A few, strategically placed, large pots of shade loving annuals?

Will the bed have a straight edge parallel to the fence?
Is the ground that isn't covered with grass also a bed?
Do the maple roots go all the way to that left corner, or only where the fence section is higher?
What's the soil like?
How wide would you like it to be?

What's the drainage like? The southwest exposure and the maple roots would usually mean the soil will be dry, but maybe this spot is actually damp. As long as it's not too damp, that would be good, because the hardest spot to plant is dry shade.

Daffodils come to mind as a lovely spring planting for that spot. Many bulbs will do fine under the dense shade of a maple because they can mature their leaves early enough. The you'd need a ground cover for summer. Geraniums? True geraniums, not pelargoniums. Geranium macrorrhizum is a carefree ground cover that can thrive in dry shade. But maybe something taller would balance the fence better.
You could put a few pots of colorful flowers on pedestals in a sea of foot-high ground cover.

Gees, CC, you ask a lot of questions ; ).

The bed would run parallel to the fence, probably straight. No, there is no other bed there. The ground is bare because that's where the enormous arborvitae used to be. The roots are closer to the right end of the fence, the left has very few.

The ground seems to have average moisture. I'd rather not get into ground covers because I already have an ongoing battle with ivy and pachysandra and would prefer not to create another battleground!

Astilbe?? I was thinking something even taller.

Daylilies?

Deer eat the daylilies. I should have mentioned the deer problem. That's why hosta is out, too. The deer don't come to my neighborhood as much as others, but there's something about daylilies that lures them in.

While thinking of a permanent planting...I'm thinking narrow hollies, and while waiting 2-3 years for the maple roots to rot, for this summer, how about 3 Ricin's,, Castor Beans. They'll screen the fence well.

Thanks, Paula. The maple roots won't be rotting because the tree is still there. Would the hollies grow as understory trees? What is that in the picture? Not the guy, the plant.

Thanks for the ideas.

Sorry 'bout that, 'tree taken down' was in mind, as referring to the maple rather than the arborvitae, my bad.
No... hollies are not the answer there. Dry shade, yes. Tough.
You might try a small Cryptomeria japonica in the corner, 'Yoshino' is a popular... if the shade is less dense there. They can take shade, and if the roots are not as dense there, being further from the maple, it might work. Tall annuals are going to be difficult as well without at least a few hours of sun, like the Castor Bean in the picture. They're easy from seed...just pick them up from the grocery store. You could also limb up the maple to let in more light.

Any shade-loving shrub would work...Kerria, Forshthia, Wegelia...Rhododendron, Azalea....Oakleaf Hydrangea...the more compact Viburnums...

Thanks, Eibren. I am not familiar with Kerria, so I will look into that. IN the meantime, I got a bunch of free ferns, so I have planted them in front of the fence. I had a heck of a time getting through the roots. I only hope the maple doesn't miss some of its roots. I also am considering cherry laurel otto luykens, or maybe schip laurel.

I planted a hydrangea approximately 7'-8' away from an established maple tree and successfully moved a burning bush about 10' away from the same maple both of which are doing well after 2 years. They both have Northwest exposure. I kept them both well watered the first year so they didn't have to compete so hard with the maple roots for water.

I think I see a rhodedendron in the left side of the picture, if you have acidic soil you could use some azaleas as well.

This year I wanted to get some things established under a silver maple tree and in talking to a local nursery I decided on lamimum, heuchera silver scrolls and some epimedium, but of course these are all smaller. I also have a hydrangea and liriope planted nearby. getting around the roots has been difficult, but so far everything seems to be co-habitating rather well.