What Herbicide Will Kill Johnson Grass?

Glyphosate remains the single most reliable herbicide for killing johnsongrass, including its underground rhizome network, but it is non-selective and will destroy most crops along with the weed. For fields planted with soybeans or cotton, selective grass-killing herbicides in the ACCase-inhibitor family, such as fluazifop, quizalofop, and clethodim, can knock back johnsongrass without harming the crop. In corn, nicosulfuron fills a similar selective role. The catch with all of these products is that johnsongrass has two lives: the visible shoots and the buried rhizomes that fuel regrowth. Choosing the right herbicide, applying it at the right time, and understanding which products actually reach the root system are what separate a temporary knockdown from lasting control.

Why Johnsongrass Is So Hard to Kill

Johnsongrass spreads both by seed and by an aggressive underground rhizome system. A single plant can produce tens of thousands of seeds in a season, and rhizome fragments as short as a few centimeters can regenerate into new plants. Research on rhizome biology shows that johnsongrass buds have no natural dormancy period at any time of year, meaning any piece of rhizome left in the soil is ready to sprout whenever conditions are right.¹ The optimal temperature for bud germination is around 30°C (86°F), but growth slows sharply below 15°C. That explains why johnsongrass surges in warm weather and why late-spring and summer applications matter most.

This dual spread system is the core problem. A herbicide that scorches the visible foliage but fails to move into the rhizomes will give you a clean-looking field for a few weeks before regrowth appears from below. The real question is not just which herbicide kills the top growth, but which one depletes the energy stored underground.

Glyphosate and Glufosinate Compared

If you are dealing with johnsongrass in a non-crop area, a fallow field, or a glyphosate-tolerant crop, glyphosate is the go-to choice. In greenhouse and field trials, glyphosate maintained excellent shoot control above 96% and was the only broad-spectrum herbicide that prevented johnsongrass regrowth from rhizomes after the plots were mowed down four weeks later.² Earlier greenhouse work confirmed that glyphosate reduced energy reserves (total nonstructural carbohydrates) in rhizomes by about 61%, helping explain why treated plants failed to bounce back.³

Glufosinate, on the other hand, looked nearly as good on the surface: it also hit above 96% shoot control. But when those plots were mowed and monitored for regrowth, johnsongrass came back in every glufosinate-treated plot.² The reason comes down to how these two herbicides move inside the plant. Glyphosate is systemic, traveling from leaves down into roots and rhizomes. Glufosinate is a contact herbicide that kills the tissue it touches but does not translocate well enough to drain the rhizome’s energy supply. Greenhouse data showed glufosinate reduced rhizome carbohydrates barely at all, while glyphosate slashed them substantially.³ Adding residual herbicides to glufosinate tank mixes did not improve the picture for rhizome control, either.

The practical upshot: if you are managing johnsongrass in a Liberty Link (glufosinate-tolerant) crop system, expect to need repeated passes or supplemental tactics, because glufosinate alone will not stop the rhizomes from regenerating.

Selective Herbicides for Soybeans and Cotton

In broadleaf crops like soybeans and cotton, you cannot spray glyphosate over the top (unless using a Roundup Ready variety), so grass-specific herbicides become your main weapons. These belong to the ACCase-inhibitor class, sometimes called graminicides because they kill grasses while leaving broadleaf plants unharmed. The postemergence herbicides most consistently effective against both seedling and rhizome johnsongrass in soybeans and cotton include fluazifop, quizalofop, and propaquizafop.⁴

Soybean trials comparing seven postemergence herbicides found that sethoxydim, fluazifop-P, haloxyfop, quizalofop, and clethodim all provided 70 to 90% johnsongrass control, while fenoxaprop and imazethapyr fell short.⁵ Among that group, quizalofop stood out in greenhouse work for its ability to deplete rhizome energy reserves by about 64%, preventing regrowth six weeks after treatment, similar to glyphosate’s performance.³

One thing to watch for in broadleaf crop fields is tank-mix interactions. When sethoxydim was combined with certain broadleaf herbicides, its performance against rhizome johnsongrass dropped, even though the same tank mix did not hurt seedling johnsongrass control.⁶ The lesson: if your johnsongrass is coming from rhizomes (which is usually the harder problem), be cautious about mixing graminicides with broadleaf herbicides before checking compatibility data for your specific products.

