Where exactly in Zone 5 are you? There is the Perennials Forum - which is quite acive - for discussing plants, and what kinds of plants look good in combination. Try your regional forums as well since zone 5 covers a lot of territory.

Hostas, astilbes, Dicentra (bleeding hearts), ferns are great for filtered shade; lilies do best in sun but will take filtered. I'd have to study up on clematis, but I think they like their roots shaded. You really have no end of choices for both perennials and annuals and some small shrubs - potentilla, spirea, roses...

Yes, definitely go over to the Perennials forum for plant combo ideas...but just a couple notes. I think that columbine and heuchera would be great with the hosta. However, I think it might be too shady there for the clematis to grow well. Additionally, clematis need something small to twine around, like fishing line, for them to go vertical. If just planted near those boxes they wouldn't naturally "climb" over them as say, ivy, might.

Patty

Thanks for the feedback! We are actually in Northern Illinois but will definitely seek additional advice from the perennial and midwest forum (unfortunately no IL) concerning combinations, forgot about them.

Was thinking the same thing about both the lilies (not in full sun) and clematis not doing that well in this location. The idea with the clematis was to install some lattice in front of the boxes (sorry, original post said trellis) to help them climb.

Clematis do prefer more sun and there are some climbing roses that can handle some shade.
Some favorites; Hostas, Astilbie, Ferns, Brunera, Bergina, Ligularia, Miosotis (Forget me Nots), Aquilegia (Columbines), Asian lillies, Day lillies, Camisafuga, Dicentra (Bleeding Hearts), Ranunculus, Digitalis (Fox Glove, Pulmonium, Lamium, Convallaria (Lilly of the valley), and Primroses. All these do well in my zone 5 shade garden. Good soil is the foundation to a good garden so I would suggest digging in lots of peat moss.

Tyhawk suggests digging in peat moss. While that may be what your soil needs, it might not be. Best to have your soil checked and see what it could use. While peat is an excellent source of organic material and helps with water retention (although some think it takes water from your plants), it doesn't add any nutrients.

Having said that, that list of shade plants would make a kick-a** garden.

Patty

Virginia creeper is a native vine that does OK in shade and would climb over that cable house in a jiffy. Virginia creeper is sometimes mistaken for poison ivy, but it is much prettier, turns red in the fall, and has a whorl of FIVE leaves instead of three like poison ivy. Sometimes the bottom leaves only get three, hence why it gets confused for PI. On a further note, the partial shade conditons in your yard are perfect for growing poison ivy, and since birds aren't allergic to the plant, they spread the seeds through their droppings. I'd learn to identify it and keep a lookout for it showing up and then pull it right away. I pull it using rubber gloves to protect my hands and then additionally covering them with plastic bags from the grocery. I put the bag over my hand and then pull the ivy, and then turn the bag inside out and cover the ivy with it, it ends up a neat little package and then you can put it in the trash.

Barefoot is right that peat moss doest add nutrients. What it does is loosen up your soil and make it easier for plants to root. It also adds moisture retention and prevents clods etc. Ive never seen a flower bed that cant use peat moss. Where I work we had excessive phosphorous problems from 20 years of triple 16 fertilizer being added to the beds. Things grew poorly and we dug out a foot of the soil and replaced it with a mix that is 70% peet and 20% soil and 10% compost. Although thats an excessive and expensive amount of peet for 3 acres of beds our plants are huge and thrive remarkably with good fertilization. Planting and weeding is a dream, you can push your hand down 8 inches without a problem. Another wonderful thing about good peet moss is that it has a neutral PH which can help normalize acid or alkaline soils.

My suggestions would be some Japanense. Maples in the filtered light just for a step-down on the front corner. There are some very pretty gold ones, not sure if they are zone 5, or even a varigated Dogwood would light up the area. You could still do your trellis with five-leaf Akebia vine, there is also a varigated form of that. Climbing Hydrangea would look nice growing up the larger tree, but they sometimes take awhile to bloom. Ferns, Hosta, Bleeding heart, and woodland plants would look very nice. I would start with the bigger trees and shrubs then pick the perennials. You could add some Caladiums and Impatiens for seasonal color.