructor at Longwood Gardens, Inc., Kennett Square, Pennsylvania. She teaches a course in horticulture mathematics. NICK E. CHRISTIANS, PhD , is University Professor of Horticulture at Iowa State University, Ames, Iowa. His previous books include Mathematics of Turf-grass Maintenance, Fourth Edition, and Fundamentals of Turfgrass Management, Third Edition (Wiley). Ann Marie Vanderzanden, PhD , is Associate Professor of Horticulture at Iowa State University, Ames, Iowa. She is coauthor of Landscape Design: Theory and Application.