If the plant you are looking for is any kind of food plant or other useful plant, you could try the "Multilingual Multiscript Plant Name Database", at http://www.plantnames.unimelb.edu.au/Sorting/List_bot.html
It gives Chinese names in both Chinese characters and what I assume is Pinyin, also Cantonese names where applicable.
For example, asparagus bean (Vigna unguiculata subsp. sesquipedalis) is given as "Dou jiao", or as "Ch'eung ts'ing tau kok" in Cantonese.
For searching a name it uses Google, narrowed to the site. If searching on a plant name of 2 or more words you should put it in double quotes.
