If You See This Pattern on Your Porch, This Is What It Means

This peculiar squiggly pattern is caused by a certain type of garden pest. Learn why they do it and how to prevent it.

If you’ve ever walked out on your deck or porch and noticed an intricate squiggle pattern on the wood, you may have wondered  what caused it. Look closely, and you’ll notice the tiny zig-zags are actually quite beautiful. So who exactly is this miniature artist, and why are they leaving a mark on your porch railing?

What Causes This Pattern?

The pattern is caused by snails! But it’s not a mucus trail from their slimy movements, as you might assume. Those look more like thin silvery lines. The small winding pattern seen here is caused by their mouths.

A snail mouth is a flexible band containing thousands of microscopic teeth, called a radula. The radula scrapes up, or rasps food, leaving this squiggly path behind as the snail slowly feasts.

The Conditions

If you see snails or evidence of snails in your yard, you most likely live in a humid climate, or a place where it just rained. Moisture is a key requirement for snails. They often hide in dark, moist areas during the day, like underneath rocks or your deck. At night, they come out to feed. That’s why you might discover the mysterious pattern in the daytime, with nary a snail in sight.

So what exactly are they eating off your porch or deck railing? Algae or mold, most likely. Herbivorous snails eat many types of plant leaves, flowers and fruits, as well as fungi and algae. So when algae or mold grows on your deck, snails gather for a feast.

But you might even find snail-eating tracks on a clean, painted wood deck or porch. Snails need daily calcium in their diet to strengthen their shells, which they usually get from feeding on limestone. But if limestone isn’t available, they’ll sometimes eat the paint right off your deck, because many paints and coatings contain calcium carbonate.

one garden snail crawling after each other on wet floor with green backgroundPhichaklim1/Getty Images

The Culprits

The snails munching on your porch are most likely common garden snails. There are more than 500 species of land snails in North America, but the garden snail (cornu aspersum) is the most common.

Land snails take advantage of whatever food they find within crawling distance. So if they can reach algae or fungi growing on your porch, chances are they’ll stop for a snack.

The Remedy

If you want to rid your deck or porch of these slimy creatures, keep it clean and dry. Clear away dead leaves (another favorite snail snack) and pine needles, which can block the sunlight and create an ideal environment for algae, mold and mildew.

Give your deck or porch a deep cleaning at least once a year with a pressure washer and deck cleaner. When it rains, ensure everything drains properly so water doesn’t pool on the wood. You can also spread salt or other types of snail bait to rid your entire garden of snails and slugs.

Erica Young
Erica Young is a freelance writer and content creator, specializing in home and lifestyle pieces. She loves writing about home decor, organization, relationships, and pop culture. She holds a bachelor's degree in Journalism and Mass Communication.