When do leaves change color?
Day length starts it. Elevation and latitude decide the date. Weather decides whether it's worth photographing.
The season, north to south
| Region | Typical peak |
|---|---|
| Interior Alaska | Sep 7 |
| High Colorado aspen | Sep 20 |
| Minnesota's Arrowhead | Sep 26 |
| Adirondacks | Oct 4 |
| Northern Vermont | Oct 5 |
| Northern Pennsylvania | Oct 11 |
| Shenandoah | Oct 13 |
| Blue Ridge, NC | Oct 17 |
| Ozarks | Oct 24 |
| Texas Hill Country | Nov 8 |
What actually triggers it
Day length is the trigger. As nights lengthen after the summer solstice, trees build an abscission layer at the base of each leaf and stop making chlorophyll. That clock is fixed by latitude and date, which is why peak lands in the same week most years.
A cold snap doesn't start the process. It can sharpen the color once it's underway.
Why yellow is reliable and red isn't
Carotenoids, the yellows and oranges, sit in the leaf all summer, masked by chlorophyll. When the green drains away they're simply revealed. They show up every year.
Anthocyanins, the reds and purples, are manufactured in autumn from sugars trapped in the leaf. They need warm sunny days to make the sugar and cool, frost-free nights to trap it. Drought, warm nights, or an early freeze each take the reds away first. That's the whole difference between a memorable year and a brown one.
Elevation beats the calendar
Peak moves about 3.5 days earlier per 1,000 feet of elevation gain. On the Blue Ridge Parkway that means a single afternoon's drive can cross a month of phenology. Miss peak in the valley and you can often drive uphill to find it — or downhill, if you arrived early.
This is the reason we forecast individual overlooks rather than counties. See the method.