you could anchor it with rebar stakes such as those in the NHZ 16-inch J-hook ground anchors 8-pack or attach large screws to the back wall of the planter box and balance the trellis on top of them.

it’s a rectangular wooden planter box on the bottom with a tall wooden trellis attached to the back. The box is a little elevated off the ground thanks to the four little legs, and the trellis ...