Entertainment and Sports celebrities abound in Greenwich, either residing or visiting; but it's not often we are so fortunate as to have the renowned horticulturalist, Dr. Michael A. Dirr, widely regarded as the leading expert on trees and shrubs for landscapes and gardens, coming here to give two lectures on April 8th.The Greenwich Tree Conservancy and the Garden Education Center are the two sponsors for his lectures, one entitled ''What's New with Virburnum and Hydrangea'' at 10:00 AM at the Garden Education Center and the second one ''In Praise of Noble Trees'' at Greenwich Country Day School at 7:00 PM (6:00 PM for Reception and book signing).Dr. Dirr is the author of ten books, including the definitive reference book ,Manual of Woody Landscape Plants. He has published more than 300 scientific papers and popular articles and his teaching, lectures and programs have contributed enormously to horticultural knowledge and appreciation. He has received the teaching and gardening awards from the American Society of Horticultural Science, American Horticultural Society, American Nursery & Landscape Association and the Garden Club of America!