Question: I have a rather large balcony and love onions. Is it possible to grow bulb onions in a container? What would the minimum dimensions of a container have to be to grow them? Could they be co-planted with something else to maximize space?

Answer: 
Yes you can grow onions in containers. You can usually buy onion sets from local stores which will produce a good size onion much faster than starting from seed. Garlic works as well. We are currently growing it in 2 inch diameter pots from a clove bought from the store.

With containers, bigger is better subject to some limitations.

* wet soil is heavy. Check the load limits for your balcony if you are above ground level
* soil much deeper than twelve to eighteen inches will not be used except by trees or shrubs.
* make sure you have adequate access to water. Even large containers dry out and in hot summer conditions regular watering is essential.
* in regards to recommended size that is dependent on a number of factors. At least twelve inches deep and eight to twelve inches wide but bigger is better.
* you should be able to intercrop with lettuce and radishes as these grow fast and should be harvested before the onions get too big.

I hope some of the other vegetable gardeners will add their ideas on what else you can grow.
I've been container gardening for a number of years, as I'm limited in the available space I have at my town center terraced house. The containers are all at the back of my south-facing house. I also have a small plastic green house. In the containers I grow onion sets, spring onions, lettuce, beetroot, asparagus, potatoes, strawberries, a gooseberry bush, an apple tree, a cherry tree, two pear trees. In the green house I have two fig trees and two goji berries.

Results are slower than in the open ground, but it provides me with an adequate supply for a period of the year. Though crops are not as heavy with the restriction of space, so you can possibly grow any vegetable you may wish and the same is with fruit. 

Best of luck, and happy gardening