Set Your Location
Spray conditions vary significantly by location. Share your location or enter a ZIP for accurate wind, temperature, humidity, and inversion risk at your field.
Most herbicide labels call for spraying in a 3–10 mph wind, with temperatures under 85°F and relative humidity above 40%. Winds under 3 mph signal possible thermal inversion — fine droplets can ride stable air off-target for miles — while over 10–15 mph mechanical drift takes over. Mid-morning, after overnight inversions break and dew dries, is usually the safest window. Dicamba and 2,4-D ester can also volatilize off the leaf for hours once temperatures pass about 85°F. Always confirm the wind, temperature, and buffer limits on your product label before spraying.
Why These Conditions Matter
The Wind Window: 3–10 mph
Calm conditions under 3 mph allow fine droplets to hang in the air and drift unpredictably. Over 10 mph, mechanical drift carries droplets off-target. The 3–10 mph range is your safe zone.
Ideal: 3–10 mph · Consistent direction
Temperature & Volatilization
Heat causes certain active ingredients — particularly dicamba, 2,4-D ester, and clopyralid — to volatilize off the leaf surface hours after application. Above 85°F this off-gassing risk increases sharply.
Critical above 85°F for dicamba/2,4-D
Humidity & Droplet Evaporation
In low humidity (under 40%), fine spray droplets evaporate rapidly in flight. Match nozzle size to conditions — larger droplets resist evaporation but may reduce canopy penetration.
Below 40% RH — use larger nozzles
Thermal Inversions — The Hidden Danger
A thermal inversion forms when warm air sits above cooler air near the ground, trapping spray droplets in a stable layer. Indicators: smoke that flattens horizontally, dust that hangs sideways, early morning mist in low spots.
Can drift miles — no label override
Best Times of Day to Spray
Mid-morning (9am–noon) is typically the best window: overnight inversions have broken, temperatures are still moderate, winds are picking up, and morning dew has dried. Evening applications carry high inversion risk.
Best window: 9am–noon in most conditions
The Label is the Law — Always
Every pesticide label specifies wind speed limits, temperature restrictions, and mandatory buffer distances. This tool provides general guidance — but the product label is the legal requirement and the final word.
Label requirements supersede all general guidance
Understanding Thermal Inversions
A thermal inversion traps fine spray droplets in a shallow, stable air layer close to the ground. Those droplets ride horizontal air movement — potentially drifting miles before settling on sensitive crops, waterways, or residences.
WARM AIR (inverted layer)↑ lid
TRAPPED COOL AIR + SPRAY DROPLETS
Fine particles trapped · drift unpredictably in any direction
GROUND SURFACE (cooled)
How to Detect an Inversion in the Field
→Smoke test: If smoke rises then flattens and moves horizontally, an inversion is present — do not spray.
→Tractor dust: Dust that hangs and moves sideways rather than rising is a clear inversion sign.
→Ground fog: Low mist clinging to drainage areas indicates stable, non-mixing air.
→Time of day: Any calm, clear evening after sunset creates near-surface inversion conditions.
Do not spray when any inversion indicator is present, regardless of what the wind meter reads.
Dicamba Application Rules — Know Before You Spray
Dicamba drift complaints have resulted in billions in crop damage claims and ongoing regulatory pressure. These are EPA label requirements — violations carry civil and criminal penalties. Verify against your specific product label before every application.
Wind SpeedUnder 10 mph at application time. No gusty, variable wind conditions. Measure at boom height, not cab height.
Temperature2026 OTT labels: no application when the forecast high is 95°F or above. At 85–94°F, no more than 50% of your dicamba-tolerant soybean acres in a county may be treated. Volatilization can occur hours after application on hot days.
NozzlesOnly EPA-approved air induction nozzles (AITTJ, TTI, AI). Standard flat fan nozzles are a federal label violation. Verify against the approved nozzle list on your label.
Tank Mix — No AMSAmmonium sulfate and other ammonium-containing adjuvants are prohibited in OTT dicamba loads — they sharply increase volatility. A volatility-reduction agent (VRA) and drift-reduction agent (DRA) are required in every application.
Buffer Zones2026 OTT labels (Stryax, Engenia, Tavium) require a 240-ft downwind buffer, larger near listed endangered-species areas. Some label-approved mitigations (hooded booms, windbreaks) can reduce it. Confirm on your label and Bulletins Live! Two.
