Top coverage

 

The first step in optimizing your sprayer efficiency and effectiveness is figuring out what kind of coverage you want, and how much liquid you need to apply to an area to achieve the kind of coverage you want.

 

Randy Zondag, Commercial Horticulture/Natural Resource Educator and Director, Lake County Extension, asks this question a lot. And 95 percent of the time, the growers he’s asking don’t have a good answer.

 

The reason they don’t know is there is no widely-used information that can help them learn how to do it. However, there is an equation that can help. The equation basically asks how big your crop is: it takes the height and width of trees and the spacing between rows and converts that information into how many gallons of liquid you need to apply per acre.

 

“It’s coverage, not volume or dose that is important in this situation,” Zondag said.

 

The nursery tree row volume equation is gallons per acre (GPA) = 10 DxH divided by S, where D is the diameter of the tree, H is height and S is the spacing between rows. For orchards, replace the 10 with 40.

 

So the optimal gallons per acre for your situation will be equal to 10 times the diameter of tree times the height of the tree. Then, divide that result by the distance between rows. The diameter used in the equation is the spread of the tree’s branches, not the trunk’s diameter. Also, the height used in the equation is the height of canopy, not the full tree.

 

“A lot of trees are planted 11 feet high, but you don’t want to spray the bottom three or four feet,” Zondag said.

 

So, if a tree’s width was 6.5 feet and its height was 11 feet, with a spacing of 9 feet between rows, the first step would be to subtract three feet from the tree’s height. That would give you an equation of 10 times 6.5 times eight (the new height), divided by nine. The optimized amount of gallons per acre for this example would be 58.

 

Zondag said most people should only be applying between 40 and 60 gallons per acre if they are doing the job right. However, the average person uses 100 gallons of water per acre to get coverage.

The effectiveness of your coverage depends on what sticks to the leaf, not the volume of liquid used on the plant. In fact, that’s where a lot of waste occurs.

 

“For years we were taught the more liquid you use, the better your coverage,” Zondag said. “That’s not true. It’s a big fallacy. Actually, the more liquid you use, the more ends up on the ground. It makes coverage worse because a lot of it rolls off the leaf and takes other droplets with it.”

 

Canopy penetration is important, because some bacterial diseases don’t necessarily infect the top of the leaf. They look for natural openings, which often occur on the underside of the leaf. Plants have breathing holes on underside of leaves called stomata. Each stomata opens or closes based on the water status of the plant and the CO2 and oxygen exchange in the plant.

 

“That is where bacteria come in, as well as through holes caused by pruning,” said Matthew Krause, Product Development Manager, Plant Disease Management for BioWorks. “This is why you need to get good spray coverage on the underside of leaf to get good disease control.”