You are confusing pelargoniums - not hardy and from South Africa - with native hardy geraniums. 2 different species with a similar seed pod and flower form so when they first arrived in the UK people thought they were also geraniums.Hardy geraniums will provide ground cover. If you go for macrorhizum forms they will be in flower - white, pale pink or deeper pink forms available - before the roses and the foliage will be scented and go red in winter for added interest. Other forms such as Rozanne will grow taller and flower later and longer and give you blue flowers to contrast with your roses. Hundreds of geraniums out there so go see what's good for you.
jack frost is a classic for under roses in rose gardens
This is a good place to start - https://www.rhs.org.uk/plants/articles/graham-rice/perennials-and-bulbs/10-hardy-geraniums and then make a list of the ones you like and see if you can find them in your local GC or can order them online.
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