Tank Mixing Herbicides: What's Safe, What Works, and What to Never Combine

Why tank mixing is worth doing correctly

Tank mixing two or more pesticide products into a single application is standard practice in professional lawn care and increasingly common among serious homeowners. The primary benefit is efficiency: one pass covers both pre-emergent weed control and post-emergent broadleaf knockdown simultaneously, saving time, fuel, and labor.

The secondary benefit is broader spectrum control. No single product covers every weed in every growth stage. A spring application combining prodiamine (pre-emergent for crabgrass) with a three-way broadleaf herbicide (Trimec or SpeedZone for dandelion, plantain, clover) addresses both warm-season annual grass weeds and established broadleaves in a single tank.

The risk is real but manageable: incompatible products can form precipitates that clog nozzles, pH conflicts can degrade one or both active ingredients before they reach the plant, and some combinations cause phytotoxicity to turf that neither product would cause alone. The protocols below prevent all of these problems.

The jar test: the non-negotiable compatibility check

Before mixing any unfamiliar product combination in a full tank, run a jar test. This is the standard compatibility protocol and takes 15 minutes.

Fill a quart jar with water at the same temperature and hardness as your tank water. Add each product in the order you plan to mix them in the tank, using proportional amounts (if you plan to use 2 oz of product A per gallon in the tank, use 0.5 oz in a quart jar). Shake gently after each addition.

After adding all products, let the jar sit for 15–30 minutes without shaking. Then observe: any precipitation (solids settling to the bottom), clumping, layering of the mixture, or excessive heat generation indicates an incompatibility. A compatible mix looks uniform, stays in suspension with gentle agitation, and shows no unusual changes.

Products that fail the jar test should not be mixed. Precipitates that form in solution don't re-dissolve — the affected active ingredients are no longer available for plant uptake, and the precipitate will clog nozzles.

• Jar test passes: clear, uniform mixture with no settling after 30 minutes
• Jar test fails: precipitation, clumping, layering, excessive heat, color changes
• Always test before first use of any new product combination
• Re-test if you change water source, add a new product to an existing mix, or change product lots

Safe and effective combinations: pre-emergent + post-emergent broadleaf

The most common and well-established tank mix in residential lawn care is a pre-emergent herbicide plus a post-emergent three-way broadleaf product. The classic version combines prodiamine 65 WDG (0.5–0.75 oz per 1,000 sq ft) with Trimec Classic or SpeedZone EW at the labeled broadcast rate.

Prodiamine is a dinitroaniline pre-emergent that prevents crabgrass, foxtail, and other warm-season annual grasses from germinating. The three-way broadleaf product handles existing dandelion, plantain, chickweed, and other cool-season broadleaves simultaneously. This combination is chemically compatible, delivers no turf phytotoxicity at labeled rates on cool-season grasses, and represents a single pass that would otherwise require two separate applications.

Dithiopyr (Dimension) and pendimethalin (Scotts Halts) are also commonly tank-mixed with broadleaf post-emergents and are equally well-established as safe combinations. Verify the specific label for each product, but these combinations have been standard practice in professional lawn care for decades.

• Prodiamine 65 WDG + Trimec Classic: standard spring combo, cool-season safe
• Prodiamine 65 WDG + SpeedZone EW: same coverage with faster broadleaf knockdown
• Dithiopyr 40 WSB + three-way broadleaf: pre-emergent with post-emergent action on some weeds
• Pendimethalin + Trimec: established combination, follow label rates for both
• Always verify turf safety for your specific grass species on both labels

Broadleaf herbicide + surfactant: when it helps and how much to use

Non-ionic surfactant (NIS) is the most common tank mix additive and one of the most impactful for improving herbicide performance. Surfactants reduce the surface tension of the spray droplet, allowing it to spread across and adhere to the waxy leaf cuticle rather than beading off. On waxy or hairy-leaved broadleaves like clover, wild violet, and oxalis, a surfactant can be the difference between partial control and full knockdown.

Standard rate for NIS with broadcast broadleaf applications is 0.25% v/v — that's about 1 teaspoon per gallon, or roughly 1 oz per 3 gallons. More is not better: excess surfactant can cause the spray droplet to run off the leaf before absorption, and at high rates can cause phytotoxicity on turf.

Some herbicide formulations — especially emulsifiable concentrate (EC) and solvent-based products like SpeedZone EW — already contain a surfactant package in the formulation. Adding additional NIS to these products is often unnecessary and can increase phytotoxicity risk. Check the label; if it says 'surfactant included' or 'do not add additional surfactant,' follow that instruction.

What not to mix: pH conflicts, incompatible chemistry, and physical problems

The most common chemical incompatibility is a pH mismatch between tank mix partners. Different pesticide formulations are stable within specific pH ranges. 2,4-D amine and MCPP are most stable and active at pH 5–7. Glyphosate (Roundup) is most effective at pH 4–6 and degrades rapidly in alkaline water above pH 7. Mixing an alkaline product with glyphosate in hard water can cut glyphosate efficacy by 30–50% before it reaches any plant.

