Author: Jason Deveau

Pressure Changes Spray Angles
When we consult a nozzle catalogue we are interested in the flow and droplet sizes produced at a given pressure. Perhaps we should also consider the effect of pressure on spray angle. We have several articles discussing the collective impact of spray overlap, nozzle spacing and boom height on coverage uniformity (Check here and here for example). However, we don’t really address the fact that fan angle is not a constant. This may be more relevant with the growing adoption of spot sprayers.
To illustrate the potential for fan angle variation, we assembled a collection of red, flat fan nozzles (‘04s) from several manufacturers. We plugged each nozzle into a spray pattern table, set the regulator at a given pressure, and photographed the spray angle and flow distribution. This process was repeated for each nozzle at seven different pressures within the manufacturer’s approved range of 20-80 psi. After digitizing the photos, we measured the spray angle using a digital protractor.
We anticipated a concomitant increase in spray angle as the pressure increased. This is not news. Anyone who has operated a sprayer has seen the spray pattern open up as the boom fills and pressurizes. Bear in mind this was only performed once (i.e. n=1), so while it illustrates trends it shouldn’t be mistaken for a rigorous scientific comparison. Further, this demonstrates a static situation and not a dynamic one where travel speed, wind conditions and the vortices from the sprayer it self will influence matters.
We saw similar trends with nozzles other than 110˚ fans, but let’s focus on 110˚s due to their current popularity.
Fan angles for five common 110 degree AI flat fans over their manufacturer-recommended pressure range
The spray angle for 110˚ nozzles ranged from 75˚ at 20 psi to approximately 143˚ at 80 psi. One nozzle failed to reach 110˚ at any pressure. Conversely, there was another that was over 110˚ at nearly all pressures. Ideally, spray nozzles should be operated around the middle of their manufacturer-recommended operating range. Three of the nozzles tested came close to 110˚ at that median pressure, but only the TeeJet AIC110-04 measured 110˚ at the middle of its recommended range (~50 psi).
Using that nozzle as an example, let’s look at the pressure, spray angle and subsequent distribution of flow along the swath at three different pressures. At 20 psi, the spray angle was 85˚. The yellow balls are floats that reflect flow as a series of cross sections of the swath. We see that aside from the tapered edges (which illustrate the need for 100% overlap between neighbouring nozzles) the distribution was fairly even. One of the priorities in nozzle design is to ensure a low coefficient of variability over the operating pressure range. In other words, the length of the swath may change, but the spray quality and uniformity in that swath is still within spec. At 50 psi the nozzle produced the expected 110˚ fan, and the spray distribution remained even. At 80 psi, the angle spread out to 125˚, spanning a greater distance, but it started to produce a less-even distribution.
Photographs of spray angle and distribution for the TeeJet AIC110-04 at the extreme low, middle and highest pressures of its recommended pressure range.
When fan angle changes with pressure, it can have significant implications. Nozzle spacing on a boom varies from sprayer to sprayer. Generally 50 cm (20 inch) centres are the standard in North America, but we’ve seen 15″ and even 10″. Nozzle spacing and boom height collectively determine the degree of spray overlap. Excessive overlap isn’t a problem, although additional nozzles do mean added expense, cleaning time and potential for plugging. Conversely, gaps in the pattern could lead to sub-lethal applications or flat-out misses. For example, in this soybean demo plot (below) we sprayed a contact herbicide at low pressure to collapse the spray pattern. You can see the alternating stripes of hits and misses that resulted from an incomplete overlap of spray.
Soybean demo plot sprayed with a contact herbicide using 110˚ air induction flat fans at 20 psi. The collapsed spray pattern did not overlap sufficiently to burn the entire crop down, leaving a striped pattern and demonstrating the poor coverage.
Nozzle manufacturers generally recommend a 100% spray overlap for flat fans. This creates sufficient overlap when the boom sways low to the ground. It also increases the degree of droplet size homogeneity under the boom as coarser and fewer droplets are generally found at the “horns” or edges of the pattern compared to the centre. In order to ensure this degree of overlap, sprayer operators should observe and consider changes in fan angle over their typical pressure range. Otherwise, the cost of poor deposit uniformity under the boom could be high.
