I have a few large pots (16" X 16" ,20" deep) that have become root bound and I didnt get around to repotting in the spring (it got to HOT to soon). They are both outside plants/trees so I was wanting to know it root pruning and repotting them in the fall will be OK or would it be better to wait till spring. FYI I live in Va Bch,VA (zone 8).
The best time to plant, move or repot tree's shrubs etc is either EARLY spring or late summer.
Early spring is because the soil is not yet really warmed up and late summer the soil should be cooling down and both times give the roots time to settle, take up more moisture while the sun is not as hot and also by both these times the plants are either not yet began growing or they are settling into there rest period.
I would have everything you require to do the job ready at hand, choose a cooler sheltered place to do the work IF possible and don't water the plants for a couple of days before you start the re-potting.
Remove the plant from the pot / planters, have the new soil ready and mix in a few handfuls of multi purpose Plant food pellets, I would also add a few handfuls of Vermiculite to the soil mix as this helps hold onto water and allows air to remain in the soil around the roots which all plants require especially if in a confined space.
Lay a large plant on it's side and use a hand fork to remove a lot of the soil from between the roots, keep rolling the plant so you can continue to remove as much soil as you can all around the root ball, After you are happy you have removed as much soil as needed, you then need to prune away some of the roots, start removing the really old damaged / broken roots and then work your way to the newer roots till you are happy that you have a good root-ball left to replant into the new soil mix. (At this time I would stand the root you have readied for re-potting into a large bucket of water to allow any remaining soil to get a good soaking and all the other roots to take up a good drink before you place the readied plant into the new soil mix)
Once the plant is set into the new fresh potting mix, make sure you give the new soil a good soak and keep the plants in a shaded and sheltered area for a few weeks till they recover, keep watering when needed (place your fingers down into the soil and feel if the soil is dry down about 2-3 inches, if dry, give a soak,or place upturned juice containers with the bottom cut off, into the soil and when you water the plants, fill up the containers and this way the water will reach the roots where needed and it is a much easier way to water shrubs / trees etc especially in a hot climate.
gradually move the plant out to the sun again but dont allow them to get sun burnt while they are recovering from re-potting.
Hope this helps you out.
Good luck, WeeNel.