Here in Episode 10 of Exploding Sprayer Myths we’ve coaxed @Nozzle_Guy back into the orchard. This is part two of a two-part mini series on airblast calibration. In Episode 9 we talked about air settings and travel speed, and now we’re tackling nozzling and coverage.
But here’s the twist: Rather than use spray math to determine the required nozzles to achieve ideal coverage, we do it backwards. This process uses ideal coverage to determine nozzles and finishes with sprayer math.
Confused? Watch the video and this surprisingly simple and versatile approach will become clear. See if you catch the subtle visual joke about “coverage” realize that to pull this off, we had to film it backwards.
Special thanks to the @RealAgriculture team, the Simcoe Research Station and Don Murdoch.
Checking coverage on water-sensitive paper with some of the Grape Growers of Ontario members in 2012Press play to hear the audio version of this article.
When an extension specialist, equipment retailer or consultant is asked to calibrate an airblast sprayer, they would be well advised to calibrate the sprayer operator as well.
Consider this: you and the operator are each investing three hours (average) to optimize the sprayer for a specific set of circumstances: the crop dimensions, density, and the weather conditions at the time of calibration. Depending on the reason for the application, you may even account for the product(s) mode of action and the pest location. This means that once you leave, the circumstances will change and the benefits of your efforts will quickly diminish.
Calibrations, like milk, have an expiry date.
There are three possible outcomes from a single, stand-alone calibration:
The operator manages efficacious applications throughout the season because the variability in weather, crop and pest isn’t significant. This is generally not the case.
Not recognizing that sprayer settings need constant adjustments (or being unable to make the changes) the operator experiences only modest results and decides calibration isn’t worthwhile.
The operator experiences failures and lays the fault with you (as the last person the touch the sprayer) and/or the agrichemical rep that sold the chemical. Few sprayer operators blame timing or spray coverage.
Explaining how to place water-sensitive paper and ribbons in an apple tree
The solution lies in the proverb “Give a man a fish and you feed him for a day; teach a man to fish and you feed him for a lifetime.” It is the sprayer calibrator’s responsibility to involve the sprayer operator and ensure they understand what is being done, why it is being done, and how to do it when you leave. Otherwise, expect to calibrate that sprayer again… soon.
Personally, I have had the most success educating and empowering sprayer operators to make their own seasonal adjustments based on a formulaic approach. Depending what you are trying to accomplish, you may not need all of the following steps, or you may perform some on your own and others as part of the education:
1) You could be working one-on-one, or you may be presenting to a large group. When it’s the latter, I like to arrive the day before to meet the host or owner of the sprayer(s). You can scope out the operation and triage the equipment so you know what parts you might need the next day. It also helps to see the space you will be working in.
2) Perform a pre-calibration inspection of the equipment with the sprayer operator. They know their equipment and can tell you about usage, history and maintenance. It also opens a dialogue between you and helps the operator to relax. Remember: from their perspective they may feel they are being judged and they will take criticisms and corrections personally. Do your best to reassure them that you are trying to make a good thing better – not to correct failings.
3) If you’re working at a large operation, educate the manager (decision maker) and the operators (drivers) at the same time. If you teach the manager, they might not effectively communicate the lessons to their operators. Likewise, if you teach the operators, they may not be able to convince the manager to let them spend money, or time, on making changes to the sprayer program. Get everyone on the same page, at the same time.
4) With the operator, perform a basic maintenance check. Specifically, confirm sprayer ground speed, evaluate pressure gauge accuracy and evaluate nozzles. Explain what you are doing, and ask the operator questions. This is where you learn about their attitude. Are they open-minded about changing how they do things? How has their efficacy been in the past? Will they spring for new parts? Do they need convincing that this process must be repeated regularly?
Demonstrating how deflectors aim air, and spray, into the target using some scrap wood.
5) With the sprayer in the crop, have the operator tie wind-indicator ribbons in the canopy (or better, use lengths pre-tied to springback clips). Explain what they are doing and why. Tell them these ribbons should be monitored, maintained and replaced season-long.
