George Hamilton discussed how control flow valves can benefit high tunnel growers during a recent webinar. Photo courtesy of George Hamilton

by Sonja Heyck-Merlin

Jonathan Ebba is a landscape and greenhouse field specialist, and George Hamilton is a fruit and vegetable production field specialist. Both are employed by University of New Hampshire Cooperative Extension and gave a presentation titled “Spraying in Tunnels: 5 Top Tips for Success” during the third and final session of the High Tunnels After Dark: 2020 High Tunnel Production Conference. The series was a collaboration between Maine, New Hampshire and Vermont Extension.

The five tips were primarily for growers who use backpack sprayers in their high tunnels, although the tips are applicable to those using different types of sprayers.

Tip 1 relates to pressure fluctuations that occur with a hand pump backpack sprayer. “As you’re pumping, the pressure goes up and down unless there is a control flow valve built into the sprayer,” Hamilton said. The effect of this inconsistent pressure is an uneven spray pattern. He suggested installing a dual compensating control valve that will compensate for lower and higher pressures.

“The way this tool works is, if you have to maintain 21 pounds of pressure, it will not allow any spray to come out if you are under 21 pounds of pressure. This way, you’re always spraying at a constant rate, so you don’t get high and lows as you are spraying down the row,” Hamilton said.

Using water-sensitive paper within the canopy of whatever is being sprayed was Tip 2. Water-sensitive paper is a rigid paper with a specially coated yellow surface which will stain dark blue as spray droplets land on it. It’s used to evaluate and monitor the distribution and density of a spray. “Using this tool, you can get immediate feedback about your coverage,” Hamilton said.

According to Hamilton, it’s important to check the distribution of your spray in a test before spraying the entire crop. He said, “Once you complete the test spray, let the surface of the paper dry, and you get a pattern of how much spray is on that surface. If the surface is saturated, you’re using too much. The ideal for most fungicides or insecticides is about 70 droplets per square centimeter.”

Ebba provided Tip 3: Using adjuvants, particularly spreader-stickers. Adjuvants are substances added to a spray which enhance its performance. “Normal water droplets sitting on a leaf bead up,” Ebba said. “Spreader-stickers work to break the surface tension, and the spray is less likely to roll off. They also help the absorption of the spray into the leaf tissue as well.”

It’s critical to check the spray label to see if the spray allows for the use of a spreader-sticker. Sometimes it may already be included. Hamilton stressed that in order to avoid leaf burn, spreader-stickers should not be used in high temperature or high humidity situations.

“There’s always the recommendation to use a small-scale trial for plant safety,” Ebba said. “Also, some of the stickers on the market are only for herbicide use, so make sure what you’re getting is labeled for the crop you’re using and the pesticide that you’re using it with.”

Tip 4 relates to body and wand position as you’re spraying. According to Ebba, the position of the wand is very important, especially if you’re dealing with a pest, such as the whitefly, which are often found on the undersides of leaves.

“If you’re using a backpack sprayer, invert the lance, so you can get up under the leaves. Then come back over the crop with a downward spray. You want to get in there and physically move some of those leaves,” Ebba said.

Tip 5 was to alternate your starting point. “If you always go down the row in the same direction, you’re always spraying at the same angle, so you get the same type of coverage,” Hamilton said. He suggested starting at the opposite side that you typically start on and go in the opposite direction. “This way, you’re facing the plant from a different direction, and you get different coverage on the pest you’re trying to control or the host plant. It will also help with overall coverage through the whole growing season.”

Hamilton and Ebba emphasized the importance of carefully reading the instructions for any substance sprayed within a high tunnel.