Sure wouldn't work in my zone where pots (and sometimes their plantings) get moved indoors for the winter! And you'd have to be really sure the pot is where you want it permanently - i.e. that you aren't going to want to move it somewhere else in a year or two... Maybe things are a lot different in your area but, for here, one of the advantages of pots is the abiliity to rearrange them/move them around each year and that wouldn't be possible if things had rooted through a hole in the pot and pavement. Maybe I'm not understanding your method completely - how do you deal with issues like wanting to move the pots elsewhere? What if the pot broke - how would you move the plants to a new pot?

First, a pot that is 30 inches tall and wide and full of damp soil with palms that are 15 feet tall isn't something that you are going to want to or be able to move easily in the first place. As we don't have freeze damage issues to worry about, breakage on these thick walled ceramic glazed type pots is not an issue. I don't permanently plant aggressively rooting plants into large pots such as this without double potting them with a cheaper plastic nursery pot inside. This methodology I use for things like Strelitzia nicholai which will ultimately break a pot with its roots. Palms, in general, are much better behaved as large growing, long term container plants without fear of roots breaking the pot.

I wouldn't be bothered doing this sort of thing for smaller pots with herbaceous or shrubby things that can be moved, or are more easily divided when necessary. Thus, I only use this for permanent large scale plantings that will stay in place permanently.

On the other hand, I have found cutting a larger drain hole at pots useful for fast growing plants such as restios, as well. I know this is all Greek for anyone outside of the Pacific Coast, but the principle applies wherever gardeners can leave containers outdoors year round, and are using plants that are known to grow such thick roots that they can block drainage at too small drain holes which are the standard when purchased. With the restios, I cut the larger drain hole, and then set the pot over a saucer. This minimizes roots clogging the drainage and letting the plant drown in its own pot. I've had this happen on plantings in pots at client's gardens where I may only check on container plantings once every two months or so. Two months of roots with no air because automatic drip irrigation keeps them flooded is usually a death sentence for most non-aquatic plants.

Does that explain it better? Also, I recently read an on-line article on container gardening in the New York Times where they discussed using large pots that would remain outdoors with plants all winter. Interestingly, the author was trying to use these large ceramic glazed pots in upstate New York, but attempting to keep them from cracking by insulating them on the inside of the pots with styrofoam. I'd think that double potting and providing a sand or styrofoam pellets layer between pots would be better, and would prevent freeze damage via soil moisture from cracking the pot. It would be extremely difficult to apply sheets of styrofoam insulation against the generally curving wall faces of most pots, and to hold it in place. Further painting the inside of the pot with a water-proofing membrane/sealer would also protect the pot from absorbing moisture and cracking during a freeze. As I don't garden where this is an issue, I haven't really explored the issues of this problem, and don't tend to move pots around seasonally, except for visual reasons as things come in and out of interest/bloom, etc. My container plantings here in California are intended to look good 365 days of the year, with no down season, even in winter.

Definitely a more thorough explanation.... and definitely it's a different world down there! We move all pots indoors for the winter - either into the unheated garage, or to the basement for the brugs. Most of the pots that go to the garage are enptied (their contents either discarded or dried for storage - e.g. elephant ears - as appropriate for the type of plant that was in the pot), but hostas will survive in the garage over the winter, even in the relatively thin plastic containers they are in. Strawberries survive the winter in the garage in the big plastic storage pots (with drainage holes we drill in then when we plant in them). We harvest leeks and baby carrots from the garage all winter from the big storage pots we grow them in. But all the pots need to be off the driveway so its clear for snow removal during the winter. I wouldn't bother trying to grow permanent plantings, e.g. trees or shrubs, in pots. We planted the hostas that were in the pots for two years into the garden a week or so ago. We'll plant new ones in the pots next spring and leave them there for a couple of years to bulk up before adding them to the garden. Different climates; different approaches... :-)

Woody, I've had good luck leaving my biggest concrete urns outside over the winter as long as the soil and perennial plant material (usually junipers or sedums) is left in place. That way, I think a lot of the snow sloughs away before penetrating the pot. The oldest one is over a decade old and has still not cracked, although I do have to freshen up the plantings every few years. Saves a lot of work for me - the only thing I have to move into the garage in the fall is the pond fish.

On the other hand, I'm not trying to grow large trees such as palms in pots. I do like your tricks of nestling the largest ones below grade and enlarging the drainage holes, David. That is certainly do-able, no matter the zone.

Here is a link that might be useful: Overwintered pots

I learned from Roger Raiche, a Californian horticulturist, to cut the entire bottom off of a huge pot when planting them in the garden, I use an angle grinder with a diamond blade and completely zip off the entire bottom of the pot . This tip has served me well over the years and I am always grateful to Roger for this tip.

When planting most pots that will be placed on a deck or stone terrace I almost always use a soil-less planting medium so as to not stain the surface with run off.

I also never place gravel at the bottom of a pot and instead simply use a piece of screen or shade cloth.