Organophosphate insecticides (chlorpyrifos, malathion) should not be mixed with alkaline materials, including some wettable powder formulations and liquid fertilizers. Alkaline hydrolysis degrades organophosphates rapidly. If you're applying an insecticide with a fertilizer, check the pH of the fertilizer solution before combining.

Wettable powders and water-dispersible granules (WDG) mixed with emulsifiable concentrates (EC) sometimes produce physical incompatibility — the emulsion breaks and the mixture separates. The jar test catches this reliably. In general, add WDG and WP products before EC products in the mixing order, as the EC carrier oils can interfere with the dispersion of particulate products.

• Never mix: glyphosate with hard, alkaline water — buffering required
• Never mix: organophosphate insecticides with alkaline products
• Caution: EC formulations with WDG or WP products — jar test required
• Caution: products specifying different minimum water volumes — dilution conflicts
• Caution: two products with the same active ingredient at full rates — risk of overdose
• Always check: both labels for explicit tank mix restrictions before mixing

Fertilizer and herbicide combinations: timing and product conflicts

Liquid fertilizer and herbicide combinations are common in commercial lawn care but require careful management. The primary concern is phytotoxicity: fertilizer salts combined with herbicide can increase the rate of foliar uptake of both products, potentially injuring turf even at labeled rates for each individual product.

The safest approach for homeowners is to apply fertilizer and herbicide separately, with at least 5–7 days between applications. This prevents any additive phytotoxicity and lets you assess turf response to each product independently.

If you intend to combine liquid fertilizer with a herbicide, use a low-salt liquid fertilizer (such as a slow-release amino acid or fulvic acid-based product) rather than a high-salt solution grade fertilizer. Keep both products at the lower end of their respective label rates. Avoid this combination during heat stress or when the lawn is under any other stress.

Dry granular fertilizer should never be mixed into a spray tank. The obvious practical issue is that it won't dissolve properly. The timing issue is that granular fertilizer needs to be watered in, while most herbicides need to stay dry on the leaf surface for 24–48 hours after application — the two applications are fundamentally incompatible in technique.

The WALNA mixing order

Mixing order has a significant effect on compatibility and product stability in the tank. The correct sequence is WALNA: Water, Agitate, Liquid concentrates, Non-ionic surfactant, Add last items (such as ammonium sulfate or pH buffers).

Start with at least half your final water volume in the tank before adding any product. Agitate with the pump or agitator running before and during product addition. Add liquid concentrates and EC formulations next. Add NIS or other adjuvants after the active ingredients are in solution — surfactant added first can create excessive foam before product has dispersed. Add any final items like ammonium sulfate (AMS) or pH buffer last.

WDG and wettable powder formulations should be pre-slurried in a small amount of water before adding to the tank — add them to a cup of water and stir until fully dispersed, then pour the slurry into the tank. This prevents clumping and dry material floating on the surface.

• W — Water: fill tank to 50% of final volume first
• A — Agitate: run pump or agitator before adding product
• L — Liquid concentrates (SL, EC, EW formulations): add while agitating
• N — Non-ionic surfactant: after liquid concentrates are in solution
• A — Add last: AMS, pH buffer, water-dispersible granules pre-slurried
• Fill to final volume with remaining water while agitating continuously

pH buffering: when it matters and how to do it

Municipal and well water varies widely in pH, and many herbicides are sensitive to alkaline conditions. In hard water (high calcium and magnesium content) with pH above 7.5, some herbicides undergo alkaline hydrolysis — a chemical breakdown process that degrades the active ingredient before it can be absorbed by the target plant.

Glyphosate is the most pH-sensitive common lawn herbicide — efficacy drops measurably in pH above 7, and hard water cations (calcium, magnesium) bind to the molecule and reduce uptake further. For any glyphosate application in hard water, add ammonium sulfate (AMS) at 1.5–2 lbs per 100 gallons of spray solution. AMS buffers pH and competes with hard water cations for the glyphosate molecule, restoring full efficacy.

For most broadleaf herbicides (2,4-D, triclopyr, MCPP), pH sensitivity is less acute but still real. A spray tank pH of 5.5–6.5 is optimal for most products. You can check tank pH with a simple test strip or pH meter and adjust with citric acid-based buffer products (LpH, Indicate 5) if needed.

• Optimal spray tank pH for most herbicides: 5.5–6.5
• Glyphosate: add AMS at 1.5–2 lbs per 100 gallons in hard water
• Check tank pH with a test strip — takes 30 seconds
• Acidify alkaline tanks with citric acid buffer (LpH, Indicate 5, pH Down)
• Add buffer before product — adjust pH first, then add herbicide