I buy cheap plants from supermarkets. Don't the hypermarche's in France have offers? Perhaps not right this minute, but in the better seasons for planting.
In spring you could throw seeds of annuals like nasturtium, cosmos, cornflower, godetia around and let them grow where they fall. In autumn you could plant winter flowering pansies (OK they don't flower in the depths of winter, but they do in autumn & spring), spring flowering bulbs.

thanks so much

What about osteospernum or gazanias? They put on a great show in the sun? Provided you keep deadheading, they will flower for a long time.