What's inside
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A boom sprayer for a lawn mower is a tow-behind tank sprayer, usually 15 to 40 gallons, with a 12-volt pump that runs off the mower’s battery and a boom or boomless nozzle that covers a 6- to 15-foot swath, letting you treat a half-acre to several acres of lawn for weeds or fertilizer in one pass instead of walking it with a backpack. The right setup depends on how much area you spray, how much weight your mower can safely pull, and whether you need even broadcast coverage or mostly spot treatment.
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How a Tow-Behind Lawn Sprayer Works
Almost every unit built for riding mowers and lawn tractors shares the same layout:
- Tank on a small two-wheel cart with a pin hitch that drops into the mower’s rear hitch plate.
- 12V diaphragm pump wired to the mower battery with clips or a plug, usually delivering roughly 1 to 2.2 gallons per minute at up to 40 to 70 psi.
- On/off switch and pressure control, sometimes a gauge.
- Spray boom with two to seven flat-fan nozzles, or a boomless nozzle that throws a wide fan from a single point.
- Handgun wand on a hose for spot-spraying fence lines, beds and individual weeds.
Well-known names in this category include Fimco, NorthStar, Chapin and Agri-Fab, along with many store brands built from similar pumps and tanks.
Sizing: Tank, Swath and Mower Capacity
Water weighs about 8.34 pounds per gallon, and that number drives most of the decision. A full tank adds a lot of weight behind a machine that was designed to cut grass, not haul liquid.
| Tank size | Liquid weight when full | Typical swath | Good fit for | Typical price range |
|---|---|---|---|---|
| 15 gal | ~125 lb | 6–10 ft (boom) or spot spray | Up to about 1 acre, small rear-engine riders | $150–$350 |
| 25 gal | ~210 lb | 8–12 ft | 1–3 acres, standard lawn tractors | $250–$500 |
| 40 gal | ~335 lb | 10–15 ft | 3+ acres, garden tractors and zero-turns rated for towing | $400–$800 |
| 60 gal and up | ~500+ lb | 12–20 ft | Pasture and acreage; better suited to compact tractors and UTVs | $600–$1,200+ |
Check the tongue-weight and towing guidance in your mower manual. Many zero-turns are not designed for heavy towing, and a sloshing tank on a slope can push a light mower downhill. On hilly ground, run half-full tanks rather than oversizing.
Boom or boomless?
A traditional boom places nozzles every 20 inches or so at a fixed height, which produces the most uniform coverage and the least drift. Its weakness is obstacles: trees, fence posts and gates all catch the boom ends, and most booms only fold manually. A boomless nozzle has nothing to snag and handles irregular yards well, but its spray pattern is less even at the edges and more prone to drift in wind. For an open, rectangular lawn, a boom is the better tool. For a yard with trees and beds, boomless or a short boom plus the handgun is often more practical.
Calibrating: The Step Most Owners Skip
Product labels give rates per 1,000 square feet or per acre, but your sprayer’s output depends on pressure, nozzle size and how fast you drive. Without calibration you are guessing, and over-applying herbicide is how lawns get burned. The classic 1/128-acre method works because 128 fluid ounces equal one gallon, so the ounces you collect equal gallons per acre directly.
- Measure a course. For a boom, mark a distance in feet equal to 340 divided by your swath width. With a 10-foot swath, that is 34 feet.
- Time the run. Fill the tank with plain water and drive the course at your normal spraying speed and throttle. Note the seconds; repeat and average.
- Collect output. Parked, with the pump running at the same pressure, catch spray from one nozzle for that many seconds.
- Read the result. Multiply the ounces collected by the number of nozzles. That total in ounces equals your gallons per acre (GPA).
- Adjust. Drive slower or raise pressure to increase GPA; drive faster or lower pressure to reduce it. Small pressure changes do much less than speed changes.
You can also check the math with the standard broadcast formula: GPA = (GPM per nozzle x 5,940) / (mph x nozzle spacing in inches). For example, nozzles putting out 0.2 GPM, spaced 20 inches apart, at 4 mph give 0.2 x 5,940 / (4 x 20) = about 15 gallons per acre. Once you know your GPA, divide the label’s product amount per acre by it to find how much product goes into each gallon of water.
Ground Speed and Coverage Math
| Swath width | Speed | Area covered per minute | Time for 1 acre (no turns) |
|---|---|---|---|
| 8 ft | 3 mph | ~2,110 sq ft | ~21 min |
| 10 ft | 4 mph | ~3,520 sq ft | ~12 min |
| 12 ft | 5 mph | ~5,280 sq ft | ~8 min |
Real times run longer because of turns, overlap and refills, but these numbers show why a modest 25-gallon tank at 15 GPA covers well over an acre and a half per fill. Keep speed steady; a mower in high gear on rough ground bounces the boom and creates streaks.
Weed Sprayer vs Fertilizer Sprayer Use
Most people buying a weed sprayer for a lawn mower are applying selective broadleaf herbicides. These products are formulated for low rates and need even coverage, which favors a calibrated boom. Liquid fertilizers usually run at higher volumes and can corrode some fittings, so rinse thoroughly afterward. If you use non-selective herbicide (the kind that kills everything) anywhere near ornamental beds, consider a dedicated tank or at least a very thorough triple rinse with a tank cleaner before switching back to lawn work. Residue in a pump or hose is a common cause of mysterious dead strips.
Ownership Realities: What Wears and Fails
- Pump diaphragms and check valves are the main wear items. Debris and dried chemical cause most failures; a clean inlet strainer prevents many of them.
- Nozzles wear and spray heavier over time. Replace them when output varies by more than about 10% from new or from each other.
- Freezing cracks pump housings. Drain the system and run pump-safe antifreeze through it before winter.
- Battery drain. Pumps commonly draw a few amps; run the mower engine while spraying so the charging system keeps up.
- Tires and axles on cheaper carts are light-duty; pneumatic tires handle bumpy lawns better than solid plastic wheels.
Safety and Drift
The product label is the legal rate and the first source of truth. Spray when wind is light, generally under about 10 mph and steady, and avoid hot afternoons when some herbicides volatilize and drift onto gardens. Use the lowest pressure that gives a full fan pattern, because high pressure creates fine droplets that travel. Wear gloves and eye protection when mixing, mix outdoors on a surface where spills can be contained, and keep children and pets off treated grass for the time the label specifies.
Which Setup Fits Your Yard
| Your situation | Suggested setup |
|---|---|
| Under 1 acre, mostly spot weeds | 15-gal spot sprayer with handgun; boom optional |
| 1–3 acres of open lawn, broadleaf control | 25-gal with a 2- to 5-nozzle boom, calibrated |
| Yard with many trees and beds | 25-gal with boomless nozzle plus handgun |
| 3+ acres or pasture, capable garden tractor | 40-gal with wide boom, pneumatic tires |
| Steep slopes or light zero-turn | Smaller tank run half full, or a backpack sprayer for banks |
FAQ
Boom or boomless?
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