More than twenty thousand years ago, a major ice age spread across the northern latitudes of North America, Europe, and Asia and all the earthworms died. For millennia, these regions had no earthworms. But fast forward to the 19^th century, when Europeans and others started emigrating to North America, bringing potted plants with them. Worms and other critters hitched a ride in the soil.

On the East Coast, the worms altered the ecosystem. They ate the fallen leaves and other organic matter that the trees in northern forests evolved to depend on for nutrition. Centuries after the introduction of worms, East Coasts woodlands are changing.

 I have heard this phenomenon discussed on gardening talk shows and read more about it in 1493: Uncovering the New World Columbus Created by Charles C. Mann. As European settlers moved across the country, the worms moved with them and made themselves at home. However, in the western U.S., worms have benefited the soil and agriculture.

If you dig a hole in damp soil, you will probably run across a worm. Maybe more than one. These critters are earthworms (Lumbricus terrestris), and they live in the first 12 inches of soil. They dine on soil microorganisms and emit droppings (known as castings) that improve the soil. They also aerate the soil. Their activities produce tunnels that improve drainage and encourage root formation. Worms do not live in compacted soil.

Night crawlers (Eisenia hortensis) also live in the soil. When the air is damp, they surface and feed on plants. They are much bigger than red wigglers. I once put two night crawlers in a worm compost bin. When I cleaned the bin the next year, there were still just two night crawlers at the bottom of the bin. Although some night crawlers from Europe and Africa are apparently good composters, these were not.

Red wigglers (Eisenia fetida, the so-called fishing worms) are the king of the composters. They are the worms you find under stones, pots, and piles of leaves. They only eat decaying organic matter.

I once put a layer of maple leaves in a compost bin and, about a week later, I had a crop of baby maples. The worms ate all the leaves but left the maple seeds.

Typically, red wigglers eat or chew 90 percent of what you give them, including newsprint, plain cardboard, eggshells, daffodil leaves, spent flowers, leftover vegetables, coffee grounds and animal fur. If it's organic matter, they will eat it. Their castings are a mild fertilizer that you can spread over a garden bed. Water it in and nutrients return to the soil.

Some gardeners add a cup or so of worm compost to the planting hole for vegetables. It gives the seedlings a boost. I sprinkle worm castings on the surface of container plants to feed them.

African red wigglers (Eudrilis eugeniae) are usually used in African keyhole gardening. They grow much longer than our red wigglers and eat more. The two species can coexist because are not necessarily interested in the same decaying material. The African red wigglers need warmer temperatures, but I have corresponded with someone in Michigan who is successful raising them in his garage.

Other critters will take up residence in your compost bins, too. Sowbugs and pillbugs will arrive first. Their manure is just as good as worm castings. You may also spot small toads, earwigs, slugs, snails, an occasional small snake (they like worms), lizards, ants, and flies. Most of these creatures coexist happily but I do try to get rid of the fruit flies.

This year I am experimenting with putting compost bins in my raised beds. My beds are not round like keyhole gardens, but I put a bin in the middle of each bed so the worms can go back and forth and hopefully fertilize the vegetable bed they are in.

I am hoping that, with this method, the bins won't need cleaning out every few years. Only time will tell.

Workshop: Join UC Master Gardeners of Napa County for “Pruning Deciduous Fruit Trees” on Saturday, January 27, from 10 a.m. to noon at Las Flores Learning Garden, 4300 Linda Vista Avenue, Napa. Learn the whys and hows of winter pruning, techniques, and tool care. Workshop is indoors but dress warmly for outdoor activities, weather permitting. Space is limited. Register here.

Library Talk: Join UC Master Gardeners of Napa County for “Growing Lettuce Year-Round in Napa County” on Thursday, February 1, from 7 pm to 8 pm, via Zoom. Learn how a seasonal field trial revealed lessons about growing lettuce in Napa County. Register to receive the Zoom link.

Help Desk: The Master Gardener Help Desk is available to answer your garden questions on Mondays and Fridays from 10 a.m. until 1 p.m. at the University of California Cooperative Extension Office, 1710 Soscol Avenue, Suite 4, Napa. Or send your questions to mastergardeners@countyofnapa.org. Include your name, address, phone number and a brief description.