Surfactants: When You Need One, Which Type, and How Much

What surfactants actually do

A surfactant (surface active agent) is a molecule with a hydrophilic (water-attracting) end and a hydrophobic (water-repelling) end. When added to a water-based spray solution, surfactant molecules orient themselves at the water-air interface, reducing the cohesive force between water molecules — this is surface tension reduction.

In practical terms: plain water on a grass leaf beads up into droplets because of high surface tension — the water molecules prefer to cling to each other rather than spread across the waxy leaf surface. Add a surfactant, and the droplets flatten out, forming a thin film that covers far more surface area. That increased surface area contact is what allows the herbicide dissolved in the solution to be absorbed through the cuticle rather than sitting on top of it and eventually evaporating.

The secondary function of surfactants is to improve uptake through the cuticle itself. The waxy cuticle layer covering leaf surfaces is lipophilic — it repels water-based solutions. Oil-based surfactants (MSO, COC) can partially dissolve the cuticle wax and act as a carrier, moving the herbicide molecule through the barrier and into the leaf tissue. This is why oil-based surfactants dramatically outperform water-based NIS on species with thick, waxy cuticles.

NIS: non-ionic surfactant — the general-purpose standard

NIS is a water-soluble, electrically neutral surfactant that reduces surface tension without adding significant oil-based penetration. It is the most widely used adjuvant in turf herbicide applications and the default choice when a label says to use a surfactant without specifying the type.

Common NIS products include Tactic (Syngenta), Class Act (WinField United), Activator 90, Induce, and any product labeled as non-ionic surfactant for agricultural or turf use. Look for a product that states non-ionic surfactant and lists polyoxyethylene as the active component. Concentration typically ranges from 70–90% surfactant actives.

NIS works well for most broadleaf herbicide applications in cool-season turf — Trimec, SpeedZone, T-Zone, and Tenacity all call for NIS when a surfactant is needed. It improves coverage and droplet spread without the penetration aggression of oil-based adjuvants, making it the safer choice for applications in warm conditions or on stressed turf where increased absorption could also increase injury risk.

• Best for: Tenacity, Trimec, SpeedZone, T-Zone, most broadleaf herbicides
• Rate: 0.25% v/v — 1 tsp (0.17 oz) per gallon of spray solution
• Products: Tactic, Class Act, Activator 90, Induce, and labeled NIS products
• Do not use for: quinclorac applications (requires MSO), or when the label specifically requires an oil-based adjuvant
• Turf safety: excellent — NIS at 0.25% adds minimal stress to cool-season grasses

MSO: methylated seed oil — the penetrant for tough-to-control species

MSO is an oil-based adjuvant produced by methyl-esterifying vegetable oil (typically soybean or canola). The methyl-esterification process produces fatty acid methyl esters (FAME) that are highly effective at penetrating waxy plant cuticles — far more so than NIS. MSO dissolves into the cuticle wax layer and carries the herbicide molecule through it, dramatically improving uptake on species that would otherwise limit absorption.

The most critical use case for MSO in turf is quinclorac (Drive XLG) applications for post-emergent crabgrass control. The Drive XLG label requires MSO specifically — NIS is not an acceptable substitute. Crabgrass (Digitaria spp.) has a thick, waxy cuticle that NIS cannot penetrate adequately. Studies show 20–40% reduction in crabgrass control when NIS is used instead of MSO with quinclorac, which explains the widespread experience of quinclorac applications that simply do not work.

MSO is also the appropriate adjuvant for many corn and soybean herbicides, some broadleaf herbicides applied in hot, dry conditions, and applications targeting weeds with thick or waxy cuticles in general. Check the label first — if it specifies MSO or a crop oil adjuvant, do not substitute NIS.

