As temperatures in Boston oscillate between seriously frigid and somewhat balmy, this is a great time to observe what I call “botanical thermometers,” namely plants whose leaves or flower petals respond to and move according to the temperature. My favorite wintertime plant thermometers: witch-hazels. 

Amazingly, four of the five extant species of witch-hazels flower during the winter (the common witch-hazel, Hamamelis virginiana, flowers in the fall). This winter flower show gets underway with Hamamelis vernalis (Ozark witch-hazel) in late December and ends with H. japonica (Japanese witch-hazel) in early April. The petals of witch-hazel flowers have multiple flex points or kinks that you can easily observe. When it is really cold, the petals roll up. When temperatures rise, the petals reflex and open up.  No one has any idea why this happens, but sometimes, a little mystery with nature is just fine!  

Pictured here are three of the almost endless hybrids created by crossing different species of witch-hazels. An (accidental) cross between a Chinese witch-hazel (H. mollis) and a Japanese witch-hazel here at the Arnold Arboretum in 1928 resulted in the amazing Hamamelis x intermedia ‘Arnold Promise’ (top image; 195-2005*B). Since then, horticulturalists have been playing cupid and creating all manner of witch-hazel hybrids including Hamamelis x intermedia ‘Jelena’ (lower right; 462-65*A) and Hamamelis ‘Brevipetala’ (a cross between Chinese and Ozark witch-hazels; lower left; 678-89*A). With more than 125 accessioned witch-hazels on the grounds of the Arnold Arboretum, now is the perfect time to see witch-hazels in flower and gauge the temperature.

Botanical Thermometers 3-9-19 by Ned Friedman