Fall Color Explained.

Leaves from the CA Native Vine Maple (Acer circinatum)

Being a Californian, I experience what people from other parts of the country call "Fall Lite". Oh yes, these people make fun of us and our lack of "real" seasons but their poorly masked envy of our mild Mediterranean paradise (and all kinds of awesome at arm's length) doesn't phase me one bit. Besides, I've learned to grab my share of fall color and get it every place I can.... from the big leaf maple lined creeks of Skyline, to the dogwoods and poplars of the sierras to the wide spectrum of garden and street trees that show off all hues from that fiery warm slice of the color wheel.

Chinese Pistache: a common Bay Area street tree.

Despite our mild version of the season, Autumn is still my favorite time of year. I soak up the changing colors with an intense joy like welcoming an old dear friend who comes around each year to usher in a time of lovely memories, scents, sounds, tastes and adventures. Like mom's minestrone wafting down the hall to a hot cup of tea by the fire with a good book, the changing of the leaves marks a sweet shift from the go-go crazy fun of summer to harvest, reflection and introspection.


So what makes leaves change color? In order to discuss color, we must first discuss the pigments responsible. Contributing Autumnal pigments from nature's paint palette go as follows:

• Chlorophyll- the green pigment that rocks photosynthesis (converting light to sugar)
• Carotenoids- such as xanthophylls or the more familiar carotene, responsible for yellows, oranges, and brown pigments (as seen in as carots, corn, apricots, narcissus, bananas, etc)- and function to absorb light energy for use in photosynthesis
• Anthocyanins- as the suffix "cyan" suggests, is the red and purple pigment that gives it's color to apples, berries, grapes, cherries, violas, cabbage, etc. They are water soluble and appear in the watery liquid of leaf cells.

Both chlorophyll and carotenoids are present in the leaves throughout the growing season. But most anthocyanins are produced in the autumn, in response to bright light and excess plant sugars within leaf cells as a means of photprotection during nitrogen translocation (or as I look at it: a kind of natural sunscreen while the plant relocating nutrients away from the leaves for storage and food).

In a nutshell, Fall color happens due to two factors: 1.) the breakdown of green chlorophyll (in response to shorter days), which unmasks the already-present orange and yellow pigments (carotenoids) while 2.) the red is from a simultaneous process going on which I mentioned above with the anthocyanins being produced in response to light.

Why are some years so much better for Fall color than others? 

Lagerstroemia (Crepe Myrtle)

It all has to do with temperature & weather during the period of season shift (but overall weather of the entire year contributes as well). Warm, sunny days and cool but not freezing nights bring on the most dramatic fall color. Reason: During these warm sunny days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions bring on production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson. Because carotenoids are always present in leaves, the yellow and gold colors generally stay consistent. So the variability of the temperature/weather create the different gradations of yellow to red to orange that change from year to year.

Ginkgo Biloba

The endless variables of weather patterns of the year will effect the timing and drama of the fall show, just like it will effect the bloom times and crops. The ideal year for a killer Autumn show is a wet rainy winter & spring, a good "normal" summer and warm sunny fall days with cool nights. Of course, us being in this mild climate we love so dearly, those cool nights are still relatively mild. Therefore no technicolor Fall madness like I've heard tell of on the East Coast. Still we've got quite a bit of lovelies that put on a good "last hurrah to summer" show. Start gazing at street and park trees or plants while hiking through the hills and you'll start to see and admire them like I do. A great one to admire (from afar) is poison oak. It decorates the landscapes of almost all plant communities in California and when in masses, creates a brilliant tapestry of color with the existing landscape.

Poison Oak in Fall

So whether or not we completely understand why the colors are the way they are, we can all sit back in admiration of nature's paint mixing party. And on a larger scale, watching the leaves drop, recycle nutrients and break down into a humusy soil layer- feeding fungi, plants and an entire ecosystem (including many critters invisible to the naked eye) while the woody tissues seal themselves off in protection from the cold... as the seasons cycle. Well it's all pretty awesome to soak in. I dig it. Happy Autumn.