• Best for: quinclorac (Drive XLG), grassy weed herbicides on waxy-leaved species, labels that specifically require MSO
• Rate: 1% v/v — approximately 1.3 oz (2.5 tablespoons) per gallon of spray solution
• Products: Helena MSO, Loveland MSO, Brandt MSO, and products labeled as methylated seed oil
• Do not confuse with: COC (crop oil concentrate) — similar but not identical; verify label compatibility
• Increased turf injury risk: MSO increases absorption for all compounds, including the herbicide's turf-toxic fraction — do not use MSO with products that only call for NIS

COC: crop oil concentrate — a close relative of MSO

Crop oil concentrate (COC) is a petroleum-derived or paraffinic oil-based adjuvant used extensively in row crop agriculture. It functions similarly to MSO in improving uptake through waxy cuticles and is often called for on the same types of products — post-emergent grassy weed herbicides, sulfonylurea herbicides applied in less-than-ideal conditions, and others.

The key difference between COC and MSO is the base oil. MSO uses methyl-esterified vegetable oil (FAME), while COC uses paraffinic mineral oil blended with emulsifiers. Both are effective penetrants. For turf applications, MSO is generally preferred when labels specify an oil-based adjuvant, because MSO's vegetable oil base is more compatible with the chemistry of modern turf herbicides. COC can be used interchangeably with MSO in many situations, but verify the herbicide label before substituting.

For homeowners, MSO is usually easier to source at farm supply stores and garden centers. COC is more commonly found at agricultural supply outlets serving corn and soybean producers. Either will work for quinclorac applications when MSO is specified, as long as you verify the COC product is labeled for that use.

Silicone surfactants: ultra-spreading agents used sparingly

Silicone surfactants (also called organosilicone or trisiloxane surfactants — product examples include Silwet L-77, Sil-Bond, and similar) are the most aggressive spreading adjuvants available. They reduce surface tension far below what NIS or MSO can achieve, creating a spray droplet that spreads across the leaf surface almost completely flat within seconds of contact.

This ultra-spreading property is valuable in very specific situations: applying herbicides to small-leaved or narrow-leaved weeds where surface contact is limited, improving canopy penetration in dense turf, and reducing the required spray volume to achieve coverage. In research applications, silicone surfactants improve uptake of systemic herbicides significantly.

In cool-season turf practice, silicone surfactants require careful handling. Their aggressive spreading means the herbicide is also absorbed more completely into the turf tissue — increasing both efficacy on weeds and injury risk on the grass. They are used at very low rates (0.05–0.1% v/v, or roughly 1/4 teaspoon per gallon) because exceeding the correct rate causes the spray to spread beyond the target area and increases phytotoxicity risk. For most homeowner applications, NIS is sufficient and safer — silicone adjuvants are a professional tool for specific use cases.

• Best for: applications requiring maximum leaf coverage, narrow-leaved weeds, canopy penetration
• Rate: 0.05–0.1% v/v — approximately 1/4 tsp per gallon (much lower than NIS)
• Products: Silwet L-77, Sil-Bond, Break-Thru, and labeled organosilicone surfactants
• Increased injury risk: aggressive spreading increases uptake into turf as well as weeds
• Avoid: in hot weather, on stressed turf, at rates above label recommendation
• General recommendation: use NIS unless you have a specific reason to need silicone spreading

Why dish soap is wrong

Dish soap (or any household detergent) is an ionic surfactant — it carries an electrical charge (typically anionic, negatively charged). Ionic surfactants are incompatible with many herbicide formulations because the charged surfactant molecule can bind to herbicide molecules, alter their chemistry, or cause the spray solution to become unstable (phase separation, precipitation, or foaming).

Beyond compatibility issues, dish soap is not formulated for plant interactions. Products like Dawn and Palmolive are designed to break down fats in water, and they accomplish this by being aggressive to lipid-based structures — including the lipid-based plant cell membrane and cuticle. At sufficient concentration, dish soap is phytotoxic. At low concentrations, it provides inadequate surface tension reduction compared to NIS.

