I recall growing up learning that an early frost in Autumn meant bright colors on the trees in the coming weeks.
Turns out that isn’t really true. And in fact, an early frost or freeze – depending on the geographic location, timing, and depth of frost/freeze can actually have a detrimental impact. The reason I never noticed this fact was due to confirmation bias (something I realize I’ve written about a lot on this website).
Then I started reading about it. Because I wanted to know when the trees might get pretty in Mississippi.
What I found out was both really cool and really disappointing.
WHY LEAVES CHANGE COLORS
We can go back to school for a second and remember that leaves change colors for a few reasons…
- Less daylight
- Cooler nights
- More sugars left in the leaf
A combination of these three factors can lead to a few things happening.
Less daylight means less chlorophyll. And recall that chlorophyll is the key to photosynthesis – converting the energy in photons into the chemical bonds of sugar molecules. It is also the thing that gives the leaf its green color. So if there is less chlorophyll, there is less green, and instead some of the other pigments of color start to show up.
And according to Michigan State University, “The extent of red color and length of time leaf color is displayed is directly related to temperature and moisture conditions that occur before and during the period when the amount of chlorophyll in the leaves is diminishing.”
Cooler nights means that the veins in the leaf itself will constrict. This limits the flow of those sugars (albeit fewer now that there is less chlorophyll) to the roots. So if it is warm, warm, warm then there is a cool snap for a few days, you may see leaves change colors a bit quicker.
Going back to the folks at Michigan State, “An abundance of sugar and light in the leaf lead to the production of vivid anthocyanin pigments, which produce red, purple and crimson colors. Yellow and gold leaf colors are produced by carotenoid pigments, which are ever-present in the leaves and are therefore less dependent on the aforementioned conditions.”
That means the three things listed above are all interconnected.
Want certain colors, look for certain trees! The Forest Service has a list of trees and their likely fall colors:
Certain colors are characteristic of particular species:
- Oaks: red, brown, or russet
- Hickories: golden bronze
- Aspen and yellow-poplar: golden yellow
- Dogwood: purplish red
- Beech: light tan
- Sourwood and black tupelo: crimson
- Red maple: brilliant scarlet
- Sugar maple: orange-red
- Black maple: glowing yellow
- Striped maple: almost colorless
PERFECT LEAF-CHANGING WEATHER
Since temperature and moisture are the main weather-related influences, look for places that are cooler with good soil moisture.
The Forest Service says that multiple warm days with plenty of sunshine followed by cooler nights is the best way to ensure bright colors. Thinking back to the explanation above, during these warmer days, sugars are produced in the leaf but the cooler nights keep the sugars from moving out. The combo of sugar+light promotes anthocyanin pigments, which tint reds, purples, and crimson. If you want yellows, the good news is that carotenoids are always present in leaves, so the yellow and gold colors remain fairly constant from year to year.
The neat part is that because temperature and moisture are always different year-to-year it means that each fall is unique in its own way. Drought, for example, can delay the onset of fall color by a few weeks. A warm period during fall will also lower the intensity of autumn colors.
The perfect leaf-colorizing weather, though, according to the Forecast Service and Michigan State includes a warm wet spring, favorable summer weather, and warm sunny fall days with cooler nights.
FROSTS AND FREEZES AREN’T GOOD
Frosts and freezes are actually color killers. Folks at the University of Wisconsin say that an early frost speeds up fall leaves and can put a quick end to any color at all. And multiple frosts or a freeze before leaves start to change color will likely make leaves to wither and fall from the tree without changing color at all.
So an early frost or freeze is actually bad for anyone trying to catch a glimpse of some good fall colors.
