What Selective Herbicide Kills Only Johnson Grass?

No single herbicide kills only Johnsongrass and nothing else, but several selective herbicides do an effective job of killing Johnsongrass while leaving specific crops unharmed. The right choice depends almost entirely on what crop or desirable vegetation surrounds the weed. In broadleaf crops like cotton and soybeans, grass-specific herbicides from the “fop” and “dim” chemical families are the go-to options, with fluazifop, sethoxydim, and clethodim providing 90% or better control in field trials. In corn, nicosulfuron is the standard pick. And in grain sorghum, the options narrow dramatically because Johnsongrass is so closely related to the crop that most herbicides harm both.

Why Johnsongrass Is So Hard to Kill

Johnsongrass (Sorghum halepense) spreads through both seeds and underground rhizomes, and the rhizomes are the real problem. A single plant can produce rhizomes that extend several feet in every direction, and fragments left behind after tillage can sprout into new plants. Research has shown that longer rhizome fragments grow faster and begin producing new rhizomes much sooner than shorter ones, meaning that even light cultivation can scatter pieces that quickly re-establish the weed.¹ This underground network is what makes a one-and-done herbicide application rarely sufficient. You need a product that is absorbed by the leaves, moves through the plant’s vascular system, and reaches those rhizomes underground to prevent regrowth.

The economic stakes are real, too. In cotton, each Johnsongrass plant per 15 meters of row reduced lint yield by roughly 30 to 43 kg per hectare, amounting to a 4 to 5% loss per weed.² That adds up fast in a field with heavy infestations, which is why growers have invested decades of research into finding selective solutions.

Graminicides for Broadleaf Crops

If you grow cotton, soybeans, or other broadleaf crops, you have the widest selection of selective herbicides for Johnsongrass. These products belong to two chemical families that target an enzyme found only in grasses, which means broadleaf plants are naturally immune to them.

The first family goes by the shorthand “fops” (aryloxyphenoxypropionates). Fluazifop and haloxyfop are the best-known members, and both have delivered 90% or greater Johnsongrass control in cotton trials that tracked results for over a year after application.³ Fluazifop-P-butyl (sold under brand names like Fusilade) is widely available for home and commercial use and is one of the most common recommendations for Johnsongrass in garden beds, landscapes, and broadleaf crop fields alike.

The second family, known as “dims” (cyclohexanediones), includes sethoxydim and clethodim. Multi-year cotton trials found that repeated annual applications of sethoxydim outperformed older arsenical herbicides and even glyphosate in reducing rhizome Johnsongrass populations over time while protecting cotton yields.⁴ Clethodim (sold as Select or Clethodim 2EC) is another popular choice in this family, and its performance improves substantially with the right spray additive, a point covered in more detail below.

Both fops and dims work by inhibiting the ACCase enzyme that grasses need to build fatty acids, so they kill Johnsongrass from the inside out while your broadleaf crop keeps growing normally. They are applied postemergence, meaning you spray them on actively growing Johnsongrass rather than treating the soil before it emerges.

Nicosulfuron and Other Options in Corn

Corn presents a different challenge. Corn is itself a grass, so ACCase-inhibiting graminicides would damage the crop. Instead, corn growers rely on ALS-inhibiting herbicides, particularly nicosulfuron (sold as Accent and other trade names). Nicosulfuron works because corn metabolizes the chemical quickly enough to avoid injury, while Johnsongrass cannot.

Field trials evaluating nicosulfuron with different spray nozzles and adjuvants achieved Johnsongrass biomass reductions ranging from about 83% to 97%, and adding a nonionic surfactant pushed that range up to 90% to 100%.⁵ Earlier Virginia field experiments also evaluated sulfonylurea herbicides for postemergence Johnsongrass control in corn under both no-till and conventional conditions, confirming that this herbicide class is the primary selective tool for corn growers dealing with the weed.⁶

One important caution with nicosulfuron in corn: certain corn hybrids, especially those with specific herbicide-tolerance traits or insecticide seed treatments, can show crop injury. Always check the product label against your specific hybrid before spraying.

