From their website:
"For this contest, we challenge our readers to combine five plants:
* one tree or shrub
* one grass or sedge
* one perennial
* one annual
* one wild-card plant of your choice
You may use multiples of the same variety, but there must be only five distinct plant varieties in your design."

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. ;-)

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". ;-)