Here’s a tip: If you are working with a large audience, keeping them focused is critical. Growers will take the opportunity to catch up with each other while you are occupied with the sprayer. They are also inclined to wander away to answer cell phones. If they are not focused, you are on a service call and are not really educating. If you feel you are losing control, single out the ringleaders or wayward students and give them jobs, such as holding tools, or placing/removing water sensitive papers. When they have a responsibility, they pay closer attention.
A convenient, weather proof calibration kit for flagging tape, clips and water sensitive paper.
6) Discuss where water-sensitive papers should go, and how they should face. Give the operator a latex glove and after you write on the back of each card (position and trial number) have them clip them in place. Tell them how much they cost, where to buy them and the benefits of using them regularly.
7) Have the operator spray the target crop using their typical set-up (i.e. ground speed, pressure, rate, air settings, etc.) Have attendees and the operator watch the ribbons as the sprayer passes. Spray from one side with both booms on and then stop to discuss results. Then spray from the other side and explore the cumulative impact.
8) The operator will be very surprised to learn they have drenched or missed the papers. They may or may not be surprised to have seen the ribbons stood straight out (indicating too much air). If you like, you can even set up papers in the next alley (or alleys) to show how much spray blew through the target. When the papers are dry enough, collect them and store them somewhere safe for later comparison. They tend to blow away, so stick them to a whiteboard with two-sided tape, or clip them there with paperclips. Explain that they can (potentially) save a lot of money and lost fill-time by improving their efficiency. Get them on-board for the big change to come.
9) Optimize sprayer ground speed, air direction (i.e. deflectors) and air speed/volume (i.e. fan speed). Then re-nozzle the sprayer using brass disc and core tips to reduce output in areas that were drenched or increase output in areas of sparse coverage. Quite often, I turn off the lowest (and sometimes, highest) nozzle positions. A piece of water-sensitive paper at the top and bottom of the canopy will confirm the wisdom in this. Label a new set of papers and have the growers position them in the same locations. Spray again. This entire process should take about 1/2 an hour and is described in detail in the Airblast101 handbook.
Tying flagging tape in trees to indicate prevailing wind and to calibrate airblast air settings.
10) The goal is 85 medium droplets per square centimetre and 10-15% coverage on 80% of the target surfaces for most insecticides and fungicides. If there are still drenches or misses, or if you’ve gone too far in a few positions, correct them and try once more. This is iterative. Make sure the sprayer operator will not be spraying in particularly hot or windy conditions, or your calibration at the top of the target can be compromised. Once you are both satisfied, work out the new sprayer output per area (e.g. US gpa or L/ha). You will have to discuss whether the operator plans to concentrate the tank mix to maintain the labelled “per area” rate (not recommended by me) or will continue to mix the tank as always and simply drive further on it (recommended by me). The later is called “Crop-Adapted Spraying“. Don’t push because it’s their livelihood, and therefore their choice.
11) The final step relies on how well you’ve earned the sprayer operator’s trust throughout this process. Once you have an output and spray distribution that you are both happy with, the operator should invest in molded ceramic tips that emit similar rates to replace the brass disc-core. Then, they must be willing to repeat the process on any crops that are significantly different to ensure they have the right settings. Sometimes only modest changes are required between blocks. Perhaps they will dedicate certain sprayers to certain blocks to reduce the number of changes required. In either case, they will have to revisit these settings as the season progresses to compensate for denser and/or larger canopies.
A few examples
The following figures illustrate three airblast calibrations in Ontario apple orchards from spring 2014. Some required one attempt; others required a few trial settings before we achieved reasonable coverage. In all three cases, the sprayer operators reduced per-area rates, bought new nozzles and planned to buy water-sensitive paper. Further, they indicated they would continue to monitor ribbons (as long as they could be seen) and would review coverage after petal-fall.
