Good answer from floral - few who advocate the concepts of so-called "companion planting" ever take the differing needs of the plants into consideration. So what sounds good in theory doesn't work in actual practice most of the time.

So it isn't a question of the root depth. Rather it is a question of nutrient and water competition vs. compatibility and perennials vs. annuals that you need to evaluate when making your choices.

Dave

I don't think you can look at soil strata as being nutritionally independent, such
that deep rooted plants get their nutrients only from deep soil and shallow
rooted plants get their nutrients only from shallow soil. In fact, soil
nutrients percolate (mostly downwards) through soil. Plant-usable
nitrogen in particular is highly water soluble. So if you have plants
whose roots overlie those of other plants, the latter will end up
nutrient-deficient.

Thanks everyone.

Floral, what kind of lettuce would you use? Would you keep it small so that it would resemble more of a grouncover? Any lettuce at full size would start to compete for resources with the brassicas no? I remember the lettuce that I grew got pretty huge and many called for 1 foot spacing. That was the romaine Paris island one

You were growing a head lettuce variety (Romaine) and yes they require spacing and can get quite large. But there are hundreds of varieties of leaf lettuces that can be scatter sown and grown like a ground cover. Look into some of the cut-and-come-again leaf lettuce mixes.

Dave

http://www.territorialseed.com/product/London_Springs_Lettuce_Mix_Seed

I grow Little Gem and various leaf lettuces. Kale, collards etc all grow large and need quite a bit of space between them. I don't see much problem fitting lettuce in between.

Brassicas tend to actually be shallow rooted. I would not plant anything too close. Lettuce under tomatoes works great though!

So-called "deep rooted" plants usually have an upper fringe of shallow roots that collect rainwater.

floral, how close are you throwing your brassicas next to one another and how close do you throw your lettuces next to those brassicas? Thanks everyone

as Peter says, brassica are in fact shallow rooted. What you want to interplant there is something that is annual or biennial, deep rooted (8 ft at maturity), mycorrhizal, and tolerates the presumed high nitrogen levels which people put under brassicas. Deep rooted things include beet (non myc.), carrot (does not like high N), chicory (perfect). You can mix them. See

http://soilandhealth.org/wp-content/uploads/01aglibrary/010137veg.roots/010137toc.html

ahappycamper - I don't 'throw' any of my vegetables. Brassicas are grown about 2 feet apart. Lettuce about 6 inches. These are all planted out as transplants, not seed. I grow the plants from seed myself but not in situ. Despite what others say about deep and shallow rooting plants, intercropping of salad and brassicas works. It is tried and tested and a common practice over here. The salad is all harvested in the summer by the time the brassicas are ready in the winter. Here's the link again

The picture shows Chard and Red Russian kale on my allotment. It wasn't taken to show intercropping hence you can't see the rows in total.