Raised Bed on Bedrock

[DeeEnvy]

I have the good fortune to be on the Great Canadian Shield but this is not an advantage if you want to put a garden in. My maximum soil depth is maybe a foot on top of the bedrock. Great for building houses.
I have read spotty information on how big the root ball is and various sizes of containers but nothing specific on how big a raised be should be keeping in mind the roots cannot go deeper than the bedrock below the bed.
What size should I have for a raised bed for depth and area for one plant?
I am starting with four rhizomes and will be building raised beds to handle two plants each. Further expansion is in my future as well.

 

[GVH_Dan]

Yeah, the crown (root ball) can get pretty big. The rhizomes can stretch out a good 5 to 7 feet away and the tap roots can go a long way down. So a raised bed should be about 8 feet deep.

OK, that's a bit excessive. Give it a couple of feet and let roots break up the bedrock.

Or there's a little document from Dupont you should check out...Farming with Dynamite

 

[DeeEnvy]

Thanks Dan.
That is what I thought.
I guess I will do a 30" high bed and 4' x 4' for each rhizome.
I should stay with beans. Much easier here.
But what's the challenge in that?