Finally figured out the difference between repellant Citronella (Cymbopogon nardus) and edible Lemongrass (Cymbopogon citratus) plants, both are from the Cymbopogon family commonly referred to as lemongrass. In Chinese, they are all called 香茅草。
I have both plants in my garden. The one that flowers now is Citronella, not Lemongrass. Here are some photos to show the difference between the two.