Application TimingApply only through R1 (beginning bloom), and only from 1 hour after sunrise to 2 hours before sunset. Never during a temperature inversion. Check your label for the exact cutoff.
Record KeepingAll three approved dicamba products require detailed application records including wind speed, direction, temperature, and nozzle type. Keep records for a minimum of 2 years.
Rain-Free Period by Herbicide Class
Rain-free periods are the minimum time needed after application for adequate product absorption. Always check your specific product label — these are general guidelines by class, not legal requirements. Systemic herbicides need more time to translocate through the plant.
| Herbicide Class / Example Products | Minimum Rain-Free | Optimal Dry Time | Notes |
|---|---|---|---|
| Contact — Glufosinate (Liberty) | 4 hrs | 4–8 hrs | Contact action — needs full leaf coverage. Rain before 4 hrs significantly reduces activity. Sunlight needed for activation. |
| Contact — Paraquat (Gramoxone) | 30 min | 1–2 hrs | Very fast uptake — most effective when absorbed before rain. Cell destruction begins within minutes of contact. |
| Systemic — Glyphosate (Roundup) | 4 hrs | 6–8 hrs | Formulation-dependent: some premium products claim rainfast in 30 min–2 hrs, generics run longer. When unsure allow 4+ hrs, more in cool, cloudy conditions. Your label is the authority. |
| Dicamba (Stryax, Engenia, Tavium) | 1 hr | 4 hrs | Check your specific label — some require 4 hrs minimum. Volatilization risk continues after rain if temps rise. |
| ALS Inhibitors — Sulfonylureas, IMIs | 1 hr | 2–4 hrs | Absorbed relatively quickly. Some sulfonylureas have soil activity that continues post-rain regardless. |
| PPO Inhibitors — Cobra, Flexstar, Cadet | 4 hrs | 4–8 hrs | Contact + some systemic activity. Efficacy drops sharply with early rainfall. Best when applied to dry canopy. |
| ACCase Inhibitors — Select, Poast | 1 hr | 2 hrs | Grass herbicides with rapid uptake. Relatively forgiving — rain after 1 hr typically maintains good performance. |
| Fungicides — Strobilurins, Triazoles | 1–2 hrs | 2–4 hrs | Systemic fungicides are absorbed quickly. Protectant fungicides (contact) need longer dry periods to form a residual film on the leaf. |
Spray Mix Calculator
Carrier rate is how much water you spray per acre (usually 10–20). Tank size is your sprayer's tank — set it to yours, even a small 3-point tank works. Enter each product's label rate and get exactly how much of every product and how much water goes in each tank — including the partial last tank so you don't over-mix — plus whole-field totals. Works in oz, pints, quarts, gallons, pounds, % v/v, and per-100-gallon rates.
Products
Match your label: most herbicides are per acre (fl oz / pt / qt / lb). AMS, surfactants (NIS/COC/MSO) and drift agents are usually per 100 gal or % v/v. Mixing two or more products? Run a jar test first.
Per tank
Calculator only — the product label is the law. Verify rates, carrier volume, and adjuvant compatibility against the label and a jar test before mixing. Add products to the tank in the correct order (see the Tank Mix Order guide below).
Sprayer Calibration Calculator
Are you really applying the rate you think? Check your actual gallons per acre from ground speed, nozzle spacing, and measured output — or work backward to the nozzle output a target rate needs. Uses the standard broadcast formula: GPA = 5940 × GPM ÷ (MPH × spacing).
Standard broadcast calibration (constant 5,940 for inch spacing). Catch each nozzle and average them — replace any nozzle more than 10% off the mean. Always confirm against your monitor and re-calibrate when you change nozzles, pressure, or speed.
Before you load the tank
How to run a jar test (compatibility check)
- Fill a clear quart jar with the same water you’ll spray with, scaled to your tank — for a 100-gal tank, about 1 quart is the right proportion.
- Add each product in your tank-mix order — conditioners/AMS first, then dry products, then liquids, then adjuvants — one at a time.
- Cap and gently invert 10 times after each addition.
- Let it stand 15–30 minutes.
- Look for clumps, gels, oily layers, crystals, sludge, or separation that won’t re-mix. If it looks bad in the jar, it will plug your boom — don’t load the tank.
Spraying near sensitive areas? Check the free DriftWatch / FieldWatch registry for mapped sensitive crops, beehives, and organic fields near you, and confirm your label’s downwind buffer before you start.
