What's the soil like under the raised area?

I grow tomatoes in an 8" raised bed -- but the soil beneath the bed is pasture soil, and there isn't a significant problem with drainage.

Unless the original soil below the bed is dense clay or rock, or there's a barrier between the soil above ground and the soil beneath, or you have a drainage problem, 8" should be fine.

I'm more concerned about the 18" wide aspect. And I wouldn't want to put more than 2 tomatoes in a 6' bed, or more than three in an 8' bed.

the bed will have a bottom and be raised and supported above the ground......like a big container. water will be able to drain through with no problems.

why would you be concerned with the 18" width?

thanks

A raised bed is a planting area which is higher than the surrounding ground. Most have walls, though some do not. In nearly all cases, roots can grow down into the underlying soil.

What you have is a container or planter, and a rather shallow one. Different situation entirely. I haven't grown in containers, so at this point I'll abdicate in favor of those who do.

But I think it will help the container people to know how many tomato plants you plan on growing in that space, and if possible the varieties.

My concern with the width is because that's an indicator of soil volume. For instance, most of the spacing I see recommended for tomatoes is 24-36". That's in the ground, not in a container. Containers are different.

Generally container capacity is measured in gallons. There are 231 cubic inches in a gallon. Assuming your 8" deep planter isn't quite full (say 7"), that means every 1' of linear space in the 18" wide planter holds 6.5 gallons. So the full 6' would hold 39 gallons. Hopefully I did the math right!

Raybo's new EarthTainer design seems to hold 24 gallons of potting mix, and he plants two tomatoes in that, so presumably 2' spacing in your 18" wide planter would be okay.

Another thing the container people will want to know is your climate. If your summers are hot, they will probably recommend you not paint your (shallow!) planter a dark color. Dark colors absorb more heat from the sun, resulting in hotter soil and, sometimes, cooked roots. With in-ground planting that's not so much an issue, since the sun only shines on the top of the soil, and the temperature below ground remains much cooler.

I grow tomatoes both in raised beds and in containers. It is my observation that determinate and "bush" varieties do quite well in containers, even somewhat smaller ones. But even then, you need as large a container as you can reasonably provide. Last year I grew SunGold, an indeterminate, in a 15 gallon nursery pot, 16 inches in diameter and fourteen inches deep, and with regular fertilizing and watering it did very well.

My raised beds are built over clay soil that drained poorly when I started my garden and is on a slope. The first year or two that I had my beds I would dig, turn, and amend the soil in the beds every time I planted in them (3 crops a year). I used rotted manures, especially horse, and home made compost. This year, for my tomatoes, I have gone back and double dug those beds to a depth of 2 feet or more. It's not as back breaking as it sounds. The soil in the beds keeps the native soil underneath moist and soft, and it is full of earthworms. Over time, the soil has become as friable as you could ever wish for. I add several inches of compost over the top of the beds in early spring each year. My soil is very low in nitrogen so I supplement with blood meal regularly too.

Tomato plant roots can go 2 feet down, and they are heavy feeders. You need to give them plenty of room to roam. Tomatoes are also very susceptible to fungus diseases. That, and root room, is why it's recommended that you give your plants a bare minimum of two feet, measured from the central stem of one plant to the central stem of its next door neighbor. You'll get far better and far more fruit if you'll do this.

thanks all for that information. guess i'll try some both ways.

I grow all of my tomatoes in containers...the deepest I can find. Even when I use something 18 deep or more, the roots end up growing out the bottom--thick enough that it can be difficult to get the pot pulled up in the fall. So I think you will have several problems...
1) Where will the roots go? 8 inches isn't much space.
2) You will have a difficult time keeping enough moisture in it.
3) You don't have room for planting your tomatoes deep, which helps with extra root formation.
4) How do you plan to support your tomatoes? It will have to be something outside of your box, as I don't think 8 inches is deep enough to give stability to anything sturdy enough to support full-grown tomato plants.

According to the "All New Square Foot Gardening" book, that will work fine. They use even shallower boxes.
I used 6" deep beds, and got wonderful tomatoes, very productive, from mine. I didn't grow in above ground closed beds, but I can tell you that the roots will grow laterally if they can't grow down, (mine did) and they stood up just fine. I was amazed at the root system when I pulled them in the fall. I used nearly 100% mushroom compost.
I'm building a couple of beds for my mom like yours for a Sun Sugar and a few short determinate tomatoes, Ida Gold, Whippersnapper, and Tumbler. I know she will have to water daily, and I'll add Azomite and Tomato-Tone when planting. I'll add some vermiculite to hers for water retention.
Don't let anyone who hasn't tried it tell you not to try. ;-) It has worked out for others. Keep reading on the net and in books. Good luck and good weather!

I have no problems growing tomatoes in raised beds either.
the big problem I have is trying to grow the tomatoes in the SAME raised beds every year... I grow them In big pots now because the soil in the beds are now infested with wilt from growing the same family of plants every year.

I tried a couple last year just to try and .. they died in the middle of the season while the ones in my big containers grew wonderful !

this year I'll be solarizing the beds ..although other plant familys did just fine

I am now a firm believer in crop rotation and not building any spot specifically for one type of plant every year