When I dug in my Berkshire garden this summer I found a host of earthworms.  That, it turns out, is bad.

I was raised to regard earthworms as the gardener’s best friend.  It’s true, these benevolent creatures (or so I regarded them then) aerate the soil with their tunnels and eat organic litter from the surface of the soil, carrying it back underground to excrete it as “castings” that are full of nutrients for plant roots.

My mother, a devoted gardener and my first horticultural instructor, always impressed on me the beneficial role that earthworms play in the garden.  Later, when I had graduated college and was studying horticulture at the New York Botanical Garden, my favorable view of these creatures was reinforced by a book written by no less an authority than Charles Darwin: The Formation of Vegetable Mould through the Action of Worms, with Observations on their Habits.  In this book, Darwin cited calculations that the population of earthworms in the average garden numbered some 53,767 per acre and calculated from his own observations that, depending on the quality of the soil, worms deposited as much as 18.2 tons of castings on the surface per acre per year.  Darwin regarded this as marvelously beneficial, which in some settings it is.  But in the northern United States these industrious creatures are a catastrophe from an ecological perspective.

For earthworms are not indigenous to the northern United State; they were wiped out by the glaciers of the last ice age.  And although there are many species of earthworms that are native to the southern parts of our country, most of the ones we find up here are introductions from Europe or Asia.  Native or foreign, though, earthworms can have dramatic effects in changing the quality of the soil, consuming and decomposing much of its organic content.

This transformation has an adverse effect on native vegetation.  Especially harmful are some of the non-native worms, which are enthusiastic and efficient consumers of organic litter on the forest floor.  This layer of fallen leaves and twigs acts as a mulch and a reservoir of nutrients for surface rooted trees, ferns and wildflowers.  Eliminating it can drastically affect the survival of these species.  Nor are the natives the only plants at risk.  I know of one gardener in Connecticut whose woodland perennial garden was overrun by the large, hyper-active Asian earthworms popularly known as “jumping worms” or “crazy snake worms” (Amynthas agrestis).  In a matter of months the shredded oak leaves with which this gardener had mulched her beds disappeared, and the soil was so over-aerated that plant roots dried out.  She no longer mulches – that just incites a worm population explosion — and periodically applies a tea seed meal-based plant food, Early Bird Natural Organic Fertilizer, that is toxic to earthworms.  In this way she keeps the population of jumping worms in check, but she will never eradicate them entirely.

“Crazy snake worm” — note prominent white “clitellum” or ring

It’s easy to introduce earthworms unintentionally to your garden, as I have learned.  The ancestors of mine — ordinary nightcrawlers (Lumbricus terrestris) — probably arrived in some truckload of compost or the decomposed manure I brought in from a horse farm.  My friend in Connecticut suspects that the Asian worms arrived in her garden in the soil around the roots of a plant shared by another gardener or container-grown nursery stock.  Often, though, worms are deliberately introduced.  Left-overs from a fishing trip are dumped into the woods or garden, or worms escape from tubs or beds in which they have been cultivated to help compost kitchen and garden debris.

Hopefully, the cold winters in my zone 5 garden will help to slow the earthworms spread, and I intend to spread the tea seed meal.   I like my woods as they are – worm-free and full of native wildflowers.