Avocados are fairly heavy feeders and as a consequence produce a fairly high proportion of surface roots compared to sinkers. In my experience, they are no more invasive than many commonly grown ornamental trees (white mulberry and sweet gum come to mind). Obviously a container will limit the spread of the roots, but it will do little to control the total amount of root mass produced - instead of spreading widely through the soil, the roots will grow around inside the container. Once it begins to become pot bound, there are two ways to deal with the plant: potting on (moving it up into a larger container after the judicious trimming of any girdling roots) or root pruning (drastic reduction in the size of the rootball once the plant has grown into the maximum sized container you are willing to deal with).

The container gardening forum or tropical fruits forum have a lot of people more knowledgeable than I in container culture.

Ryan

Mine do not have outrageous roots. But Hass avocado trees get very, very big. They have normal sized roots for big trees. Containing the roots would probably assure a broken planter and no avocados, because the roots of avocados are very sensitive. You cannot plant beneath an avocado tree, for instance, if you want avocados. The feeder roots at the soil surface, down to about a foot deep, should never be disturbed. They also have to be kept cool, which would be hard to do in a planter.
Renee
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What you would need to do would be to keep the top growth small in proportion to the smaller root system. I recently went to a lecture by a guy from Dave Wilson (wholesaler of fruit trees) and he said you could do that--keep an avocado trimmed down to about 8-10'. Of course, you are going to be out there trimming it frequently.

Having said that, you want as big a planter as you can possibly give it, like 24"x24" or bigger, and as Renee said, you need to figure out a way to keep the root system cool and moist, perhaps with an outer white pot to insulate the inner one. Long term, train the tree into an umbrella shape so that the top growth will shade the container.

Of course you are not going to get the yield you would get from a large tree, but if a few Avocados is enough, well there you go.

And if you can return it and get a Reed instead of a Hass...the Avocado growers have a saying: "Hass you sell, Reed you eat."

Sean, you can also try a dwarf growing tree like 'Little Cado' which supposedly does better in a pot. Even if they are not as tastey as Hass, I figure anything homegrown will taste way better than store bought.

I have been vascilating all year whether to plant an avocado tree due to size issues and cold tolerance.

Thanks Hoovb for that info, I just might plant one and try to keep it small.