Making the most of paraquat as a contact herbicide

Paraquat has contact-only actionParaquat is a contact herbicide. It is not systemic, meaning that it does not move inside plants. Only green tissue covered by paraquat spray is affected. The image opposite shows the results of an experiment in which droplets of paraquat solution were applied to a single leaf. The intense colours indicate that paraquat has remained in the treated leaf and has no systemic action.

Paraquat’s unique profile

In addition to the contact-only action, paraquat has another rare property: it is deactivated immediately on contact with soil. It cannot be absorbed by roots or affect following crops. These features mean that paraquat can be used more flexibly and with much more precision than other herbicides. For instance, although glyphosate is an invaluable tool in the fight against weeds, it is systemic. (See how paraquat supports glyphosate in insuring against resistance


The inherited ability of a plant/weed to survive a dose of a herbicide normally lethal to that species.

Authoritative On-line References and Resources The International Survey of Herbicide Resistant Weeds monitors the evolution of resistant species and assesses their impact. All confirmed instances of new cases are listed.
here.) Spray readily moves from the point of contact throughout the plant. Sometimes, of course, this is a benefit, particularly in the control of perennial weeds


Weeds that return year after year. Some die back in the winter but their roots remain alive and shoots reappear in spring; some don't die back and grow in size and stature the next season.

Authoritative On-line References and Resources

The International Weed Science Society represents individual associations around the world.
. Other herbicides may move through the soil to be absorbed by roots. Unless there is good selectivity, the result to crops such as fruit and vegetables is dead or damaged plants.

Many crop uses

Paraquat, however, can be used in between rows of vegetables, in orchard and plantation crops and in vineyards to control weeds quickly and in all weathers. Its speed of action and rainfastness are famously second to none. Paraquat cannot penetrate mature bark, so trees can be sprayed right up to the base. In wide-row field crops such as peanuts and potatoes, paraquat’s contact-only action means that it can be used early post-emergence to help keep the crop weed-free before the canopy closes. The young crops soon grow away from any damage, which is confined to leaves receiving the spray. In pastures, paraquat can be used to improve the quality of the sward. A correctly timed paraquat spray will control weeds while allowing perennial grasses


The leaves are "narrow" as opposed to the "broad" leaves of broadleaved weeds. Also called 'monocots' having one seed leaf opposed to 'dicots' having two seed leaves.

Authoritative On-line References and Resources

The International Weed Science Society represents individual associations around the world. 
to regrow and reassert their dominance.

Saving the soil

Only shoots are affected by paraquat. Being non-systemic it does not reach roots and other growing points, and is deactivated in soil. Therefore, roots of annual weeds


Weeds that complete their life cycle within one growing season, or year. From seed to flowering to seed before the year ends.

Authoritative On-line References and Resources

The International Weed Science Society represents individual associations around the world.
remain for sometime, anchoring the soil and helping to resist soil erosion


Displacement of solids (soil, mud, rock and other particles) usually by the agents of currents such as, wind, water, or ice by downward or down-slope movement.

Authoritative On-line References and Resources This site brings together reliable information on soil erosion from a wide range of disciplines and sources. It aims to be the definitive internet source for those wishing to find out more about soil loss and soil conservation.
. Similarly, less aggressive ‘soft’ perennial grasses can be allowed to regrow to similar effect.
Figure 1. Accumulated soil losses at three sites over four yearsThe Paraquat Information Center has many examples of how the use of paraquat has very significantly reduced soil erosion. Some results from a long-term experiment on several sites in the Philippines are shown in Fig. 1. Due to its contact-only action, paraquat was markedly superior to traditional methods of weed control, which relied on hoeing and cultivations, in reducing soil erosion.

For best effects …

For best effects, good coverage of weed leaves by paraquat spray is essential. However, run-off


The occurrence of surplus liquid (like rain) which originates up-slope and is collected beyond the ability of the soil to absorb it. The surplus liquid then flows away over the surface to reach the nearest surface water (pond, lake, river).

Authoritative On-line References and Resources

US Geological Survey's Water Science School
of excess spray solution would be wasteful, so care must be taken to apply the herbicide correctly.
  • Calibration: Any type of sprayer must be properly calibrated in order to apply the recommended amount of active ingredient. This involves taking into account ground speed, pressure and the specific nozzles being used. Pressure must also be regulated to avoid spray drift.
  • Nozzles: Flat fan nozzles will produce the most even distribution of spray droplet sizes. This is important because if droplets are too large they will bounce-off leaves and be wasted and if they are too small they may drift off-target. Nozzle shields are advisable to ensure no drift onto crop plants causing unsightly lesions.
  • Wetter: a non-ionic surfactant, mixed typically at 0.25% on a volume basis, is generally the best adjuvant for use with paraquat. The surfactant will allow spray droplets to wet and spread on the surface of leaves, giving good contact.
You can read more about best practices in spraying paraquat here.


The brand name of the leading paraquat product is Gramoxone.