Got some citronella plants （驅蚊草） from Mongkok the other day. For HKD 20, they are a good little herb to grow at home. The smell is pleasant and in fact if you go to Thailand for holidays, you would notice the smell immediately.
If you think they are going to drive mosquitoes away as they claim in the flower market, you are probably out of luck. As most studies indicates that the plant is ineffective in mosquito repelling.