Several nozzles shut off, spray re-distributed. Targets still drenched in two locations with a 24% savings in spray mix.Three successive re-calibrations were required. Output was reduced in the first trial, but poor coverage in position 3. Top nozzles turned off and spray re-distributed in trial 2, but a gust of wind reduced coverage at the top of the tree. Bottom nozzles turned off and spray redistributed to top nozzles for a 40% savings in spray mix.Output reduced in all nozzle positions and sprayer fan speed reduced. The high humidity greatly reduced droplet evaporation and increased the spread on the papers. In this case, it was decided not to reduce output any further to account for anticipated growth and the high humidity. There was a 27% savings in spray mix.
Conclusion
So, the next time you calibrate an airblast sprayer, be sure to teach the sprayer operator (and audience) what you are doing and why. Involve and engage them. Answer their questions. Encourage them to perform the same calibration for each significantly different block and make mid-season changes. With luck they will only call back to report success and savings, and not to condemn your efforts, or worse: to ask you to re-calibrate their sprayer!
Droplet size influences droplet behaviour. The following table lists the pros and cons to changing droplet size when overall spray volume (e.g. L/ha) remains constant.
Relative Spray Quality
Pros
Cons
Coarser Droplets
Lower drift potential because they resist deflection by wind and evaporation from heat and low humidity.
Lower droplet count may reduce coverage.
Greater mass means they move ballistically, propelled at higher speeds by pressure for greater distance.
May fall out of the spray before reaching the top or centre of the canopy.
Coarser droplets do not penetrate dense canopies as easily as finer droplets.
Redistribution due to bounce, shatter or run-off may either improve or compromise coverage.
Redistribution due to bounce, shatter or run-off may either improve or compromise coverage.
Finer Droplets
Higher droplet count may improve coverage (if they arrive at the target).
Higher drift potential from wind, and evaporation from heat and low humidity.
Finer droplets penetrate denser canopies better than coarser.
Finer droplets move unpredictably and require optimal air settings to direct them to the target. Sprayer design and air settings will determine if it is optimal for nearby or distant targets, but it is rarely if ever both.
Finer droplets move unpredictably and require optimal air settings to direct them to the target. Sprayer design and air settings will determine if it is optimal for nearby or distant targets, but it is rarely if ever both.
It is preferred to use nozzles that create coarser droplets at higher rates (to compensate for fewer droplets) in the higher boom positions. They are more likely to stay on course to the tops of the trees, and when they miss, many fall out of the air rather than contribute to drift.
Learn more about strategies to reach the top of a canopy here.
Finer droplets have very little mass and therefore very little kinetic energy. This means they slow quickly (imagine throwing a feather) and require entraining air to carry them to the target. Finer droplets also evaporate quickly, particularly on hot and dry days (i.e. unsuitable Delta T conditions). If employed, they should be distributed in the lower-middle portion of the boom where they have the least distance to travel and are most likely to be intercepted by canopy.
Boom distribution
Unlike a broad acre boom sprayer, where each nozzle emits the same rate, an airblast boom can distribute spray unevenly. For a curved (axial) boom, the rule of thumb is to produce 2/3 of the overall volume from the top 1/3 of the boom. This compensates for the distance and greater proportion of canopy it is intended to cover.
A vertical (tower) boom positions each nozzle roughly the same distance from the target, and if that target is a hedged canopy, the spray can be distributed equally over the boom. Research has demonstrated that there is no appreciable advantage to one spray shape over another (e.g. flat fan, hollow cone, full cone) other than the spray quality they produce.
In extreme cases, operators might elect to “fire hose” spray to the tops of canopies using high pressures. This is achieved by using streaming nozzles or removing the swirl/whirl/disc plate in a disc-core combination nozzle in the top few nozzle positions. Given the heavy demand on the pump and the inaccuracy of the method, this should only be considered when air fails to reach the tops of trees.
Learn more about nozzling an airblast sprayer here.
Spray coverage and diagnostics
It’s well understood that spray coverage has a negative correlation with tree height. The irony is that in large nut trees the upper portion of the canopy produces much of the harvest. Taken collectively, this may explain why pest activity is also highest in the upper canopy. When choosing a spray volume and boom distribution, the metric is threshold coverage in the top 1/3 of the canopy. This requires us to define threshold coverage.
