By Jane Mangold, Montana State University Extension Invasive Plant Specialist

Introduction

Two grass plants leaning against the front of a pickup truck.

Invasive (left) and native (right) Phragmites found in Gallatin County in 2025. Photo: Noelle Orloff, MSU.

Phragmites australis was featured in the 2014 September Weed Post and this post revisits the species to provide additional identification information and an update on its status in Montana. Two lineages of P. australis occur in Montana – one native (sub-species (ssp.) americanus) and one introduced and invasive (ssp. australis). Native and introduced Phragmites habitats overlap and include wetlands (both freshwater and brackish), along lakes, streams, and rivers, and near springs. Invasive Phragmites may be more likely to occur in disturbed sites like roadsides and railways, construction sites, and near agricultural fields.

Status in Montana

The first populations of Phragmites were found in Hill and Blaine Counties in 2014. At that time, Phragmites was under review for listing as a noxious weed and was subsequently added to the statewide list in 2015. Since then, the populations in those counties have been managed with fire and herbicide and have decreased from about 4 acres to 0.2 acre (~200 stems). In more recent years additional populations have been found in the following counties: Missoula (2020), Gallatin (2023, 2025), Lincoln (2025), and Flathead (2025, 3 populations). These infestations have been or will be managed and are being monitored annually for treatment as necessary.  

Identification

Two stems of a grass with the one of the left having fewer leaves and being reddish in color compared to the one on the right with leaves and being green.

Leaf sheaths and stem of native (left) and invasive (right) Phragmites. Photo: Jane Mangold, MSU.

 

Distinguishing between native and invasive Phragmites is difficult, and multiple traits should be considered. The September Weed Post from 2014 includes a table that lists key features to examine to differentiate between native and invasive Phragmites. This post focuses on comparisons of leaf sheaths and inflorescences of the native and invasive sub-species.

The leaf sheath on native Phragmites is loose and tends to fall away from the stem while those on invasive

A close-up of two leaves, the one on the left is lighter green and narrower than the one on the right.

Leaves of native (left) and invasive (right) Phragmites. Photo: Jane Mangold, MSU.

Phragmites adhere tightly to the stem. Once the leaf sheath falls away, the stem may turn a reddish color, therefore stems on the native are inclined to be redder than the invasive. The stem is smooth on the native while it is ridged on the invasive, but this is most easily noted by touching versus looking at the stem. The leaves of native Phragmites are generally light, yellow-green while those on the invasive are dark green. Inflorescences of the invasive are denser than the native and may be a different color. Colors of leaves and inflorescences change over the course of a growing season. Differentiating between the native and invasive is easiest after mid-summer and into winter.

A close-up of two grass inflorescences with the one on the left being a lighter color and less dense than the one on the right.

Inflorescences of native (left) and invasive (right) Phragmites. Photo: Jane Mangold, MSU.

Additional photos and descriptions can be found in the MSU Extension publication “Watch Out for Phragmites” and “Identifying and Reporting Invasive Phragmites” from the University of Minnesota.

Further Information

For more information about this month's weed post, contact Extension Invasive Plant Specialist Jane Mangold. Past posts are available in the Monthly Weed Post Directory.

This weed post is also available as a printable PDF (217 KB).