Finally, a small break for me from the grind of teaching and grant-writing. Time to catch-up a bit!
This photograph was made last year on November 5.
I haven't checked on this plant the past couple weeks, but when I last glanced at it, it seemed to be colouring nicely again this year, so it might be worth looking for if you are a local and able to visit the Garden later in the week. I just had a look--it has lost almost all of its foliage already, though a nearby plant still has a little colour. This specimen is in the Alpine Garden, near the pond in the Asian continental bed.
Not all specimens of Enkianthus campanulatus in the Garden colour as nicely this one. This particular plant gets exposed to enough sunlight that it produces significant amounts of anthocyanins (the pigments responsible for the red colouration) in its leaves during the autumn. Exposure to sunlight is typically a benefit to plants; in general, the more light the greater the rate of growth (with exceptions). However, as chlorophyll molecules are broken down in the autumn, sunlight is thought to become somewhat detrimental to the plant. The light (and associated increased temperature) from the sun increases the rate of oxidative reactions, thereby binding nutrient ions so that they can't be resorbed into the overwintering tissues of the plant. There is some evidence that the autumn production of anthocyanins in leaf tissue helps bind the molecules responsible for oxidative reactions (i.e., anthocyanin is an antioxidant), permitting the resorption of the valuable mineral nutrients such as nitrogen.