Have you ever looked out at your garden and thought “I’m SURE I didn’t plant flowers THAT color”? We received some photos from a local gardener who said his blue garden phlox seemed to be changing to a pink color between morning and afternoon. Although you might think that the flower colors just looked different in the changing light or because they were aging and fading, he rightly assumed that something else was going on. A little sleuthing into the pigments that give plants their colors explains how this color shift happens.
The plant pigments that affect flower color include alkaloids, carotenoids, and flavonoids. Anthocyanins are flavonoid plant pigments responsible for blue–purple to pink flower colors. Some factors that influence the anthocyanin content of flower petals include:
Cooler morning temperatures that increase anthocyanin content and produce darker colors, and hotter afternoon temperatures that reduce anthocyanin content, producing lighter colors, can explain the mysterious shift from blue to pink in our gardener’s phlox.
Plant pigments and temperature also play an important role in the leaf color changes we see every fall. Carotenoids and the flavonoid anthocyanins—in addition to chlorophylls—are leaf plant pigments. As leaves lose chlorophyll and much of their carotenoid pigments, the remaining carotenoids give some leaves their yellow fall color. As with flowers, temperature and light also affect leaf colors. Cold, sunny days help increase the anthocyanins, which can give leaves varying shades of red—or orange—if enough carotenoids are present. The anthocyanins and the remaining chlorophyll make many fall leaves brown rather than red.
For more information, see Leaf Pigments by Harvard Forest.
Article written by Debbie Green, Extension Master GardenerSM Volunteer.