What are lichens? They are neither plant nor animal.

If that doesn’t make them inscrutable enough, there is also this: A lichen might, at first glance, be mistaken for an errant wad of chewing gum or a misplaced splatter of paint.

But they are very much living creatures and are thought to be one of the earliest land-dwelling forms of life. They are among the most widespread, too, present on every continent, covering an estimated 8% of the planet’s land. They inhabit even Antarctica and the harshest deserts, including some places where plants and animals cannot thrive.

Closer to home, you may find them happily taking up residence on your wooden garden bench or picket fence, stone walls or other rock surfaces, or on the trunks and branches of trees and shrubs.

These subtle beauties offer plenty to look at in every season. So don’t try telling the scientists who study them that your garden is winding down.

It’s always lichen season, say Jessica L. Allen and James C. Lendemer, authors of the new book “Urban Lichens.” But fall is the time when these organisms can really command our attention, after the visual distraction of fall leaves fades.

The book aims to get us to notice — and try to identify — the major types and most familiar species of lichen.

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“It’s not an onerous task of microscopy and chemistry,” Lendemer said. At a recent National Park Service educational event he attended, about 20 high school students unfamiliar with lichens were given lichen-covered branches and asked to make observations.

“They picked up on the things we’d use to identify them — the main growth forms and diagnostic features,” he said. “And they picked out the basic types, even without knowing the terms for them. They intuited a lot.”

So can you. Ready for a backyard lichen quest, before some of them are hidden by snow?

If it’s not a plant, what is it?

Allen and Lendemer have been on many such quests together since they met in 2012 at the New York Botanical Garden. Allen, now an assistant professor of integrative plant biology at Eastern Washington University, was pursuing her doctorate in a joint program with City University of New York.

At the botanical garden, where he is an associate curator, Lendemer oversees the Western Hemisphere’s largest lichen collection. He is also an assistant professor at CUNY.

The two are forever on the trail of these composite symbiotic organisms, which they describe in the book as “an intensive cooperation between a fungus and an alga or a cyanobacterium, and sometimes all three.”

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Most of a lichen’s structure is the fungus. The alga lives with it, and in return for shelter it provides photosynthesis, producing sugars that sustain the fungus. But the two are not alone.

“In many ways, lichen are miniature universes,” Allen and Lendemer write, as a diverse community of bacteria, non-lichen fungi, nematodes and tardigrades (also known as water bears) live in and on a lichen.

Many other creatures also rely on lichens, from moths whose larvae use them as food to hungry caribou, deer and moose. Hummingbirds and flying squirrels incorporate bits of lichen into their nests.

And take note, gardeners: Lichens help with soil formation by accelerating the breakdown of rocks. They perform nutrient-cycling, too, as the cyanobacteria in them fix nitrogen from the atmosphere, converting it into a more usable form.

And when lichen falls on the soil and breaks down, Allen said, “it’s a little packet of fertilizer — a fertilizer application.” (It’s also a lichen-hunter’s prize: “Fallen branches often harbor a wealth of lichen,” she said, so keep an eye out during garden cleanup for any to set aside and examine with a magnifying lens.)

Many plants, including epiphytic ones, depend on lichens for humidity and moisture.

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“They’re like sponges in the environment, soaking up moisture quickly and releasing it slowly into the area,” Lendemer said. “Without lichens, the forest is drier and sadder.”

Lichens are highly sensitive to pollution, making them excellent indicators of air quality, and of habitat quality in general. The industrial revolution’s impact reduced lichen diversity, especially in cities, until the clean-air legislation of the 1960s and 1970s gradually made even urban areas hospitable again.

So a lichen-rich landscape is reason to celebrate.

Types of foliose lichen you’re most likely to see on a tree in the garden are the common greenshield (Flavoparmelia caperata) and the rough speckled shield (Punctelia rudecta), standouts even in the snowy days of winter. (Margaret Roach / The New York Times)

The three types (and many colors) of lichens

Although they have no roots, lichens need a home base, called a substrate, to attach to. Common substrates include stone, wood or bark, and soil, but lichens can live on anything.

