Pampas grass is a widely planted perennial grass that is native to Argentina, Brazil and Chile. The grass grows up to 10-feet tall and 6-feet wide at maturity, producing pinkish or silvery-white plumes. Pampas grass works well as a perennial border and helps prevent coastal erosion, but it is invasive in California and Hawaii. Wild forms of the grass yield vast quantities of seed and so enable the grass to take over landscapes quickly.
Glyphosphate is a broad-spectrum, nonselective herbicide sold under various trade names, such as Round-up or Glyphomax. Glyphosphate kills pampas grass by inhibiting the synthesis of amino acids and other metabolic processes the grass normally carries out for survival. Glyphosphate moves from the foliage to the root and underground storage systems. Typically, greater surface contact with the foliage enables more of the herbicide to reach the rhizome. Remove any dead culms from the previous years’ growth that may hinder contact with the herbicide. Wait until the plant produces 12 inches of new growth before applying glyphosphate as a foliar spray. Cut the foliage to the ground when the grass is completely dead. Repeat the application in fall if regrowth occurs.
Glufosinate ammonium is a nonselective herbicide sold under the trade name Finale. Glufosinate ammonium is chemically similar to glyphosphate but works more rapidly, often presenting effects in two to three days. Glufosinate ammonium does not travel to the roots or underground storage systems and poses less risk of permanent injury to nearby plants than glyphosphate. However, pampas grass respouts readily, so the chemical only temporarily kills the plant by burning back the foliage. Gardeners use glufosinate ammonium for short-term control of small tufts until mechanical removal is convenient. Glufosinate ammonium performs best when applied during the “boot stage” in fall when the plant concentrates on seed development.
Hexazinone is a broad-spectrum herbicide sold under the trade names Pronone and Velpar. Hexazinone is frequently used to control grasses in reforested areas and rangelands. Hexazinone is a water soluble substance that requires moist soil to enter the root system. It performs best in sandy soils that are low in organic content, clay and silt. Hexazinone does not bind to soil particles like other herbicides and remains mobile in the soil up to six months, so it has the potential to spread to nontarget plants and leach into local waters and kill algae. Thus home gardeners should use the less mobile liquid formulas at the manufacturer’s suggested application rate.