For autumn blooms in part shade/sun, Japanese anemones, cimicifuga and wood aster (Aster divaricatus or now Eurybia divaricata) are all showstopping performers once established. Hosta plantaginea can also be a spectacular late summer bloomer when grown in the right conditions.

Fall: Cardinal flower, Monkshood and hardy plumbago are three of my favorite, especially the bright red of the cardinal flower combined with the yellow of rubeckia.

Spring: To me, the daffodils are the stars, especially if you plant early, middle and late blooming bulbs in quantity. The other star in part sun are the foxgloves. I also love the way redbuds look.

Summer: June and July are the weakest time in my garden. I'm still hunting for a show stopping bloomer in my part sun garden, but I haven't found one yet.

Winter: The shrubs have it here--witch hazel and daphne.

I completely forgot to include seasons other than fall in my response above.

Spring: Bulbs such as Scilla siberica, Chionodoxa sardensis, Eranthis hyemalis and variants of Crocus tommasinianus can be easily established and naturalized for a spectacular spring show in part shade. Many other "minor" bulbs would also fit this niche nicely. For perennials, Dodecatheon meadia and Gillenia trifoliata are not often seen but can really put on a good show.

Summer: For early summer, martagon lilies are outstanding in partial shade. Don't be off put by the expense; you are paying for a reliable perennial not just a disposable bulb. They take a few years to really get going and may not bloom well for the first season or two. For later in summer, many other hybrid lilies will do quite well in part shade. Stick with the orienpet hybrids and avoid the orientals if you want good perennialization. Many of the orienpets are also heavily and pleasantly fragrant. One of my favorites is 'Conca d'Or' and it's pale yellow color can really light up a shaded area.

Winter: Not a high gardening point in my area, so I will let others chime in here.

I should mention, daysquid, I'm just a little south of you inside the 495 Beltway so we have similar conditions. Based on your Gardenweb handle, can we assume you are assoicated with the Naval Academy?

Kimka: I'm actually not associated with the Naval Academy but that is an excellent guess. I used to live in Los Angeles and I like to fish. I ended up fishing with some real salt of the earth men. One day we saw squid -- they said that was a very lucky and rare occurance because squid is great bait. I just liked the sound of Day Squid. Back to gardening, it's great to know that you have similiar conditions. I can use all the help I can get. Where are you exactly?

Spring: Dicentra 'Gold Heart'

Oh yes cool plant guy, I adore Bleeding heart Gold Heart, especially in front of a bank of regular Bleeding hearts with their darker green leaves.

daysquid, I'm in Silver Spring, just over the DC line in a little neighborhood that is surrounded by Rock Creek Park called Forest Glen Park. Hence the part sun, part shade, open shade, dappled shade, dense shade gardening since I have no full sun anywhere. You'd be surprised what you can grow in part sun if you are willing to settle for less flowers than in full sun.

I'd love to have you come to what we call a shovel tour some day. This involves having a few garden buddies over for a garden tour and I dig up a few things that people are interested in to take home to plant. I'm thinking of having one in fall once the summer heat goes away.

If you haven't dropped in on the Mid Atlantic Gardening forum here at Garden web, you should. Most of the time we are a fun bunch although the forum has been a bit quiet of late. Usually we have a great big informal plant swap in the Spring and a smaller one in the fall (it isn't really a swap because there is no quid pro quo involved; we just give plants away to each other.) If you don't have any plants to put in, you bring chocolate (or other food).

kimka, your shady spot seems very similiar to my conditions. We live on a wooded slope. The slope presents more challenges.

I would love to attend any and all events. I'll definitely visit the Mid Atlantic Gardening forum. I'm happy to bring food or drink - and enthusiasm. I'm very eager to learn and talk gardening.

Suggested Plants: Where and when do I buy these? Should I be visiting the garden centers in the fall for a good selection or if I have my heart set on something -- should I just order online? And what resource?

And Tulips -- could they work in part sun?

Thanks again to all!

coolplantsguy: Dicentra 'Gold Heart' is very nice. What about the other seasons? What other cool seasonal selections can you share?

ispahan:can you recommend any martagon lilies of these in a dark purple, light pink, or blue/violet? Thanks.

