Garden Tips: Not all earthworms are beneficial

I started working for Washington State University on April Fools Day in 1980. The first question posed to me by a local gardener was the best way to kill earthworms.

Was this a joke?, I thought.

I gasped at the heresy of killing these beloved beneficial organisms.

I had yet to learn how terribly bumpy a lawn could become from the activity of nightcrawlers, the type of earthworm causing the problem.

Earthworms are a group of soil-dwelling invertebrate animals called annelids. They have brown to reddish-brown cylindrical segmented bodies. They are considered beneficial because they feed on decomposing organic matter found on and below the soil.

As they burrow through the soil, they help increase and mix soil nutrients, reduce compaction and improve aeration.

So why would anyone want to kill these helpful creatures? Not all earthworms are the same. The one that causes bumpy turf is the common nightcrawler, Lumbricus terrestis Linnaeus. It is a large non-native earthworm, reaching a length of 3 to 6 inches. These nightcrawlers maintain permanent deep (up to 12 feet or more) vertical tunnels. As they feed on organic matter, they also ingest soil. As a result, they excrete soil enriched by the organic matter they ingested. This excreted fecal material is called castings.

To maintain their tunnels, nightcrawlers deposit their castings on the surface in small mounds. Researchers estimate that nightcrawlers are capable of depositing up to 20 to 25 tons of soil on the surface, per acre per year. Unlike the earthworms that meander through the soil in search of food and do not maintain permanent tunnels or create mounds, nightcrawlers are not effective at enriching and mixing nutrients in the soil.

Nightcrawlers live longer than you might think, with an average life span of six to nine years. They are also hermaphroditic, having female and male organs. They mate and breed most actively in the cooler weather of spring and fall, breeding about every two weeks and producing two to 20 offspring each time.

With each worm capable of producing more than 100 offspring each year, it is easy to see how easily a population increase, especially when conditions are favorable.

Common nightcrawlers are sensitive to high temperatures, doing best when the temperature is about 70 degrees. This explains why their most active times for creating mounds of castings are in the spring and fall. They go deeper in the soil and are less active when temperatures are warmer or colder. They also need adequate soil moisture, with 30 percent soil moisture being optimum. Precipitation or watering is favorable to their activity.

There are no pesticides recommended for controlling nightcrawlers, because it would also kill beneficial organisms and other earthworms. However, there are some things that can be done to help lessen the hazard that their mounds create:

▪ Use a dethatching or power rake that is set just low enough to rake apart the mounds, but not deep enough to actually remove lawn thatch.

▪ Use a lawn roller to flatten the mounds, followed by core aeration to relieve the compaction created by the roller. Core aeration alone may also help level a moderately bumpy lawn.

▪ Because moist soil is conducive to nightcrawler activity, avoid frequent shallow watering. Allow the surface of the soil to dry some between watering.

This story was originally published April 17, 2016 at 10:40 PM with the headline "Garden Tips: Not all earthworms are beneficial."