Watch Out for Prickly Lettuce, the Invasive Weed Appearing All Over the Country

Prickly lettuce, a common invasive weed across the U.S.

Photo: Marielle Anzelone

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This is the first post in a new series called "Backyard Botany," which will look at plants in the environment, with profiles of natives and exotics, discussion of invasive plants and other issues of the day.

As a botanist, I look at plants. I don't mean the roses and daffodils in your yard, but wild, naturally occurring plants. This unplanted greenery -- like goldenrods, oaks and sumacs -- spreads around the landscape due to fortuitous interactions with (usually) birds or wind. There are lots of spots to view these plants. Woodlands, wetlands and meadows are obvious choices. Yet if your daily life doesn't take you near a natural area, look just outside your door. Built environments have wild plants, too. Think of cracks in the sidewalk, clumps of soil at the curb, weeds in the lawn.

In that spirit, I'd like to encourage you to get to know the nature around you by paying attention to your foliar neighbors. That song from Sesame Street would be appropriate here: "These are the plants in your neighborhood...they're the plants that you meet when you're walking down the street, they're the plants that you meet each day."

Unlike many animals, plants don't have eyes and cute escapades to grab your attention. But don't let the lack of vertebrae fool you -- these trees, shrubs, grasses, forbs and ferns have fascinating stories to tell. Of course, they also play an important ecological role, especially native plants.

First let's clarify these definitions of "native" and "exotic." Typically, a "native" plant is one that naturally occurs in a region without human assistance. So a native plant must be defined by a place and grow there through natural phenomenon.

In New York City, for example, native plants can be trees, shrubs, forbs (typically called, incorrectly, "wildflowers"), graminoids (grasses, sedges or rushes, collectively), ferns or woody vines. New York City's native flora has evolved in the five boroughs over the last 13,000 years or so, after the glaciers retreated northward. Some plants that are native to the Big Apple are found up and down the Eastern seaboard, like red maples. Others, like threadleaf sundew, have a more limited range of distribution. Roughly 3,000 plants co-mingle in New York City, about two-thirds of which are native.

Given my work as an urban conservation biologist, I can be a native plant snob. I'm going to work against type with our first plant profile, a true global weed: prickly lettuce (Lactuca serriola).

From Eurasia, prickly lettuce did not originate in the U.S. It is then considered an exotic that has naturalized. Prickly lettuce is found in disturbed areas such as farmland, roadsides and urban lots throughout the United States. This time of year, it's an easy plant to run into during casual walks around your neighborhood.

One of the first things you would notice is its height -- up to 6 ft., it's remarkably tall for a non-woody plant. It is a biennial forb, meaning it takes two years to grow from a seed to a flower that then produces seeds. As the common name implies, each leaf has short prickles along its margin and the underside of the central vein. The base of each leaf has a pair of angular lobes that clasp the stem.

Both the stems and leaves of wild lettuces (we have several species native and not) exude a milky sap. Of course, you don't notice it in your Caesar salad because it was bred out of the cultivated species we eat. This latex exude serves to thwart would-be herbivores. In spite of that, caterpillars of some moth species and white-tailed deer have been known to munch on the leaves.

The leaves are interesting for another reason: they turn to follow the sun during the day. This tracking helps the leaves capture as much energy from the sun as possible. Thus another common name for L. serriola is "compass plant." Who says plants don't move?

While the height of the plant may wow you, the flowers may not. They are not fragrant or unusually large. They do find fans in local bee populations, however. Panicles of pale yellow blossoms entice the buzzing visitors with nectar and pollen rewards. As members of the Asteraceae family (formerly Compositae), what you would call a single flower is actually many tiny flowers aggregated. Think of a sunflower and the edible seeds -- those only come from the fertile "disk flowers" in the center. The outer petals are in fact flowers themselves. They are the sterile "ray flowers." Unable to create seed, these ray flowers support reproduction by advertising the presence of the disk flowers to passing pollinators. In the case of prickly lettuce, it has about two dozen fertile ray florets, meaning there is no prominent "center" as in a daisy (absence of disk flowers) and the ray flowers have both sterile (petals) parts and reproductive parts.

The seedhead of prickly lettuce is fun for kids. As with many other members of the Aster family, it has wind-borne seeds that are good for blowing. The seeds can be more stubborn than dandelions, so be sure to blow hard!

Marielle Anzelone is a botanist and native plant landscape designer, and is the founder of NYC Wildflower Week. Learn more about Wildflower Week.

Check out this exclusive TDG video of Marielle Anzelone talking about Wildflower Week and the importance of protecting, and exploring, urban nature:

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