This might look like a good idea, but you need to include the energy costs from building all those tiny robots. Fixing them when they break down. Building a strong enough roof to hold them, 6 inches of topsoil and your crop. Irrigation and fertilizer and pesticides for that crop. Storage space for that crop for the better part of a year including some form of climate control to minimize spoilage etc etc.
Honestly, sectioning off the mid west and transporting the food to you by trains is actually a much better idea. You can still use robots but you get to use really big and cheap ones that do a simple job and get regular maintenance by people that know what their doing. You can also minimize the contamination from air pollution etc.
PS: Moving 2 weeks supply of food around the world by boat can easily take less energy than driving to a 5 miles to the super market to pick it up. If you want to be as environmentally friendly as possible, move to a big city and become a vegetarian. Living in the countryside feels more natural but it's much much worse for the planet.
Once robots are growing the food, a lot less of it will be imported. Even with the cost of maintenance, robots will be cheaper than human labor, eliminating a major reason for importing.
I guess if you think of robots as they are today, it may seem like they'd break down a lot. But agricultural robots will eventually be more like insects and, like many living things, cheaper to replace than repair.
