Asparagus in planter bags?

I've got a quick question about planting asparagus in patio planter bags.

On sites that sell them in kits with crowns and bags they have them with the deep potato type bags. I've got quite a few of these but also a few that arent as deep and range between 1 and 2 and a half foot deep .... how deep do these need to be planted and would I be better using a deeper potato bag?

Or as these will be in place for a while would I be better buying a 'uuuge pot to them in and if this would be better, how big? .... I've got 3 crowns.

It's all concrete here Yorkie. Everything I grow is either in a pot, planter or in the greenhouse 

It's worth a try if you want to spend the money I suppose but asparagus in a bag is a bit like rhubarb in a tub.

Deep roots needing LOADS of water but good drainage.

To give you an idea of the size of asparagus roots, have a look at this link and scroll down a bit:

http://www.asparagusgardener.com

I had an asparagus bed of 48 crowns for about ten years until the plants were spent.  When I dug up the bed it was like huge octopus tentacles growing deep into the ground.  This is how asparagus rejuvenates itself every year and makes it so yummy.  It's a long term commitment and I doubt whether you're going to have any long term success with them in a bucket or a bag.  If you consider the effort and the payback you might as well buy a few spears from Sainsbury when you need them.  When my asparagus bed was in full flow you've got no idea the pleasure that you get for a few weeks of the year but it takes up alot of land and the asparagus beetle will find you and he's a real pain.  Beautiful insect that I bet you've never seen before but it will totally decimate your crop.  It will find you and they don't sell derris dust any more which is the only effective way of killing off asparagus beetle.  For all the effort/expense of growing a few crowns of asparagus in a bag/bucket you'd get more pleasure growing daffodils instead