I like big herbaceous plants. When the only space I had to garden in was a Toronto backyard about 20′ by 20′, I grew spikenard, glade mallow, pokeweed and royal fern.
I have lust in my heart for gunnera (out of the question in Quebec) and western skunk cabbage (Lysichiton americanus), (possibly possible with snow cover). I am especially covetous of umbrella leaf (Diphylleia cymosa), from the southern Appalachians but considered hardy to our Zone 4. Although I am, of course, officially only interested in North American native plants, I can see throwing my scruples aside if I ever encounter a really robust variety of red-leaved ornamental rhubarb.
I added seedling glade mallows to the swale planting last year. I am hoping my little pokeweeds will make it through this brutal winter. Yes, I do know how annoyingly fecund pokeweeds are further south, but at the extreme northern edge of their range, only very, very sharp drainage allows their roots to survive the winter.
Spikenard seeds are stratifying in the fridge right now; they may not germinate until next spring (2016). Prairie dock (Silphium terebinthinaceum) and giant coneflower (Rudbeckia maxima) are two big-leaved species I will be sowing this spring.
Big herbaceous plant fill an important design niche at the farm: they take the place of foundation shrubs anywhere snow comes off the roof. We have a big clump of goatsbeard (Aruncus dioicus) doing shrub duty on the downhill side of the shed roof.
Big-leaved plants provide contrast in texture. So many of the grassland plants for sunny meadows and prairies are, naturally enough, grasslike — fine textured and vertical — or they have non-descript bitty leaves, that the few prairie species with big, bold leaves are valuable design components.
Pokeweed does not have individually big leaves; it is just a potentially big herbaceous plant with fine presence in the fall when its stems turn burgundy and the fruit ripens. Poke and spikenard are shrub-sized perennials that should be considered whenever one is thinking about adding native fruiting shrubs to attract birds.