Why Leaves Are Changing Color Early: What Drought Stress Means

Nearly 89% of Colorado's land area was in drought in early September, and the aspens noticed before most people did. By the second week of the month, Colorado Public Radio reported that fall color in the state was running roughly a week ahead of its usual schedule. In Minnesota, where more than half the state sat in severe drought, forest officials were already seeing some trees drop leaves a month early.

So if the maple in your yard went orange while the rest of the street is still green, the short answer is this: early color is usually a stress signal, not a sign that autumn has arrived ahead of time. The tree is shutting down because keeping its leaves has become too expensive.

What does it mean when leaves change color early?

Start with how the normal process works. Leaves are green because of chlorophyll, the pigment that lets a plant turn sunlight into energy. As days shorten, trees take that as their cue to stop making it. In the words of a Climate Central explainer, shorter days prompt plants to ramp down photosynthesis, which reduces chlorophyll and reveals orange and yellow pigments that were there all along, masked by the green.

That schedule is set by the calendar, and the calendar does not change year to year. (The change in day length is actually fastest around the equinox, which is why the light seems to vanish so quickly in late September; we covered that in a separate explainer.) When one tree jumps ahead of its neighbors, something other than daylight is driving it.

The University of Wisconsin's horticulture extension puts it plainly: trees showing color early are much further along in the process than their green counterparts. Usually, the cause is either recent or chronic stress issues, leading to trees shutting down early.

Why drought pushes trees to quit early

Photosynthesis costs water. Leaves open tiny pores to take in carbon dioxide, and every time they do, water escapes. In a wet summer that trade is cheap. In a dry one it starts to look like a bad deal, and a tree with dry roots can cut its losses by shutting the leaves down and letting them go.

Dan West, lead entomologist at the Colorado State Forest Service, told CPR that this year the dry conditions were doing more of the work than the season itself.

"Really this year we're seeing a lot more effects from the drought than we are just from that autumnal kind of seasonal change."

Dan West, Colorado State Forest Service, speaking to Colorado Public Radio

The result is not always a better show. Drought-stressed trees may grow fewer and smaller leaves, and those leaves can brown and fall before the colors fully develop. NOAA's SciJinks education site warns that a drought or an early frost could make the leaves fall off of the trees before they've had time to change colors.

Brian Schwingle, a forest health program consultant at the Minnesota Department of Natural Resources, added a wrinkle that matters for anyone trying to read their own trees. Speaking to CBS Minnesota, he said severe drought can lessen the brilliance of some colors. It can also cause some trees to drop their leaves or change colors early — or later, depending on the species.

That last clause is the reason a neighborhood can look patchy in a dry year. Different species respond differently, so the same drought can turn one tree early and leave the next one green.

Is it bad if one tree changes color before the others?

Not necessarily fatal, but worth a look. A whole region turning early in a drought is a weather story. A single tree turning early while its neighbors of the same species stay green is more likely a story about that tree.

Wisconsin's extension lists the usual suspects beyond drought:

• damage to the trunk, which reduces the tissue that moves water and sugars;
• a declining root system, where dying roots can no longer support the full canopy;
• tough sites near streets, driveways and buildings, with heat, compacted soil and cramped root zones;
• recent construction nearby, a common trigger for early color.

The extension's advice is to check the trunk for wounds and for visible wood-rot fungi, and to bring in a trained arborist if a structural problem is suspected. A tree that colors early year after year is telling you more than one that does it once in a dry summer.

Does climate change make leaves change color earlier?

Here the honest answer is: it pulls in both directions, and scientists are still sorting out which way wins where.

Climate Central notes that fall nights have warmed in 212 of 243 U.S. locations it analyzed, by an average of 2.7°F from 1970 to 2023, and warmer fall nights can delay the start of leaf color. But the same group points to a 2023 study finding that warmer temperatures before the summer solstice lead to leaves shutting down about two days earlier per 1°C of warming. A hotter early summer means trees finish their growing work sooner.

Layer on drought, which is its own early trigger, and you get seasons that are harder to predict. Climate Central calls autumn "the neglected season" in climate research, which is a fair description: spring leaf-out has decades of careful study behind it; fall color has far less.

West was blunter. We're moving now into our new normal, he told CPR. Whether that holds for every forest is an open question, but the pattern this September (early color in Colorado, warnings of duller color in drought-hit Minnesota) fits a picture in which water, not just daylight, sets the timing. Readers who followed our coverage of how flash droughts develop will recognize the mechanism: a few hot, dry weeks are enough to change what the landscape looks like.

Can you do anything about it?

For a yard tree, the practical lever is water. The tree-care company Davey advises owners to water heat- or drought-stressed trees once a week until October, saturating the soil under the drip line (the ring below the outer edge of the canopy), and to keep mulch no more than 3 inches deep, spread wide and away from the trunk. None of that will reverse this year's color, which is already set. It may decide how much leaf the tree has to work with next spring.

For leaf-peepers, the lesson is simpler. In a dry year, go early, and do not assume the forecast map tells the whole story. The tree two doors down already did.