Tank Mix Order Guide
Getting the mix order wrong causes clumping, gelling, clogged nozzles, reduced efficacy, and crop damage. Always do a jar test first.
1
Water — Fill Tank 50–75%
Fill the spray tank halfway to three-quarters with clean water.
Starting with water ensures even dispersion and prevents incompatibility reactions.
2
Water Conditioners & AMS
Add ammonium sulfate (AMS) at 8.5–17 lbs per 100 gallons. Add pH buffers if water is hard or alkaline. Exception: never add AMS to an OTT dicamba load (Stryax, Engenia, Tavium) — the label prohibits it; use the required VRA + DRA instead.
AMS binds calcium and magnesium ions that tie up glyphosate and other weak-acid herbicides.
3
Start Agitation
Begin continuous mechanical or hydraulic agitation before adding any pesticide products.
Agitation keeps dry particles suspended and prevents settling and gelling.
4
Dry Formulations (WP, WDG, DF)
Add wettable powders and water-dispersible granules first among the pesticide products.
Dry formulations need the most water volume and agitation to disperse properly.
5
Liquid Flowables & Suspension Concentrates (SC)
Add suspension concentrates and flowable formulations. Shake containers first.
Flowables disperse better in water that already contains dissolved dry products.
6
Emulsifiable Concentrates (EC)
Add EC formulations dissolved in oil-based solvent with emulsifiers.
ECs create an oil-in-water emulsion; position minimizes compatibility risks.
7
Water-Soluble Liquids (SL, S)
Add soluble liquid formulations. These include many glyphosate formulations.
Soluble liquids mix readily and are least likely to cause compatibility issues.
8
Adjuvants, Surfactants & Crop Oils
Add NIS, COC, MSO, drift reduction agents, defoamers, and compatibility agents last.
Adjuvants affect the surface tension of the entire spray solution.
9
Top Off Water to Final Volume
Fill to target volume. Continue agitation. Begin spraying as soon as possible.
Mix only what you will spray that day.
✓ Always Do
Jar test first — combine small amounts in the correct order in a clear jar. Wait 15 minutes.Shake containers — flowables and SCs settle in storage.
Triple-rinse empties — that rinse water is product you paid for.
Record everything — products, rates, order, weather. Required for most certifications.
Never Do
Never mix concentrates directly — violent reactions, gelling, and equipment damage.Never skip agitation — some products separate in under 5 minutes.
Never add dry products to an oily mix — always add dries before ECs.
Never assume compatibility — even familiar mixes can react differently with a new water source.
Sprayer Tips & Tricks
Nozzle Selection Makes or Breaks Your Application
Air induction nozzles (AITTJ, AI, TTI) produce larger, air-filled droplets that resist drift and are required on most dicamba labels. Flat fan nozzles give better coverage for contact herbicides but produce more fines. Dual-fan nozzles split the pattern for better canopy penetration.
Rule of thumb: In wind over 8 mph or humidity under 50%, move up one nozzle size from your standard.
Speed & Pressure — The Drift Multipliers
Higher pressure creates smaller droplets, and bumping pressure up meaningfully increases the share of drift-prone fine droplets. Keep pressure at the low end of the nozzle manufacturer’s recommended range.
Target: 30–40 PSI for most flat fans, 40–60 PSI for air induction. Boom height: 18–20 inches above target for 110° nozzles.
Carrier Volume — More Water, Better Coverage
University extension trials generally find higher carrier volumes (15–20 GPA) improve both contact and systemic herbicide performance. More water means more droplets reaching the target and better canopy penetration.
Contact herbicides benefit most from 15+ GPA. Systemic herbicides are more forgiving at 10–15 GPA.
AMS — A Low-Cost Performance Booster
Ammonium sulfate binds hard-water cations that antagonize glyphosate, and improves glyphosate performance on tough weeds like waterhemp and Palmer amaranth even in soft water. Never add AMS to an OTT dicamba load — the label prohibits it.
Rates: 8.5–17 lbs dry AMS per 100 gallons. Always add AMS before herbicide.
Spray Timing — Weed Size Matters More Than Speed
Spraying 2-inch waterhemp works far better than spraying 6-inch waterhemp with the same product and rate. Scout fields and prioritize those closest to the weed size threshold.
Target weed height: Most POST herbicides are labeled for 2–4 inch weeds.
