Achnatherum brachychaetum

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Authors: Karan Rawlins, Hillery Reeves and Kaylee Tillery at the Center for Invasive Species & Ecosystem Health, University of Georgia

Contents


5387065
Taxonomy
Kingdom: Plantae
Phylum: Magnoliophyta
Class: Liliopsida
Order: Cyperales
Family: Poaceae
Genus: Achnatherum
Species: A. brachychaetum
Scientific Name
Achnatherum brachychaetum
(Godr.) Barkworth
Scientific Name Synonym
Nassella brachychaetum
(Godr.) Barkworth
Stipa brachychaeta
(Godr.) Barkworth
Common Names

punagrass, shortbristled needlegrass, needlegrass, punagrass

Overview

Appearance
Achnatherum brachychaetum is a densely tufted perennial, with shallow roots and erect stems to 3.3 ft. (1 m) tall. Foliage is tough and unpalatable to livestock, except for young shoots. It is present almost year round. It is often associated with alfalfa fields. The dense clumps can mechanically interfere with mowing.
Foliage
The leaf blades are linear, 0.04-0.07 in. (1-2 mm) wide, 16-24 in. (40-60 cm) long, flat or slightly involute, and rigid. Feels rough to touch. The sheaths are open and glabrous, but with ciliate hairs along the upper margins and especially at the auricles. The ligules are membranous.
Flowers
Achnatherum brachychaetum flowers in the Spring and Summer. The panicles are nearly pyramidal 8-12 in. (20-30 cm) long and just pass the tops of the blades. Spikelets with 1 floret. The glumes are glabrous and nearly equal, ranging from 0.24-0.31 in. (6-8 mm) long.
Fruit
The seed have many hairs. Awns are straight or bent and up to 0.8 in. (1.8 cm) long. Modified spikelets called cleistogenes can be found inside the stems above the nodes of flowering shoots and at the stem base.
Ecological Threat
Achnatherum brachychaetum can be found in disturbed sites. It is often seen in alfalfa fields and nearby drainage ditches, dairies, and horse pastures.

Reproduction

Reproduces by seed. Cleistogamous seed develops within the leaf sheaths at the bases of plants in early summer and matures by late summer. Cleistogamous seed is hard-coated and can persist in the soil for at least 2 years.

Range

Achnatherum brachychaetum is native to Argentina and Uruguay. It has naturalised in the United States and Spain. Achnatherum brachychaetum is a noxious weed in New South Wales, Australia.

Management/Monitoring

Achnatherum brachychaetum is a shallow rooted perennial bunchgrass found primarily in alfalfa fields or along irrigation ditches. It is native to Argentina, where it is a serious problem in alfalfa. It is believed to have arrived in contaminated alfalfa seed. Therefore, prevent new infestations by planting only certified alfalfa seed. If it is economically feasible, shift crop rotations to other annual or perennial crops where tillage can be utilized for weed management. Infested areas should be monitored for at least two years since seed are viable for that length of time.

Manual Control
  1. Mature clumps are very rigid, but may be pulled or dug. Clumps should be removed, dried, and possibly burned.
Mechanical Control
  1. Achnatherum brachychaetum may vegetatively reproduce from the crown, but is not stoloniferous or rhizomatous. It also has the ability to produce seed cleistogamously which allows it to persist in alfalfa fields. Frequent mowing can eliminate aerial seed production. However, vegetative regrowth will rapidly occur, and viable, cleistogamous seed will be produced. Tillage is an effective control method.
Chemical Control
  1. When Achnatherum brachychaetum is found in alfalfa, fall applications of either sethoxydim (0.45 lb ai/A) or clethodim (0.25 lb ai/A) provided 70-80% control, 8 months after treatment. Fall applications (late October) were more effective than spring (early March) applications. Applications in both the fall and spring resulted in 95% Achnatherum brachychaetum control, 8 months after treatment.
  2. A tank mix application of sethoxydim (0.45 lb ai/A) and thiazopyr (0.50 lb ai/A) also provided 95% control in the fall.
  3. Glyphosate (2% v/v) to individual plants resulted in excellent control (99%) in the fall up to eight months after treatment. Alfalfa plants surrounding the Achnatherum brachychaetum were also killed. Glyphosate may be an excellent spot application treatment for Achnatherum brachychaetum along irrigation ditches or non-crop areas.
  4. Regrowth from treated clumps has been observed with most herbicide treatments. Larger clumps will be more tolerant than seedlings or small clumps.
Biocontrol
  1. There are no biocontrol agents for Achnatherum brachychaetum, and there is little information regarding any plant/insect interactions.
Management Control
  1. Crop rotation that allows the use of both tillage and selective grass herbicides is the best approach for eradicating Achnatherum brachychaetum. Infested areas should be monitored for several years and new seedlings should be aggressively managed to prevent new clumps from forming.

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