When the first volume of HPP was released thiry-one years ago, it quickly became a leading resource in the field of horticulture. It was recognized as one of the best 75 books in the last 75 years by the American Horticulture Society. With each new edition, the book has gained in respect and popularity and has since been simply described as the bible of perennial books. The fourth edition differs significantly from the third edition (2008). Dr Armitage has added dozens of new species and hundreds of new cultivars from breeders throughout the world. His comments on morphological characteristics of each species are scientifically sound while simple to understand by any reader. However, the real value of his writing is in his thoughts concerning a plant's garden worthiness. Other significant additions have been the detailed discussions of changes in plant nomenclature and in-depth dialogue on the problem of invasive plants. The new edition provides easy-to-read information for over 500 genera, species and cultivars used in the gardening and landscape trade. It also includes Dr. Armitages's interesting and occasionally controversial thoughts on the Internet, pronunciation of plant names, differences in gardening in the South and North, along with many other topics of interest. The book is written for professionals and gardeners in the United States and Canada but people throughout the world have also found it useful. 1109 p.