Q:

How to develop fine roots in potted plants

I wanted to know the best way to encourage lots of fine feeder roots for a healthy plant. I started a couple of lemon trees from a drink cup seed last year as an experiment. I used a wicking system to start them and one seed had about half the room of the other. They both started very well and grew, but the one that had twice the volume of dirt as the other grew twice as big in the same time frame. 
When I talk about the amount of space, I mean that I used two 20oz soda bottles with the tops cut off, inverted and set in the bottoms with strings dangling down into water. one bottle I cut off the top third and the other bottle I cut almost in half.
However, the roots in these and many other of the plants I repot, tended to grow straight down with a tap root. Of course I did get some branching roots, but I don't get anything as pretty as when I buy a potted plant from a nursery or store and remove the pot to find there are almost as many fine roots as dirt. I just feel like these fine feeder roots are more healthy than the just having the large stringy roots. I'd like to know how to encourage them to form.
I lost one of the lemon trees for an undetermined reason. The other grew a couple of feet tall, but started to die off when I moved it outside and slowly acclimated it to the sun last summer. It's inside for the winter and only has one leaf left. I don't have a lot of hope for it and I'll start over soon. The eventual goal is to have one lemon tree in a large pot that I'll move inside for the winters and outside for the hot and humid SC summers.
I know you are supposed to move up pot sizes gradually, to prevent the roots from taking off and not using all the pot space. Also, so that when you water the pot doesn't hold more water than the plant can uptake, leading to root rot.
On the other hand. I started two plants in exactly the same manner and conditions, just separate container sizes and the larger container size did better. It wasn't leggy or anything. As a matter of fact, the smaller container was too small and I ended up having to move it to a bigger container because it was looking sickly. It recovered in the larger pot, but ultimately died.
Any help you can provide me in encouraging fine feeder roots will be greatly appreciated. I'm just not sure how to balance it. If smaller containers encourage more fine roots, but you have to disturb them by repotting to a larger container, it seems counter productive.

A:

Tree roots are genetically programmed to grow outwards, and downwards in a branching structure together with the tap root. This allows it to maximize the use of the usually infinite space available to it. If the roots hit an obstacle, they divert around it. If they hit air, as in a cliff face, they stop growing and signal the plant to send another root out from the base.
Plants grown in pots therefore become root bound as they hit the impervious sides of the pot, and start to circle around in a futile attempt to circumvent the obstruction.
Air pruning pots take advantage of the signal sent by roots that meet air to start growing new roots, and this results in a much denser and healthier root structure.  For example, Air-pot.com claims citrus growers in California can get fruit in 3 years instead of 5 with their pots.  Grow bags may have the same advantages.
However, if you grow anything in air pruning pots, the plants readily dehydrate so it's best to set up a drip irrigation system for them.

A:

Roots branch by pruning just like the top of tree branches because of pruning. If you never prune the branches, they will tend to become long and lanky. Likewise with roots. Root pruning is what develops lots of fine 'feeder' roots close to the trunk. 
When the tree is young, you pop it out of the pot, and slice off the outer portions of the root 'ball'. Use a small gardening cultivator or a root hook to scrape out the roots and remove the soil from the outer portions of the 'ball'. Cut off all the roots flush (or nearly so) to the new surface(s). Then it can be put back into the same pot or a different one with some additional soil. Alternatively you can use a small pruning saw to just cut soil and roots away in one step, but the cuts of the roots aren't as 'clean'. 
Several fine roots will grow from the cut root ends and new roots will branch off the cut root further back in the undisturbed soil. Repeat each spring for a few years, then only every second spring, keeping the tree for ever more in the same, or same sized, pot. Be a little more aggressive if you are wanting to reduce the size of the root mass. A little less aggressive if you want it to ultimately be larger.
Planting the tree in a pond basket, colander, air pot, or Anderson flat for a few years is the carefree way to do this development. When a root tip escapes through a hole it gets 'air pruned' which, in turn, causes branching along the root back inside the container. So you can just let one these pots do the work for you, in effect.
If you are wanting to be rid of tap roots and have a root pad that is just a few inches deep, you will first need to unpot and progressively thin the root 'ball' for a few years. Then you can take it easy with the carefree way for several years. That is, if you aren't wanting your tree in a pretty pot.
On the other hand, if your tree is so large and heavy that it is impractical to remove it from the pot, you can saw around the tree inside the perimeter of the pot (I find a keyhole saw works well for this), then dig out the peripheral soil and severed root ends, replacing with fresh soil. I recommend that you use the same 'mix' for replacement as this will minimize potential problems with uneven water distribution in the pot.
Lastly, bear in mind that it is easy to keep a tree with deep roots in a deep pot of heavy soil stable. However, this becomes more and more difficult the shallower the root pad. Movements of the trunk then more easily lift and tear the delicate single cell hairs that take on most of the mineral nutrients and water (of course, this is part of why many species have tap roots). It can easily become necessary to wire the tree into a rigid pot to keep them stable in the soil/pot (ala bonsai).

