You need to have at least 12 inches deep of soil for something big like a tomato. So you can either achieve that above the landscaping fabric, or you can "double dig" down into the soil at the bottom of your raised bed to allow the roots easy access into the ground. I did the landscape fabric thing and I had a hosta sprout right up through it. And weed wise, I think if you put 12 inches of soil on top of the weed, it's probably gonna die. You'll have more trouble from weeds seeded at the top of the soil. The exception may be if you were planting on top of grass, or in an area where other plant that spreads via roots may try to invade your space. For example you may have heard NEVER to plant Mint in the ground. That's cause it spreads via the roots. So if you put your raised bed next to a mint patch, that mint would move right on in to the nicer neighborhood.

Long story short, I don't think the landscaping fabric does all that much. But if you want to try it, just make sure you have enough soil depth on TOP to support the plant.
Does that help?
Susan

Thank you so much for your reply Susan !!!

"For example you may have heard NEVER to plant Mint in the ground."

Well that is what I did. hmmm. I have a BIG MESS. That is one of the reasons why I want to do raised beds. I thought I might be able to smuther the mint by doing raised beds. Between the beds I would have something to cover the landscaping fabric. I could spray the whole garden but I really do not want to do that. Do you have any suggestions how to get rid of the mint. It is rampant !!

Again Susan, thank you so much for your help. I really appreciate it.

Rolf

I don't have any suggestions on the mint, but when we started our SFGs we lined the area with cardboard. Then hubby placed the garden liner and we filled it up. Ours are 8" high and I've planted tomatoes for the past two summers. They have grown very nicely. (Too bad the wilt got them the first summer.)

Good luck and use what you have on hand, always. It keeps things much more practical.

RJB-you do not need 12" of Mels mix to grow tomatoes. That will cost you a mint. You only need 6"-its all I have ever done and have the pictures to show it. The reason is simple: with all the nutrients in 6" of Mels mix, plant roots dont need to travel further and further down for water and nutrients. You would maybe be correct in a regular traditional garden, but not the SFG. Really-its all you need if you do it right.

Here is a youtube about the single stem method for growing tomatoes in a square foot garden.
There is also a section in the book about this method along with planting tomatoes horizontally.

Here is a link that might be useful: Growing Single Stem Tomatoes Vertically in a Square Foot Garden

On the first set of beds I did about 4 years ago I made them 8" deep and put landscaping fabric on top of them. I did this to keep the bermuda from coming up into my beds. I originally had a yard full of bermuda and even though I had killed it all off with Roundup, it still kept coming up from the rhizomes down deep.

The problem is, that the bermuda and other invasive plants will just follow the water right up into your bed. They found a way to come right in the side of the fabric, between the staples.

End result? I still had weeds and bermuda in my beds and I had this annoying landscape fabric in the way was well.

I rebuilt my beds last year and used 12" redwood and did NOT use landscape fabric. I grew some fabulous daikon and carrots this year that I'd have never been able to grow in those old beds.

"On the first set of beds I did about 4 years ago I made them 8" deep and put landscaping fabric on top of them."

I meant "put landscaping fabric on the bottom of them"