Perhaps not perceivable, but you can remove the other potential causes to narrow the difference down to a genetic factor and make a reasonable assessment. But the caretaker can't be held responsible for something's innate inability to thrive. Those issues are statical outliers. Take a random assortment of mint plants and most will respond to the same care with only superficial differences.

And we could be here all night and the next morning trying to establish a metric for when life ceases to become worth living.

[Which is Alhaitham's idea of a good time, granted.] I wouldn't give up over onions.