I have an artificial border against a driveway which has limited depth (about 20-30cm, 8"-12"); I wanted a line of plants with good volume (perhaps 1m/yd plus in height) and some nice colour, so Hydrangeas would have been ideal. Unfortunately, the drive gets the full sun in the afternoon and after a short trial and some hot weather it seemed unlikely they would last, so I transplanted them to more shaded locations.
Now I am looking for a replacement, but I don't really have an idea what would have the right combination: relatively shallow roots, about 1m in height, nice colours, tolerant of full sun and low maintenance.
The location is Brussels which is apparently zone 8b. The soil is quite heavy but I can mix in some compost if necessary. I am not sure how it drains. The border has a wall on one side and a concrete bottom at least, so it could be sealed, but on the other hand, the soil seems relatively dry.