Besides the poisonous issue, in zone 5 castor beans are probably not going to do well except in the hottest, sunniest site.

Not sure if it's what you're looking for but Hosta 'Sum and Substance' is a very big, very architectural plant. Otherwise some ferns might work; ostrich fern can take some sun, grows very upright and gets up to 4-5 feet tall when happy--which usually means lots of water but even in ordinary garden soil with regular watering it can easily reach 3-4 feet. Plus they would quickly take care of the "en masse" requirement...

How about a pot of clumping bamboo? something that does not grow too tall, and has some architectual interest in the culm for color or growth shape?

Thanks for the suggestions!

Johnnie, I actually have ostrich fern and Sum and Substance. The ostrich fern hasn't done very well in that area, maybe too dry? Even in the other areas where it seems happy it's only 2' high or so. Maybe I was sold ostrich fern and it was a different type!

Bamboo is an interesting idea ccokoball, I'll look more into types that might work.

Try Miscanthus giganteus (aka floridus, aka sacchariflorus)..This is a giant grass which is tough and makes quite a statement. Should be perennial in your zone.

Thanks Don! I have that along my back fenceline. I'll think about transplanting some this spring. The back clumps are pretty wide though, it might swallow my patio! That kind of height behind it would be amazing though!

I like the grass solution. Bamboo is also a grass, so why not it too....

And, if it is in a container, you could get by with running bamboo. The running kinds are much more cold hardy. In a huge pot, it could not go anywhere. Some kinds are very full and also do not grow to be giants.

If containers are good for you, try some of the nandina domestica, with clusters of white flowers, then red berries, and even a flush of fall color. You can possibly try it in the ground in your zone, but it also makes a good container plant. It is one of my favorite plants, and never gets too wide unless you choose to let the new cane growth stay.

How about Fatsia Japonica
http://images.search.yahoo.com/search/images;_ylt=A0geu76a_a9JMvEAmRBXNyoA?ei=UTF-8&p=fatsia%20japonica&fr2=tab-web&fr=yfp-t-501-s

I have grown it in the south of the UK, in Libya and now where I live in South Africa/Cape Town
In sandy soil in heavy clay like soil, on chalky soil and in shade or sun Its tough and I love the big leaves

Have you given Agaves a thought? For sculptural impact there isn't anything better than a perfectly symetrical agave in a large pot. Some varieties to consider are
Agave desmettiana (also Variegated and limited spines)
Agave Americana
Agave Deserti
Agave Americana Medio-Picto Alba
Agave Attenuata (This one has no spines)
Agave vilmoriniana (This one also has no spines)

They are all extremely drought tolerant so they'll forgive you if you forget to water them once in a while. I've seen them grown in outdoor malls here in wide, shallow pots held high on top of pillars. They must be watered with a small drip line or something. They look amazing, creating a complete 180 degree span of leaves over four feet in diameter.

I doubt any agaves will do well in zone 5 in partial shade. BTW when does it get sun--morning, mid-day or afternoon to evening? It can make a big difference in what will do well in that location.

A few more suggestions:

Plume poppy (Macleaya cordata) is a hardy but VERY dramatic perennial that can take partial shade. Just be aware that it can be invasive.

Hydrangea quercifolia is a beautiful woody shrub with very large leaves.

Aralia spinosa and A. elata are dramatic and very tropical-looking, but the viciously spiny stems and suckering tendencies may make them unsuitable.

How about a split leaf philodendron? It has a nice tropical look, is quite hardy, although in zone 5, you will need to protect it. I have one that is about 3 years old, 5' tall and 4-5' wide. The beauty of this plant is the trunk; cut old leaf stalks, the ends dry and fall off, leaving a round, brown scar on the trunk. Just beautiful!

Janie

Here is a link that might be useful:

Split leaf philodendron sounds good to me, but it will not take a zone 5 winter even with protection. You will have to bring in inside or in a greenhouse for winter.

How about Angelica gigas?

Petasites japonicus var. giganteus is a favorite of mine - invasive, though, but it can be mowed down or pulled up pretty easily. I saw a huge one at Longwood Gardens, about 4' high with a spread of about 5', huge imposing leaves, it's a wonderful plant, with flowers in earliest spring that look positively alien!

Here is a link that might be useful: a pic of a petasites

I would try variegated northern sea oats, a drought-and shade-tolerant upright grass that is stunning in a mass planting. Has showy bronze dangling seed pods in late summer.