Alfred Rehder (4 September 1863 in Waldenburg, Saxony – 25 July 1949 in Jamaica Plain, Massachusetts) was a German-American botanical taxonomist and dendrologist who worked at the Arnold Arboretum of Harvard University.

[1] Rehder broke off his attendance at the gymnasium in Zwickau in 1881 and did not pursue university studies, instead working for three years as an apprentice under the tutelage of his father.

During this time, he was involved in the creation of the Brocken Garden for Alpine plants, initiated by Albert Peter in 1890 on the highest mountain of the Harz range.

In 1895, he was appointed associate editor of Möller's Deutsche Gärtner-Zeitung (published in Erfurt), Germany's premier horticultural journal, for which he wrote numerous articles.

At about this time, he was also introduced to Liberty Hyde Bailey of Cornell University, who asked him to prepare the text on woody genera for the Cyclopedia of American Horticulture (4 vols., 1900–02).

Rehder created the first system of isothermic zones for the United States that related average winter minimum temperatures to the hardiness of specific plants.

[8] He was co-author, with Ernest Henry Wilson, of Plantae Wilsonianae: An Enumeration of the Woody Plants Collected in Western China for the Arnold Arboretum of Harvard University During the Years 1907, 1908, and 1910 (3 vols., 1913, 1916–17) and A Monograph of Azaleas: Rhododendron subgenus Anthodendron (1921).