that depend on the crops you want grow in understory

You need to understand that there is no free lunch - even in gardening. Just because you see space between plants, that doesn't mean it's a good idea to fill it in. A short season crop like lettuce is fine - it will be harvested before tomato roots spread too much. But plants do compete with each other - and not just for sunlight. They compete at the root level as well. You're told to space plants not just for how big they grow above ground, but for how much root space they need.

Celery like a lot of water, and tomato roots will be sucking water out of your soil like crazy once they get big. It's not that you can't get away with squeezing in a few plants among plants like tomato - it's just that there is a cost.

Its big enough for two rows of peppers/tomatoes/potatoes/etc,

Surely not all at the same time in that small bed? Not enough room for those much less anything else.

2 rows of peppers would maybe allow some interplanting of shade tolerant crops but then you'd have to figure out how to adjust for the very different nutrient needs of peppers vs. lettuce or spinach and the very different water needs of peppers vs. celery.

Tomatoes would shade out most any of the things you list and potatoes would smother them. So something has to give. Try 2 tomato plants at one end, two pepper plants at the other end and some carrots and lettuce or spinach in between.

Even Sq. Foot gardening doesn't allow for cramming something into every square inch. :)

Dave

I think that if you water enough for the celery then your tomatoes will split.

You might be able to squeeze some lettuce or spinach in, as long as you pull them out again early. But they will only be enough for one meal.

Peas have tiny root systems -- you could put some of them in. Again, a small crop.

I'd recommend strawberries, nasturtiums, or goosefoot, if you like it (I do :). I think those would produce enough to be kindasorta worthwhile, and without harming your other plants.

-- Nila, who grows a lot of things crammed together.

I think if you think in terms of growing "up" you might at least solve the shading issue to some degree. I stake my tomatoes with 12' high bamboo poles... I know, I know. you need a ladder to harvest!... but I thin out the foliage on the plants as they grow, especially down in the lower area, and this leaves quite a bit of space for sun to penetrate at most hours save for high noon. All my varieties are indeterminate, and I get an excellent harvest throughout the growing season growing in this way. If I'm disciplined about managing both tying and pruning, I'll end up with 12' high tomato towers, with the fruit concentrated above 3' off the ground. In a good season, the plants will reach the top and fold over and continue growing back down towards the ground. Anyway... enough about my tomatoes... this method leaves a lot of space at ground level. If you can deal appropriately with the root and water requirements of your various plants I think you could really cram a lot of vegetables into your limited space.