I wouldn't put it past a determined cat to knock the whole thing over.
I'm pretty sure Sibley in her prime could have. I know my mom's cat Bart could, because he did knock something similar over. Mom's cat Jesse would probably climb up and over somehow, she's small enough she couldn't budge the plant but she's a good climber and has gotten herself into odder places. My Arwen wouldn't bother probably, she's not interested in catnip. Now, put a mouse in there and all bets are off - Arwen will manage something.