In part sun/part shade areas of my garden, here's what blooms & when. Altho' some of these are said to prefer full sun, they do extremely well with 2-3 hours of dappled sun:

Spring (April-May): columbine, pulmonaria, brunnera, hellebore, Jacob's ladder, astilbe, ferns

Summer (June/July/August): cushion spurge, globeflower, beardtongue, sweet william, hardy geranium, cardinal flower, Japanese iris

Late summer/early fall: toad lily, crocosmia, snakeroot, Japanese anemone

Other than hellebore/Lenten rose, not much puts on a show here where winters are generally severe. That said, lambs ear/stachys 'Countess Helen Von Stein' has lovely foliage from February thru December. Lady's mantle & Japanese sedge also offer foliage interest through the cold months.

You have gotten some great suggestions from others, so I'll give you some plants not already mentioned. My garden is quite a bit farther north than you, so check that these plants will do well in your area.
Spring (which is late April - early June for me, but would be earlier for you):
Dwarf crested iris, Iris cristata, is happy in full shade where it blooms well.
Not a great photo and a bit past its maximum bloom, but Veronic Georgia Blue blooms well in part shade and spreads to fill available space without being a thug and overwhelming its neighbors. For about 2 weeks in May there is a bright blue river here in mostly bright shade.
{{gwi:202703}}From May, 2011

Clematis HF Young is one the few early blooming clematis that I have found easy to grow. Geurnsey Cream is another one that does well for me in part shade. This HF Young gets a fair amount of sun, but would probably bloom as long as it gets 5 hours of sun.
{{gwi:265354}}From June 5, 2012

Summer:
Hydraneas are long bloomers, and though they are shrubs I find they do well in my borders. This one is the common variety Endless Summer which does bloom for me here in zone 4 all summer, but I also have H. arborescens Annabelle as well as a bunch of H. paniculata varieties. Annabelle blooms from early July until frost for me, though by now the flower heads are green, and the paniculatas (like Pinky Winky, Limelight, Pink Diamond, etc) start in mid to late July and are lovely until hard frost. Being further south, you would want to check with folks local to you as to your best long-blooming varieties. It is joined here by Clematis Ville de Lyon, one of my favorites. Many other clematis will bloom in part sun, as long as they get at least 5 hours of sun, particularly if the shade is relatively bright.
Both plants were only in their second year here.
{{gwi:265355}}From garden photos 07

H. All Summer Beauty
{{gwi:265356}}From garden photos 07

I know that you asked specifically about flowers, but since you said you are new to gardening, I will stretch your boundaries a bit. Many of my part-shade show-stoppers are foliage plants and are stunning all season. Some of the shrubs and small trees have multiple seasons of interest that might include spring flowers, late summer berries,and/or and fall color.

Japanese forest grass, Hakonechloa macra, has both golden and variegated varieties. I find my gold plants light up a dark spot with chartreuse foliage and in part shade are a lovely gold color.
One of the gold varieties of Hakone grass on the left side.
{{gwi:258831}}From June 2010

Heuchera, Heucherella, Tiarella, and hosta will provide some flowers, but for most, the real showstopper will be the foliage. This is relatively tame combination, but the colors and contrast among these can be stunning.
{{gwi:265357}}From June 5, 2012

In mid-August the berries on the pagoda dogwood (Cornus alternifolia) are at their maximum color, though this was taken just a bit earlier than that. The bright red stems (which hang on after the berries are gone), the deep purple berries, and the many birds who come to eat the berries make this a great small tree in August, though the tiny white spring flowers and the peachy fall colors are nice as well. I am not sure if you are too warm for this plant to do well, but if so Kousa dogwood has large red fruit in the late summer as well as more traditional spring dogwood flowers and late red fall folliage.
Cornus alternifolia
{{gwi:240157}}From garden photos 07

Rhododendrons and azaleas are stunning plants in late spring, and there are also some of the deciduous azaleas that bloom in summer. Many of the deciduous azaleas have beautiful fall color.