If ribbons and leaf movement represent the feedback mechanism for air settings, then water sensitive paper (WSP) is the choice for spray coverage. Placement in tall trees can be tricky, given that we are most concerned with coverage at the top, but this can be overcome by mounting the WSP on telescoping poles. Papers can be oriented horizontally to represent a leaf, or curled around the pole to give panoramic coverage and emulate a nut. Beware over-blowing in the lower canopy, which creates a shingling effect where leaves cover one another (or the WSP) and block coverage.
Fluorescent dyes and kaolin clay show spray coverage in situ, but there are drawbacks. Few growers will spray dye and come back at night with a black light to examine targets. Further, a target sprayed with dye or clay cannot be sprayed a second time, which means the grower can’t adjust the sprayer and try again in the same canopy. And finally, it’s very difficult to determine if there is more or less coverage with clay or fluorescent dye.
Learn more about how to use water sensitive paper here.
WSP is fast, cheap and effective. With the exception of drench applications, the most demanding spray application (e.g. contact fungicide) should produce a spray coverage pattern of 85 drops per cm2 and 10-15% total coverage on 80% of the targets. This threshold comes from collective research and experience in many horticultural crops, and should true hold for tree nut.
Be prepared to make changes to your sprayer calibration to compensate for tree height, canopy density, and weather conditions throughout the season. The feedback from water sensitive paper is far more accurate than shoulder-checks and leaf residue. It takes some time and effort, but it’s well worth it. Coverage is King.
What others have done
Researchers like Brad Higbee (Paramount Farming Co.) and Ken Giles (UC Davis) have explored spray coverage and efficacy from different sprayer configurations to combat Naval Orange Worm in almond. What follows is a summary of their observations. This information comes from their presentations and conversations with Brad.
Ten years of trials spanned travel speeds of 3-6.5 km/h, volumes of 1,400-2,150 L/ha, and sprayer-generated airspeeds (measured at source) of 80-290 km/h. They looked at efficacy, residue levels and WSP coverage both in leaves and on the nuts themselves. When comparing sprayer configurations, the target almond tree was divided into four levels:
Level 1 = 1.8 m to 2.5 m (Lower canopy)
Level 2 = 3.0 m to 3.7 m
Level 3 = 4.2 m to 4.9 m
Level 4 = >5.5 m (Upper canopy)
Many configurations were tested, but the following figure shows the top four. Of those not shown, most notable are the Bell 206 helicopter (280 L/ha at 50 km/h) and the Curtec AC 1000 Cross-Flow tower.
A. Air-O-Fan low profile axial D-240 (Also used Air-O-Fan 232). B. Progressive Ag two-head 2650 electrostatic air-shear with 4 m tower (Also used 4.9 m three-head and 5.5 m four-head). C. Blueline Accutech 10-head air-shear tower. D. Low-profile axial airblast with two Sardi-style fans on mast. Upper fans set to 70% overall fan speed and spray volume. Axial fan and nozzles set to 30%
Here is a summary of their observations:
Spray coverage and residue deposition was weakest in upper half (Levels 3 and 4) of canopy. Tower sprayers tended to provide more uniform coverage across vertical levels. For low-profile axial sprayers, most of the residues were deposited in the lower half of the tree.
The Air-O-Fan low-profile axial had the highest overall residues. But, above 3.7 m there was severe drop off in coverage. PTO-driven sprayers seemed as effective as engine driven. Incremental improvements were observed on this sprayer when using multiple banks of booms, full cone and hollow cone nozzles.
The Progressive Ag tower provided the highest residue deposition above 3.7 m and modest deposition in the lower canopy. While tower sprayers tended to provide more uniform coverage, the Progressive Ag was not significantly better than the Air-O-Fan overall.
Aerial application (280 L/ha) combined with the Air-O-Fan low-profile axial sprayer (1,870 L/ha) did increase residues in the upper canopy, but did not result in greater damage reduction relative to the Air-O-Fan alone.
Slowing the Air-O-Fan low-profile axial sprayer from 4 to 3.2 km/h resulted in 30% more coverage and 47% higher residue deposition overall.
