McMahon's most enduring contribution was his Calendar, the most comprehensive gardening book published in the United States in the first half of the nineteenth century. Thomas Jefferson would often follow the Calendar step-by-step when planting tulips in his flower beds and sea kale in his vegetable garden. The 648-page Calendar was modeled on a traditional English formula, providing month-by-month instructions on planting, pruning, and soil preparation for the various horticultural divisions - the Kitchen Garden, Fruit Garden, Orchard, Nursery, etc. Hardcover, 648 pages, plus index. Appendix lists 3,700 varieties.