- Operate nozzles around the middle of the manufacturer-recommended pressure range. However, just because a nozzle is rated over a range of pressures does not mean the angle is constant.
- Lower pressures are a greater concern than higher pressures. 30 psi is the absolute lowest pressure for operating a 110˚ air induction flat fan; the ideal operating range for these nozzles is 50-70 psi.
- If nozzles are not maintaining the recommended 100% overlap at your preferred pressure range, then consider switching nozzle rates, and adjusting pressure and boom height.
This work was performed with Victoria Radaukas, 2015 OMAFRA application technology summer student.
February 1, 2022
Coverage is King
We’ve often heard the adage “Coverage is King” but what does that mean, exactly? It means that in order for your spray application to yield acceptable results, a threshold amount of the active ingredient in your tank must end up on the target. But at what point have we achieved sufficient spray coverage without wastefully over-applying to the target? What does good coverage look like?
Let’s manage expectations right here at the beginning of the article: There is no single, definitive answer because it depends on the nature of the application. In other words, you have to understand which factors are relevant to your specific situation before you can understand what success looks like.
Let’s highlight some of those factors:
Transfer Efficiency, Catch Efficiency and Retention
This relates to the spray’s ability to span the distance from nozzle to target (transfer efficiency) get intercepted by that target (catch efficiency) and then deposit a biologically-active residue on the target surface (retention).
- First, the spray must reach the the target location. This may be the soil, or it might be the underside of a leaf deep in a plant canopy. The degree of success will depend on the droplet size(s), distance to the target and the environmental conditions.
- Then the droplets have to be retained by the target surface and not bounce or slide off. Difficult-to-wet surfaces such as fruit, stems and waxy vertical leaves may be more easily covered with finer droplets and/or formulations that include activator adjuvants (e.g. surfactants).
- Then the deposit must stay wet long enough to be absorbed by the tissue, or leave a hardy residue on the surface that can withstand weathering (e.g. precipitation, sun, and even bacteria) long enough to encounter the pest. More on this below.
Mode of Action
This relates to where spray must deposit (or relocate to) in order for it accomplish it’s objective. Here are a few examples of how products might work. Read your pesticide label to determine your situation.
- Some products require contact. Insects must touch them, either via a droplet landing on them or as they move through a deposit. Similarly, certain fungicides must contact fungal hyphae on the plant surface. A few products are designed to drench the target, as is the case with oil-based miticides.
- Some insecticides must be ingested. That may be in the form of a surface deposit or in plant material that has absorbed the chemistry. Similarly, some fungicides are absorbed by plant tissue.
- Many herbicides are mobile (i.e. systemic). They may be drawn up through the roots, or enter the cytoplasm via leaves and travel to the growing points on the plants, or move through the xylem. Others are contact, staying relatively close to the original deposit.
The sprayer operator should consider these factors when planning the application and when evaluating the resulting coverage. So how do we visualize coverage? Some operators look for the shine on leaves, or a cloudy residue once the spray has dried. That’s better than nothing, but we recommend water sensitive paper (WSP), which is still the most versatile and economical way to visualize coverage.
WSP can be purchased from most retailers that carry spray equipment. It is available in three sizes, of which the 1” x 3” size is the most common. It can be folded and clipped to a plant surface, or placed on the ground. We’ve written several articles on how to use it (such as here and here and in pretty much a third of the articles on Sprayers101).
There are two metrics that must be evaluated when assessing coverage on water sensitive paper:
- the area of the target that has spray on it, and
- the distribution of the droplets over that area.
Let’s use a metaphor to explain:
The Battleship® / Coverage Metaphor
Imagine the boats in this Battleship® game are the insect pests, and the board they’re on is a leaf. The white pegs represent the spray deposits. In this first image, we see 100% coverage and a very high deposit density. Sure, we got every boat, but this is literal and figurative overkill. There’s no need to completely drench the target in order to control most pests. When you spray a target past the point of run-off, you are not adding more pesticide to the target – you are displacing what was already there. The surface will not exceed the concentration of product you sprayed (with the possible exception of mixes that include certain adjuvants). While additional volume can improve coverage to a point, there is a diminishing return.
