I'm in Wisconsin, too! Looks like a perfect place for shade plants that like it on the dry side...hostas grow anywhere. Sweet woodruff would fill in under and around them. Maybe an astilbe (a. chinesis take less water than others) in the corner next to the stairs. If you can dig there, I'd amend the soil with compost. If not, just add some to each planting hole. They'd have to be watered the first year, but after that, that should be okay under a box elder.

This is a difficult spot because anything growing here will have root competition with an established tree, reduced water and reduced lighting. Try:

Pachysandra

Lamium

Pumonaria

Ivy (Hedera)

Vinca

You might get samples of anything you think you might like, put them all in, give it a year to see what thrives, then go with one of the plants that does best.

1 or more large containers might be nice, along with some hanging plants off the railing in ones of those flower boxes attached to it...

Hostas, Astilbes, sweet woodruff(creeping ground cover) bleeding hearts (they have different kinds if you don't like the traditional pink one), columbines, ferns, some irises will deal with some shade, brunneras, pulmonaria, lamium(another ground cover), lupines, Heuchera(they are really pretty they have tons of colors! ranging from light pink to orange to green to dark purple black. they are also known as "coral bells" even though they don't always bloom a pink color) there are also tiarella, heucherella

If you don't care about flowers someone had mentioned something in the Hosta forum about daylily foliage which I never thought of before. The foliage does look kind of nice they don't bloom in deep shade but they will grow and who knows, there might be enough light in that area for them to bloom.

One of the things you can do is add soil and mound it up a little if you do try to put in a garden. How much sun does this spot get if any at all? There are a bunch of possibilities for your little spot. You could even get a whiskey barrel and put some shade loving plants there if you are too worried about roots.

good luck

Here is a link that might be useful: heucheras and shade plants

I would try lamium that stuff lets off seeds and they will spread nicely around the tree trunk. My neighbors have two large walnut trees and the only thing that will grow under them is hostas. They love to garden too and it's a shame nothing else will grow.