Lettuce in a box

I have a few lettuces growing in an old dishpan in a glassed-in porch. The dishpan is about 9" x 14" x 4" and it has half a dozen small mesclun-type lettuces in it. They grow about fast enough in short winter days on a chilly porch for a small green salad for two every week or two. I'd like to increase the production next winter by placing a large, plastic-lined box on the patio table, aiming at about four times the surface area and two or three generous salads a week. Does anyone have any ideas on how deep my box must be? I'd like to minimize the weight on the table but not make the box so shallow that it won't grow healthy lettuce.
Kathy
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Kathy wrote:

than 6" deep. -aem
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Success doesn't depend on the depth of the container, but rather temp, water and fertilizer. I grew many kinds of lettuce in the greenhouse in a couple of 8 ft sections of plastic gutter. Put a small layer of gravel in the bottom and fill with potting mix. Either predrill holes in the bottom for drainage or install a small section of downspout in the center that runs to a bucket. Put caps on the ends. The gutter can be cut to any size and can be hung or set up like a window box on the porch. You can expand the setup to your liking. Gary

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