Options in Corn

Corn presents a different challenge because corn itself is a grass, so ACCase inhibitors like fluazifop and quizalofop would kill the crop along with the weed. Instead, corn growers rely on ALS-inhibiting herbicides, primarily nicosulfuron and primisulfuron, which are selective enough to spare corn while controlling johnsongrass.

Field trials at multiple sites showed that nicosulfuron was generally more effective than primisulfuron at controlling rhizome johnsongrass. The most consistent results came from 50:50 split applications, spraying half the total rate followed by the second half about two weeks later, rather than a single early or late pass.⁷ Greenhouse work confirmed that nicosulfuron prevented regrowth when applied to foliage, whereas primisulfuron-treated plants often regrew from the base.³

Interestingly, nicosulfuron did not dramatically reduce carbohydrate levels in the rhizomes the way glyphosate or quizalofop did, yet it still prevented regrowth in greenhouse trials. The mechanism seems to involve sustained inhibition of new growth rather than outright starvation of the rhizome, which may explain why split applications work better than a single shot: the second spray catches any buds that start to break after the first.

Getting the Timing Right

Herbicide timing is at least as important as herbicide choice. The general rule is to spray johnsongrass while it is young and actively growing, ideally at the three- to five-leaf stage. Research on multiple herbicides at different growth stages confirmed this pattern clearly. Nicosulfuron at a reduced rate provided 90% or better control of both seedling and rhizome johnsongrass when applied at the three- to four-leaf stage. Fluazifop-P maintained 90% or greater control of rhizome johnsongrass up through the five-leaf stage. Clethodim needed the higher label rate to hit 90% control of rhizome johnsongrass and only achieved it at the three- to four-leaf stage.⁸

Seedling johnsongrass was more forgiving on timing: fluazifop-P controlled seedlings at 90% or above through the seven-leaf stage, and clethodim did so even at the eight-leaf stage at reduced rates.⁸ The gap between seedling and rhizome response makes sense: rhizome-origin plants have a bigger energy reserve underground and can absorb more punishment before dying. When dealing with a field dominated by rhizome johnsongrass, earlier applications matter more, and cutting herbicide rates to save money is riskier.

Translocation studies also shed light on a point that sometimes confuses growers. There is a common belief that you should wait until johnsongrass is tall so the herbicide will “move down” into the rhizomes more effectively. Radiotracer work with one ALS-inhibitor showed that the pattern of herbicide translocation from the treated leaf did not differ between plants at 10, 30, and 60 cm heights.⁹ In other words, waiting for taller plants does not improve translocation, but it does give the rhizome network more time to expand and store energy, making control harder.

Pre-Emergence Herbicides

Stopping johnsongrass before it emerges can reduce the load that postemergence sprays have to handle. Trifluralin, a dinitroaniline herbicide incorporated into the soil before planting, has a long track record against johnsongrass seedlings. Greenhouse work showed that johnsongrass seedlings were killed when their first internode grew through trifluralin-treated soil.¹⁰

Rhizome pieces are harder to stop with pre-emergence herbicides because they can sprout from deeper in the soil profile. When small rhizome segments were planted within a trifluralin-treated soil layer, nearly all of them were killed. But when longer rhizome pieces were planted below the treated layer, the kill rate dropped dramatically, with only about 11 to 19% dying.¹¹ Longer rhizomes also survived at higher rates than short ones even within the treated zone, likely because they had more stored energy to push through. This means trifluralin works well as a seedling barrier but cannot be counted on alone to suppress established rhizome infestations, especially in fields where tillage has spread rhizome fragments deep into the soil.

The Growing Problem of Herbicide Resistance

Johnsongrass has developed resistance to several herbicide classes, which is reshaping how weed scientists think about long-term management. Three resistance patterns are worth knowing about.

ALS-inhibitor resistance has appeared in johnsongrass populations in European corn fields. Researchers in Greece found populations that were highly cross-resistant to four different ALS-inhibiting herbicides: foramsulfuron, nicosulfuron, rimsulfuron, and imazamox. Even rates well above the recommended label dose failed to control these populations. However, those same resistant populations were fully controlled by ACCase-inhibiting herbicides (propaquizafop and clethodim) at all rates tested, including rates as low as one-eighth the recommended dose.¹² This finding offers a clear backup plan for corn growers with ALS-resistant populations, though it also highlights the danger of losing an entire herbicide class in a single field.

