You never get a second chance to make a first impression. This old truism is making things difficult for the recently discovered native populations of Phragmites, also known as common reed or phrag.

The phrag we all love to hate is an invasive tall grass that is becoming the dominant plant along the upper edges of our salt marshes, growing so thickly that it crowds out any other plants, including cattails, sedges, wild flowers, and woody shrubs.

It can also spread into the salt marsh itself, replacing the low short salt meadow cord grass that usually dominates. It is widely believed that changes due to our many and varied uses of the landscape are responsible for this spread; excessive nutrients and reduced tidal flushing are two of the most frequently cited explanations.

New studies of the genetics of phrag are giving us another explanation for its invasiveness. We now recognize three genetically distinct subspecies of phrag: a native population that occurs in most of the United States and southern Canada, an introduced population that is found everywhere, and a Gulf Coast population that appears to be intermediate between these two.

The native phrag is not invasive. It does not spread and come to dominate a salt marsh. If the phrag you are familiar with is spreading, then it is the introduced subspecies.

Fortunately, there are other ways to distinguish the native phrag. As with any two plants that look similar, a combination of several characteristics are best.

At this time of the year, one of the easiest ways to tell the two apart is the color of the flowerhead, which for the introduced phrag is still purplish while the native flowerhead has already aged into a tannish color. The color of the lower stem is also diagnostic, with the introduced phrag having a tannish stem while the native phrag stem is reddish. But fall arrives on Sunday and both of these distinguishing marks fade as the fall progresses.

The persistence of the leaf sheath is another distinguishing field mark. These leaf sheaths wrap completely around the stem during the growing season, and tend to stay on the plant into the fall.

The native phrag leaf sheath tends to fall off the stem at this time of the year, while the introduced phrag stays attached and is often difficult to pull off. This difference is especially easy to see on grass stems from a previous growing season; the dead stem of the introduced phrag will tend to have its leaf sheaths attached to the stem while the native phrag will have dropped them.

The native phrag stem — without its leaf sheath — is also smooth and shiny, contrasting to the rough and dull texture of the introduced phrag. The native phrag also tends to have clusters of dark spots on its stem.

The native subspecies of phrag has been here for thousands of years. Various studies have documented its presence at archaeological sites and in historic sediments. Native North American tribes have used phrag to make a wide variety of things, including arrowshafts, cigarettes, flutes, whistles and matting.

A study published in 2004 described the native phrag and gave it a scientific name, Phragmites australis americanus. A workshop on native phrag was held on August 29 at the Woods Hole Research Center. The Vineyard was well represented at that workshop – nine of us were there. We learned about this new subspecies and visited a salt marsh in Falmouth where we got to see it growing alongside the introduced phrag.

While no native stands of phrag have yet been found on the Vineyard, we have plenty of the invasive phrag. Too much of it by most accounts. A first impression of phrag is that it is bad because it is displacing native plants and blocking views of the water.

The recently discovered native subspecies of phrag, which is not invasive, will definitely need a second chance to make a first impression.

Robert Culbert is an ecological consultant living in Vineyard Haven.