Electrostatic treatments did not perform well on WSP (small droplet size was suspected), but they were among the best in residue deposition at full volume and “delivered surprising residues at high speeds/low volumes”.
Brad has done remarkable work studying the impact of several sprayer configurations. While many were tested, there are still more that might be considered.
Canopy management
When all else fails, we are left with only one alternative: canopy management. Hedging and pruning the trees to create sprayer clearance opens canopies to spray (and light and air) and is a critical part of crop protection.
Learn more about the benefits of canopy management here.
Topping trees to bring them to a manageable height to improve coverage and reduce drift may be the only viable option for protecting the crop. I acknowledge that a great deal of nut production takes place in the upper third of the canopy, and it is beyond the scope of this article to discuss production and yield economics. However, when the crop is left unprotected, the yield quality is negatively impacted and it has been shown that a reduction in harvest weight is offset by the improvement in overall quality.
Where plants are very old and overgrown (such as macadamia), it is highly recommended that the orchardist engage a local crop expert and discuss a strategy for canopy management. There are many benefits, including:
Improved harvest quality
Fewer refills (saving time and water)
Less time to spray means more timely applications
Potential chemistry savings
Savings in gas, noise and equipment wear and tear
Potential for reduced off target spray drift
Summary
Spraying large nut trees is a challenging proposition. A number of inter-connected factors are involved and an operator must address all of them make spraying as efficient and as effective as possible.
Adjust sprayer air settings first, using canopy penetration as your guide to travel speed.
Distribute the 2/3 of the volume and coarser spray quality to the top 1/3 of the boom.
Consider an air-assisted vertical boom configuration to improve coverage uniformity and reduce drift.
Use water sensitive paper for critical coverage feedback and make changes based on that feedback.
Develop a canopy management strategy to improve spray coverage and yield quality.
I’ve studied spray applications in a diversity of crops, both broad acre and specialty, but perhaps nothing is as challenging large tree nut canopies. Australia’s macadamia orchards can form >10 metre high, >4 metre deep canopy walls! So in writing this article I face the opposite situation I normally encounter when advising on airblast sprayer settings.
In my region, fruit orchard, cane, bush and vine crops are typically sprayed with airblast sprayers. Over the years, through breeding and crop management, these operations have densified. The idea is that smaller, uniform crops can be managed, protected and harvested more efficiently. The ratio of quality fruit to planted area goes up, and input costs go down.
However, our aging fleet of sprayers are overpowered relative to the target. This means much of what I do involves demonstrating to sprayer operators what sufficient coverage looks like, and then teaching how to restrain sprayer parameters to achieve this ideal coverage as efficiently as possible.
So, are there any commonalities?
Yes! The need to understand what “good coverage” looks like, and the parameters that affect it, is universal to any airblast operation. Assuming the operator already has product choice and pest staging well in hand, there are three major factors that influence the quality of the spray application: The sprayer settings, the geometry of the target and the environmental conditions.
In theory we can discuss each of these factors individually, but in practice they interact with one another. It is wrong to adjust one factor without considering the other two. This is also why you should be wary of anyone that tries to sell you a sprayer by demonstrating it in an empty lot on a calm day! Always calibrate a sprayer in the planting, in weather conditions you would normally spray in.
Air volume and direction
Air adjustments are perhaps the most impactful changes you can make to your operation. The air stream created by the sprayer not only conveys the spray solution to the target, but opens the canopy and exposes leaf surfaces to the spray. In order to achieve adequate coverage, the volume (and speed) of sprayer-generated air must be sufficient to span the distance from sprayer to target, and then displace the volume of air in the canopy while depositing the spray.
I admit to a bias when it comes to air shear systems. These sprayers utilize sprayer-generated air to atomize the spray liquid as well as convey it. As such, you cannot easily adjust the air without affecting spray quality (aka average droplet size or VMD). My preference is an arrangement where nozzle selection allows you to control spray quality independent of air settings. In any case, adjusting air settings requires the operator to “see” air.
