Ground covers help to keep the weeds down, and also serve as a living mulch shading the soil from the drying sun. Windflower makes a very pretty short cover at this time of year. It will gradually spread over the years. The plants die back in the summer heat, but by then other perennials will be large enough to take up the space.
The blue scillas are a loose ground cover that are best at giving a shot of bright blue in the springtime. These ones are planted in among some variegated periwinkle which will bloom a little later.
Primulas planted about 12" apart can form an almost impenetrable mat. Great for keeping the weeds out. The leaves will stay all summer and, often, they will rebloom in the fall.
Pulmonaria is also very thick. It will self-sow quite freely, but unwanted plants are easily pulled out.
They need to be in shade as hot sun will burn the lovely spotted leaves.
Here's an all native ground cover of wild garlic. I found some seed heads that still had some seeds, so I tossed them around in some bare spots hoping to start some new patches.