A red maple in mid-October in Troy. Photo by Dana Wilde

We normally begin taking care of the plants on the deck about the end of September to protect them from the inevitable cold. This year, we barely mentioned it until the second week of October.

All last month, the waking-up temperatures on our home meteorology center (about the size of a cellphone on the kitchen counter) registered 50s and 60s, with a handful of 40s. The first two weeks of October have been all 40 and 50 degrees. No sign of frost.

The driveway, meanwhile, is collecting rivers of acorn-smelling brown dead leaves, as usual. Ash and birch trees are always the first to go. Some dull brown maple leaves are mixed in, too, but this year the “fall foliage” apparition has been generally late. The sumac, which usually turns as early as August, only started scintillating around the first of October. The old red maple got its first patch of scarlet last week. When we moved to this house in the mid-1990s, it was usually on fire in late September. That has not happened for years.

How conditions combine to create sharper or duller fall colors is as uncertain to scientists as to farmers, tour agents and backyard naturalists. The common wisdom is that some arcane mixture of moisture, cold and dwindling sunlight (plus a tree’s location, such as exposed in a bog) in different measures leads to different timings and intensities of color.

Leaves are green in summer because they are flush with chlorophyll, a pigment that soaks up light the tree uses to photosynthesize into energy. (To a tree, light is food.) Chlorophyll absorbs red and blue light, and reflects back green light, making the leaves green. Chlorophyll is a somewhat unstable compound, and so the tree has to manufacture it continuously all summer. To do this, it needs light and warmth.

In autumn, light is receding. This happens because the Earth is tilted north to south with respect to the sun. In June, the Northern Hemisphere is leaning full tilt (about 23.5 degrees) toward the sun. Days are long, the sun is high at noon and its rays are relatively direct.

As the Earth keeps chugging, the North Pole leans slowly away from the sun. Days get shorter and the sun’s high point gets lower, so there is less sunlight for plants to use to make chlorophyll. And shorter days with sun rays at lower angles means less warmth.

Tree leaves contain other pigments besides chlorophyll. Carotene absorbs blue and blue-green light and reflects back yellow light, and anthocyanins absorb blue, blue-green and green light, and reflect back red. As chlorophyll levels diminish, carotene and anthocyanin levels stay the same or even increase, and so less green light, and more yellow and red light reflects from the leaves. They turn red, yellow, orange, copper and purple based on the measures of carotene and anthocyanins in them. A red maple has a lot of carotene and its leaves turn scarlet. A sugar maple has more anthocyanin and its leaves turn red and orange.

The sun runs pretty much the same course in the sky year after year, decade after decade, millennium after millennium. By late August and early September, trees are sensing the lowering angle and quantity of sunlight, and begin to close down operations. Whether this is purely mechanistic or involves some kind of consciousness, I do not want to argue about right now. The production of growth hormones slows, and leaves begin the process of senescence: A layer of dead cells builds up between the leaf stem and the twig, and eventually that layer becomes brittle and breaks, and the leaf falls. By the last week or so of September, a dry, withered look creeps over fields and woods.

Trees follow the sun. Among humans, such devotion would be deemed religious. To every thing, there is a season. Plants submit to the sunlight, and the withering of the woods and fields comes on the same elemental schedule regardless of other conditions. But the intensity of the colors that follow has complicating causes: Moisture in the ground from summer rains and from wetter or drier conditions in September and October, and how and when sharp cold strikes.

This year, through the second week of October, there has been almost no sharp cold. In fact, except for the unchanging change in sunlight, summer stayed through September and has lingered into mid-October. If you can remember autumns 20, 30, 40 years past, you know this is not normally how it works. Fall is getting pleasantly, alarmingly warmer. And if you have any intellectual maturity, you know about the inevitable causes. You may wonder if you have any responsibility to take care of them.

Dana Wilde lives in Troy. You can contact him at naturalist1@dwildepress.net. His book “Winter: Notes and Numina from the Maine Woods” will soon be available from North Country Press. Backyard Naturalist appears on the second and fourth Thursdays each month.