ACCase-inhibitor resistance has also been documented, though the pattern is different. Researchers found johnsongrass populations in summer dicot crops (like sunflower and soybean) that were cross-resistant to the FOP-type ACCase inhibitors, including propaquizafop, quizalofop, and haloxyfop, with resistance levels eight to twenty-five times above normal susceptibility for some products. However, clethodim and cycloxydim, which belong to the DIM subgroup of ACCase inhibitors, showed only a mild shift in susceptibility with resistance factors of just two to three times normal.¹³ For growers who have been relying on FOP herbicides, clethodim may still work, but this is a narrowing window.

Glyphosate resistance in johnsongrass has also been confirmed. A study of a resistant biotype from Texas found no target-site mutations, meaning the weed’s glyphosate target enzyme had not changed. Instead, resistant plants accumulated roughly half as much glyphosate in their aboveground tissue at seven and ten days after application compared to susceptible plants.¹⁴ The researchers suspect an altered metabolic fate of glyphosate within the plant. The resistance level in this biotype was not extremely high, but even moderate resistance can translate into control failures at standard field rates, especially in dense stands.

Crop Rotation as a Herbicide Strategy

One of the most underappreciated tools for johnsongrass control is rotating crops specifically to rotate herbicide chemistries. A long-term study found that johnsongrass actually became the dominant weed after four years of continuous corn treated with the same triazine and urea herbicides year after year. But when the field was rotated into a corn-cotton-cotton-corn sequence that introduced trifluralin and MSMA in the cotton years, the johnsongrass infestation was effectively controlled.¹⁵ The rotation worked because the cotton-year herbicides attacked johnsongrass from a completely different angle than the corn-year herbicides, preventing any single biotype from prospering.

Given the expanding herbicide resistance problem, rotation is not just an economic nicety; it is becoming a necessity. A grower who plants Roundup Ready soybeans every year and sprays glyphosate every year is selecting for glyphosate-resistant johnsongrass. Alternating crops, and the herbicide programs that go with them, disrupts that selection pressure.

When Control Pays for Itself

Herbicide programs are not free, and johnsongrass does not always reach densities that justify spraying. Researchers developed an economic decision model for soybean growers that balances the cost of a postemergence herbicide application against the soybean yield loss that johnsongrass would cause if left untreated. The key insight is that the threshold at which control becomes profitable depends on johnsongrass density, soybean row spacing, herbicide cost, and expected soybean price.¹⁶ Soybeans planted in narrow rows tolerate johnsongrass better, because the crop canopy closes faster and shades the weed, so the threshold density before spraying pays off is higher than in wide rows.

For glyphosate-tolerant soybeans specifically, an economic comparison found that growers could increase their return by planting a glyphosate-tolerant variety and spraying glyphosate whenever the expected yield from a conventional variety treated with a graminicide program would fall below a calculated break-even point. The break-even yield differences ranged from about 67 kg/ha less to 202 kg/ha more than what would be achieved with glyphosate, depending on the graminicide used.¹⁷ In plain terms, glyphosate in a tolerant variety tended to be the most cost-effective option, but the margin depended heavily on the specific graminicide costs and local soybean prices. With glyphosate resistance now emerging, those calculations are shifting, and growers may increasingly need to factor in the cost of a second herbicide mode of action or a follow-up application.

Biological Control Research

Chemical herbicides are not the only avenue being explored. Researchers have investigated Gloeocercospora sorghi, a fungus that causes a disease specifically on johnsongrass and related sorghum species, as a potential bioherbicide. The challenge has been producing enough fungal spores to be practical at field scale. Laboratory work optimized a liquid culture medium for rapidly growing conidia of this fungus, using butter bean brine combined with mannitol and carboxymethylcellulose as key ingredients.¹⁸ This kind of work is still in the early stages, and no commercial bioherbicide for johnsongrass is widely available. But for areas where chemical resistance is narrowing the options, or for organic operations where synthetic herbicides are off the table, biological approaches could eventually become part of the toolbox. For now, though, the practical herbicide choices remain chemical, and the emphasis is on using them wisely enough that they keep working.