Not sure about the plants actually. But you should visit the AVA website. I think they have something on plants. I would reckon that if you are not bringing in plants with the intention to sell/cultivate for sale, then it should be fine.
In any case, I would recommend checking it into your luggage and declaring at the customs.
P.S. I re-read your title and realised you said "take away". You might want to check with HK customs as well if you are intending to collect wild samples yourself. If you are buying from a shop, then it's probably fine.
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