Unless the label specifically asks for a drench, this is too much coverage.
In this second image, we’ve covered about 15% of the target area, which is reasonable. However, note the lack of distribution. You can see that we’ve missed quite a bit of the leaf. If our pretend pests are sedentary and if this was a contact product, then we’ve missed. If this was WSP we would advise the sprayer operator to note how much space there is between the deposits. Could a pest such as an insect or small weed easily fit between the deposits?
15% coverage is good, but the distribution is bad.
In this third image, we are still covering about 15% of the target, but now the spray is distributed more evenly. Some of you are likely noticing that we missed a pest. That observation reminds me of one of my favourite exchanges from the movie “Christmas Vacation” where Clark finally got his house illuminated, but his father-in-law only sees the problems: “The little lights aren’t twinkling.” “I see that and thanks for noticing, Ed.”
15% coverage, distributed evenly. Deposits may have some pest activity beyond the edge of the residue (light red circles).
Yes, we still missed a pest, but spraying is playing a game of odds. You want enough spray to increase the odds of controlling a pest, but not so much to waste spray (and money and time). This image represents an ideal coverage situation. If this pest moves, or this pesticide redistributes even a little, it will affect the pest.
Plus, we should not discount the threshold of influence that lies around pesticide residue. Imagine a small circle around each droplet (illustrated here as light red haloes) where active ingredient may redistribute beyond the initial deposit to affect an adjacent pest. Perhaps even more importantly, deposits do not spread on WSP the way they do on actual plant tissue, so WSP always gives an underestimate of the potential coverage.
In this last image, we see that red deposits have been introduced. This represents a disease control program where an earlier (white) application retains some residual activity when next application (red) is applied. The second spray application almost never lands on top of the first, giving much more protection on the target. For those keeners out there, note that we got that last pest!
In the case of many disease management programs, subsequent sprays tend to fill in gaps left by previous sprays. If timing is prompt, residual activity will see you through.
If you Absolutely Need a Number…
So, what if you’ve read all this but still insist on a firm number to define adequate coverage? We’ll reiterate that there’s no universally-accepted threshold of deposit density or area covered. It would be nice if pesticide labels included this information, but they don’t.
We’ll stick out necks out and say that in general practice we see excellent results when we achieve 85 discrete deposits per cm² as well as 10-15% surface coverage on at least 80% of the water sensitive papers in a spray application. If you can manage this, it should give satisfactory results in most situations.
Ontario Agriculture Conference – 2022
For a really in-depth conversation on the topic of coverage, check out our presentation from the 2022 Ontario Ag Conference. We tried to deliver a fun and memorable demo at the end of this presentation to show how different droplet sizes might contribute to coverage. Enjoy.
January 18, 2022
‘Twas the Night Before Christmas – Parody
Here’s our rendition of the Christmas classic: ‘Twas the Night Before Christmas… but with a bit of a sprayer-winterizing twist. You can hear us read it in the audio bar below, read it yourself, or watch as the talented Bridgette Readel (@bmreadel) reads it to you (end of article). Happy holidays from Sprayers101!
