Although not specifically considered a groundcover, hardy geraniums are a classic pairing with roses. There are many selections that will spread or ramble to cover the ground and the range of flowers colors available (blues, purples, pinks and white) are an excellent compliment to roses - for your Cecille, I'd pick something intense to contrast, like 'Salome', 'Ann Folkard' or 'Rozanne'. The benefit of using geraniums is that they share the same cultural requirements roses do (rich, fertile soil, regular watering, lots of sun) compared to ill-thought out choices like lavender.

If you plant several young geraniums and mulch the area well between them and the rose, you should be able to keep weeds down to a minimum. Plus, both the roses and the geraniums will appreciate the benefits of a good mulch.