Adjuvant Selection — NIS vs. COC vs. MSO
Adjuvants are helpers added to the tank so spray sticks to and soaks into the leaf. NIS (nonionic surfactant) reduces surface tension at 0.25% v/v. COC (crop oil concentrate) penetrates waxy cuticles at 1% v/v. MSO (methylated seed oil) is required on most dicamba labels and provides both penetration and retention.
When in doubt, read the label. Using COC when the label calls for NIS can cause crop injury.
Tank Cleanout — Prevent Carryover Damage
Dicamba, sulfonylureas, and fomesafen bind to tank walls and hoses. A simple water rinse will not remove them. Clean thoroughly between every product change.
Cleanout protocol: Drain → Rinse → Fill 10% with water + tank cleaner, agitate 15 min, spray through boom, drain → Repeat rinse.
Spray Records — Your Legal Shield
Detailed spray records are legally required in most states and are your first line of defense if a neighbor alleges drift damage. Records also let you compare product performance across fields and years.
Record every application: Date, time, products, rates, carrier volume, nozzle, pressure, speed, wind, temp, humidity, field, crop stage, weeds, adjuvants, applicator license.
Boom Height — Every Inch Counts
Doubling your boom height above the target quadruples the area your spray can drift into. On rough fields, slow down — boom bounce at speed launches drift-prone fines.
Measure it: 18–20 inches for 110° nozzles, 24 inches for 80° nozzles.
Resistance Management — Rotate or Lose
Waterhemp, Palmer amaranth, and marestail now have populations resistant to 5+ herbicide groups in parts of the Midwest. Rotating modes of action is the only way to preserve the chemistry that still works.
Practical rotation: Use PRE residuals (Group 15 + Group 14 or 27) to reduce POST weed pressure.
Nozzle Type Comparison
Droplet size classification follows ASABE S572.3. Always verify against the specific product label.
| Nozzle Type | Droplet Size | Drift Risk | Coverage | Best For | Dicamba Approved |
|---|---|---|---|---|---|
| Air Induction (AITTJ, AI, TTI) | Ultra Coarse – Extremely Coarse | LOW | Moderate | Dicamba, 2,4-D, systemic herbicides, high-wind conditions | ✓ Yes |
| Turbo TeeJet (TT, TTJ) | Coarse – Very Coarse | LOW | Good | General POST herbicides, fungicides, balanced drift/coverage | Some models |
| Dual Fan (Guardian Air, TADF) | Coarse | MED | Very Good | Contact herbicides, canopy penetration in thick stands | Check label |
| Extended Range Flat Fan (XR) | Medium – Coarse | MED | Excellent | Contact herbicides, fungicides, insecticides in calm conditions | ✗ No |
| Standard Flat Fan (TP, DG) | Fine – Medium | HIGH | Excellent | Low-wind contact apps, maximum coverage needed | ✗ No |
Source: ASABE S572.3 standards. Dicamba approval based on the EPA-approved 2026 OTT labels for Stryax (formerly XtendiMax), Engenia, and Tavium. Always verify against your specific product label before application.
Seasonal Spray Calendar — Corn Belt & Upper Midwest
Application windows by month for corn and soybean producers. Timing and conditions shift year to year — use this as a planning reference and always confirm current conditions before application.
April
Burndown & Pre-Emerge Corn
Burndown glyphosate/2,4-D on cereal rye and winter annuals. Pre-emerge corn (atrazine, acetochlor). Variable temps — watch for frost. Inversion risk high on cold, calm nights.
PRE BURNDOWNMay
Pre-Emerge Beans & Post Corn
Pre-emerge soybean programs (PPO + chlorimuron). POST corn V2–V5. Wind and temp stabilizing. Watch for rapid temperature swings — prime inversion risk period in cool, dry springs.
PRE POSTJune
POST Soybeans & Residual Layby
POST soybean V1–V3 before canopy closure. Dicamba window (V2–R1). Waterhemp pressure peaks. Hot, windy days — afternoon applications high risk for dicamba volatilization.
POSTJuly
Corn Fungicide & Late POST
Corn fungicide window VT–R1. Late POST soybean before R1. Peak heat risk. Early morning applications critical — dicamba often prohibited above 85°F. Scout for escape weeds.
POST FUNGICIDEAugust
Soybean Fungicide & Layby
Soybean fungicide R3 if disease pressure warrants. Monitoring for late season escapes. Minimal herbicide opportunities as canopy is closed. Still watch inversion risk on warm, clear nights.
