Dennis was hired as an Assistant Professor of Horticultural Science at NCSU in 1979 with responsibilities in teaching, and research responsibilities in peach breeding and genetics. He was promoted to Associate Professor and Professor in 1983 and 1988, respectively. His efforts in peach breeding have led to the development of numerous peach cultivars that are widely grown in the southeastern U.S., including ‘Contender’, ‘China Pearl’, ‘Intrepid’, and ‘Carolina Gold’
In 1998, his research efforts shifted to ornamental plant breeding and genetics, focusing on Cercis (redbud) and Buddleja (butterfly bush). He has made considerable progress in developing improved ornamental forms of Cercis, focusing on the manipulation of genes controlling plant architecture, flower color, and leaf color. His redbud introductions include 'Ruby Falls', 'Merlot', 'Whitewater', 'Pink Pom Poms', 'Flame ThrowerTM', and Golden FallsTM. His work in Buddleja has focused on sterility, compact growth habit, and expansion of the color palette. His butterfly bush releases include 'Blue Chip', 'Miss Ruby', 'Miss Molly', 'Purple Haze', 'Pink Micro Chip', and 'Blue Chip Jr'. In 2013, the American Horticulture Society awarded him the Luther Burbank Award for extraordinary achievement in plant breeding.
Dennis served as the Horticultural Science Graduate Program Director for 11 years. He was Director of the JC Raulston Arboretum from 2005-2008. His teaching responsibilities have included Tree Fruit Culture, Basic Agricultural Genetics, Gardening with Herbaceous Perennials, Plant Propagation, and Breeding Asexually Propagated Plants. He has twice received the North Carolina State University "Outstanding Teacher" award. He is now an Emeritus Professor in the Horticultural Science Department, continuing his efforts in redbud and sweetgum breeding.
Dennis is an avid gardener. His home landscape includes a 2,000 square foot herbaceous perennial border, and a diverse collection of woody shrubs and trees. Dennis has been married to Georgina Werner for 52 years.