Fall colors: Here's the Science behind the Reds, Golds and Oranges

Garden Friend - I thought you might like to read my garden article that ran in The Denver Post about fall leaf color changes.

Link to The Denver Post article, click HERE. The full article is below.

 

Yellow, gold and orange leaves are fairly consistent from year to year due to other pigments (specifically carotenoid) that hang around in leaves despite the weather. (Betty Cahill, Special to The Denver Post)

By Betty Cahill | Special to The Denver Post

October 17, 2023

Winter will be here soon enough, but enjoy those gorgeous fall leaf colors along the Front Range before any snowflakes arrive.

Reach back and recall your grade-school days and the simple science behind deciduous tree leaves changing color every fall. If you said it’s a result of shorter days and less sunlight, which allow tree leaves to take a winter break from all that work they do converting water and carbon dioxide into sugars and oxygen (known as photosynthesis), then you pass with flying colors — pun intended!

Dried, almost burned-looking outer leaf edges are often caused by the plant’s inability to take up enough water during tough dry summer conditions.  (Betty Cahill, Special to The Denver Post)

There are other processes going on as well. It all has to do with leaf pigments.

In order for the whole photosynthesis process to happen, leaves require help from chlorophyll, which is what gives leaves their green color. The natural substances that make up leaf cells (pigments) visibly become more noticeable each fall as chlorophyll production wanes from less sunlight.

Technically, the fall leaf pigment colors are wired into the green leaf color we see all summer. The dazzling fall colors are simply hidden because of the extensive dominant amount of green pigment (chlorophyll) generated during the summer.

Environmental and plant genetic factors can also affect the pigments in leaves, which play an important role in fall leaf color intensity and duration.

What’s weather got to do with it?

Many warm, sunny days and cool, not-freezing nights allow the anthocyanins (colored molecules produced by leaves) to shine through with all those brilliant shades of crimson, purple and red. Days like this allow more sugar to be produced in the leaf during the day while the cool nights result in a gradual closing of the sugar-producing leaf veins, preventing the sugars from completely moving out.

Yellow, gold and orange leaves are fairly consistent from year to year due to other pigments (specifically carotenoid) that hang around in leaves despite the weather.

Fall moisture helps leaves stay colorful for a longer period of time, conversely, drought conditions while leaves are losing their chlorophyll pigments lead to brown leaves and early drop.

An autumn purple ash puts on a dazzling display in Denver. (Betty Cahill, Special to The Denver Post)

Which trees drop their leaves the soonest in fall or earlier?

Easy answer: Look around where you live. Trees that are environmentally stressed or unhealthy from drought stress, lack of consistent watering, experiencing pest insect or disease issues often start losing their leaves the earliest, even in the summer months.

It’s easy to spot trees, shrubs and perennial plants dropping leaves that are stressed by leaf scorch. Dried, almost burned-looking outer leaf edges are often caused by the plant’s inability to take up enough water during tough dry summer conditions.

Parts or entire sections of the root system may not be able to meet its water demands. So in addition to scorched leaves, branches or whole sections of a tree can appear dead. Other causes of leaf scorch include heat stress, too much fertilizer, excess moisture and poor root growth as a result of the tree growing close to pavement, or damage from nearby construction and root disruption from tilling.

The remedy to leaf scorch, although not immediate, is proper year-round water management that meets the plant’s needs.

What’s the deal with trees that don’t drop their leaves in the fall?

Oak tree leaf loss — specifically with white, pin, English and red oaks — has its own fall leaf story to tell. In a nutshell, these oaks, along with beech and hornbeam trees, hold on to most or some of their leaves during the winter and then shed them in the spring. This process is called marcescence — pronounced “mar-CESS-enss”.

The reason for this phenomenon is all about the cells in leaves separating from the end of the leaf stem where it is attached to the twig (called the abscission zone). Most trees have the type of cells that drop their leaves in the fall while marcescent trees don’t utilize the abscission cells to drop their leaves until the spring when the new buds form.

Enjoy “Colorful Colorado” at its most colorful!

Resources

Aspen Fall Colors: csfs.colostate.edu/aspen-fall-colors/

Science of Fall Colors: fs.usda.gov/visit/fall-colors/science-of-fall-colors

When Oak Leaves Fail to Fall: internationaloaksociety.org/content/when-oak-leaves-fail-fall

Leaf Scorch: planttalk.colostate.edu/topics/insects-diseases/1400-12-leaf-scorch-trees-shrubs/

Betty Cahill speaks and writes about gardening in the Rocky Mountain Region. Visit her site at http://gardenpunchlist.blogspot.com/ for even more gardening tips.