If the soil is hard and not mixed in with that on top, I don't think they will grow very deep. They will go deeper in loose soil. I had some in potting mix big pots that had roots to the bottom. The pots were at least two feet tall. The roots will go where they are happy.

I've linked below to what I consider to be the definitive article on tomato root structure, depth, etc. Just scroll down to the Tomato section and you'll see that there's two kinds of root structures , tap root and fibrous, one wants the latter, and the roots can extend down and out many many feet.

Note that this article was written in 1927. Most of the basic information about plant growth and more was done in the years before genes were Ided, molecular work was done, etc.

Carolyn

Here is a link that might be useful: Tomato Root Structure

Tomatoes in my tilled gardens will often send the tap root down 18" or more because it is great soil well amended for years. Pulling the dead plants can sometimes take the ATV.

But that doesn't mean you have to have a bed that deep. ;) Tomatoes in my raised beds will go as deep as the bed allows (mine are 8" and 12" deep beds) with the tap root and then run it laterally for another 10-12 inches. You go to pull them up and the 2 foot long tap root just pops right out like a 2' long mole tunnel. ;)

But it's the fibrous roots that are vital to the plant as the article Carolyn linked indicates, so your 12" deep bed will be plenty.

Dave

Yet again, Dave is right. ;-) The tomatoes will grow down 1 ft to your "bad soil," then hopefully start growing sideways along the top of it. If this bad soil is clay, they will for sure grow along the top of it. If it's a light/porous soil (but still bad...), they may grow into it. You could put some type of gardening cloth/covering on the bottom of the raised bed, if it's easily done at this stage. If allowed to do so, tomato plant roots would probably easily grow 2-3+ ft deep. For example: My seedlings grow about 5-10 rings around the bottom of their cups (probably about 6-7 inches deep) before they get transplanted/potted up to their final homes (I try not to let them grow like this for too long, so they don't get root bound. I'll pot up if absolutely necessary). When I remove the plants/soil from the containers at the end of the growing season, they are growing rings around the bottom of an 18 inch deep container & about halfway have it filled with just roots. Anyways, like Dave says, I think your 1 ft deep bed will be fine if you keep a mineral rich (with compost or whatever you prefer) mix in it. - Steve

My clay soil is so bad and so hard that roots of most plants can't grow in it. When I pull my tomato plants up at the end of the season I see they are hardly any longer than they were when they were in the four inch square pots I use for my seedlings. I have a rototiller that is supposed to till to eight inches deep, but my soil is so hard that it can only till to 4 inches, and that only after we get a rare rain during the winter. And that is after I put a more powerful 8 horsepower engine on it and carbide tipped tiller blades. I am not kidding, I bought a Bosch Brute 66 pound electric jackhammer (the big powerful one that costs $1300.00) and a clay spade to use for planting my fruit trees and tomatoes. It had a blade about eight inches long and half an inch thick made out of tool grade forged steel, and over the course of the years I have worn two inches of the metal away digging holes. What makes matters worse is my garden area in on a steep hill and water just runs off it so the only way I can keep water near my plants is to make circular dikes around them to keep the water next to the plant until it can sink in. I get very little help from mother nature as the previous three years were all drought years averaging about a total of seven inches of rain per year. I put seventy cubic yards of compost on my garden over the years but it hardly made a dent in the soil structure. During the winter rains it turns into sticky mud, and during the rest of the year it turns hard as a rock and cracks and crevices develop in it, some seem to go all the way to China.

Since you can effectively transplant the root ball over 18" deep, it is a moot question.