Well, I really don't understand why this a concern of yours. Are there lots of roots exposed, or something?
To answer your question about landscaping fabric...there's no way that it will stop tree roots.
If I was renting the place, i'd go ahead and build the boxes in the spot you've selected, then plant them.

EG

I am not sure that there is a good answer to your question because some plants are just not going to grow well near a black walnut tree because of the toxins.

http://www.extension.umn.edu/yardandgarden/ygbriefs/h407blkwal-tox.html

I wouldn't let that stop me from trying though and I would probably just put landscape fabric on the bottom and maybe set the boxes on cardboard. It would probably be OK for one season anyway before the cardboard breaks down completely. I have ornamental plants growing near my walnuts and they are OK, but vegetables probably need a barrier from the ground below.

do you have any other place to put your sfg? or is this the only space you have?? just curious ~Medo

Thanks all for your thoughts!

Yes, this is the only place I can plant. It is a typically smallish downtown Toronto yard, with several trees shading the (Eastern) 2/3 of it. That leaves the back (Western) 1/3 open for planting. I've actually decided to do fewer beds because I have become less optimistic about my sunlight situation (and my budget for buying soil).

The research I've done suggests that the highest concentration of Juglone is in the roots of the tree, and if I can keep the roots from coming in contact with the soil around my sensitive plants (as well as keeping the beds away from the dripline and clearing them of any Walnut tree debris), I am hoping that even the sensitive plants should be ok.

The whole yard is pretty dense with tree roots (maple, cedar...) which could also creep into my beds and, while not poisoning my plants, they could crowd their roots and feed on their nutrients. I've been told that as I water my garden, the tree roots are likely to rise towards the surface of the yard where the moisture is, rather that growing down.

I hope I don't sound too paranoid, but I figure I should try to do this right the first time.

Have you considered containers?

ok, i get you!! LOL
besides containers, i use plastic on the bottoms of my beds, because i have quack grass, its the most GOD awefull stuff i have ever seen!! and it grows thru anything!! but not thru my plastic. so of course the roots won't come up thru there. :')
also there is the idea that maybe you could clip back your tree limbs? then maybe so much wouldn't fall on your plants??
~Medo

Have you called your local extension? I'm guessing this is a pretty common problem in your area. I wish I had better information for you, sorry.

If that was something I felt I *had* to do, I'd look more at digging down to put a real root barrier along the outside edges going ~18" straight down. That approach will even stop bamboo.

i know, i had to dig this up, sorry, but i have one too!! we got one cut down, i have one more out back, and now two pecans, those i want to keep, because apparently they are not native, my fil says they are alot bigger than native ones. they look small to me!! LOL walnut tree's make me irritated because they make those things fall all over the ground, and we have to pick them up!! and if you don't keep up on them, WHOA baby!! HUGE mess!!! LOL ~Medo