The Grain Sorghum Problem

Grain sorghum is where selective Johnsongrass control gets genuinely difficult. Johnsongrass is a wild relative of grain sorghum, and the two are so closely related that most grass-killing herbicides cannot tell them apart. The ACCase-inhibiting graminicides that work beautifully in cotton and soybeans will damage or kill grain sorghum. A recent trial in India confirmed this directly: both quizalofop-ethyl and fluazifop-P-butyl caused phytotoxicity in sorghum, while only tembotrione (an HPPD-inhibiting herbicide) provided acceptable Johnsongrass suppression without serious crop injury.⁷

Even with tembotrione, the Johnsongrass control efficiency in that trial was around 71%, well below the 90%+ you can achieve with fops or dims in broadleaf crops. The development of ALS-inhibitor-resistant grain sorghum varieties, such as Inzen sorghum, has opened the door to using ALS herbicides in sorghum, but this strategy comes with its own complications. A four-year study in Texas and Arkansas found that Johnsongrass density and soil seedbank size dropped dramatically when herbicide-resistant sorghum was combined with multiple management tactics, including preemergence and postemergence herbicides, desiccant application before harvest, harvest weed seed control, disking, and follow-up graminicide treatment of regrowth.⁸ In other words, herbicides alone were not enough in sorghum; stacking several practices together made the real difference.

Herbicide Resistance Is Already Here

One of the biggest threats to selective Johnsongrass control is the weed’s ability to evolve resistance to the very herbicides growers depend on. This is not hypothetical: resistant populations have already been confirmed for both major herbicide classes used against it.

In Greek cornfields where ALS-inhibiting herbicides like nicosulfuron had been used repeatedly, researchers found Johnsongrass populations carrying a point mutation that made their ALS enzyme insensitive to the herbicide. Resistant plants were homozygous for a specific amino acid substitution (Trp-574 to Leu) in the ALS gene.⁹ Follow-up molecular work confirmed the same mutation pattern, with resistant individuals consistently carrying the altered gene while susceptible plants did not.¹⁰ Because Johnsongrass is tetraploid (carrying four copies of each chromosome rather than two), resistance can establish quickly once a favorable mutation appears in even one gene copy.

The ACCase-inhibiting fops and dims are not immune to this problem either. Researchers documented Johnsongrass populations in summer broadleaf crops that had developed target-site resistance to ACCase inhibitors through a different mutation, the first confirmed report of this specific resistance mechanism in the species.¹¹ If you have been using the same graminicide on the same field for several years and notice declining effectiveness, resistance should be high on your list of suspects.

The practical takeaway is to rotate herbicide modes of action whenever possible and not rely on any single chemical year after year. Pairing chemical control with mechanical and cultural methods helps slow resistance development.

Application Tips That Actually Matter

Herbicide performance on Johnsongrass is surprisingly sensitive to how you spray, not just what you spray. Two factors stand out in the research: adjuvants (spray additives) and application timing relative to Johnsongrass growth stage.

Adjuvants are substances added to the spray tank to improve herbicide uptake. For clethodim, field experiments showed that crop oil concentrates, methylated seed oils, and certain blended surfactants significantly improved control of rhizome Johnsongrass compared to standard nonionic surfactants.¹² The mechanism is straightforward: Johnsongrass leaves have a waxy cuticle, and oil-based adjuvants help the herbicide penetrate that barrier. Older research on dalapon (a now less common herbicide) found that even a 1% surfactant concentration improved Johnsongrass control at both moderate and high temperatures.¹³ Similarly, nicosulfuron performance in corn jumped when a nonionic surfactant was included, with Johnsongrass biomass reduction climbing from as low as 83% to as high as 100%.⁵

Timing matters because Johnsongrass is most vulnerable when it is actively growing and has enough leaf area to absorb a lethal dose, yet has not grown so tall that it shades lower leaves or begins diverting energy to seed production. For most postemergence graminicides, the label recommends treating when Johnsongrass is 12 to 18 inches tall. Spraying too early misses late-emerging flushes; spraying too late lets rhizomes store enough energy to survive. In heavy infestations, a split application (an initial spray followed by a second treatment a few weeks later on regrowth) often outperforms a single heavy dose.

Rope-Wick Applicators for Mixed Stands

When Johnsongrass grows taller than the surrounding crop, a rope-wick or wiper applicator offers an elegantly simple form of selectivity. The device applies a concentrated herbicide solution (usually glyphosate) only to the vegetation it physically touches. Because Johnsongrass often towers above cotton, soybeans, or other crops, passing a wiper over the canopy at crop height applies herbicide to the Johnsongrass without contacting the shorter crop below.