‘Twas the night before Christmas and all through the farm
No sprayers were stirring, not even a Dramm
The nozzles were stored in a bucket with care
In the hopes that the herbicides were no longer there
The impeller spun freely in its little bed
We can’t spray next year if the controller is dead
I took off my gloves and my PPE cap
As the sprayer settled down for a long winter’s nap
When from under the hood there arose such a clatter
I sprang back to the cab to see what was the matter
Away to the ladder I flew with a flash
Tripped over clean filters and swore as I crashed
The light from my smartphone held up from below
Gave a luster of mid-day so I could behold
Some movement above as my vision did clear
Eight mice moving ‘round in the engine and gears
With a flash of their tails, so lively and quick
I knew that our cat wasn’t doing the trick
More rapid than eagles my curses they came
I remembered my checklist with each point by name:
- I’d drained all the places that water could hide
- I’d used RV antifreeze throughout the inside
- I looked closely for leaks from tank lid to sump
- I checked the spray tender and both transfer pumps
- I cleaned the outside and cab wall to wall
But I’d forgotten to mouse-proof… dash it all
And then came a scratching from up near the roof
The cat was responding, no longer aloof
Down from the rafters she came with a bound
She coiled as she listened to each squeaking sound
Her claws sprang to life from the tip of each foot
Her fur was all matted with ashes and soot
Her eyes, how they twinkled, her visage so scary
She grinned like the cat that had ate the canary
She leaped over me and into the fray
The mice scampered past, running every which way
One ran up my leg and jumped onto to a shelf
And I screamed when I saw them in spite of myself
Then the cat reappeared strutting all ‘round my head
And I lay there, and knew there was nothing to dread
As I climbed to my knees, I felt I had shirked
Steel wool and some traps would have probably worked
Then the cat rubbed against my legs with her nose
I reached out to pet her, as I slowly rose
And that’s when she bit me and hissed like a kettle
I jumped up too fast and my head hit the metal
As I swayed back and forth the cat ran out of sight
I’m getting a dog by the end of the night
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December 1, 2021
Adventures in Lecturing – Turn Off PowerPoint
Harvest is mostly done and growers want to hear what we’ve learned and what’s coming next. Lecture season is upon us once again.
In 2021 we’re still finding our way through virtual conferences and hybrid models, but I like to think we’re slowly returning to the in-person format. Just last week I gave my first in-person talk in 20 months. It felt wonderful after having spoken into a dead-eyed camera for so long. Half-way through my lecture I remembered a lesson I learned a few years back and spontaneously decided to go off-script.
Let me explain.
In 2016 I was invited to present at the 40th annual Tomato Days conference in Southern Ontario. I knew what I wanted to say, but didn’t have a decent slide deck for that particular topic. I’d have to pull one together.
I work hard on my presentations. I employ lots of imagery (I create all my own illustrations). I get persnickety about fonts, white space and slide transitions. I try to tell a story that educates and hopefully, entertains. Prideful? Perhaps. But if you’re willing to sit on a hard chair for an hour, I’m going to do my best to make it worth your while.
I finished the slide deck, drove three hours to the conference, handed my USB data key to the organizers and sat down to wait my turn. It was a clear, bright winter morning and I saw that the pavilion we were in was more-or-less windows and a roof. It was so bright, in fact, that none of the 150 attendees could see the projector screen!
I watched sympathetically as the first speaker spent 30 minutes trying (and failing) to verbally describe his graphs. I cringed as the second speaker pantomimed her illustrations in some kind of brave, interpretive dance. Then it was my turn.
I decided I wasn’t going down that road.
When the moderator brought up my talk, I turned the useless projector off. I asked the squirming and disinterested audience:
Q. “What’s the most terrifying thing you can do to an academician?”
A. “Take their Power Point away.”

For the next 30 minutes we had a discussion about spray coverage. No props. No slides. The audience slowly warmed up to the new format. They shared experiences. They debated. They asked questions. I became more facilitator than speaker.
When our time was up I think everyone was pleased. Sure, I missed a lot of my key points and never really addressed the subjects I thought I would, but who cares? Everyone learned something.
For me, I learned that speakers should abandon the script every now and again. It’s not always ideal since we’re there to teach and structured visuals are often required. But, the next time you’re asked to speak, consider the possibility of using your time to engage your audience and establish a dialogue… not just talk at them until the moderator gives you the 5-minute warning.
I have a colleague who does this masterfully. Whenever he is the last speaker on the agenda, and the previous speakers have discourteously gone over-time and whittled his time in half, he jumps straight to his take-home slide. He leads a quick discussion with the audience and becomes a hero. The moderators are now back on schedule and no one is late for lunch.
