The simplest answer is they burrow deep down into the soil. Normally worms can't move if their body temperature gets very cool. If the soil they are in freezes, worms can’t move and they die. So the worms that survive northern winters are usually the ones that have buried themselves deepest in the soil.
First, there are more than 20,000 different species of worms around the world and they belong to several different animal groups. More than 4,400 of them are various types of earthworms. But only about 30 species of earthworm are found in the United States.
Worms live in decaying trees, in bark, under rocks as well as along rivers, near springs and in ponds. Their favorite place to live, however, is burrowed inside the earth’s rich soil.
During the winter months they burrow deep within the earth until the surface warms again during the spring. During the warm summer months, worms stay closer to the tops of soil where they create tunnels to wiggle in and out of. These tunnels are extremely important for plants as they create a path for water and air, which is essential for the survival of plant life.
Although worms can’t survive freezing temperatures, they do lay eggs that are encased and protected by very small cocoons. These cocoons can make it through the winter to survive as baby worms once temperatures warm up.
Other earthworms, such as the common nightcrawler, often used as fish bait, live close to the surface in warm weather and down deep in cold weather. When winter hits, these worms burrow down below the frost line (the level below the soil surface in which groundwater freezes).
Nightcrawlers can burrow to depths of six feet or more. When they burrow down below the frost line, they nest in small chambers at the bottom of the tunnels they dig.
Once worms burrow far enough underground, the soil stays at a fairly constant temperature that keeps the worms warm through the winter. This warmer soil down deep acts like a warm blanket and insulates the worms from the cold.
Nightcrawlers don’t really hibernate like some animals do in the winter. If there is a warm spell during the winter, night crawlers will occasionally come back up to the surface for a while until it gets cold again. When spring returns, worms make their way back to the surface.
Worms don't have lungs; instead, they breathe through their skin, as long as it stays moist. To keep their skin moist through winter, they release fluid and mucus that coats their body for whatever time needed. If the worm’s skin dries out, the worm will die from suffocation. While worms need moisture to survive, too much moisture can be fatal. If too much water is present, it takes the place of oxygen, which will cause the worms to flee to the surface. Once on the surface, worms will be exposed to sunlight. If worms remain in the sunlight for too long, they can become paralyzed and they will die.
Under ideal conditions, scientists estimate the average lifespan of earthworms that survive winter at four to eight years, while the most common garden varieties live only one or two years.
Worms eat so much that they typically produce excrement equal to their own weight every 24 hours.
They feed off of leaves and dead grass, which contain organisms that provide a healthy diet of bacteria, algae, and fungi. Worms feast on dirt as well, especially if they live deeper inside the earth. Worms also eat plants, fruits and vegetables.
Although you cannot see them, worms do have mouths. The earthworm mouth is called the prostomium. The worm’s mouth is actually big enough and powerful enough to grab a leaf and drag it around.