I think that sounds like a really nice idea, and there are lots of trailing plants that would look good in that kind of a setting, and wouldn't compete with the boxwood.

the first thing that leaps to mind is Creeping Jenny Lysimachia nummularia. (I can't remember how to post pictures, but if you do a google search for images you can find them.) They are very easy to grow, very attractive, and are winter hardy here in Central Ohio. The 'gold' variety is an especially pleasing bright chartreuse. And you can find them everywhere, for really cheap. When you first plant them, tuck the long trailers all around the inner rim of the pot, and they'll quickly root and send more trailers over the edge.

Creeping thyme would be another nice trailing plant. They don't spread as fast and far as the lysimachia, but they have a wonderful scent and with enough sun, they have dainty little flowers. I'd look for one of the more colorful gold or varigated varieties, just to give you the color contrast with the boxwood. Most garden centers and even some box stores should have a decent assortment of thymes.

Not winter hardy, but certainly easy to grow are trailing Tradescantia. Back in the 70's it seemed everybody had a pot of Wandering Jew, T. zebrina, which have beautiful metallic purple, silver and green leaves. But now there are all sorts of varieties. I have a beautiful light green and white varigated one, and there's a variety called "Blushing Bride" which is two shades of green and bright pink. They don't live thru the winter, but if you take a bunch of cuttings in the fall, you can root them easily in water and carry them over the winter in pots or in water as houseplants -- and put them out again in the spring. (My rooted cuttings usually get too leggy, so I pinch the ends off in late January and root *those* too, to plant out in spring.)

Sweet potato vines may get a little bit long for your barrels, but they are beautiful, and are easy to find. Mine have never flowered noticeably, but you can easily find them in chartreuse and a almost-black dark purple that really stand out. There's a varigated green-cream-pink variety out there, too.

For years, I grew hanging baskets of varigated Vinca major on my front porch. (Can you tell I like varigated plants?) I would pinch them back when they got about 2" tall, and they'd split to make two vines. I'd keep pinching the growing tips here and there, and by mid-summer, they would have formed a beautiful curtain that trailed to the edge of the porch. If you cut the vines back to 3-4" in the fall, I bet they would overwinter, too.

Those are just the ones I've grown that leap to mind. Do a google search for trailing plants for part shade and see what you like. Or go to the garden center and see what they have. And don't forget to check out the section where they keep groundcover -- some of those are nice trailers that will work well for you.

Here's just a few other plants to consider:

trailing coleus
fuchsias
petunias -- might need more sun
vining nasturtiums -- might need more sun to flower
trailing lobelia
calibrachoa ("million bells")
trailing begonia
vining gernaium
dichondra
ivy -- lots of varieties and winter hardy, too!

Have fun!

Thank you! You gave me so many great ideas, it was difficult to decide - but I went with the vinca major. I do think it will winter over, which is always a good thing. Thanks again!

No problem -- enjoy your plants!