Since “Tomato Days”, I now try to do this once a year. I never know when the mood will take me, but when it does I give the audience a choice: They can hear my canned presentation or I can shut it down and we can have a conversation. To date, given the option, every audience has opted to go off script. It’s scary, it’s fun and like I said earlier, everyone learns something.
I challenge you to try it the next time you’re lucky enough to be in front of an audience in person.
November 29, 2021
Air-Assisted Spraying in Greenhouse Ornamentals
The aesthetic value of ornamental plants requires a near-zero tolerance for insect pests, which cause up to 10% of crop losses per season. Controlling them with insecticides is a difficult proposition:
- Key pests such as thrips, aphids and whiteflies tend to feed on the underside of leaves – a notoriously difficult surface to target because of it’s orientation relative to the spray nozzle (see image below).
- Other pests, such as mealybugs, are found on stems. Stems are hard-to-wet plant surfaces because spray tends to run off. Further, as the plant canopy grows and densifies, these surfaces are buried deep inside, out of line-of-sight.
- The insecticides available for closed environment spraying must be compatible with biological controls and are therefore “softer” chemistries. Examples include soaps, oils and entomopathogenic fungi. These products require contact with the pest and are at best translaminar, so coverage becomes critical for performance.
Whitefly on the abaxial laminar (under-leaf) surfaces of Poinsettia.
Spraying for Insects
The planting architectures and canopy morphologies are highly variable in ornamental greenhouses. Perhaps they are young plants with sparse canopies, densely packed in pots on raised tables. Perhaps they are mature, hanging plants with dense canopies. Perhaps they are something in between.
Crop canopy morphology and planting architecture are highly variable from operation to operation.
Ideally, each combination of canopy morphology, planting architecture, pest and chemistry would have a specific sprayer designed to optimize coverage and efficiency. This is economically unrealistic. Instead, many producers utilize technologies that rely on high water volumes and hydraulic pressures to “drench” targets indiscriminately. Others employ highly manual methods that allow the operator to aim the nozzle in relation to the canopy on a case-by-case basis, but still rely solely on water to distribute the insecticide.
Typical application technologies in ornamental greenhouses. The backpack sprayer (left) with its manual pump is inexpensive and the operator can aim the nozzle more accurately. The trailed tank-and-handgun (right) utilizes higher hydraulic pressure and water volume in an attempt to improve the work rate. Both rely solely on water and hydraulic pressure to distribute spray.
These technologies have their place, but the reliance on hydraulic pressure and carrier volume has drawbacks:
- High water volumes lead to higher humidity in closed environments which may favour disease.
- The inevitable run-off creates waste water that may require treatment before leaving closed environments.
- High carrier volumes dilute an already “soft” chemistry and hydraulic pressure doesn’t always improve canopy penetration or coverage uniformity.
Air-assisted spraying can be a viable alternative (and an improvement) over these approaches. Stationary or mobile, many ultra-low volume sprayers already employ air to capitalize on the mechanical advantage offered by smaller and more numerous droplets. Finer droplets have very little mass, so they must be directed and carried by air currents to get them to the target. Sufficient air energy will also displace the air within the target canopy and physically expose otherwise hidden plant surfaces to the spray.
The upshot is that air can partially replace water as a carrier and it has the potential to improve coverage uniformity throughout the target canopy.
Testing Air-Assisted Spraying
We chose to test this assertion in a chrysanthemum nursery. Our objective was to compare the coverage from the grower’s conventional hydraulic gun to that of a customized backpack mist blower.
Crop Canopy and Architecture
The crop canopy wasn’t fully mature but still represented a very dense target. In order to compare canopy penetration the canopy was divided into three depths: The Top exterior, the Middle (8″ from ground) and the Bottom (just above the pot soil). Each treatment area contained 8×2 plants and a buffer of three plants was maintained between treatments. We made three sprays (reps) for each condition.
Sprayers
Several attempts were made to redirect and redistribute air from a commercial backpack mist blower. The goal was to create an air outlet that would distribute the same air speed over a long and narrow swath. Air is highly compressible and early attempts using baffles, straightening vanes and variable outlet sizes were unsuccessful. A compromise was reached by reducing the swath to about plant-width (40 cm). This was confirmed by spraying water on dry pavement and measuring the width of the swath. While not ideal, the operator could span the full 75 cm plot width by shifting the outlet back-and-forth laterally while spraying. There are videos below that show examples of both applications.