In my region, I advise tying 25 cm lengths of flagging tape at the top, middle and bottom of the far side of the upwind tree. Then, drive past with the air on and the spray booms off. If the ribbons stand straight out, the sprayer is over-blowing and the operator can drop to a lower fan gear, reduce the tractor RPM’s (if using a positive displacement-style pump) or drive faster. If the ribbons don’t move, the opposite steps can be taken. If the ribbons still won’t move, the sprayer is under-powered, it’s too windy to spray, or the canopy is too large.
Let’s explore that last point. In the case of a canopy as large as macadamia, it is unlikely a low-profile axial sprayer can produce sufficient air volume to displace all the air in the canopy – particularly at the top of the tree. In this case a more humble goal would be to move the leaves at the trunk, indicating that the sprayer is managing to drive the air to the centre. To monitor this, an observer wearing safety goggles would have to stand at the far side of the upwind trunk and (while being very careful of flying debris) watch for leaf movement.
This becomes increasingly difficult to monitor as the target gets fuller, higher, and farther away from the sprayer. Consider the macadamia trees in the following figure:
The observer will have difficulty seeing leaf movement at the top of either the taller or shorter tree, but we can safely assume there will be less movement as a function of height. Since our goal is uniform penetration throughout the canopy, we must somehow compensate for this differential. Consider the following figure which extrapolates the path between the sprayer air outlet and the tree:
In this figure we have divided each side of a low-profile axial sprayer into halves. The bottom half of the air outlet must produce enough air volume to displace area X. I realize I’m mixing area and volume, but bear with me. For the taller tree, the upper half of the outlet must produce enough air to displace 2.5 times the area versus the bottom half. Given that it is a single air outlet, this means inconsistent coverage.
Comparatively, the shorter tree requires a more uniform air distribution. While this improves matters, there are further challenges. Sprayer-generated air slows and disperses proportional to distance, requiring more air to compensate. Also, orchard wind speed increases with elevation, increasing the potential for interference and dispersion. So, the taller the tree, the harder it is to achieve uniform canopy penetration.
Spraying shorter nut trees with a low-profile axial sprayer is possible. The sprayer would require a large fan (≥1 m diameter), an aggressive fan blade pitch and a high fan speed. Air deflectors and air separation vanes would also be needed to segregate and focus the air. And travel speed would play a significant role.
Travel Speed
Travel speed should be considered as function of air penetration. A slower travel speed (~2 km/h) facilitates the displacement of stagnant canopy air with sprayer-generated air. Further, a slower travel speed reduces the wake effect that can suck finer droplets from the swath.
It may seem counter-intuitive, but slower speeds can result in greater productivity. There is no need to increase the volume sprayed per hectare, so additional refills are not an issue. Further, improving spray coverage at slower speeds may prevent the need for an additional “clean-up” application later on, saving time and reducing environmental impact. Time lost to slower travel speed can also be reclaimed with more efficient loading practices.
Learn more about travel speed here and productivity here.
Directed Sprays and Off-Target Deposition
When the height of the target tree exceeds alley width, or when branches overgrow alleys, many low-profile axial sprayers suffer from line-of-sight issues. Lower branches/leaves block the upper canopy and too many nozzles target the lower canopy. See the figure below.
One option is to direct spray vertically to ensure the swath reaches the top of the canopy. In this case it is hoped that droplets remain Coarse enough to fall from the swath and penetrate the canopy, or blow laterally with prevailing wind (left side of figure). This unadvisable strategy is unlikely to achieve consistent results and greatly increases the potential for drift.
Alternately, the top of the swath can be vectored directly at the top of the tree, but it must pass through canopy to reach it (right side of figure). This strategy increases the potential for drift, risks missing a portion of the upper canopy and is also unlikely to yield consistent results.
Ideally, we would use a sprayer design that brings the air (and nozzles) closer to the target. Hypothetically, there are several possible configurations, but in practice their success will be hampered by boom sway and roll (from sloped plantings or uneven alleys) and pressure drop restrictions (from boom height). Here are a few possibilities:
A. A vertical boom with a tapered inflatable bag to convey and redirect the air laterally (typically one-sided). B. An axial sprayer topped with a ducted tower with vertical booms, terminating in either a second axial fan or one-sided cannon. C. An axial sprayer with a vertical mast with a series of Sardi-style nozzle/fan assemblies distributed along the height.
