Pesticide Drift from Ground
Table of Contents
- Why be Concerned About Drift?
- Factors Affecting Drift
- Equipment to Reduce Drift
- Techniques to Reduce Drift
- Buffer Zones
- Legislation and Liability
- What to do if You Suspect Drift Damage
- Additional Resources
This Factsheet explains causes of spray drift, ways to reduce spray
drift, key steps if pesticide drift is suspected and the legal responsibility
of those who spray pesticides.
Pesticide drift is the aerial movement and unintentional deposit
of pesticide outside the target area. There are two forms of pesticide
- Particle drift is the movement of pesticide
droplets or solid particles outside the area being treated.
Coarser droplets move short distances and fall close to the point
of release. Finer particles (i.e. less than 200 microns) can remain
suspended on air currents for long periods of time and can be
carried far outside of the target area. For example, a 100
micron droplet takes 11 seconds to fall three metres in still
air, and will drift more than 20 metres in an 8 km/h wind.
- Vapour drift is the movement of pesticide
vapours outside the area being treated. Vapour drift is invisible
and can have a considerable impact. Vapours are created when spray
droplets evaporate both at the time of application and for some
time after the spray has dried on plant or soil surfaces. The
potential for vapour drift is more a product of the volatility
of the active ingredient, the formulation (e.g. esters) and environmental
conditions (e.g. hot and dry) than the equipment used.
Why Be Concerned About Drift?
The potential impact of pesticide drift includes:
- less product being deposited on the target, resulting in
- financial loss associated with wasted pesticide and time
- the risk of injury or damage to human health, susceptible plants
(e.g. adjacent crops), non-target organisms (e.g. wild and domestic
animals, pollinating insects), the environment or property (see
Figures 1 and 2).
There are also legal responsibilities and liabilities associated
with all these issues.
Figure 1. 2,4-D drift damage on grape leaves.
Figure 2. Glyphosate drift damage on a tomato
All spray operators need to be concerned about pesticide drift
and must make efforts to mitigate drift in all forms. While some
level of drift either during or after an application will always
occur, understanding the factors that affect it can minimize its
Factors Affecting Drift
The factors that cause drift are a combination of:
- environmental conditions
- spray practices and, to some degree
- the pesticide's physical and chemical properties.
The key environmental conditions are: wind velocity and direction,
temperature and relative humidity. The most important spray practices
involve the spray quality produced by the sprayer, the distance
between nozzle and target, and the travel speed of the sprayer.
Each factor is described below.
Wind Velocity and Direction
Air movement will always result in some spray being carried outside
the target area. The amount of spray drift increases significantly
with increasing wind speeds. To minimize drift:
- spray when wind speeds are light to moderate and moving away
from sensitive environmental areas
- never spray during periods of dead calm because spray remains
suspended in the air until the wind changes and potentially carries
- change sprayer settings if the wind increases during spraying
or halt the job until conditions improve. High or changeable wind
not only increases the chance for drift, it may also compromise
the application by making coverage inconsistent.
See Table 1 for general spraying recommendations related to wind
conditions. Visible indications, such as the movement of smoke or
rustling of leaves, only provide general guidance. Use a proper
wind gauge when spraying to measure and record wind speed at the
level of spray release.
Temperature and Relative Humidity
In general, do not spray when relative humidity is lower than 40
per cent and air temperature is above 25°C. This improves target
coverage and reduces the chance of drift due to temperature inversions
Table 1. Wind Conditions and Spraying
||0-2 km/h (0-1.25 mph)
||May lead to vapour drift where finer droplets
remain suspended in the air, prone to evaporation and drift
long after spraying is completed
||Smoke rises vertically
||Do not spray
||Wind direction is unpredictable and may indicate
||Direction keeps changing
||Do not spray
||2-3.2 km/h (1.25-2 mph)
||Suitable for spraying
||Direction shown by smoke
|Light to gentle breeze
||3.2-9.6 km/h (2-6 mph)
||Ideal for spraying
||Leaves rustle, wind felt on face, twigs in motion
||9.6-16 km/h (6-10 mph)
||Higher wind speeds pose the most obvious risk
of drift through, around or over the target
||Small branches move, raises dust
||Spray with caution or do not spray
Do not spray during periods of dead calm. These periods may occur
in early morning or late evening, when the temperature is usually
cooler and the relative humidity is typically higher. This combination
of factors can result in drift-sized droplets staying in the field.