Field trials comparing rope-wick applicators in cotton found that glyphosate-treated plots contained about 83% fewer live Johnsongrass shoots than cultivated control plots within a few weeks of each application. Cotton yields in the treated plots averaged roughly 81% more seed cotton than the controls.¹⁴ This approach turns a non-selective herbicide like glyphosate into a functionally selective one through physical placement rather than chemistry. It is particularly useful in situations where you need to deal with herbicide-resistant Johnsongrass that no longer responds to the selective graminicides.

Biological Control Research

The idea of using pathogens as “bioherbicides” against Johnsongrass has been explored for decades but remains largely experimental. Three fungi native to Florida, Drechslera gigantea, Exserohilum longirostratum, and E. rostratum, showed promising results in greenhouse trials. A mixture of the three pathogens caused 83 to 100% disease severity across seven grass species including Johnsongrass, and none of the 52 nontarget plant species tested were harmed, including 21 crop species.¹⁵ A separate study tested Bipolaris species isolated from Johnsongrass in North Carolina and found that one isolate caused 52% mortality to annual Johnsongrass in Mississippi field conditions with minimal damage to corn or grain sorghum.¹⁶

The catch is that none of these have made it to commercial availability. A review of Johnsongrass diseases noted that some of the proposed biocontrol agents later turned out to cause disease in at least some cultivars of grain sorghum. Others ran into the rhizome problem: they damaged the above-ground plant, but the Johnsongrass simply regrew from its underground reserves.¹⁷ The concept of using multiple pathogens together, to broaden the spectrum of attack and reduce the risk of the weed evolving resistance, is sound in theory but has not cleared the practical hurdles of mass production, formulation stability, and field reliability.

Choosing the Right Herbicide by Situation

Because the answer depends entirely on context, here is a practical breakdown of which selective herbicides fit which situation:

• Cotton and soybeans: Fluazifop-P-butyl, sethoxydim, or clethodim applied postemergence. Use a crop oil concentrate or methylated seed oil adjuvant for best results. Plan for a follow-up application on regrowth from rhizomes.
• Corn: Nicosulfuron applied postemergence with a nonionic surfactant. Check hybrid compatibility before spraying, and rotate to a different mode of action in subsequent years to slow resistance.
• Grain sorghum: Tembotrione in conventional sorghum, or ALS herbicides in ALS-resistant sorghum varieties. Pair with mechanical and harvest-time tactics for meaningful long-term reduction.
• Gardens and landscapes: Fluazifop-P-butyl (Fusilade) or sethoxydim (Vantage) sprayed directly on the Johnsongrass foliage. These will not harm flowers, shrubs, or broadleaf ground covers.
• Mixed grass stands: No truly selective chemical option exists that kills Johnsongrass but spares other grasses, since they share the same vulnerable enzymes. A rope-wick applicator with glyphosate is the best workaround when the Johnsongrass grows taller than desirable grasses.

Whichever product you choose, the label is the law. Rates, re-entry intervals, and crop rotation restrictions vary by product and by state, so read the fine print before you mix anything in the tank.

When Johnsongrass Is Growing Among Other Grasses

The hardest scenario for selective control is a lawn, pasture, or native grassland where you want to remove Johnsongrass without killing the surrounding turf or forage grasses. The biology works against you here: every selective herbicide that kills Johnsongrass targets metabolic pathways shared by all grasses. There is no chemical that sees Johnsongrass as different from bermudagrass, fescue, or bahiagrass at the molecular level.

Your options in this scenario come down to timing, placement, and patience. In warm-season pastures, you can sometimes exploit the fact that Johnsongrass emerges and grows vigorously earlier in the spring than some other warm-season grasses, creating a window where spot treatment with glyphosate hits Johnsongrass while the desirable grass is still dormant or just beginning to green up. In cool-season grass settings, the reverse timing can work: treat in late summer when Johnsongrass is at peak growth but cool-season grasses are semi-dormant and more tolerant of incidental contact.

Mowing alone will not eliminate Johnsongrass, but frequent mowing at a low height can reduce its ability to photosynthesize and weaken the rhizome reserves over time. Combining repeated close mowing with occasional spot herbicide applications on the most persistent clumps is a realistic long-term strategy for pastures and rights-of-way. The four-year integrated management study in grain sorghum showed the same principle at work in a crop setting: stacking multiple tactics drove Johnsongrass density down far more than any single herbicide program could.⁸ Persistence matters more than product choice when you are fighting a perennial weed with an extensive rhizome system.