Valerian (Centranthus ruber) is a great plant for season-long bloom and for attracting birds and butterflies. It tolerates drought extremely well (I've seen it growing in cracks and crevices of rocks with almost no soil to hold moisture.) Perhaps your valerian is flopping because your amended clay soil is overly rich for this plant. However, you can trim back your plants to keep them from becoming floppy. I usually trim mine back a time or two each growing season, partly to keep them compact and partly to prevent them from self-seeding everywhere. Wait until the first major flush of bloom is past peak, then cut the plants back by a foot or so. Blooming side shoots soon develop.