I'm a bit fuzzy and tired, so maybe this comes out weird, but here goes . .

What could you do to engender enough empathy for a single shrubbery (not nature in the abstract) that you could then set it against some act of cruelty or sadism and get a public reaction?

I don't think it's possible, even for the most sympathetic of hearts, to feel empathy for plants. They're just too different, and that's not the plants' fault or ours, we just don't recognize signs of distress from plants on an instinctual level. Me for example, if a dog has trouble walking, a bird trouble flying, a fish trouble swimming, I'd recognize it in their movements and can guess to a good extent how injured they are, if they're feeling pain or showing signs of fatigue, and sometimes I'd even go as far as to assume their emotional states (though I will readily claim that even though I can't tell if a fish is scared or upset, I can tell in a dog). Plants don't communicate distress in ways that I readily recognize, so I pretty much always overlook any potential that they're feeling it, and even when I know plants can signal distress and learn to recognize it, I'm not gonna have the level of emotional reaction I'd have if I were witnessing a puppy in distress. I feel it's safe to say I'm not alone in this dilemma.

Is there a way to feel empathy for flora without personifying it?

Maybe the goal instead of empathy should be fascination? If we can't appreciate them on an emotional level, we can appreciate them on an intellectual level, and that intellectual appreciation and fascination can lead to an emotional drive to admire them and want good things for them. I wish Hubski had a botanist. I listen to podcasts about plants from time to time and they're pretty cool things. I've always appreciated plants, but learning what I have in the past few years, my appreciation for them has grown in leaps and bounds and while dogs and birds and lizards will always take top place in my heart, affection wise, there's plenty of room for a good tree.

If you really wanna tickle your philosophical funny bone, there are discussions out there from everything from plant communication, to perception, and even cognition. But even more mundane stuff, like how mangroves are super specialized to thrive in brackish waters or how multiple times over, various branches of plants independently evolved to be carnivorous. Wanna really get into things? Read up a bit on root/fungal networks.

Sometimes though, more simply, we can just appreciate that they're often really, really nice.