This variegated dogwood is one of my favorites, with green and white foliage and in winter bright red stems (as long as I prune 1/3 of it annually to get new stem growth.)
{{gwi:226218}}From June 2010

A final shrub that is stunning in the late fall, Mount Airy Fothergilla, which has amazing orange, gold and red very late fall color for me here. The white, bottle brush spring flowers are lovely as well, with a light honey scent, though that is a nice bonus rather than a show-stopper. I think that a border with this and Kousa dogwood with spring bulbs, a few May-blooming perennials, and summer blooming clematis would make a great border with all season interest.

nhbabs: What a wonderful post. Thank you for all your suggestions. I'm going to put hydrangeas in one of my part shade gardens for sure! We live on a wooded slope with three areas for gardening -- only one of which is flat. None of them are especially large. As much as I love the dogwood, we already have a mature dogwood, and it's too bad it isn't varigated. Your suggestions have definitely broadened my horizons and the pictures are very helpful. Thank you.

nhbabs: I'm an idiot. I thought variegated dogwood would be a huge tree. After doing some research, I see that it is a manageable shrub. What variety do you have and can you tell me what other plants are in that picture? I really love that collection of plants.

Hey, Daysquid, you're a newbie, so be kind to yourself. There are also variegated dogwood trees, particularly some of the Kousa dogwoods. I can only take partial credit for this arrangement since the flowers in the middle seeded themselves here, but I also like it. Serendipity is one of the pleasures of gardening for me.

The fine textured plant at the bottom is periwinkle, and I wouldn't recommend it since it spreads quickly and is difficult to get rid of. It can smother more delicate plants. A good substitute which isn't such a thug is cranberry which has slightly smaller shiny green leaves and will grow fine in average soil in part shade. The large leafed plant is common edible rhubarb (and I harvest some of the stalks in the spring and early summer for pies.) The tall grassy stuff behind the rhubarb is the foliage of Siberian iris which has purple-blue flowers, though they also come in white and yellow. The flowers in the center: the pinker one on the left is foxglove, and the one with the purple and white flowers is lupine, both of which self-seed in various places around my garden. Both the lupine and the foxglove are biennials, so have a rosette of leaves the first year and flowers the second, after which they usually die, though occasionally one will bloom another year.

My dogwood came without a tag, but is an older variety of probably Cornus alba since I've had it about 20 years. A smaller (though not small) variegated one is Cornus alba 'Ivory Halo', supposed to get to about 5'. There is also Cornus alba 'Cream Cracker' which is supposed to be about 3'x3' and resistant to leaf spot which sometimes makes variegated dogwoods look a bit tattered in the late summer. There are others as well, including some that are green and gold variegated if that appeals to you. You could ask in the shrubs forum if anyone grown any of the variegated dogwood shrubs and see how large they have gotten and if there have been disease issues.

Here's another part shade photo, though at this time of year it's between flowering, but I wanted to show the cranberry which is a tidy green mat all year. Starting in the middle of left side: foliage of Rhododendron 'Checkmate', Iberis sempervirens, hosta, cranberry, hosta 'Brim Cup', Hydrangea 'Endless Summer', and clematis 'Ville de Lyon' across the bottom.
{{gwi:227202}}From June 2010

nhbabs made mention of 'Tiarella'. I give this plant a two thumbs up as a spring 'knockout'. Both the foliage and flower are stunning. Here is a picture of one of my "Sugar and Spice" from this past spring:

'Tiarella' looks great! Thanks.

I'm headed to a huge garden center just outside of DC tomorrow -- any more suggestions? I'm so excited for this outing!

Just one, daysquid.

Take me with you!

Take an iPad or cell phone with browsing capacity so that you can look things up when you are at the nursery rather than relying on the tags. For shrubs I also have an app from Michael Dirr who is a shrub guru, formerly a prof at U of GA. Having the references available helps keep me from buying too much that is impractical.

Have a great time!

SunnyBorders -- You'll just have to come to Annapolis for breakfast. Poached eggs over softshell crabs on cornbread. And then we'll head to the nursery.

And nhbabs recommends we go with ipad in hand. I've got us covered!

All are welcome! To bad we can't do a virtual fieldtrip with the ability to taste (the softshells) and smell the roses.