Several iterations of the air outlet design.
Through trial and error, the outlet was held above the canopy at a height and angle that optimized air penetration. If the outlet was held too far away, there was insufficient air energy to penetrate the canopy. If held too close, too much spray-laden air would escape the canopy. These attempts were performed at a comfortable walking pace to account for dwell time (E.g., the longer the outlet remained stationary over a canopy, the deeper it penetrates).
With the gravity flow set to “1” and moved as it would be used during spraying, we measured walking pace and timed how long it took to spray a known volume. The application volume was 1,250 L/ha (~133 US gal./ac).
The grower’s conventional sprayer was used according to their typical practices. Walking pace and flow rate were measured to establish application volume for both sprayers.
By timing walking pace and performing a timed output test, the application volume was 2,400 L/ha (~256 US gal./ac) for the conventional sprayer.
Coverage Indicator
Coverage was quantified using dye recovery and fluorimetry. The process is described in detail in this article and this article. Basically, a known concentration of Rhodamine WT dye is applied to the plant. Sprayed leaves are collected from key locations in the canopy and placed in labelled containers with a known volume of water. Later, that water is analyzed in a fluorimeter and the data is normalized by leaf weight (or in this case, leaf surface area) to account for the volume used and the size of the leaf sampled.
Dye pooling on leaf surfaces following an application using conventional methods.
Relative size and number of leaves sampled from each canopy depth.
In addition to dye recovery, we also used water sensitive paper as a qualitative indicator. Papers were placed at the Middle depth facing into and away from the direction of travel and sprayed with both methods. This was used as a visual check to ensure spray went where it was intended, but it also provided insight into how spray might deposit on the leaf surface. As an artificial collector, water sensitive paper does not behave like a leaf surface, but it is helpful for relative comparisons.
There were obvious visual differences in how spray deposited on water sensitive papers located in the middle of the canopy. The mist blower had far less drenching and an even distribution of finer deposits compared the the conventional method. From left to right: Mistblower, facing sprayer travel direction. Mistblower, facing away from sprayer travel direction. Conventional sprayer, facing away from sprayer travel direction. Conventional sprayer, facing sprayer travel direction. When comparing these papers, remember that the mist blower was using approximately half the volume of the conventional method.
Results
As mentioned previously, dye recovery was normalized by spray volume and leaf area for each condition. The results align with inferences made in the above image. Spray coverage can be highly variable which often leads to statistically insignificant results, but the mean-dye-recovered does demonstrate clear trends. The top of each canopy received a similar dose of dye for each condition. This comes as no surprise and is typical of any overhead application into a canopy. However, the air-assisted condition resulted in more than 2x the dye in the middle of the canopy and more than 10x the dye at the bottom compared to the conventional method.
Bars represent standard error.
When considered as a percentage of overall dye recovered, we see that the dye deposited was more uniform in the air-assisted condition. 16% of total dye recovered in mid-canopy in the air-assisted condition canopy versus 7% in the conventional condition. 13% at the bottom on the air-assisted condition versus 2% at the bottom of the conventional condition.
Conclusions
Based on this study, there is compelling reason to consider air-assisted applications in closed environments. Canopy penetration and coverage uniformity was improved in the air-assisted condition. In addition, there is potential for reduced water volumes, which mean less contaminated run-off and lower humidity levels in closed environments.
Future work would require a better-engineered sprayer than the prototype used here. Further, while improved coverage often improves spray efficacy, it is not always a direct correlation. An efficacy study comparing crop damage and pest counts should be performed to confirm that this method of application represents a positive return on investment.
This research was performed with Dr. Sarah Jandricic, OMAFRA Greenhouse Floriculture IPM Specialist. Thanks to Schenk Farms and Greenhouses Co. for collaborating in the study.
November 19, 2021