Water foxtail (Alopecurus geniculatus)

Common Names: marsh foxtail, knee-joint foxtail

Description:

Water foxtail (Alopecurus geniculatus) is a low, wetland grass native to Eurasia [1][2][3]. Its North American status is uncertain ? most authorities treat it as an introduction that likely arrived with ship ballast in the 1800s [3]. MISIN tracks it as a non-native grass of wet meadows, ditches, and shorelines; it is recognized by its bent, kneeling stems and soft, cylindrical flower spikes [1].

Habit:

Water foxtail is a small perennial grass, only about 2 to 24 inches (5 to 60 cm) tall, that grows in tufts on wet ground [1]. Its most telling feature is the way it grows: the stems lie along the ground at the base, bend sharply upward at the joints, and take root where those bent "knees" touch the wet soil ? the source of its other name, knee-joint foxtail [1]. It grows only in wet places [1].

The flower head is a soft, dense, cylindrical spike ? like a slim, blunt pipe-cleaner ? about 3/4 to 2 3/4 inches (1.5 to 7 cm) long and roughly 1/4 inch (4 to 6 mm) wide, often tinged gray or purple [1]. Each tiny flower carries a short awn (bristle) that bends outward as the head dries, giving the spike a slightly whiskery look [1]. It is easiest to identify when these spikes are up, in the warmer months.

At-a-glance cues:

1. A low, tufted grass growing only in wet ground [1]
2. Stems that lie flat, bend upward at the joints, and root where the bends touch the soil [1]
3. A soft, dense, cylindrical flower spike about 3/4 to 2 3/4 inches long, often gray- or purple-tinged [1]
4. Short awns (bristles) that bend outward from the drying head [1]

Leaves:

1. Color: green to gray-green [1]
2. Blade: flat, about 1 to 5 inches (2 to 12 cm) long and 1 to 7 mm wide, rough to the touch on the upper surface [1]
3. Ligule: a thin membrane about 2 to 5 mm tall where the blade meets the stem [1]
4. Auricles: absent (no ear-like flaps clasping the stem) [1]

Stems:

1. Habit: soft and herbaceous, growing in tufts; the stems trail along the ground and bend upward at the joints [1]
2. Rooting: takes root at the lower bent joints (nodes) where they touch wet soil [1]
3. Length: about 2 to 24 inches (5 to 60 cm) [1]

Flowers:

1. Arrangement: a dense, spike-like head (a tight cylindrical panicle) about 3/4 to 2 3/4 inches (1.5 to 7 cm) long and 4 to 6 mm wide [1]
2. Color: often tinged gray or purple [1]
3. Awns: each floret has a short awn (3 to 6 mm) that bends outward on the drying head [1]
4. Bloom time: the warmer months (spikes present in late spring and summer) [1]

Fruit and seeds:

1. Type: a dry grain (caryopsis), the seed about 1 to 1.5 mm long [1]
2. Dispersal: spread by seed ? carried in water, wet mud, and on animals, boots, and equipment [1][3]

Habitat:

Water foxtail grows only in wet places ? an obligate wetland plant [1]. Look for it in wet or flooded meadows, marshes, ditches, shallow water, and along the shores of ponds, lakes, and streams [1][3]. It does well on wet clay, mud, silt, or peat [3].

It is native to Eurasia, from Scandinavia to the Mediterranean and east to the Russian Pacific coast [3]. In North America it is naturalized across the Great Lakes region and much of the northern United States and Canada, though whether it is truly non-native here is uncertain [1][3].

Reproduction:

Water foxtail spreads in two ways, and a homeowner is most likely to notice the first. It reproduces by seed, and the seed travels in water, in wet mud, and on animals, boots, mowers, and equipment [1][3]. In the past it is thought to have hitchhiked long distances in ship ballast [3].

It also spreads on the spot: wherever a trailing stem bends and its joint touches wet soil, that joint puts down roots and starts a new tuft [1]. That lets a single plant creep outward into a small patch over time. For control, the timing that matters is simple ? remove plants before the flower spikes ripen into seed.

Impact and Damage:

Water foxtail is not a hazard to people or pets. It is not a toxic plant, and no source reviewed for this sheet reports it harming livestock or causing skin reactions [1][3].

Its documented impact is modest. It is a small grass of wet, often already-disturbed ground ? ditches, wet meadows, and shorelines ? and where it establishes it adds to the wet-grassland layer [1][3]. One assessment concludes that it is probably too uncommon to cause significant economic harm and has probably had no significant effect on native plants in its range [3]. MISIN tracks it as a non-native wetland grass worth watching rather than an aggressive invader, so early removal of new patches is mostly about preventing it from becoming established in high-quality wetlands [3].

Similar species:

Several grasses with soft, cylindrical flower spikes are mistaken for water foxtail. Its combination of bent, rooting stems and a strictly wetland home is the best way to tell it apart [1].

Meadow foxtail (Alopecurus pratensis) is the closest look-alike ? a very similar foxtail grass ? but it is taller and grows upright in drier hayfields and meadows, rather than trailing and rooting at the joints in wet ground like water foxtail [1].

Shortawn foxtail (Alopecurus aequalis) shares the same wet habitats and soft spikes, but its awns barely reach past the tip of the flower, so its head looks smooth ? unlike the short, outward-bent awns that give water foxtail a whiskery spike [1].

Timothy (Phleum pratense) also has a dense, cylindrical head, but it is a stiff, upright grass of dry fields and roadsides with a rougher, bristly spike, not the soft head and bent, wet-ground stems of water foxtail [1].

Monitoring and rapid response:

Signs to watch for:

1. A low, tufted grass in wet meadows, ditches, or shorelines, with stems that bend and root at the joints [1]
2. Soft, cylindrical flower spikes about 3/4 to 2 3/4 inches long, often gray- or purple-tinged, in the warmer months [1]
3. Short awns that bend outward from the drying head [1]

What you can do:

1. Hand-pull or dig small tufts, taking the whole plant ? including any stem joints that have rooted into the mud ? so it cannot regrow [1]. Wet soil makes this easier, and small patches are worth removing while they are small.
2. Remove plants before the flower spikes ripen into seed. Since it spreads mainly by seed [1], pulling or cutting before the spikes mature is the key to keeping it from spreading.
3. Bag and dispose of seeding heads rather than composting them, and clean mud and seed off boots, tools, and mowers before leaving a wet site so you do not carry it to new ground.
4. For a natural area crew: focus on small, outlying patches in high-quality wetlands before they seed, and revisit across the season to catch late growth. Because impact is modest, prioritize protecting intact wetlands over chasing well-established roadside-ditch stands.

When to call a professional:

1. Large or well-established stands in standing water or sensitive wetland, where hand-removal would churn up the site
2. Any wetland grass you cannot confidently identify ? several foxtails look alike, so a photo-backed report is worth more than a guess
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Native Plant Trust, Go Botany. Alopecurus geniculatus (marsh foxtail). gobotany.nativeplanttrust.org
2. Global Biodiversity Information Facility. Alopecurus geniculatus L. gbif.org
3. Smithsonian Environmental Research Center, NEMESIS Chesapeake Bay database. Alopecurus geniculatus. invasions.si.edu

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