When the wind picks up, these spray droplets can move away from
the target area, possibly causing injury to adjacent non-target
areas. This is of particular concern with certain herbicides when
they drift onto sensitive neighbouring crops.
Hot and dry conditions increase drift because droplets rapidly
evaporate and become fine droplets, vapour or particles of concentrated
pesticide. On hot days the land also warms, giving rise to upward
convection currents that carry droplets above the crop canopy during
spraying. Lateral air movements then transport these particles as
far as several kilometres from the target area.
Hydraulic nozzle tips are classified by their spray patterns, flow
rates and average droplet size; this is called "spray quality".
When the operating pressure changes, the average droplet size also
changes. For example, a nozzle that produces a medium spray at low
pressure would produce a finer spray when pressure is increased.
Generally, finer droplets provide better coverage but are more
likely to evaporate and drift. They slow down quickly once they
leave the nozzle opening, take a much longer time to get from the
nozzle to the target and do not penetrate dense canopies without
air assistance. Coarser droplets resist evaporation and, because
they have more momentum, are not easily deflected by wind. But they
are more likely to bounce and run off the target and provide less
Consider these two options carefully when selecting nozzles, operating
pressures and travel speeds.
Droplet Size Classification
The droplet size classification system (very fine to extremely
coarse) uses "volume median diameter" (VMD), which is
a manufacturer's term to describe the average droplet size produced
by a nozzle (see Table 2). This system was developed for flat
fan nozzles spraying into still air under specific
conditions and should not be used to classify cone-pattern nozzles,
such as those produced by disc-core or disc-whirl nozzles. Generally,
cone-pattern nozzles produce finer particles than most flat fans
used in horticulture because they operate at high pressure, encounter
shear from air assistance and are sprayed over larger distances.
Distance between Nozzle and Target Area
Once released, spray droplet size decreases rapidly through evaporation
and/or volatilization. As the release height above the target or
crop canopy increases, the potential for spray drift also increases.
The same holds true for the distance between an airblast nozzle
or vertical boom and the target.
See the impact droplet size and environmental conditions have on
particle drift. Developed by researchers with the United States
Department of Agriculture, "DRIFTSIM" is an easy-to-use
computer program that predicts drift distances of spray droplets
under a wide variety of conditions. It can be found at www.ars.usda.gov/services/software/download.htm?softwareid=252.
Table 2. Droplet Size Classification
||Volume Median Diameter (microns)
||highly susceptible to drift - not advised
||attaches to under-leaf surfaces, but tends to
remain in the air
||lands on stems and narrow leaves
||lands on large flat surfaces, like broadleaf weeds,
but may bounce
||wind tolerant, but will run off crop
Pesticide's Physical and Chemical Properties
Formulation and Adjuvants
While drift-reducing adjuvants are available, check to ensure they
are compatible with the pesticide in use. If not compatible, they
can change the spray quality. This could damage the crop, make coverage
uneven and/or reduce canopy penetration. Test a small area before
adopting on a large scale. As well adjuvants can break down and
become ineffective when used in tanks with paddle or hydraulic agitation.
It is preferable to use a product already formulated to reduce
drift. See Additional Resources at the end of this Factsheet for
a list of publications that provide more detail on chemical product
For example, low volatility amine formulations of acidic herbicides
are less prone to vapour drift while ester formulations have a higher
likelihood. If a drift-prone product must be used the product label
will include additional precautions. For example, avoid spraying
dicamba dimethylamine (a highly volatile amine) during high temperatures,
since vapour drift can occur at high temperatures (> 25°C)
within one to two days after application. Even the slightest drift
from dicamba can cause major damage to nearby sensitive crops.
