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 grown onions in containers for several summers. They do grow quite well, but I have noticed that they don't grow near as large as they do if grown in the ground. The pots I've used are about 15" in diameter, and I've grown about 4 -5 onions in them, depending on how large you want the onions to get. It's nice too, because you can use the onion tops for salads and such! I've also grown them around a pepper plant (with the pepper plant in the middle) and it doesn't seem to affect the growth of either, so it's a win/win situation!