Choosing the right time to spray can be tricky. Our gut tells us that spraying when it’s windy is wrong. The experts tell us that spraying when it’s calm is wrong. So when can you actually spray?
I’ve always advised my clients to spray in some wind, because it has a few advantages. The main one is that wind helps disperse the spray upward and downward, diluting the spray cloud fairly rapidly. Another advantage is that winds tend to be reasonably steady in their direction and velocity (or at least that can be forecast), so downwind areas can be identified and potential impacts are known or predictable. It helps if it’s sunny, because that improves the dispersion of the cloud even more.
First, let’s define “windy”. The classic wind scale is the Beaufort Scale, intended for the sea, but also used on land. The upper limit for spraying is probably Force 3 or Force 4, with upper limits of 20 – 25 km/h or so. The Beaufort Scale calls these “Gentle or Moderate Breezes” (they had to save the alarming words for hurricanes), and the scale provides good visual clues such as what wind does to flags, leaves, or dust.
Spraying under breezy conditions can be done fairly safely if you follow specific steps. The idea is to understand what the risks are and to manage them.
The cornerstone is to use a low-drift spray and match it to a pesticide that will work well with larger droplets. But there are other important aspects to consider. Below are the top six to think about:
- Choose a herbicide that can handle large droplets. Glyphosate products are well suited to coarse droplets. But glyphosate commonly has contact actives in the mix, like CleanStart or Heat or bromoxynil, and these are less likely to perform well with big droplets than those that contain Group 2 and 4 mixes. Actives with soil activity also have more tolerance for larger droplets.
- Use a low-drift nozzle and operate it so it produces a Coarse to Very Coarse spray quality, as described by the manufacturer. Some of the new labels call for Extremely Coarse to Ultra-Coarse sprays, and to achieve these you may need to purchase new nozzles. Low-pressure air-induced nozzles operated at about 50 – 60 psi will generally be very low-drift, but lower drift models are available. Produce a finer spray, either by increasing the pressure or moving to a finer tip, only if you use contact modes of action.
- Keep your boom low. For low-drift sprays, you need at least 100% overlap, which is for the edge of one nozzle pattern to spray into the centre of the adjacent pattern. In other words, the spray pattern should be twice as wide as your nozzle spacing at target height. For most nozzles, a boom height of close to 20 inches is enough to achieve this overlap. An automatic boom leveling system is very helpful.
- Maintain reasonably slow travel speeds. These reduce the amount of fine droplets that hang behind the spray boom, and they also make low booms more practical.
- Know what’s downwind and what harms it. When you have a choice, avoid spraying fields that have sensitive areas downwind such as water, shelterbelts, pastures, people, etc. If you can’t avoid being upwind of these areas, make sure you check and obey the buffer zone restrictions on the label. These will also give you an idea if the product is harmful in water or on land, or both.
- Let the weather help you. Take the wind from the side if you can. Going straight into the wind creates a lot of extra drift. Spray when the sun shines if you have a choice. Early morning, late evening, or cloudy days increase the distance that drift moves. When it’s sunny, the drift cloud disperses quickly and causes less damage.
If you feel that drift is unavoidable and someone might be impacted by it, talk to those people first. It’s one of the most important things you can do.