Equipment to Reduce Drift
There are several ways to minimize the potential for spray drift.
Some involve adjusting equipment and others relate to the application
method. All require that the spray applicator be both aware of the
conditions that promote drift and be willing to make changes while
While there are many types of sprayers used in agriculture for
the ground application of pesticides, the two most often associated
with drift are the airblast sprayer and the horizontal
boom sprayer. Minimize spray drift by adjusting these sprayers
to produce a coarser spray quality at a minimal effective distance
from the target. How to make these sprayer adjustments are outlined
An airblast sprayer is a vehicle-mounted or vehicle-drawn device
that uses a high-speed stream of air to carry the spray from air-shear
or hydraulic nozzle tips into the target. The potential for drift
can be reduced by:
- adjusting settings to produce the minimal effective air speed
throughout the season. Depending on the crop, air should just
move the leaves on the far side of the canopy. Full air early
in the season is rarely appropriate
- increasing droplet size by
- using lower pressures (within the range indicated by the
- using air-induction nozzles or
- switching to disc-core (or disc-whirl) nozzles that produce
a coarser spray quality
- using deflectors to channel air into, not over or under, the
target (see Figure 3)
- using towers to reduce distance-to-target and direct air into
the target (see Figure 4)
- switching to a tangential (Figure 5), recycling (Figure 6),
multi-duct (Figure 7), or multi-fan sprayer, all of which reduce
- using foliage sensors that turn boom sections on and off to
match the size and shape of the canopy.
Horizontal boom sprayers
A horizontal boom sprayer is a vehicle-mounted or vehicle-drawn
device that sprays through a series of nozzles attached to a boom
that extends outward on one or both sides. The potential for drift
can be reduced by:
- maintaining a minimal effective boom height. This height is
nozzle-dependant, but generally 50 cm above the top of the target.
Follow nozzle manufacturer's guidelines
- using low-drift nozzles, such as air-induction, drift-guard,
turbo tee jet, etc.
- using drop-arms to bring the nozzles closer to the target crop
and match the contour of the canopy surface (Figure 8)
- using the appropriate spray angle. Wider spray angle nozzles
(e.g. 110°) create smaller droplets than nozzles with narrower
spray angles (e.g. 80°). Although smaller droplets generally
increase the chance of drift and reduce penetration in dense canopies,
the wider-angle nozzles allow the boom to lie closer to the target
- using air-assist to direct spray into the canopy (Figure 9)
- using shrouds to create physical barriers to drift (Figure 10)
Figure 3. Airblast sprayer with deflectors.
Figure 4. Airblast sprayer with towers.
Figure 5. Tangential sprayer.
Figure 6. Recycling sprayer.
Figure 7. Laser-scanning sensor-controlled air assisted
sprayer. (Photo courtesy of Dr. Heping Zhu, USDA Application Technology
Research Unit, Ohio)
Figure 8. Drop-arms on a horizontal boom.
Figure 9. Air-assist sleeve on a horizontal boom.
Figure 10. Shrouds on a horizontal boom.
Other categories of sprayer
Some sprayers are not easily categorized and can use a combination
of the previously described modifications. Examples of these other
sprayer categories include:
- vertical boom sprayers. Similar to a horizontal boom, but running
vertically and used to spray crops such as canes, bushes and trellised
- boomless sprayers, which use a cluster of nozzles or rotary
discs to create the spray pattern. Often the cluster nozzles create
large-sized droplets that can reduce spray drift, but the rotary
disc can also produce very small droplets and create a drift problem
- wiper/wick systems (Figure 11), which make mechanical contact
between crop and pesticide by wiping the stems and leaves in the
target area. Because there are no spray droplets involved with
this method, particle drift is not a problem.
Figure 11. Rope-style wiper/wick system.
Techniques to Reduce Drift
There are many techniques to minimize drift when using the equipment
previously described. Equipment modification on its own is not enough
to reduce incidents of drift. Spray application is only as good
as the spray applicator's methods. Review the best practices below
to help control drift.
