Short version:
Amend, amend, amend.

use those coffee grounds! Green will love them!

Add compost! Edible landscape is the new big thing. Best wishes for a great garden.

it's too bad you can't build a raised bed there. i think herbs/greens at ground level will get splash off from cars, occasional trampling by humans and dogs (along with dogs marking/urinating and if cats are around possible pooping in the garden)

i wonder if you could slightly mound it up towards the middle and just plant items around the edges that will protect your greens in the center. so the outside stuff would just be edibles for show and the stuff in the center could be used for the cafe.

i also think fruit trees are a good option because everything will be well off the ground...

> The only space near the cafe is a strip about 1 1/2 meters wide and 15
> meters long, between the sidewalk and the street. That's with a 1/2
> foot of grass space left on all edges of the garden bed for soil
> spillover.

I'm confused about this part. It makes me imagine:

Sidwalk
Six inches of grass
4.5 feet of vegetables
Six inches of grass
Curb
Street

Is that correct? If so, then I would strongly (ever so strongly) urge you to remove all of the grass. Grass is an aggressive weed, and you will spend the rest of your life weeding it out of the vegetables. If you want to leave a six-inch edge free of vegetables, I would suggest instead planting a low perennial ground cover, like creeping thyme.

As a separate issue, I would also strongly advise against eating anything grown right next to the street, and especially urge against eating anything raw. What about growing plants suitable for flower vases instead? You could demonstrate the _idea_ of urban gardening, and display flowers and other decorative plants in vases on the tables, without any risk of inflicting food-borne illnesses on your patrons.

Editing to add: "Plants suitable for flower vases" could include edible flowers and attractive edibles - perennial herbs, chives, daylilies, and, to fill space the first year, nasturtiums.

This post was edited by chickenfreak on Sat, Jun 8, 13 at 21:52

Whether you'd need to remove soil or not depends greatly on the type of soil you already have. You will need to amend and fertilize virgin soil however. I agree I would take out all the grass. Though instead of plantig thyme which would cost money, I'd use stones as a border. You should be able to find bricks or broken concrete ("urbanite") for free at construction sites or Craigslist or freecycle.

I grow some of my veggies on my parking strip and I think it can be fine for consumption. But it depends on how much traffic your street gets. In a crowded commercial area you may want to consider something other than crops that are eaten raw.

Another thought I had is to plant things that people recognise and will tend to steal anyway. Raspberries and tomatoes are always stolen when grown next to the sidewalk and that may be desirable for you since it would be a demo garden after all.

The grass edge is actually now marked out at 1 foot all the way around. I had hoped this patch would absorb runoff and filter soil from hitting the streets and sidewalk, and double as a relatively clean place for gardeners to work from.

I can not mound the soil in any way because will promote loss of soil, and all the good amendments! Edging with some sort of protection is not a bad idea, but I think I will leave a lot of the perennial development of the garden to another year. Right now I want to get the garden growing something, and for it to look good.

As for the edging, that sound pretty important. Would rock edging really do that much more than plastic garden trim/edging? Sinking rocks into the edge sounds like a ton of work, and I don't think we're ready for something so permanent.

So far I'm gathering that I don't so much need to replace all the soil, as add some good compost (the cafe just began their 3-bin compost 2 weeks ago) and maybe some coir as required.