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Title: Fate of pre-emergence herbicide applications sprayed through containerized hydrangia canopies

item Derksen, Richard
item Altland, James
item RENNECKER, JOSHUA - The Ohio State University

Submitted to: Journal of Environmental Horticulture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2012
Publication Date: 6/1/2012
Citation: Derksen, R.C., Altland, J.E., Rennecker, J.C. 2012. Fate of pre-emergence herbicide applications sprayed through containerized hydrangia canopies. Journal of Environmental Horticulture. 30(2):76-82.

Interpretive Summary: Preemergent herbicide are used in potted shrub production to reduce incidence of weeds that could reduce plant health and increase water and nuturient use. However, delivery of these herbicides through a canopy to a substrate surface are not well understood. A better understanding of the fate of these herbicides could lead to more efficient delivery and reduce off-target contamination. A series of field trials were conducted using a motorized greenhouse boom to apply a fluorescent tracer spray mix over the top of potted hydrangea plants (50 cm tall). Spray delivery parameters that were evaluated included spray volume, droplet size or spray quality, and air-assisted delivery. Test revealed that on average, only 5% of the applied rate of tracer actually reached the target surface under the canopy. Foliar analysis revealed that most of the spray was retained in the upper-third of the canopy. Spray coverage across the substrate surface was also affected by the canopy. Treatments in this study produced only 2-10% coverage and 17-41% spots/sq. cm on the WSP on the substrate surface. However, further studies are necessary to determine if this level of coverage and spot density will provide the desired level of control. On average, the air-assist treatment as used in these trials would be less effective than the non-air-assist treatments because it produced the lowest mean deposits on the substrate surface as well as the lowest spray coverage. Slowing air speed and increasing droplet size would reduce the chances of spray reflecting off the substrate surface. The spray deposit results by spray volume or spray quality used in these trials were inconsistent. While the Medium spray quality XR8002 and XR8004 treatments demonstrated a potentially greater density on the ground targets under the canopy than the Very Coarse treatments, the higher spray volume, Very Coarse spray quality, AI11003 treatment produced greater spray deposits and spray coverage on the ground targets. These results demonstrate the need for improved methods to delivery herbicides to substrate surfaces under a canopy. High volume and larger droplet size delivery will help producers make more efficient applications which could reduce overall herbicide use and hand-weeding requirements during the production cycle of potted nursery plants.

Technical Abstract: Preemergent herbicides should be delivered to the soil surface for maximum effect. Spraying through a canopy above the soil surface represents a challenge because of the filtering effect of the canopy on the spray stream and because of the additional distance created between the soil surface and nozzle by treating above the canopy. The objective of this work was to determine the effect spray quality, spray volume, and air delivery had on delivery of sprays to the soil surface through a potted hydrangea canopy. Petri dishes and Water Sensitive Paper were placed on the substrate surface of potted hydrangeas (var. Pinky Winky) to collect spray material falling under the canopy. Eight targets were used for each plant and were place around the circumference of the pot in the four cardinal directions. Different sizes of TeeJet flat fan Extended Range (XR) and Air Induction (AI) Nozzles were selected to provide 187 and 374 L/ha application rates with Medium (XR) and Very Coarse (AI) droplet spectrums. A specially designed, five-port, air-assist delivery device was used to make air-assisted delivery applications using TeeJet XR8001 flat fan tips. Treatments were applied over the top of a 3 x 5 arrangement of potted plants at a speed of 4.0 km/h. Spray deposits on foliage sampled from the top of the hydrangea canopy had 8-10 times higher spray deposits than foliage from the middle elevation and the targets on the substrate surface. Surface coverage under the canopy ranged from 2-10% and average spot density ranged from 17-41% spots/cm on Water Sensitive Paper targets. Overall, the AI11003 used to make Very Coarse spray applications at 374 L/ha produced the highest mean spray deposits and coverage on the soil surface. The air-assist sprayer produced the highest deposits in the canopy but the lowest deposits and coverage on the substrate surface. The air from the air-assist sprayer likely reflected spray back up off the substrate surface. At the 187 L/ha application rate, the XR8002 nozzle producing a Medium spray quality produced higher spray coverage and deposit than the AI110015 nozzle producing a Very Coarse spray quality. On average, only about 5% of the applied dose actually reached the intended target (substrate surface) across all treatments and approximately 50-60% of the spray material was accounted for on the foliage. Producers can use this information to understand how little spray actually reaches the substrate surfaces if it has to travel through a canopy. Larger droplet sizes and higher spray volumes will help ensure better delivery through the canopy.