Paraquat is a fast-acting, non-selective, post-emergent herbicide used to kill a variety of weeds and unwanted plants.
It is among the strongest herbicides, and it is considered to be highly toxic, so its use is strictly regulated in many countries, with some even banning it due to safety concerns.
Paraquat is not sold on its own but is found in some herbicides.
One of the most common brands that contain paraquat is Gramoxone. Other herbicides that include paraquat in certain amounts are Para-SHOT, Devour, Ortho Paraquat CL, and Blanco.
Before using any herbicide, it is important to check the label to see if it contains paraquat and follow strict safety precautions to avoid harm.
In this article, we will focus mainly on the usage, application, and restrictions of paraquat as contained in Gramoxone herbicide.
Contents
Active Ingredient
Paraquat is a chemical compound belonging to the bipyridinium family that has non‑selective properties.
As the active ingredient in Gramoxone herbicide, it constitutes up to 30.1% of the total formulation. The other ingredients account for 69.9%, bringing the total to 100%.
Gramoxone SL 2.0 contains 2.0 lbs (0.91 kg) of paraquat cation per gallon, which is equivalent to 2.762 lbs (1.25 kg) of paraquat dichloride per gallon.
Mode of Action
It is a contact herbicide that acts by disrupting photosynthesis in the plant.
When sprayed on the leaves, it penetrates the leaf surface and reaches the cells. Inside the plant cells, it diverts electrons from the photosystem, stopping the process altogether. This oxidative stress causes the affected plant tissue to die and desiccate quickly.
Due to its contact mode of action, you must make sure that the entire surface is thoroughly covered with herbicide, because it does not move through the plant, so only the areas that are covered in herbicide are affected.
When to apply it
For great results, you should apply this herbicide to weeds as soon as they emerge, while they are still small.
Weeds between 1 and 6 in (2.5 and 15.2 cm) tall are easiest to handle, while bigger weeds can be more difficult to control.
If the weeds have been grazed or mowed and have lost most of their green foliage, allow them to grow to 2 and 4 in (5.1 to 10.2 cm) before spraying if possible.
Is it rainfast?
This herbicide becomes rainfast within 15 to 30 minutes. After 30 minutes of application, even if it rains, its effectiveness will not be affected.
It remains effective in many types of weather. Cold temperatures below 55°F (12.8°C) and cloudy skies might slow down how fast it works, but they do not make it less effective.
What are the recommended application areas?
This herbicide can be applied in various row crops, specialty crops, and vegetable crops.
It is effective for weed control in crops such as alfalfa, corn, almonds, and cotton, as well as potatoes, tomatoes, and many more.
| Acerola | Alfalfa | Alfalfa (Dormant Season) |
| Almonds | Apples | Apricot |
| Arborvitae | Arracacha | Arrowroot |
| Artichoke, Chinese | Artichoke, Globe | Artichoke, Jerusalem |
| Ash | Asparagus | Atemoyas |
| Avocados | Bananas | Barley |
| Beans, adzuki | Beans, asparagus | Beans, black |
| Beans, broad | Beans, garbanzo | Beans, Kidney |
| Beans, lablab | Beans, lima | Beans, moth |
| Beans, Mung | Beans, Navy | Beans, Pinto |
| Beans, rice | Beans, snap | Beans, tepary |
| Beans, urd | Beans, wax | Beechnut |
| Biriba | Blackberries | Blueberries |
| Boysenberries | Brazil nut | Broccoli |
| Butternut | Cabbage | Cacao |
| Calamondins | Canistels | Canna, edible |
| Cantaloupe | Carrot | Cashew |
| Cassava | Catjang | Cauliflower |
| Cavalo broccolo | Chayote fruit | Chayote root |
| Chemical fallow | Cherimoyas | Cherries |
| Chestnut | Chickpeas | Chinese cabbage |
| Chinese waxgourd | Chinquapin | Chufa |
| Citron melon | Citron, citrus | Clover |
| Coffee | Collards | Conifers |
| Conservation Compliance Programs | Conservation Reserve | Corn |
| Corn, field | Corn, field (grown for grain) | Corn, field seed |
| Corn, popcorn | Corn, seed | Corn, sweet |
| Cotton | Cotton, Southern | Cotton, Western |
| Cowpea | Crown vetch | CRP |
| Cucumber | Currants | Custard Apple |
| Deciduous Trees | Dry beans | Dry Peas |
| Easter lilies | Eggplant | Elderberry |
| Elm | Endive | Escarole |
| Fallow land | Federal Set-Aside | Feijoa |
| Field beans | Figs | Filberts |
| Fir | Garlic | Gherkins |
| Ginger | Gooseberry | Gourds, edible |
| Grapefruit | Grapes | Grasses |
| Grasses, for seed | Groundcherries | Guar |
| Guava | Hickory nut | Hops |
| Huckleberry | IIama | Jaboticabas |
| Kiwi fruit | Kumquat | Lemons |
| Lentils | Leren | Lespedeza |
| Lettuce | Lime | Loblolly pine |
Weeds Controlled
With its strong action, it effectively targets a broad spectrum of weeds, from broadleaf grasses and perennial weeds to the hardest-to-control species like barnyardgrass, Johnsongrass, ryegrass, and sicklepod.
