This is achieved by positioning the nozzles correctly and ensuring that the droplets are the right size. Bigger droplets are heavy and more likely to run off the leaves than adhere to them. On the plus side, they are less subject to wind drift and evaporation.
Very fine droplets, on the other hand, are easier to get into inaccessible places, but can float away on air currents, like mist, because they are so light.
The best option is therefore a combination: an air-assisted mist sprayer that breaks up the droplets, making them very small while forcing them into areas not easily reached by conventional sprayers. The blast of air also moves the leaves around on some crops, exposing the undersides.
The smaller the nozzle aperture and the higher the pressure, the tinier the droplets. But both factors require careful attention. Some farmers are tempted to opt for very fine droplets for better penetration, but lose much of the spray mixture if there is a slight breeze. You can see how much is lost in the early morning when the sunlight highlights the fine droplets rising above a land.
If you are not using an air-assisted sprayer and the droplets are too fine, they lose momentum, decreasing penetration. Larger droplets, by contrast, are thrust into the foliage but move less. Again, a compromise is necessary.
When to spray
The time of day is also important. Hot air rises; we see the effect when smoke rises. But when air becomes cooler and denser, it sinks, and we see the result of this when smoke from evening fires hugs the ground rather than rising high as it does during the day.
The mass of cool, dense air keeps the smoke at low levels. You can make use of this by spraying in the evening or night, because any fine droplets are likely to sink instead of rising and being lost as would happen in the heat of the day.
Be especially mindful of this when spraying a herbicide that can harm crops some distance away – damaging a neighbour’s crop can be very costly!
Although mornings are usually windless and a good time to spray, you must take dew into account. Spraying increases the size of dewdrops on leaves, and these could roll off, taking the chemical with them. This is less of a problem in summer rainfall areas, as the dry air means that the dew point is low and dew-free mornings are more common.
In most cases where herbicides are applied, it is usually safer to opt for a much lower pressure and larger droplets to prevent spray drift, but be careful to get the correct overlap. I use tapered jet nozzles, which have slits with a smaller gap at each end, and apply less water at the outer edges.
These nozzles are designed for an overlap and apply a uniform amount over the entire spray width. If the jet were not tapered, double the product would be applied in the overlap.
Finally, remember to set the boom at the correct height for the ideal overlap.
Bill Kerr is a vegetable specialist and a breeder of a range of vegetables.