The correct way to say it is: Where is the food planted? We don't really plant food. We plant food seeds, which grow into food. But usually if I were planting food seeds I would say to my friends "I am going to plant some vegetable, herb or fruit seeds".

This depends as both can imply slightly different possible meanings. 1. Where the food is planted. - This could mean a location or place where you plant the food. (Like a farm, or garden) For example: 'Where are you? I'm where the food is planted.' 2. Where are the food plants. - This sounds like a question about the plants, literally 'what is the location or place of the food plants?' or 2. Where is the food planted? - This is a re-ordering of the first sentence, again asking about the location where you plant the food. For example 'I have some fertiliser for the soil, where is the food planted?' Either way, I would consider using 'crops' instead of 'food plants' if you mean plants that you can eat, as it's a more natural way of saying it. Hope this helps!