When utilizing any container for crops it is important to know what you want to sprout. For example, although spinach and green onion will work well I certainly wouldn't want to try a potato. Root vegetables (ie potato, carrot etc) need more depth for growth than most containers will allow. Yes, onion can be considered a root vegetable. Stick with the simple to grown green onion for success. Pick something that has a hardy root system but doesn't require a lot of room to grown. This will make plants such as okra, brussel sprouts and spinach flourish in containers. They have strong sturdy roots and they grow completely above the soil.
Containers are a very benefical way to keep crops gowing no matter the season. It is important to be familar with the crops you want to grow, the amount of space they require to flourish and the enviorment that best suits the need. With a little research, and perhaps a small amount of trial and error you can put delicious fresh food on the table any time of year.