I would think you'd need a really large container if the desire would ultimately be to maintain the shrub/trees & perennials beyond just planting them for a contest and treating them as throw-away annuals. I really haven't planted anything with my shrubs although I have had mixed perennials (eg., sedum at the base of a clematis).

But for those who might be interested in something like this, perhaps one could try a knockout rose (shrub and continual bloomer) with a group 3 clematis (perennial summer bloomer) crawling through it, surrounded by a series of clumps of blue fescue (grass), perhaps interplanted with a creeping annual (maybe lobelia or alyssum) and viners like sweet potato vine around the edges as the wild card.

They have some interesting large planters downtown here with japanese hollies and impatiens or saucer magnolias and ivy, etc., but they haven't gotten carried away with it. ;-)

I think there may be many trees and shrubs you can grow in a moderately large planter. Fruit trees, including bananas, do well for years in pots. I've got a large Norfolk Island pine that's been in a pot for 8 years and annually doubles as my patio Christmas tree. Hibiscus and Plumeria both do extremely well in containers. All of mine are in pots no larger than 24 " diameter.

Well - I have 20 shrubs and a plum tree in containers - the oldest ones being containered for 11 years, along with dozens of perennials including woody vines like clematis (1), wisteria (1), honeysuckles (2). I am definitely one who doesn't have a problem with trees & shrubs, etc., in containers and have had to constantly battle people here on GW about the alleged impossibility of doing such. ;-).

But I guess I do have a problem with trying to maintain perennials WITH the shrubs over a long haul, which this contest seems to be suggesting (or perhaps this contest doesn't care about long term). Ie., it almost suggests putting together an annual-like "combination planting", but using trees/shrubs, perennials, etc., as the combining plants.

I have some crocuses in my plum container and as earlier noted, had some sedum crawling along the top of my clematis container, but I do know that there will come a time when you would need to divide a perennial. I have seen half-barrels with cannas in the center with daylilies planted around the edge. But it might be tricky to try to also jam some grasses or sedges and annuals in with that and a canna isn't even a shrub. LOL

But then I guess that is why it is called a "challenge". ;-)