In 1639 he began to keep a day-book of work undertaken in his garden at Hanerau in Holstein. Writing in German, he entered details of the plants and seeds he purchased, the names of the suppliers and the prices paid, the dates of planting and sowing, the weather conditions and the subsequent successes and failures. 

There are impressive lists of varieties, particularly of tulips, hyacinths, crocuses, anemones, ranunculuses, apples, pears, and melons. Fruit trees were his main interest (he was commissioned to buy fruit trees for King Frederik III and Queen Charlotte Amalie), and he recorded his tasting notes of many varieties. 

After his death in 1690 his son, also Paul Klingenberg, a distinguished lawyer, continued to add to the day-book at his residence Højriis in Jutland. He was principally interested in ornamental plants, particularly the newly imported exotics, and to assist in their cultivation he installed a hothouse of the very latest design. 

For some years after the death of Paul Klingenberg the younger the day-book was in a private collection in Denmark, but by 1765 it had disappeared, and its whereabouts remained unknown until Dr. Annie Christensen found it in the Danish Public Record Office. Realizing its great importance as a source of garden history, she transcribed it, translated it into Danish and wrote introductory chapters and a commentary on the text.  

Quotations from “The Garden Day Book of Sir Thomas Hanmer” (written in 1659) and previously unpublished contemporary plant drawings from Danish sources have been used to complement the material in the day-book and help to give the reader a more complete picture of gardening in the seventeenth century.