The perception that dish soap works as a surfactant in herbicide applications comes from the fact that adding any surfactant usually improves on no surfactant. A small amount of dish soap does reduce surface tension somewhat and can improve droplet spread compared to unsurfacted water. But it underperforms NIS in efficacy, introduces ion compatibility risk, and can cause turf spotting or injury — particularly in hot conditions. Proper NIS is inexpensive (a quart costs $10–15 and lasts for years at normal use rates) and universally compatible with turf herbicides. There is no practical reason to substitute.

When the label specifies the surfactant type vs. when it does not

Label language for adjuvants falls into four categories, each requiring a different response.

Category one: the label specifies no surfactant and does not mention one. This means the formulation is designed for use without a surfactant — the product may contain built-in adjuvants, or the application method does not require additional spreading. Adding a surfactant is not necessarily harmful but may not be labeled and can increase injury risk in some cases. Respect the label's silence.

Category two: the label says to use a surfactant or adjuvant without specifying type. Default to NIS at 0.25% v/v. This covers most three-way broadleaf herbicides and many general turf applications.

Category three: the label specifies MSO, crop oil, or a specific adjuvant type. Use exactly what is specified. This is where quinclorac, most post-emergent grass herbicides in corn, and some turf-specific products fall. The label specification reflects real chemistry — substituting will produce worse results.

Category four: the label provides a menu of acceptable adjuvants (MSO or NIS, for example). Choose based on the target weed and conditions. For tough, waxy-leaved weeds in favorable temperatures, use MSO. For general broadleaf weeds or conditions where injury risk is a concern, use NIS.

• No adjuvant mentioned: use none — the product is designed without it
• 'Add a surfactant': NIS at 0.25% v/v
• 'MSO required' or 'crop oil concentrate': match exactly as specified
• 'MSO or NIS': choose MSO for waxy targets in ideal conditions, NIS for stressed turf or broadleaves
• Always read the adjuvant section of the label before mixing — requirements are often in the 'Directions for Use' section, not the front panel

Surfactant rates and cool-season turf safety

Surfactant rate is not arbitrary. Too little provides inadequate coverage improvement. Too much increases the phytotoxicity of every compound in the spray solution, including the herbicide. The standard rates are research-derived from efficacy and selectivity optimization — follow them.

For NIS at 0.25% v/v: this equals 0.0025 gallons per gallon of spray, or 1 teaspoon (approximately 0.17 oz) per gallon. At this rate, NIS improves coverage and absorption without materially increasing herbicide uptake into cool-season turf beyond the selectivity threshold for any standard turf herbicide that calls for NIS.

For MSO at 1% v/v: this equals 0.01 gallons per gallon, or 1.28 oz (approximately 2.5 tablespoons) per gallon. MSO at this rate penetrates waxy cuticles effectively on target grassy weeds. In cool-season turf, 1% MSO is within the safe range for most established grasses under normal growing conditions (60–80°F, adequate moisture). In hot, dry weather above 85°F or on drought-stressed turf, even standard MSO rates can cause transient turf discoloration or injury. When conditions are marginal, consider reducing MSO to 0.5% v/v and assess the lawn before full broadcast application.

For applications in hot weather (above 80°F), the practical guidance is to avoid oil-based adjuvants when possible or reduce rates. Heat accelerates cuticle absorption and can push uptake past the selectivity threshold even on established, healthy turf. Morning applications on calm, moderate-temperature days consistently produce the best efficacy-to-safety ratio for surfactant-requiring herbicides.

• NIS standard rate: 0.25% v/v — 1 tsp per gallon
• MSO standard rate: 1% v/v — 1.3 oz (2.5 tbsp) per gallon
• COC rate: typically 1% v/v — same as MSO; verify label
• Silicone surfactant rate: 0.05–0.1% v/v — 1/4 tsp per gallon maximum
• Hot weather (>80°F): reduce oil-based adjuvant rates by 50% or delay application
• Drought-stressed turf: avoid MSO and COC entirely until turf has recovered with irrigation