- Use nozzles that produce the desired spray quality while minimizing
- Operate at the minimally-effective distance between the nozzle
and the target. Be sure to confirm spray coverage by placing water-sensitive
paper in the hardest-to-reach portions of the canopy. Do not operate
booms beyond the recommended height or pressure range specified
by the nozzle manufacturer.
- Do not use high pressures when making herbicide applications.
Lower pressure reduces the proportion of driftable fines.
- Where possible, use products less prone to drift.
- Correct calibration is key to ensuring all nozzles are discharging
the correct output with the optimal spray quality. The process
of calibration also includes confirming an ideal travel speed,
minimal effective distance-to-target and the correct positions
for deflectors, air-curtains and shrouds.
- Calibrate at least twice per season or whenever changes are
made to the spray equipment.
- Research shows that downwind shelterbelts (also called windbreaks)
can greatly reduce pesticide drift. Contact Conservation Ontario
to help plan a windbreak project, including the design, sourcing
the right tree species and sourcing potential funding.
- When spraying outside rows using an airblast sprayer turn off
the outward-facing boom and consider only spraying inward on the
last two downwind rows. Be aware that spraying from only one side
- Driving at a reasonable speed improves canopy penetration and
reduces the potential for drift. As travel speed increases, spray
can be diverted backwards into upward wind currents and vortices
behind the sprayer. This increases variability in spray deposit,
which is generally undesirable as it adds to drift. This effect
is amplified when driving into the wind because the shearing effect
increases the number of driftable fines, even when using coarser
- Reduce airblast airspeed (particularly early in the season when
there is not enough foliage to warrant the extra air) to greatly
reduce the potential for drift. To achieve this:
- reduce the power take-off speed (gear up - throttle down)
- use a lower fan gear or
- use a hydraulic motor to drive the fan.
Be sure to confirm coverage using water-sensitive paper and be
aware that erratic wind can make spray coverage uneven.
- Develop "Awareness Zones". For established plantings,
create a map of the crop and surround it with a 1.0 km spray drift
awareness zone. This is NOT a buffer zone. Survey the awareness
zone and identify any sensitive areas that could be affected by
spraying: e.g. neighbouring sensitive crops, endangered native
flora and fauna, waterways and wetlands, foraging bees and sites
of human habitation or activity. Prepare site-specific operating
instructions (including any of the techniques already mentioned)
for spraying near any sensitive area falling within the awareness
For example, notifying neighbouring greenhouse growers of an early
morning pesticide application gives them the opportunity to close
the vents, thus avoiding the possibility of drift into the greenhouse.
Generally, a buffer zone is the downwind distance separating the
point of direct pesticide application from the nearest boundary
of a sensitive habitat (see Figure 12). Look on the pesticide label
for any buffer zone requirements.
Health Canada's Pest Management Regulatory Agency has an online
spray drift calculator that allows applicators to modify the labelled
buffer zones as required by the product label, based on weather
conditions, the category of sprayer or droplet size. For more information,
see the Buffer Zone Calculator at www.hc-sc.gc.ca/cps-spc/pest/agri-commerce/drift-derive/calculator-calculatrice-eng.php.
Legislation and Liability
Anyone using pesticides is responsible for their safe application.
Where drift damages adjacent crops, insurance adjustors generally
ask the following questions:
- Was the damage to the applicator's own crop? If so, it is unlikely
that there will be coverage under any insurance policy
- Was the damage to a neighbour's property? If so, the applicator's
liability policy may respond
- Was the product being applied according to label directions?
The Pesticides Act requires that licensed spray applicators
carry a specialized liability insurance policy that provides appropriate
coverage for their business. Operators who work on a "for hire"
basis (e.g. a licensed spray applicator) or away from their own
farm operation will need additional coverage.
A normal farm insurance policy may not cover spraying done by licensed
spray applicators or those done in share-cropping arrangements.
It is important to consult with an insurance broker or agent to
ensure there is adequate coverage for any type of operation.