“They seem to have arrived at the perfect way to live on land,” Lendemer said. “They end up all looking pretty similar as a result, because it’s a recipe for success.”

There are three types of lichens, and figuring out which type you’re looking at is the first step in identifying it.

Crustose lichens are completely attached to the substrate they grow on; no lower surface can be seen, and to remove the lichen means removing some substrate, too. Foliose species have lobed, leaflike bodies, or thalli; you can see both their upper and lower surfaces, which are different colors. Fruticose lichens have thalli that are branch-, cup- or club-like, imparting the look of tiny shrubs.

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Lichens also come in various colors, the result of chemicals secreted by the fungus and not found anywhere else in nature. “Lichens produce the full rainbow of visible colors that we can see, and beyond,” Allen said.

“A rainbow of color in the forest,” Lendemer concurred.

Those substances serve various functions, including providing the lichens with protection from ultraviolet radiation, and some have biomedical or bioactive properties.

Where to look for them

The key to lichens’ happiness, Allen said, is the long-term stability of the surface they grow on. Areas of the garden where you don’t disturb the soil frequently — perennial beds, for example — are more likely to have soil-dwelling species of lichen.

Allen recommends starting your backyard hunt at eye level, on the bark of trees and shrubs. Look for gray or brownish patches — or even splotches of yellow, a hallmark of goldspecks (Candelariella) and candleflame (Candelaria concolor).

Next, look at areas around the base of woody plants, where extra humidity may support lichen. And have a careful look in the cracks and crevices of unpainted wood furniture and between panels of wood fencing.

Stone walls are common substrates as well, and concrete can be home to lichens like sidewalk firedot (Caloplaca feracissima), “a very lovely yellow-to-orange one that literally grows under your feet,” Allen said.

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But even with lichens that you can see from a distance: Get closer and use a magnifying lens. “Then the magic really happens,” she said, “to reveal patterns, variations in color and some crustose types that are otherwise hidden.”

What’s in a name?

There are more than 25,000 known lichen species globally, with about 300 to 400 new ones identified every year. In New York City alone, there are about 120 — a sharp increase from a century ago, thanks to cleaner air.

Despite what Lendemer calls their “often-humble common names,” like Old Gray Dust, Board-Dweller and Curly Biscuits, lichens can have lofty — or positively celestial — associations. The biblical manna from heaven was a lichen.

And sometimes even the formal Latin names can be fun.

Although Lendemer remains in New York and Allen is in Cheney, Washington, they continue to do fieldwork together. Regular trips to the Southeast, the area of greatest lichen diversity in the United States, have led to a joint discovery of two previously unknown species there.

They named them for two famous daughters of the South: In 2019, they dubbed one Hypotrachyna oprah, to honor Oprah Winfrey, and in 2015, Japewiella dollypartoniana was their nod to Dolly Parton.

Life expectancy: extra-long

A final piece of advice: Don’t ask a lichenologist how to “fix” the lichen on the bark of your trees.

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Lichens do not harm their substrates, or take nutrients from the trees or shrubs they attach to. Look, but do not disturb them; take a photo, not a sample. Live and let live.

Because live, they do — a long life and then some. “If you can see a lichen without magnification, and it’s a couple of inches across, it’s probably 20 or 30 years old,” Allen said. “Lichens keep growing until something around them shifts.”

That decades-old colony you’re looking at? Think back even further, she said, to the parent colony that came before it, on a nearby tree, and the parent before that, “forming a long, unbroken lineage back in time.”

Lichens counter the traditional narrative, the pair agreed.

“They do things differently, which I think is why so many people are attracted to them,” Lendemer said. “With animals, you can see how many generations there have been. But with lichens, it’s nonlinear, and not how we’re taught to see the world.”

Celebrate the difference, the lichenologists say. Celebrate their staying power.