The Dirt on Earthworms

Peter Hatch knows good dirt when he sees it.

Hatch recently retired after 34 years as director of gardens and grounds at Monticello, the historic home of Thomas Jefferson, author of the Declaration of Independence and America’s third president. Hatch led the interpretation, restoration, maintenance and conservation of what he calls Jefferson’s crowning achievement, a 1,000-foot-long vegetable garden carved by slaves out of a hillside on Jefferson’s beloved estate overlooking Charlottesville, Va.

Because of his work at Monticello, Hatch was asked to be an adviser to first lady Michelle Obama’s White House kitchen garden. Speaking recently at the Atlanta History Center about his time at Monticello, Hatch brought up the White House garden. “It has really good dirt,” he said, “with an abundance of earthworms.”

Earthworms? What do earthworms have to do with good garden soil?

Plenty.

Earthworms are eating machines. They shred organic matter and break up hard soils by tunneling through them and, in effect, tilling them. As a result, they create subterranean paths that aerate the soil, increase its water-holding capacity and provide environments for beneficial bacteria to flourish, which helps to break down organic matter in the soil. In the process, earthworms move organic matter and microorganisms through the soil and produce pin-head sized excrement, called “castings,” which are an excellent organic fertilizer.

What does this mean for plants? It aids their root growth, which helps to increase the production and quality of fruits and vegetables.

To help gardeners better understand the benefit of earthworms in their vegetable and flower beds, we asked Ramoa Hemmings, an assistant horticulturalist at the Atlanta Botanical Garden, how to attract them to home gardens and what might be going on with them when we see them in the ground.

Tom Oder: How can I increase the number of earthworms in my garden?

Ramoa Hemmings: They love organic matter such as decaying leaves, manure piles and rotten logs. Mulching in the fall with dried leaves or pine bark chips is a great way to encourage them into your garden in the spring.

Should I mulch at other times of the year or do things in other seasons to attract them?

Yes. You can also mulch in the spring to attract them. Once they have made it to your garden, they will begin their life and eventually reproduce. They just need the right conditions to thrive and flourish.

Does this mean they like dark places?

Earthworms love the dark because they are light-sensitive. In fact, to see the tunneling of most earthworms you have to watch for them at night. This is when they forage for leaf litter and organic matter, which they take into their tunnels.

Do I need to add anything to my garden soil for earthworms to eat? And, by the way, what do earthworms eat?

Nope. Earthworms eat a variety of organic items, including leaves and remains of plants and animals.

I've heard worm castings make good fertilizer. What is it about worm poop that makes it so good for plants?

The stomach acids in earthworms make common plant nutrients such as nitrogen and phosphorus as well as some microbes available for plants to uptake. Some forms of the nutrients already found in the soil are unavailable to the plant until they are eaten by worms.

Wouldn’t it be easier to just buy worm casting fertilizer at the nursery rather than go to the trouble of trying to attract earthworms?

Although you can purchase worm casting fertilizer, you are losing some of the other benefits that earthworms bring to the soil, such as improving air circulation by creating underground tunnels.

Can I buy earthworms and put them in my vegetable beds?

Yes. But, be aware that most worms that are for sale are used in vermiculture, which is indoor worm composting. These worms are different from earthworms and won’t survive very long in the outdoor garden because the right food and temperatures are not available to them.

Do earthworms regenerate if I am digging in the garden and cut one in two with a hoe or shovel?

Rarely, but it depends on how the worm was cut. If the tail is cut off, earthworms can regenerate a new one, although this requires a lot of energy on their part. If the head is cut off, there is no evidence of a worm forming a new one. Either way, if the worm dies, its body will decompose and add organic matter to the soil.

Is there any place in the United States where earthworms don’t live naturally or won’t live if I buy some and release them into my garden?

Not really. As long as there aren’t temperature extremes (as in the deserts in the Southwest), earthworms will live in all parts of the country. Interestingly, most of the earthworms found in the United States are not native. They are “exotics” that originated in Europe, Asia, Africa and South America. Some species have caused understory loss in wild ecosystems such as forests in various parts of the country, thus doing more harm than good in these instances.

Why do I see earthworms on the ground after a rain?

Since earthworms breathe through their skin, too much water in the soil after a heavy rain can drown them. They head to the surface to get air.

Sometimes I see them on my driveway or the street, shriveled up and dead. What’s up with that?

These creatures have no sight. The dead ones you see on streets, driveways and sidewalks found their way to pavement after escaping from moisture-saturated soil but didn’t make it back to the ground before the sun came out. Consequently, they dried out and died.

Do earthworms have natural enemies?

Only a few, primarily birds, people who use them for fish bait and gardeners who rototill the soil and fertilize with chemicals. Rototilling or disturbing the soil by other mechanical means destroys earthworm burrows and cuts the worms into pieces that may not regenerate whole worms. Adding fertilizer literally throws salt onto the wound because chemical fertilizers have salts that will “burn” the worms and make the soil uninhabitable for them.

And here are some fun facts that Hemmings offered about earthworms:

• One earthworm can produce 1/3 pound of fertilizer a year.
• In an earthworm-rich garden, that translates into 50-75 pounds of fertilizer each year in a 10x20 foot plot.
• Some worms bring minerals up from as far as 8 feet below the soil surface.
• The slime or mucus made by an earthworm helps keep its skin moist so it can breathe and also helps it move smoothly through its burrow.
• Some earthworms have been known to live up to 2 miles underground.

So, the next time you see an earthworm in your garden don’t think of this as a lowly creature you should pity. Think of it as friendly underground farmer working silently to naturally improve your soil and your vegetable harvest or the beauty of the blooms in your flower bed.

And if you put a few in a can for fish bait and take them to take to a nearby lake or pond, don’t think you’re doing the natural area a favor by dumping any of the lucky unused ones on the ground by the water’s edge. That’s one way some scientists who study the ecology, biology and biogeography of earthworms think these voracious non-native eaters may have made their way into some of the nation’s forests.