In the Garden Articles

In Praise of Earthworms

by Joan S. Bolton

Spring is well underway. The hills are greening up, thanks to last month's rain. Shrubs and trees harmed by the winter freeze are budding out. Springtime bulbs are blooming everywhere.

Below ground, there's new life as well, with earthworms burrowing through the earth. Your first hint of the brown wrigglers may be when you start spring planting. Scoop up a shovelful of dirt, and witness the soil still damp and teeming with worms.

How Earthworms Help

Seldom seen -- except after a rain -- earthworms are industrious creatures that contribute to our gardens from the ground down. Their digging improves the soil structure. Their castings release nutrients to plants. And both their digging and castings help to control runoff and erosion.

"They do huge amounts of benefits to the soil," said Lynn Moody, an associate professor of earth science and soil science at Cal Poly. "They eat organic matter, or sometimes they will eat the bacteria off the organic matter. They're ingesting all that stuff as they burrow through the soil. A lot is excreted... Their castings help increase soil aeration, (which is) passage of water into and through the soil, and passage of oxygen into the soil and passage of carbon dioxide out of the soil."

Added Chuck Ingels, a UC Cooperative Extension farm advisor in Sacramento County, "The castings provide a very readily available source of nutrients that weren't available when the organic matter was in the raw form."

Where Do They Come From?

Worldwide, there are more than 7,000 species, and they ingest soil at a rate of two to 30 times their body weight per day. About the only places that lack earthworms are hot, arid deserts and very cold lands with permafrost.

On the Central Coast, most gardens are already home to at least a few earthworms. Much of the year they're dormant, curled up in little balls below the surface, encased in protective chambers lined with mucus, Moody said. But come spring, with late rains and warming soil temperatures, the earthworms begin stirring.

As to what kind you have: earthworms are most often identified by three broad categories, rather than by species.

Field worms, or shallow dwellers, are common here. They live a year or two in the top 6 to 12 inches of soil, create lateral tunnels and excrete their castings within the soil.

"They are responsible for horizontal movement of nutrients and organic matter," Ingels said.

Field worms are most active in spring, while the soil is moist and the temperatures are moderate. Come summer, with the temperatures heating up and the soil drying out -- provided you don't irrigate -- they burrow a few inches deeper and shift into a resting phase. They perk up in fall with any early rains, then rest again during winter, with colder temperatures.

Nightcrawlers, or deep dwellers, dig vertical tunnels several feet deep. They slink up at night to gather food, slide down to eat and digest it, then slink back up to deposit "middens," or little piles of castings and plant residue on the surface.

These big, beefy worms may live three to four years, provided they're not collected for bait fishing. They can also cover a lot of ground -- one study claims they can travel overland as much as 63 feet in a single night.

"If there's no irrigation and it's dry in the summer, they're not so active. They tend to spend their time down low where there's more moisture," Ingels said. "You see their middens more in the winter and spring. When there's a lot of water, they tend to come up more."

The last of the three types of earthworms, the redworms, are debris lovers that thrive in leaf litter and compost bins, not in the soil.

How to Encourage Earthworms

Notably inhospitable conditions include fields under heavy tillage and new tract housing, where the machine-compacted soil can be difficult for earthworms to penetrate.

"The people who make homes, once they've compacted the soil for the homes, they don't decompact the soil for the plants," said Ingels.

If that's the case in your garden, Ingels advises using a rototiller or backhoe to break up the top 2 to 3 feet of your soil. Mix in plenty of organic material and gypsum to improve the soil structure. The organic material will also provide food for the worms.

Or build raised beds, which you can fill with new, loamy soil.

If you've already spotted at least a few earthworms, you'll have to shake things up in order to entice more.

That's because your existing population of earthworms is whatever your current conditions can support, Moody said. So tossing a handful of new earthworms under a bush won't do much of anything.

But you can improve conditions to encourage the existing worms to prosper and multiply.

Start by applying a layer of compost or other loose, organic material 2 to 4 inches thick over the planting area. Lightly mix the material into the top few inches of soil. Don't dig too deeply: the more you tear up the soil, rototill it, or cultivate it, the more earthworms you'll kill and the more burrows you'll destroy.

Add water on a regular basis to moisten the mulch, but not enough to make it soggy.

As the organic material breaks down, add more to maintain the layer at least a couple of inches thick. The mulch will help retain moisture in the soil and moderate the soil temperature. .

You can also buy worms or import worms to speed up the process.

"I suppose if you had just reconditioned your soil, by adding compost or careful tillage, and you didn't have a whole lot of earthworms, there may be some benefit to introducing them," Moody said. "Be careful what kind you get. Ask if they came from compost or came from the soil."

Ingels also advocates "baiting," or importing worms.

"Dig up your earthworms in one area, about a square foot of soil, then move them to another area," he said. "Then the new area should be amenable to earthworms."

Beware of Chemicals

Certain garden chemicals, including some commonly applied on lawns, may be lethal to earthworms.

"The single most important thing that homeowners can do is minimize their use of pesticides and insecticides," Moody said. "Earthworms will come out at night, or after irrigation, when the soil approaches saturation. They will pull out organic matter from the top of the soil back into their burrows. These things (chemicals) will leach into the soil."

Insecticides containing carbaryl, such as Sevin, and fungicides containing copper sulfate are considered to be extremely hazardous to earthworms. Avoid any old Benomyl or Benlate that you might have used to treat powdery mildew, too. The EPA halted the sale and distribution of the popular fungicide in 2002.

Read directions of all garden chemicals carefully. If you do apply them, do so during months when your earthworms are in a resting phase, to limit the damage.

"If you have to use chemicals, keep them on the plant, not the soil," Ingels added.

Earthworm Facts

Earthworms have no teeth. Instead, they grind up their food in their gizzards.

Earthworms have no lungs. Instead, they breathe through their skin.

At first glance, it's difficult to tell their heads from their tails.

Look for the clitellum, or thick ring that's somewhere around their middle. The shorter end, which may be a little more bulbous or stocky, leads to the head, while the longer end, which may be more pointed, leads to the tail.

Also look for the direction it's heading: earthworms tend to move forward, not back.

When earthworms do move, they don't slither like snakes. Instead, they expand and contract the up to 150 segments of their body to make headway.

Each of those segments is covered with little stiff hairs, which help them grab onto the soil to move forward. Those bristles provide defense, too. When birds try to tug earthworms out of their burrows, their hairs enable them to hang on for dear life.

Seeds of Wisdom

Earthworms prefer food that's high in nitrogen. Spreading an inch-thick layer of lawn clippings will shade the soil, moderate the soil temperature and provide a high-nitrogen food source for your earthworms.