In the following figure we see how two possible arrangements might work. On the left is a vertical boom with a tapered air assist system. This provides the shortest distance-to-target for each nozzle and in moving laterally, the air will more easily penetrate horizontal limbs. It also reduces the potential for drift.
On the right is a novel arrangement proposed by Dr. Ken Giles (UC Davis, California). A Sardi-style fan and nozzle assembly is elevated above the canopy from an axial sprayer. His intention was to create air and fluid interaction to generate turbulence that could improve uniformity and decrease drift. He proportioned 70% of the overall spray to the top fan, and the remaining 30% from the ground. Working in almond, he saw more even coverage distribution compared to a low-profile axial sprayer and noted it reduced off target drift. For a target as tall as macadamia, additional fans would likely be required.
In Part 2 we discuss Droplet size, Boom distribution, Spray coverage and diagnostics, California research and Canopy management.
Are you considering shelling out for a tower extension for your airblast sprayer? Spray towers are an excellent investment, but they warrant special consideration. Towers move the air and nozzles closer to the target compared to the curved booms on a conventional airblast sprayer. When the distance-to-target is reduced, the odds of droplets reaching the target are improved. That means less pesticide drift and more deposit in the plant canopy.
Be Aware: Nozzles need a minimal distance from the target to create an optimal spray pattern, so do not get too close.
Many growers report savings when switching from conventional airblast to towers. The towers are more efficient at depositing the spray, so they have to reduce their typical sprayer volumes to prevent run-off. We worked with one apple grower that switched from a conventional sprayer to one with a tower. His lake-side orchard was plagued by wind, and his conventional sprayer had a relatively small fan diameter (~2 feet) that couldn’t compete. Traditionally, the grower used higher spray volumes to compensate. His new tower sprayer had a larger fan (~3 foot diameter) but perhaps equally import was that the tower reduced the distance-to-target. As a result, he was able to reduce his spray output by more than 200 L/ha while improving his overall coverage! That represented considerable cost savings and reduced environmental impact.
Towers may provide better coverage than conventional sprayers in orchards with horizontal scaffolding. The tower sprays between branches, penetrating more easily, while the conventional sprayer has to spray through them. Concept from K. Blagborne, British Columbia.
While there are many benefits associated with towers, they are not suitable for all situations:
Towers must be taller than the highest target (e.g. treetop)
Towers should be used on level ground. Towers will roll on the vertical axis (i.e. tip left and right) on uneven ground, potentially missing or over-shooting targets
Towers must be able to clear netting, trellises, or an overhanging canopy.
The perils of towers on uneven ground. For towers to be effective, the tower must be at least as tall as the target. When the target is only slightly higher than the tower, some sprayer operators install an additional nozzle body on the top deflector plate to extend the reach.A home-grown airblast sprayer with tower. PVC ducts, sheets of plastic, a squirrel cage blower and grower ingenuity. While it looks suspect, and difficult to clean, it reputedly works very well in highbush blueberries.
Occasionally, we have discovered areas along tower outlets where there is reduced air flow. You can usually feel these “dead zones” with your hand (beware flying debris), but it’s better to observe short ribbons attached to the nozzle bodies as described in our articles about adjusting air direction and speed/volume. In low fan gear, watch to see if any ribbons flag or appear slack from a lack of air, you can “borrow” air by re-positioning neighbouring deflectors. If that’s not possible, try replacing the conventional nozzles in the dead zone with air induction nozzles; coverage should improve in that zone because pressure propels coarser droplets further than finer droplets. We’ve seen significant improvements using this technique in high density orchards.
In the end, if a tower will fit in our operation, we suggest it’s a worthwhile investment that will make coverage more consistent, reduce off-target drift and possibly reduce the volume of spray needed per hectare.
Towers come in many shapes and sizes. Orchards aren’t the only good fit for towers; grapes, bushes and canes can also benefit from small towers.
