COLUMBIA, Mo.—Despite their association with fall colors, you’ll want Jack Frost and Indian Summer to stay away if you’re hoping for vivid autumn leaves.

According to folklore, Jack Frost turns leaves red and purple when he pinches them with his icy fingers. In fact, a hard freeze is the last thing you want for colorful fall leaves, said Stephen Pallardy, a University of Missouri forestry professor.

On the other hand, the warm days of “Indian Summer” send trees the wrong message, leading to less-than-spectacular reds and purples.

“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.

“We have the potential for good fall color in most of the state where we hadn’t had a severe drought,” he said on Oct. 12.

Timing is everything. 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 serve as solar collectors, absorbing sunlight for energy, the process we call photosynthesis. During winter it’s too cold and dry for deciduous leaves to carry out photosynthesis, so the trees go dormant. Rather than spending resources keeping 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, and 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 signals leaves to produce red and purple pigments called anthocyanins. Scientists aren’t certain exactly why they appear, but anthocyanins seem to protect leaves in several ways. 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.

Dry spells during the summer also make for a muted fall display and an early leaf drop. This is happening across parts of southern Missouri, where the region’s abundant white oaks will be too parched to provide their usual late-autumn flourish.

Leaves finally fall off when a layer of weak cells forms at the base of the leaf. Wind and gravity finish the job, then people get to rake the leaves that fall in their lawns.