Repotting tall columnar with very shallow roots

[fanaticactus]

In repotting most of my cacti into a pumice/DE/grit mix, I know I'll encounter more than one question. This one deals with an 11" tall, 2" wide Trichocereus grandiflorus hybrid with a very small (in comparison) root system. The bulk of the thick roots (and they are healthy) occupies only about 2", with a few longer fibrous ones extending about 4". My question is: What kind of pot should I use that will not tip over because of the height of the Tricho, yet will not be so big in volume that I'll have 6" or so of substrate--way too much for the size of the root system? Should I consider a low, shallow, wide "cactus dish" type of container whose width will prevent it from tipping over from being top-heavy? I realize I would somehow have to prop up the column until the new roots grow wide and strong enough to "anchor" the cactus in place.

[DaveW]

With tall Cerei the weight of the pot/soil has to counterbalance it's height so all the conventional rules about over potting usually go out the window. Also most Cerei are gross feeders unlike the small globular cacti, therefore more soil than normal does not seem to matter. In any case in a tall pot excess water sinks down to the bottom below the roots due to the perched water table effect, therefore is not so important as in a similar width shallow pan.

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Having a had a bedded in Trichocereus put on over 12 inches growth per year when it got going I can confirm that free root run or extra root room means extra growth, and cacti in small pots are Bonsai'd therefore grow slower.