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Fate of pre-emergence herbicide applications sprayed through containerized hydrangea canopies

Derksen, R. C., Altland, J. E., Rennecker, J. C.
Journal of environmental horticulture 2012 v.30 no.2 pp. 76
Hydrangea, air, application rate, canopy, container-grown plants, droplet size, droplets, herbicides, leaves, nozzles, pesticide application, seedlings, soil, spray coverage, spray deposition, spray volume, sprayers, spraying, weeds
Preemergent herbicides are applied to the soil or potted-substrate surface to prevent weed seedling establishment. 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. The objective of this work was to determine the effect spray quality, spray volume, and air delivery had on delivery of sprays to the substrate through a potted hydrangea canopy. Petri dishes and water sensitive paper were placed on the substrate surface of potted hydrangeas (H paniculata 'DVPpinky')to collect spray material falling through the canopy. Eight targets were used for each plant and were placed 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 liters/ha (20 and 40·A -1)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 (2.5mph). No irrigation was applied either before or after treatment applications. 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 liters/ha (40 gal·A-1)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. On average, only about 5% of the spray actually reached the intended target (substrate surface) across all treatments and approximately 50-60% of the spray material was accounted for on the foliage. Larger droplet sizes and higher spray volumes will help ensure better delivery through the canopy.