Mount Mansfield

Cows graze in a Stowe farmfield as Mount Mansfield, ablaze with color, looms in the distance.

In the height of summer, a canopy of green shades the mountains and countryside, but lurking beneath the surface of every leaf is a pop of color just waiting to burst forth as the days get shorter and the nights get longer.

As sunlight fades, production of chlorophyll — the green pigment that allows plants to make food from carbon dioxide and water — decreases. Without the shades of green the other color pigments take center stage. Whether those colors will be vibrant or muted, come early or late, and just how long they’ll remain depends on a delicate dance between the skies and the trees.

The cloudier-than-usual start to September sped up the transformation, painting 15 to 20 percent of the mountainsides with vibrant reds, yellows, orange and maroon.

But with more sunshine this week, the process has slowed as trees are able to produce more food, and the chilly nights keep the sugars from traveling too far from the leaves.

On the flipside, the dry stretch of weather over the next week could stress the trees out, speeding up the color-change once more.

Progression is expected to be slow for the next few weeks, so “I wouldn’t buy a plane ticket to Vermont just yet,” said Mark Isselhardt, maple specialist with University of Vermont Extension and Proctor Maple Research Center.

“It’s all about these little pushes and take aways,” said Weathering Heights meteorologist Roger Hill of Worcester.

Areas where the soil remains moist, such as swamps or near streams, offer better conditions for photosynthesis and less stress, creating more vibrant colors.

But if there is too much rain after the dry period, where the leaves are crinkly and fragile, they may fall sooner.

“Different species act differently,” Hill said. “I’ve already noticed leaves falling off the birches,” and the season has barely just begun.

Bouts of frost will do the same, as freezer burn blocks the sugars from circulating.

The longer the temperatures remain above freezing, the better the foliage will be.

As of right now, the rest of September is calling for normal to below-average rainfall, which shouldn’t negatively impact the leaves, and cycles of warm weather in the coming weeks paint a promising picture.

“What I haven’t seen yet is the ferns dying out, and that usually happens in August, which means the soils are still wet from the spring,” Hill said. “Once we start seeing the ferns die out, that’s when the colors will change quickly.”

Models, which are based on observations over the last few years rather than weather patterns, predict Lamoille County will peak around the second week in October.

Caterpillar army

Fall foliage is as difficult to predict as the weather in Vermont, but after this spring and summer brought long bouts of rain, conditions should be prime for vibrant foliage across the state — except where masses of 16-legged soldiers set up camp.

Where an army of forest tent caterpillars inched their way up sugar maples this spring, taking the lives of many broad green leaves in Waterville, Belvidere, Wolcott and Elmore, swaths of brown were left in their place.

Damaged trees tried to bud back out late in the season, but as summer passed, and days became shorter, for some trees, replenishing their troops wasn’t worth the effort. They’d all succumb to winter soon anyway.

In those areas, the autumn leaves will be smaller, colors muted and less lush.

Enough forest tent caterpillar moths were captured at the end of this season to predict another year of spring defoliation and muted fall tones next year as well, but the outlook isn’t all dark.

The good news is that 2018 should be the last year of defoliation from these creatures for the next decade or two. The population of forest tent caterpillars is expected to crash as a result of natural controls, including viruses and a native parasitic insect called the “friendly fly,” or Sarcophaga aldrichi.

And trees that weren’t defoliated should have a good show of color, Isselhardt said.

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