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.

We also appreciate these features: The optional trellis supports ... Secure the lattice panel to the back by driving screws through the panel legs into the planter box. Screw shelf boards to ...