FUNGICIDESept–Oct
Harvest Aid & Cover Crop Burndown
Harvest aid soybeans (paraquat, sodium chlorate). Cover crop burndown post-harvest. Cool temps return — inversions common in calm fall nights. Watch for early frost timing and label restrictions.
BURNDOWNFrequently Asked Questions
The ideal window is 3 to 10 mph. Below 3 mph, calm air allows thermal inversions to trap fine droplets near the ground — they drift unpredictably. Above 10 mph, mechanical drift carries product off-target. Above 15 mph, most applications should stop. Dicamba labels legally require winds under 10 mph. Always measure at boom height, not in the cab.
An inversion occurs when warm air traps cooler air (and your spray) near the ground in a stable, non-mixing layer. Spray droplets can drift miles horizontally. Look for: smoke that rises then flattens, dust that hangs sideways rather than dispersing, or ground fog in low spots and drainage areas. Inversions are most common on calm, clear evenings after sunset. If you see any of these signs, do not spray — no label override exists for inversion conditions.
Under the 2026 OTT dicamba labels, no application is allowed when the forecast high is 95°F or above. Between 85 and 94°F, you may treat no more than 50% of your dicamba-tolerant soybean acres within a county. Above 85°F, dicamba can volatilize off the leaf surface hours after application — a secondary, invisible drift separate from mechanical drift, and it can travel farther. Check your specific product label.
Mid-morning — 9 a.m. to noon — is typically the best window. Overnight inversions have broken as the sun heats the ground. Temperatures are still moderate before afternoon peaks. Morning dew has dried. Winds are picking up into the ideal 3–10 mph range. Avoid evening applications: inversions form quickly after sunset on calm, clear evenings.
Rain-free periods vary by product class. Contact herbicides like glufosinate need 4+ hours. Glyphosate is formulation-dependent — some premium products claim rainfast in 30 minutes to 2 hours, generics often need 4–6. Dicamba labels typically require 1–4 hours — check your specific label. ALS inhibitors need 1–2 hours. See the rain-free period table above for a full breakdown by class.
Below 40% relative humidity, fine spray droplets evaporate in flight before reaching the leaf — reducing efficacy and increasing drift-prone particles. Below 30%, switch to larger droplet nozzles (coarse or very coarse classification). This tool will flag low humidity conditions in the factor breakdown.
Yes — it is a federal label requirement. All three EPA-approved dicamba formulations require air induction nozzles that produce ultra-coarse to extremely coarse droplets (ASABE classification). The AITTJ, TTI, and AI nozzle series are most commonly approved. Standard flat fan nozzles are a violation. The approved nozzle list is printed on your product label.
It depends on timing and amount. Check the rain-free period for your specific product (see table above). If you can complete the required dry window before rain arrives, you can proceed. The risk is if rain arrives earlier than forecast — always build in a buffer. Some herbicides need 6+ hours to translocate adequately.
Enter your carrier rate (gallons per acre), tank size, and each product's label rate — per acre, per 100 gallons, or % v/v — into the Spray Mix Calculator above. It returns exactly how much of every product and how much water goes in each tank, the partial last tank, and totals for the whole field. Always confirm against the product label.
Divide tank size by your carrier rate. At 15 gallons per acre a 1,000-gallon tank covers about 67 acres; at 20 GPA it covers 50 acres; at 10 GPA, 100 acres. The Spray Mix Calculator shows acres per tank automatically.
Ammonium sulfate is typically used at 8.5 to 17 pounds per 100 gallons of spray solution — the higher end for hard water or tough weeds like waterhemp and Palmer amaranth. For a 1,000-gallon tank at 17 lb per 100 gallons, that is 170 pounds. Add AMS before the herbicide. Exception: never use AMS in an OTT dicamba load (Stryax, Engenia, Tavium) — those labels prohibit ammonium adjuvants because they increase dicamba volatility.
Use the broadcast formula GPA = 5,940 × GPM ÷ (mph × nozzle spacing in inches). Enter ground speed, nozzle spacing, and measured nozzle output in the Sprayer Calibration Calculator above to get your actual gallons per acre, or work backward to the nozzle output a target rate needs. Catch each nozzle for one minute and replace any more than 10% off the average.
Multiply the per-acre rate by the acres one tank covers (tank size ÷ carrier rate). At 32 fl oz per acre and 67 acres per tank, that is about 2,133 fl oz, or 16.7 gallons, per tank. The Spray Mix Calculator converts and totals this for every product automatically.