BACKGROUND One theoretical advantage of using unmanned aerial vehicles (UAVs) to spray pesticides for maturing corn is that the strong downwash penetrates canopies. However, only few studies have been conducted to examine in-canopy downwash characteristics. This paper investigated the downwash by a six-rotor UAV in mature cornfields. 3D wind speeds in corn canopies and an open area were measured, and comparisons conducted. RESULTS The downwash by the UAV resulted in in-canopy maximum wind speeds. Z-dimensional downwash was sensitive to all factors, whereas the X- and Y-dimensional downwashes were related to layers and crop positions. Meanwhile, when comparing with the downwash between a 2 m hovering position and the optimal flight parameters, the X-dimensional and Y-dimensional motion time of top-layer downwash generally advanced by 3.8 s and 1.6 s, whereas both motion time and the strength of the Z-dimensional downwash were impeded by ≈2.2-s hysteresis at middle layers and ≈4.5-s time reduction, respectively. Thus, combined with distributions, the corn on the left or right might not be sprayed sufficiently. Furthermore, under the convergence requirement error of 0.01, the overall correlation of the model was ≈0.846 in terms of the Z-dimensional downwash and ≈0.55 and 0.61 for the X- and Y-dimensions, respectively. CONCLUSIONS The selection of operation parameters should mainly consider the Z-dimensional downwash. The optimal operation parameters were a height of 2 m with a speed of 4 m s-1 . Meanwhile, the canopy effect could influence the uniformity, motion and strength of downwash. Predictions could be achieved before operation. © 2022 Society of Chemical Industry.