I usually plant my greens in a 6" deep pot, but I also pick them young. Realistically roots will grow as deep as you let them, but my idea of ideal is no more than a foot of root length, 8"-10" would more than likely be the best.

Peaceful_Warrior, Roots is a fairly comprehensive treatment on the subject. You will get a 'feel' for what is involved in this interesting aspect.

Huh?....I want my last hour back....what does an Oxford/Scotish report on drought stricken native soils:
""Root elongation, water stress, and mechanical impedance: a review of limiting stresses and beneficial root tip traits""
have to do with the depth of the lettuce roots IN A CONTAINER

The great thing about growing lettuce in a container is that they'll do well in a more shallow container. Over the summer, I grew some lettuce in a window box that was probably only about 5-6 inches deep, and it did very well. Spinach should be relatively easy to grow in a container as well, but I'm not sure about kale. With vegetables like tomatoes and cucumbers you would want to use as deep a container as you can use, since the depth will help to making sure the plant stays healthy.

This is sort of applicable, a copy/paste job from something I left at another thread. I just came across it a few minutes ago in research & remembered this thread:

It's easy to make the assumption when you notice a particular plant in the wild, in the landscape, or even in pots, tends to have a shallow root system, that this is a genetic predisposition, but it's not. Roots are opportunistic growers, only following patterns because of cultural influences. Change the cultural influences & the patterns change as well.

When growing in containers, there is no need to try to accommodate plants that set their roots in the top few inches of the soil where they naturally occur if you are growing in an adequately aerated medium. Plants that favor their roots growing in the upper parts of the soil do so because of two things - aeration primarily, but also nutrition. As long as a container soil is well-aerated (read that to say free of perched water), even plants that are normally considered extremely shallow-rooted in situ, will happily colonize the entire soil mass of even very deep containers. Because of the very open (well aerated) soils I use, I see it as regular as clockwork.

Even though you usually can't see them - roots are the heart of the plant, and you MUST keep them happy if you have any hope of keeping the rest of the plant happy.

Al

Tapla is right. Roots will grow to what ever size you let them, and this is not only a good thing. Aeroponic systems have to deal with this problem, the root systems just keep growing and growing, eventually it prolongs production of crop in many vegetables.
For lettuce, and others, I like to find a happy medium. Something big enough to acomadate robust root growth, but small enough to force the plant into less root growth eventually. For head lettuce, a 6" dia x 8" h works well, for leaf I would go a little bigger, and for microgreens (lettuce, spinach, arugula, others) a 2' L x 5" H works well for me, but I pick them very young.

Another myth is that roots grow toward a water source or nutrients. Roots are incapable of growing through areas insufficiently moist, even to reach a perfect root environment. Roots simply grow well where conditions are favorable, and poorly or not at all where they are not.

Also of interest: Some time ago I read a study that was undertaken by Dr Alex Shigo, PhD, during which an interesting bit of information was confirmed. I believe the plant used as an example in the study was a mature red maple. I don't remember the exact numbers, so I'm going to pick numbers that are in the right neighborhood so you get the gist. He and his students discovered that in spring, when water was abundantly available near the surface, a very high % of roots under a given size (the fine feeder roots that do the gathering of water and nutrients) was located in the top 6" of soil; I want to say about 90%. By summer's end, when water was scarce near the surface, the paradigm had shifted to the point that 90% of the same size roots were found at a depth of 15" or greater.

What occurred was, the fine roots at the surface died as conditions became unfavorable;, and later in the growth cycle the tree generated roots deeper in the soil under conditions more conducive.

Al

That is really interesting Al, do you think that all plants naturally do that? My thought is, that if the top roots are normally replace by lower ones, then in containers we can prevent this and make them grow all over permanently. I have seldom looked at the roots of my plants, but the few that I have seen look as they are supposed to: medium sized white roots with plenty of fine hairs all over (this due to a well drained media similar to yours), but if we can induce more that would be great.

Roots grow where conditions are favorable and die where they aren't. In heavy soils that have areas of the soil that remain waterlogged for more than a few hours, fine roots begin to die. The longer the roots are deprived of air, the larger the size of the roots that die, and the greater the negative impact on root function and metabolism. When air begins to return to the saturated portions of the soil, only then can root regeneration begin. This cyclic death & regeneration of roots is extremely expensive from the perspective of energy outlay because a very high % of the energy that would have gone toward fruits, blooms, leaves and branch extension, or even energy that might have been stored in roots and cambial tissues has to be 'spent' replacing the roots that have died.

Obviously, and all else equal, a soil that remains well-aerated from top to bottom at container capacity (when fully wetted) HAS to offer a considerable advantage to the plant for a number of reasons, not the least of which is it saves the plant from wasting energy on building roots destined to succumb to the soggy conditions accompanying heavy soils.

Al

Getting the Most from Your Garden by Organic Gardening Magazine.
On page115 spinach needs a max space of 36 inches wide & 48 inches deep. I believe you can grow it well in 25% of that 10" X 12" or 3.5 gallon pot.
I have not done it, but I know persons who have.

When a plant, let's say a tomatoe, has been transplanted to it's final home, let's say a 10 gal pot filled with 5-1-1, you want to keep the soil "moist where the roots are", as I beleive I've heard Al and others say before. Initially, the outer and lower reaches of the pot will retain some extra moisture because the roots have not colonized there and are not drinking up the available solution. I've been trying to primarily fertigate the root zone of my newest transplants, while paying slightly less attention to the outer and lower reaches of the pots. Is there a good rule of thumb to how fast the roots are moving through the 5-1-1? How does this rate vary from plant to plant?

PJ

There is no rule of thumb because under certain cultural conditions growth would be near nothing, and under ideal conditions growth could be extremely rapid - several inches or a 5-10% increase in mass per day in vigorous growers.

The rate varies from plant to plant according to the vigor with which the plant is genetically endowed and the influence the grower's ability to provide as close to perfect cultural conditions as possible has on the plant('s ability to grow).

Al

Thanks Al. I will assume, for now, that the conditions that I am providing fall somewhere in between....hopefully closer to ideal! They all have great temps and sunshine, a favorable fertigation schedule, and an excellent medium to live in. Now if I can be wise enough to understand what they are telling me, I think I'll be in good shape! It's amazing to think that the root systems could be expanding at such a rate!