COLUMBIA, Mo.—The recipe for cooking up a spectacular fall display isn’t for beginners. You need all the right ingredients at the right time and at the proper temperature.

Prolonged drought and other stresses in southwest Missouri left many trees coming out of the summer oven with dull, brown leaves. On the other hand, a mild late-summer drought in central Missouri offers the potential for spectacular red and purple leaves in some trees—but to complete the mix the region needs sunny days and cool, dry nights during October, says Steve Pallardy, a University of Missouri forestry professor.

“The best conditions for fall colors are cool but not freezing temperatures—night temperatures in the 40s and day temperatures in the 60s,” Pallardy said.

According to folklore, Jack Frost turns leaves red and purple when he pinches them with his icy fingers, but freezing temperatures stop the chemical processes that create colorful autumn leaves. On the other hand, an unseasonably warm autumn sends trees the wrong message. Is it still summer, or what?

Falling temperatures and shorter days tell trees that winter is coming. In deciduous trees—the kind that shed leaves in the fall—the arrival of autumn triggers furious preparation for lean times ahead.

During the warm months of the year, leaves absorb sunlight for energy, the process we call photosynthesis. During winter it’s too cold and dry for deciduous leaves to carry out photosynthesis. Instead of expending effort keeping their leaves alive through winter, deciduous trees get rid of the old leaves and grow new ones in the spring.

Changing leaf color is a side effect of the process. First, production of chlorophyll—the pigment that gives leaves their green color—slows to a halt. The existing chlorophyll breaks down, revealing previously masked yellow pigments called carotenoids. Meanwhile, the tree moves nitrogen and other important chemicals from the leaves into the stems.

In some species, including sugar maple, sumac, sassafras and some oaks, autumn triggers production of red and purple pigments called anthocyanins. Sunny autumn days promote anthocyanin production, possibly to shield precious nutrients from the destructive effects of sunlight as they evacuate into the stems.

If there are a lot of cloudy days during this critical phase, the tree will produce smaller amounts of anthocyanins and the leaves won’t be as colorful.

Leaves finally fall off when a layer of weak cells forms at the base of the leaf. Wind and gravity finish the job, so just as we put our lawn mowers away for the year, it’s time to break out the rakes.

For more information, see the MU Extension guide “Autumn Colors” (G5010), available for free download.

The Missouri Department of Conservation has information about fall leaves throughout Missouri at https://mdc.mo.gov/blogs/discover-nature-notes/how-do-fall-colors-come-be.