I am not a huge proponent of perennials. I find they require too much maintenance for a very short show and then, even the foliage disappears for the winter. There are of course exceptions. Peonies for example are worth whatever effort is required for even one day of bloom but of course they do last 10 days to 2 weeks in the garden and are stunning in a vase (see Reggie Darling on the pronunciation). The foliage too is quite handsome for most of the summer and useful in arrangements with other flowers as well.
The perennials I consider indispensable are those that have evergreen foliage. Perhaps the best of these are the Hellebores (Helleborus species).

They exhibit handsome leathery foliage, appreciate shade, and furnish the garden with much-needed greenery when it is most welcome .
I particularly like them planted at the feet of deciduous shrubs…
and at the base of large trees where not much else will grow.

Planted in masses, they make an effective ground cover.
Even in northern climates where there is snow cover for most of the winter, their flowers popping up through the melting snow is a sight to behold.
I do hope you include some in your landscape.
© All photos & text 2011