| Amaranth palmer | Annual grasses | Annual weeds broadleaf |
| Annuals winter | Barley little | Barley Volunteer |
| Barnyardgrass | Beggarweed Florida | Bluegrass |
| Bluegrass annual | Brachiaria | Broadleaf weeds |
| Broadleaf weeds (annual) | Brome rescue | Bromes |
| Cereals Volunteer | Cheatgrass | Chickweed |
| Crabgrass | Cutleaf groundcherry | Dogfennel |
| Downy brome | Fiddlenecks | Foxtail giant |
| Foxtails | Goosegrass | Grasses |
| Grasses (Annual) | Groundsel | Harvest Aid |
| Henbit | Japanese Brome | Johnsongrass seedling |
| London rocket | Panicum fall | Perennial weeds |
| Perennial Weeds (suppression only) | Pigweed | Preharvest aid to desiccate |
| Prickly lettuce | Prickly pear | Purslane |
| Ragweed common | Red rice | Redmaids |
| Ryegrass | Ryegrass Italian | Sedges |
| Shepherd’s-purse | Sicklepod | Small grain Volunteer |
| Sowthistle | Spiderwort | Spikeweed |
| Tansymustard | Texas millet | Weeds Annual |
| Weeds Broadleaf | Weeds woody | Wheat Volunteer |
| Wild oats | Witchweed |
Is a Surfactant Required?
According to the herbicide label, a nonionic surfactant or crop oil concentrate is required when using it. Otherwise, its performance may be reduced.
If you choose to add a nonionic surfactant for ground or aerial applications, it must contain at least 80% surface-active agent and be applied at a minimum rate of 0.25% v/v (2 pints/568 mL per 100 gallons) of the finished spray volume.
When using a crop oil concentrate, select a non-phytotoxic crop oil concentrate or methylated seed oil containing 15-20% approved emulsifier. Apply it at 1.0% v/v (1 gal/3.785 l per 100 gal/ 378.5 l) of the finished spray volume for ground applications. For aerial applications, use 1 pint (473 mL) of crop oil concentrate per acre.
Do not use crop oil concentrate when applying this herbicide to cotton crops.
Compatible Tank Mixes
The following PSI herbicide can be used as a tank mix with Paraquat for improved burndown. However, you must ensure that you follow each solution’s restrictions, limitations, and usage rates as stated on the label.
| AAtrex® Herbicide | Atrazine |
| Bicep Magnum® | Bicep II Magnum® Herbicide |
| Bicep Lite II Magnum® Herbicide | Boundary® 6.5EC Herbicide |
| Canopy® Herbicide | Caparol® 4L Herbicide |
| Cotoran® Herbicide | Lorox® Herbicides |
| Lorox Plus® Herbicide | Princep® Herbicide |
| Sencor® Herbicide | Sinbar® Herbicide |
| Spike® Herbicide |
How to Apply It
First things first, before even preparing anything, you must ensure that the weather conditions are favorable. You want to apply the herbicide on a calm, dry day to prevent the solution from drifting in the wind or being washed away by rain.
If the weather is fine, then:
1. Read the label
You must first read the label and the Safety Data Sheet (SDS) to understand how to equip yourself, handle it properly, mix it, and follow the application procedures.
2. Wear Protective Equipment (PPE)
When working with this herbicide, you need to wear proper protective equipment while applying it:
- Long-sleeved shirt and long pants
- Shoes plus socks
- Protective eyewear
- Chemical-resistant gloves made of: Barrier laminate, Butyl rubber (≥ 14 mils), Nitrile rubber (≥ 14 mils), Neoprene rubber (≥ 14 mils), Natural rubber (≥ 14 mils), Polyvinyl chloride (PVC) (≥ 14 mils), Viton® (≥ 14 mils)
- A NIOSH-approved particulate respirator with any N, R, or P filter (NIOSH approval number prefix TC-84A)
- Or a NIOSH-approved powered air-purifying respirator with an HE filter (NIOSH approval number prefix TC-21C)
When mixing and loading the herbicide, you need all of the above, plus a chemical-resistant apron and a face shield.
3. Prepare your workspace accordingly
Ensure you are in a well-ventilated area and have a clean, flat surface for mixing the herbicide.
4. Gather the necessary materials
Have the following items ready: a clean mixing container, measuring tools, water, surfactant, other herbicides, or other diluents.
5. Measure the herbicide
Follow the label instructions to measure the correct amount of herbicide for the area you plan to treat.
6. Mix the solution
- Fill the spray tank halfway with clean water or another approved carrier, such as clear liquid fertilizer.
- Add a nonionic surfactant to the tank.
- Stir gently but continuously.
- Add dry formulations first, followed by liquid formulations.
- Add the herbicide itself.
- Finally, add the crop oil concentrate or methylated seed oil.
- Fill the remainder of the container with water or the approved carrier.
