Small carpet grass, arthraxon, jointhead grass, hairy jointgrass (Arthraxon hispidus)
Family: Poaceae
Life Form: Annual grass
Tier: 2

History & Introduction

Arthraxon hispidus was first documented in the US in the 1870s and has since been widely introduced across North and Central America, as well as the Caribbean. It is native to East and Southern Asia, as well as Australia, where it is considered threatened. 

Arthraxon was first reported in 2017 by Vicki Bustamante from Third House Nature Center in Montauk, NY as she found it along a mile-long stretch of a trail within Montauk County Park. Currently on Long Island it has been sighted in two locations — Montauk County Park and Gabreski Airport.  In both cases it has been found invading open trails with partial to full sunlight; either at the edges or directly in the middle of the path. These trails are also regularly or semi-regularly wet and are nearby water. In its native range it is a common plant found along streams, in damp meadows and alongside crops among other wet places. In New Jersey it has been found invading a tidal wetland that was later replaced by phragmites, and in Texas it can be found in orchards and roadsides. 

Identification & Biology

Arthraxon is a warm season (C4) grass, flowering late in the summer whose seeds are easily transported via water, animals and mowing equipment. It does not tolerate dense shade.

• Habit: grows 30-60cm in height, has a decumbent growth habit (lying along the ground, with tips pointing upward).
• Stem: glabrous (smooth), slender, branched, and has many nodes. Upright stems of grow up to 18 inches tall and may root at the nodes at the base of the stem where they contact the soil surface.
• Leaf sheath: shorter than internode and covered with wart-like granules
• Ligule: membranous, 0.5-1mm long, with fine hairs along the margin. 
• Leaves: ovate-lanceolate, 2-4 cm in length and 0.8-1.5 cm in width, base cordate and encircling the stem. Leaf edges are hairy and appear wavy.
• Racemes: slender, 1.5-4 cm long, clustered or digitate on the top of culm
• Spikelets: sessile, ovate-lanceolate, 3-5 mm long, grayish green or somewhat violet, with a basally twisted awn that is 6-9 mm long while the petiolate spikelets are degenerated and pin-like. 
• Fruits: are oblong caryopsis.

Phenology: Leaves emerge late spring to early summer, flowers and fruits emerge late summer and are present September through November. 

Look-alikes:

• Deer tongue grass (Dichanthelium clandestinum) – has more fuzzy stems, leaf base does not fully encircle the stem, leaf edges are less wavy.
• Japanese stiltgrass (Microstegium vimenium) – leaf bases do not encircle the stem, distinct silver midrib and leaves are rough if rubbed in one direction.

Damage & Concern

After a disturbance such as a flood or soil movement, Arthraxon can easily get established and develop a large monoculture in three to five years. As an annual, the plant seeds yearly, and spreads vegetatively as well. Little is known about the plant’s ability to survive in the seed bank. Where it has been found invading on Long Island, it is directly competing with approximately 15 rare plant species. 

Management

Manual: For small infestations, hand pulling may be feasible. For larger infestations, seasonal mowing has been found to reduce population size and vigor. The most optimal time to mow is while Arthraxon is in flower but before seed set. This appears to occur in early to mid-September on Long Island. Herbaceous matter should be bagged, solarized, and disposed of in the trash.

Chemical: Several general use herbicides approved for wetlands have shown to be effective in controlling Arthraxon. Repeated applications may be needed to reduce plant densities. It is important to evaluate infestations to understand if chemical control is necessary and/or feasible.  Always follow the directions on herbicide labels.

Biological Control: 9 fungal species have been found to associate with the genus Arthraxon. One such species, Bremia graminicola Naumov, may be host specific to Arthraxon hispidus but more research is needed to understand its ability to be widely used as a biocontrol. 

Please contact LIISMA with photos and location coordinates if you believe that you have seen this plant.