What farmers are spraying depends entirely on the season, but if you’re seeing activity right now, you’re likely witnessing one of a handful of routine applications: herbicides to prevent weeds, fertilizers to feed crops, fungicides to protect against disease, or insecticides targeting specific pests. Each time of year brings a different set of priorities, and understanding the seasonal rhythm of farm spraying can help you make sense of what’s happening on the fields around you.
Spring: Weed Prevention and Fertilizer
Spring is the busiest spraying season on most farms. Before crops even emerge from the soil, farmers apply pre-emergent herbicides to stop weeds from competing with young plants. These products work by creating a chemical barrier in the top layer of soil that kills weed seedlings before they break through. Common active ingredients include pendimethalin, prodiamine, and dithiopyr, which target grassy weeds like crabgrass. Lawn care companies use many of these same chemicals on residential turf, so if you see spraying in both farm fields and neighborhood lawns around the same time, that’s why.
Nitrogen fertilizer is the other major spring application. Corn, wheat, and other grain crops need heavy nitrogen feeding early in the season, and farmers often apply it as a liquid spray rather than spreading dry granules. This is typically what you’re seeing when a sprayer moves across a field that already has small green plants growing in rows.
Timing matters enormously in spring. For corn, certain herbicides must go on before the plants reach 8 to 12 inches tall. For soybeans, the window is tied to specific growth stages. Once soybeans start flowering, several common herbicides can no longer be legally applied. Farmers are often racing the calendar during this period, which explains why you might see sprayers running from dawn to dusk.
Summer: Fungicides and Insect Control
Once crops are established and growing through June, July, and August, spraying shifts toward protecting plants from disease and insects. Fungicide applications are common on corn, soybeans, wheat, and especially fruit crops. Orchards may be sprayed on a 10- to 14-day cycle throughout the summer, with shorter intervals during rainy stretches since moisture promotes fungal growth. Brown rot on stone fruits and scab on apples are two of the primary targets.
Insect pests also peak in summer. Farmers scout their fields and spray insecticides when pest populations cross economic thresholds, meaning the bugs would cause more crop damage than the spray costs. This is targeted work, not blanket coverage. In orchards, summer cover sprays handle multiple threats at once: a single pass might address both fruit-damaging beetles and caterpillar-type pests like the Oriental fruit moth.
Fall: Harvest Aids and Desiccants
In late summer and early fall, you may notice spraying on crops that look nearly ready to harvest. These are harvest aid chemicals, applied to speed up the drying process so crops can be combined cleanly. Cotton is a prime example. Farmers apply defoliants to strip the leaves off cotton plants, making mechanical harvest easier and reducing moisture and debris in the picked cotton. Desiccants work even faster, killing and dehydrating leaves within one to several days.
Paraquat is one of the more commonly used desiccants. It rapidly dries out leaves but can stick them to the plant rather than causing them to fall off, so the timing and rate have to be precise. Farmers typically wait until 80% or more of cotton bolls are already open before applying it, because spraying too early prevents remaining bolls from developing. Sodium chlorate is another option that can function as either a desiccant or a defoliant depending on how much is applied.
On grain crops like wheat and sunflowers, a similar drying application helps even out moisture across a field so the whole crop can be harvested at once rather than in stages.
Winter: Dormant Sprays on Orchards
If you live near fruit orchards, you’ll see spraying even in winter. Dormant sprays are applied to bare trees between late fall and early spring to knock out overwintering insects and fungal spores before they become a problem. The typical program involves three products applied at different times: copper around late November, sulfur in early January, and horticultural oil at least two weeks after the sulfur.
Horticultural oil is a highly refined petroleum product (up to 99.9% pure) that smothers insects and their eggs by coating them in a thin film. Sulfur handles fungal diseases like apple scab and peach leaf curl. Copper acts as both a fungicide and bactericide. These three cannot all be mixed together safely. Copper and sulfur should never share a tank, and neither should sulfur and oil, because the combinations can damage tree bark.
What Organic Farmers Spray
Organic farms spray too, just with a different set of approved materials. Copper-based fungicides are the backbone of organic disease management, used on everything from tomatoes to cucumbers. Several formulations of copper hydroxide and copper octanoate carry organic certification. Insecticidal soap, made from potassium salts of fatty acids, handles both soft-bodied insects and powdery mildew on crops like squash and lettuce.
Biological fungicides are increasingly common in organic systems. These contain live bacteria (primarily strains of Bacillus) that colonize plant surfaces and outcompete disease organisms. They work preventatively rather than curatively, so organic growers apply them on a regular schedule before diseases appear. The products are broadly effective against leaf spots, blights, mildews, and root rots, though they generally require more frequent application than synthetic alternatives.
How Weather Dictates Spray Timing
Farmers can’t spray whenever they want. Wind, temperature, and atmospheric conditions all dictate when it’s safe to pull the trigger. The ideal wind speed for spraying is 3 to 10 mph, a gentle, steady breeze that moves the spray predictably. Above 10 mph, too much product drifts off target. Surprisingly, very calm conditions under 3 mph can be just as risky. Light winds are erratic and shift direction without warning, making drift unpredictable.
Temperature inversions are the other major hazard. These occur when a layer of cool air near the ground gets trapped beneath warmer air above it. Spray droplets released during an inversion don’t disperse normally. They hang in the cool air layer and can drift long distances, sometimes miles, before settling. Inversions are most common in the early morning and late evening. Farmers typically wait for temperatures to rise at least 5 degrees after sunrise before spraying, which breaks up the inversion layer.
Buffer Zones Near Homes and Water
Federal rules require farmers to maintain an Application Exclusion Zone around active spray equipment. For ground sprayers using fine droplets, that zone extends 100 feet in all directions from the spray nozzle. For sprayers producing medium or larger droplets from a height above 12 inches, the zone shrinks to 25 feet. If anyone enters that zone, the applicator must stop spraying immediately, even if the person is on a public road or neighboring property. This rule was updated in 2024 to clarify that the exclusion zone applies regardless of property boundaries or easements.
Many individual pesticide labels impose additional buffer distances from water bodies, schools, or residential areas that can be significantly larger than the general exclusion zone. These label-specific requirements are legally binding. If you’re concerned about spraying near your home, the product label is the controlling document, and applicators are required to follow it exactly.
Ground Sprayers, Planes, and Drones
The equipment you see tells you something about the application. Tractor-mounted boom sprayers are the most common setup for row crops like corn and soybeans. They run nozzles along a horizontal bar that can stretch 60 to 120 feet wide, covering large acreage efficiently while keeping the spray close to the ground where drift is easier to manage. Ground sprayers provide better coverage of the upper canopy of tall crops like corn.
Aerial application by plane or helicopter covers ground faster and works on fields too wet for heavy equipment. It’s common for fungicide applications on large acreage and for mosquito or pest control over wide areas. Drones are a newer option gaining traction, offering high efficiency and precision for smaller fields or hard-to-reach areas. They’re particularly popular in specialty crops and orchards where maneuvering a full-size aircraft is impractical.