7. Prepare the sprayer
Make sure your sprayer is clean and in good working condition. Calibrate it according to the manufacturer’s recommendations to ensure the correct output.
| Parameter | Nozzle Type | |
|---|---|---|
| Flat Fan | Flood | |
| Maximum Size | 8 | 15 |
| Spray Pressure (at nozzle) | 30-50 psi | 30-50 psi |
| Maximum Nozzle Spacing | 30” | 40” |
| Direction of Spray Pattern | Down | Down |
| Maximum Speed | 10 mph | 10 mph |
| Spray Overlap (at each edge) | 30% | 50% |
8. Apply the herbicide
Apply the mixture evenly, making sure to cover the entire surface of the weeds for the best results.
Application Rates
The application rate depends on the crop you want to treat. Each crop has its own recommended rate, which can also change based on the type of application you choose or the tank mix combination you plan to use.
| Crop | Gramoxone Rate per Acre |
|---|---|
| ALFALFA (New seedlings – California only) | 1.0–2.0 pt |
| ALFALFA (No-till or conventional planting) | 2.5–4.0 pt |
| ALFALFA (Dormant season on established plantings) | 2.0–3.0 pt |
| ALFALFA (Dormant season tank mix with Velpar® L herbicide) | 1.0–2.0 pt |
| ALFALFA (Between-cuttings treatment in established plantings) | Approximately 1.0 pt |
| ALMONDS (Directed Spray) | 1.25–4.0 pt |
| ARTICHOKE (Globe, Directed Spray) | 2.5–4.0 pt |
| ASPARAGUS (Preplant or Preemergence, Broadcast/Banded Over-Row) | 2.5–4.0 pt |
| ASPARAGUS (Preemergence to established plantings, ≥2 years old) | 2.5–4.0 pt |
| PEAS, DRY | 1.2–2.0 pt |
| BERRIES (Blackberries, Blueberries, Boysenberries, Currant, Elderberry, Gooseberry, Huckleberry, Loganberry, Raspberries) | 2.0–4.0 pt |
| CACAO | 2.0–4.0 pt |
| CASSAVAS & YAMS (Puerto Rico only, Shielded Post Directed Spray) | 2.0 pt |
| CHEMICAL FALLOW (Wheat and Annual Crop Rotations – various timings) | Approximately 2.0–4.0 pt (rate varies with weed height) |
| CLOVER AND OTHER LEGUMES (Established, Region A) | Approximately 2.0–3.1 pt |
| CLOVER AND OTHER LEGUMES (Established, Region B) | Approximately 1.0–2.0 pt |
| CLOVER AND OTHER LEGUMES (Fall-seeded stands) | Approximately 1.0–2.0 pt |
| Other Legumes (e.g., velvetbean, lespedeza, lupine, sainfoin, trefoil, vetch, crown vetch, milk vetch) | Approximately 0.75–1.2 pt |
| CORN – Field, Popcorn, Sweet Corn, Seed Corn (Preplant/Preemergence) | Varies by weed height: 2.0–2.5 pt (weeds 1–3″), 2.5–3.0 pt (weeds 3–6″), 3.0–4.0 pt (weeds >6″) |
| CORN – Tank Mixes for No-till/Reduced Till | Similar to preplant rates (approximately 2.0–4.0 pt) |
| CORN – Postemergence Directed Spray / Harvest Aid | Approximately 1.0–2.0 pt (Harvest Aid Broadcast: 1.2–2.0 pt) |
Strategies for Controlling Herbicide-Resistant Weeds
Some weeds can develop resistance to paraquat dichloride, especially if the same herbicide is used repeatedly. This can cause certain weed types, like horseweed, ryegrass, and goosegrass, to become harder to control.
To manage resistance, check the fields before applying herbicide to identify the weeds present and their growth stage.
It is also good to avoid using the same herbicide more than twice per season unless mixed with another type.
Another method is to combine herbicides with other strategies like crop rotation, mechanical weed control, and cover crops.
If the weeds are still hard to control, use a mix of herbicides with different modes of action and apply them at different times.
Restrictions and Safety Measures
Livestock must not graze in treated fields or eat treated plants because Gramoxone SL 2.0 is very toxic and can cause serious harm or even death. This product cannot be used in homes or public places like gardens, schools, parks, golf courses, or playgrounds.
For preplant and preemergence applications, it should not be applied to soils without clay minerals, such as peat, muck, pure sand, or artificial planting materials.
To control weeds better, seedbeds and plant beds should be prepared early so weeds and grasses have time to grow before treatment. Disturbing the soil as little as possible during seeding or transplanting can help stop new weeds from growing.
If the herbicide is sprayed over plastic mulch before planting, transplants may get damaged if they touch the treated surface. To prevent this, there should be enough rain or irrigation to wash off the product before planting.
Also, any weeds and grasses that grow after application will not be controlled, and unless stated otherwise, some crops may also be affected.
Daniel is an experienced farmer and agricultural technician specializing in soil management, irrigation systems, crops management, and the safe and effective use of herbicides, pesticides, insecticides, fertilizers, and other agricultural chemicals. 