Alfred Rehder

Today is the birthday of the Arnold Arboretum taxonomist and dendrologist Alfred Rehder, who was born on this day in 1863. 

A dendrologist studies trees, and Rehder was the top dendrologist of his generation. Rehder learned about horticulture from his father, who was an amateur gardener.  He worked at several botanical gardens around Germany. 
At the turn of the 20th century, Rehder was sent to the US to study American grapes - which were resistant to phylloxera - the disease that was caused by aphids, and that was threatening to obliterate wine production in Europe. There was no better place for Rehder to conduct his research than Harvard's Arnold Arboretum. It was a fortuitous assignment for Rehder, who ended up meeting the director of the Arboretum - Charles Sprague Sargent. Sargent recognized Rehder's intelligence and diligence. He persuaded him to stay on and gave him the excellent assignment of compiling a bibliography of everything written about woody plants published before 1900.  It resulted in a five-volume, 3,789-page work. 
Rehder accomplished much during his time at Harvard. He launched a quarterly botanical publication known as the Journal of the Arnold Arboretum, and he came up with a system to identify geographic zones based on the relationship between temperature and the hardiness of specific plants. Rehder's work helped establish what we know as the USDA Hardiness zone maps.