Paraquat wipes out superweeds

Paraquat being applied by a weed wiperParaquat has been found to be an effective ‘fire fighting’ treatment for large glyphosate resistant Palmer amaranth, sometimes called a ‘superweed’. Researchers at the University of Georgia, USA, have found that applying paraquat through weed wipers and similar implements can very effectively kill Palmer amaranth plants up to 5ft (1.50 m) tall1. While paraquat is very effective on smaller weeds, glyphosate’s systemic action generally makes it a better option on large weeds. However, where there are glyphosate resistant weeds an innovative approach is needed. Paraquat has been approved for application through weed wipers to control weeds in peanuts in Florida, Georgia, and in North and South Carolina. Since 2005, Palmer amaranth (Amaranthus palmeri, also known as Palmer pigweed) has become an increasing problem for farmers in Georgia and neighboring states2,3. It has become common for gangs of laborers to be used to remove weeds with hoes and machetes often at a cost of over $100 per acre ($250/ha).

Benefits of using paraquat through weed wipers

  • Prevents spread of glyphosate resistant weeds
  • More effective application of late fungicides
  • Easier harvesting
  • Much cheaper than hand-weeding
Weed wipers have been developed to apply glyphosate to tall weeds standing above crop canopies. They work on the principal of a wick, herbicide solution being absorbed by rope, sponge or carpet-like materials which then wipe the leaves and stems of weeds by direct contact. University of Georgia researchers tested a number of alternative herbicides through weed wipers of various kinds. They found that paraquat proved to give a surprising and exceptionally high level of control, and was clearly the best option. Treatment costs around $4 per acre ($10/ha) for the herbicide. As paraquat is a non-selective


A chemical product used for eliminating all types of weeds (annual and perennial grasses and broadleaved weeds).

Authoritative On-line References and Resources An invaluable source of contemporary information about herbicides and weeds from Iowa State University.
herbicide, it is crucial that the crop is protected. In peanuts, a major crop for growers in the southern US states, this is easily achieved as the crop only reaches heights of 12 to 18 inches (30 - 45 cm). Good herbicide coverage is important to successful control with an estimated 50% of the weed surface needing to be wiped. This is achieved by the wiper pressing down on the weeds and using a tractor speed of no more than 5 mph (0.45 m/sec). A weed wiper with a carpet on a contra-rotating drum gave the best results in field trials.
Equally important is that paraquat needs to be applied before seeds start to form, within two weeks of flowering. Although controlling weeds late in the season means that competition will already have reduced crop yield, several important benefits from destroying large Palmer amaranth still remain. Foremost is the prevention of seeding. With the possibility of half a million seeds being produced by each surviving plant, any small glyphosate resistant populations can soon get out of hand.
Untreated control plot infested with Palmer amaranthPlot wiped with paraquat 9 days earlier
                              A good tip to enhance levels of control is to wipe weeds with paraquat solution in the late afternoon or early evening. This allows some movement of paraquat in the treated weeds under conditions of lower light or darkness, so that by the time photosynthesis resumes the next day, weeds are hit with full force.  (Read about paraquat works here.) Paraquat applied through weed wipers can also be used to control Florida beggarweed (Desmodium tortuosum) and research is now focusing on potential applications in soybean and cotton.


  1. South-East Farm Press 21 January, 2011
  2. South-East Farm Press 18 October 2010
  3. International Survey of Herbicide Resistant Weeds


  1. Images are courtesy of Dr Eric Prostko, University of Georgia.
  2. The brand name for the leading paraquat product is Gramoxone.