Section 29 of the act requires anyone who uses a pesticide that
causes an adverse effect to notify the Ontario Ministry of the Environment
(MOE). Should there be any impairment to the quality of life or
environment in the use of a pesticide, charges could be laid against
the applicator under Section 4.
Consult the Blue Pages in the local telephone directory or visit
the Ministry of the Environment's website at www.ontario.ca/environment
to locate the nearest district office. After hours, contact the
Pollution Hotline at 1-866-663-8477.
Figure 12. Defining buffer zones for common application methods.
What To Do If You Suspect Drift Damage
There are a series of steps to follow when damage caused by pesticide
drift is suspected.
Diagnose the problem
- Eliminate other possible causes for damage, such as disease,
insects, nutrient deficiency, herbicide carryover and environmental
- Herbicide drift, for example, often causes distinct patterns
in the affected crop. Greater damage is usually found near the
source, diminishing with distance. In contrast, patchy damage
is often caused by poor soil pH.
- Confirm there is evidence of a spray application. Look for wheel
tracks, weed symptoms, boom patterns and overlap on the headlands.
Look for spray evidence in neighbouring fields, lawns and ditches.
Contact the appropriate people.
- Talk with neighbours and/or nearby spray applicators. Ask what
was sprayed, when it was applied and who performed the application.
- Contact the nearest regional MOE district office or call the
Spills Action Centre at toll free: 1-800-268-6060. (Ministry locations
are available at www.ontario.ca/environment
or in the Blue Pages of the telephone directory). MOE Environmental
Officers can visit the site to take samples of plants and soil
- Affected parties should contact their respective insurance adjustors.
Document all details.
- Collect spray records for review. This includes products sprayed,
environmental conditions during all applications and details about
- Photograph suspected damage and date each photograph. Do this
several times throughout the season.
- Document yield loss from the damaged area and an undamaged area.
Choose a similar planting (e.g. same age, cultivar, rootstock,
etc.). For perennial crops (e.g. vineyards, orchards, asparagus,
berries) document the effects for several years after the damage
The methods and materials described in this Factsheet can reduce,
but do not eliminate pesticide drift. In areas near sensitive crops,
all reasonable precautions must be taken. Follow the pesticide label
instructions to mitigate drift. Extremely low, and often invisible,
amounts of spray drift can be very damaging to sensitive crops or
habitats, so it is essential to know and follow both the pesticide
label and the manufacturer's directions for proper use of the spray
- Best Management
Practices: Pesticide Storage, Handling and Application (Order
No. BMP 13).
Pesticide Education Program: Grower Pesticide Safety Course Manual.
- OMAFRA Factsheet, Six
Elements of Effective Spraying in Orchards and Vineyards
- OMAFRA Factsheet, How
Weather Conditions Affect Spray Applications (web only)
- OMAFRA Factsheet, Calibrating
- OMAFRA Factsheet, Adjusting,
Maintaining and Cleaning Airblast Sprayers
- Field Crop Protection
Guide, OMAFRA Publication 812
- Guide to Fruit Production,
OMAFRA Publication 360
- Guide to Weed Control,
OMAFRA Publication 75
- Crop Protection
Guide for Ginseng, OMAFRA Publication 847
- Guide to Ginseng
Production, OMAFRA Publication 848
- Crop Protection
Guide for Nursery and Landscape Plants, OMAFRA Publication
- Guide to Nursery
and Landscape Plant Production, Publication 841
- Turfgrass Management
Recommendations, OMAFRA Publication 384
This Factsheet was authored by Dr. Jason S.T. Deveau, Application
Technology Specialist, OMAFRA, Simcoe, and Denise Beaton, Crop
Protection Lead, OMAFRA, Guelph. Kristen Callow, Weed Management
Lead, OMAFRA, Ridgetown, Leslie Huffman, former Apple Specialist,
OMAFRA, Harrow, Mr. John Purdy, private consultant, Mr. Robert
DeBrabandere, farm claims specialist and staff of both the Ministry
of the Environment and the Ontario Pesticide Education Program
made valuable contributions.