Plant combinations can be absolutely infinite but the end result must be a successful combination of a focal point plus secondary points of interest that add up to a pleasing whole.
So how do you choose what goes with what?
I can suggest 2 good ways: create or copy!
Creating is a wonderful way to experiment. Try combining "like" colours for a soothing effect (white, cream and pale yellow) or opposing colours for drama (orange and royal blue).
Mix the design basics - evergreens, shrubs, perennials, grasses and annuals - for a result that incorporates colour, texture and height.
Research tried and true methods. Use the Internet or books. There are plenty of sites as well as books that show you great techniques and examples.
The other method is to copy. See what others have done and move your plants to resemble their combinations or buy new ones that replicate their ideas exactly.
I have been gardening for close to 30 years and have either created or copied many an idea for my own garden. I am happy to share some of those combinations with you now.
Here's a dramatic spring picture: Tulips 'Maureen', 'Estella Rijnveld' and 'Queen of the Night', blue grape muscari and a variegated geranium.
White daffodils with a few coloured tulips in the background, surrounded by grape muscari and blue flowering periwinkle.
Try self-seeding 'Love in a Mist' (Nigella damascena) with Coreopsis verticillata "Moonbeam'.
How about blue grasses like Helictotrichon sempervirens (Blue Oat grass) with ornamental kale and cabbages?
When I open my front door in the autumn, this is one of my favourite views: Sedum 'Autumn Joy', pennisetum (an annual grass) with my mugho pine and tall perennial grass Miscanthus sinensis.
Always lovely: ornamental kale beside a big rock, periwinkle, lamium and sedum 'Autumn Joy', being touched from above by Miscanthus sinensis.
Yellow daylilies with a dark "eye", surrounded by white shasta daisies.
Here are yellow lilies that are just opening with dark red daylilies and Gaillardia (Blanket flowers)
One of my perennial borders from the early 90's: Peonies, Sedum 'Autumn Joy', lilies, Salvia nemorosa "Ostfriesland', stachys byzantium (lamb's ears), Dictamnus albus (Gas plant) and Heuchera (coral bells).
Simple but beautiful: Shasta daisies and roses.
Another combo in a previous border: Nepeta faassenii (Catmint), Sedum "autumn Joy', Artemesia 'Silver King', Roses, Blue Oat Grass (Helictotrichon sempervirens) and Penstemon 'Husker Red'
I'll check the photo archives soon and put together Diary post #2.