When we bought the five-acre lot in 1968, where our house now stands, it was an open field, mowed for hay each year. After a year or so, the man who did the mowing no longer wanted the hay, so we planted several hundred trees in the field. Nearly all of them were eaten by field mice, but the area was quickly invaded by seeds from a host of local plants.

Over the next decades these invaders produced a textbook example of the process of “old-field” succession; in fact, the field served as a laboratory for my ecology classes for several years. As successive groups of students recorded the process, weeds replaced the grass; woody shrubs like multiflora rose, poison ivy and honeysuckle vines replaced the weeds; and fast-growing, scrubby trees like locust, mulberry and box elder replaced the woody shrubs. By the time my grandchildren began to appear in the late ‘80s, the former hayfield had become the “Great Forest,” where they took their first walks and began to learn lessons of plants, birds and insects that they still cherish.

Toward the end of the 1990s I began to notice a change; the trees had matured enough to create a continuous canopy, and in the shaded area under it the multiflora rose was dying out. This pleased me because it is an alien species, native originally to Asia, and specializes in crowding other plants out, and I had hopes of introducing some native wildflowers into the area. But here and there I began to notice round, dark-green leaves that I did not recognize growing close to the ground. They proved to be biennials; the following spring they shot up rapidly to a height of three or four feet, and produced clusters of small white flowers. Each flower had four petals, arranged symmetrically in the form of a cross. Upon seeing this, a switch clicked on in my head; my memory reached back 52 years and retrieved the words “Cruciferae: mustard family.”

I had grown up thinking mustard was yellow stuff that came in little urn-shaped jars with “French’s” molded on them, and was intended to be spread on hot dogs. It was not until I was 20 that I was disabused of this idea; the thing that did it was a course in field botany. There I learned that “mustard” was the name of a family of plants whose flowers all have that particular shape. It includes familiar garden vegetables such as cabbage, broccoli, radishes and turnips; it also includes several common weeds. At the time I took the course, the most familiar of these weeds was “black mustard,” the yellow flower that covers unplowed cornfields each spring. This was one of many plants whose Latin names we had to memorize; Brassica nigra still flashes through my mind whenever I see it.

In the old classical method which was still being used when I learned botany, you began by memorizing the general characteristics of the plant families that grew in your area. Then when you found a new kind of plant, you put it in its proper family and began to narrow it down by color, size, smell, taste (sometimes dangerous in those days before safety regulations!), or other peculiarities such as thorns or leaf shape. Eventually, with help from knowledgeable friends, or as a last resort, by poring through reference books, you could find the genus and species names. It was a strongly tactile process and tedious to learn; but once learned, it stuck with you. Thus when I found the white-flowered mustard plant, all of that flashed through my mind like the microfiche reader in the library at high speed.
I had already observed the shape and texture of the leaves, the size, color and shape of the flowers, and the form of the seed pods, so it seemed automatic to crush a leaf and smell it. It gave off the aroma of fresh garlic, which clicked another memory switch: it was garlic mustard. I had seen it five years earlier on a trip with the Audubon Society.

In Europe where it originated, garlic mustard is a common weed of old fields and roadsides. It is a nuisance, but is eaten by various insects and animals which keep it under control. It probably came to America like many other weeds, either accidentally in hay and bedding on ships that transported livestock, or was brought intentionally for use as a medicinal plant or salad herb. It seems to have been noticed for the first time by botanists in New England in the 1860s, though it may have been there much earlier. Like all mustards, it produces thousands of tiny black seeds which travel by sticking to the fur of animals; and like many alien species, it had no natural enemies in this country. So it multiplied rapidly, turning up in patches all over the country except in the dry regions of the Southwest.

This spring the garlic mustard has completely covered my woodlot, producing a dense growth that reaches above shoulder height. It has crowded out many other plants, including several small trees I planted last spring. And it hasn’t stayed in the woods; it has come up in the garden, in flowerbeds and borders all over the yard. I have spent the spring mowing it back and uprooting it, but like Lewis Carroll’s oysters, it just keeps coming. Battling it seems as futile as trying to bail out a boat with a teacup.

At a time when native plants all over the country are struggling to avoid extinction because of habitat loss from development and pollution, invasive alien plants are more than a problem; they have become a disaster, the last straw. If they have any value, it may be as a metaphor; perhaps they are a warning to us. After all, we too are alien invaders; and look what we have done to this continent since our arrival 500 years ago.