Plants for sunny gardens need at least 6-8 hours of full sunlight. If they get sufficient water, they will thrive on full sun from dawn to dusk. Many of them will grow in part-sun part shade as well. They will usually flower later and have fewer blooms in shady places.
If your perennial is not flowering well, try moving it to a sunnier place. Since the plants last for many years, often the garden grows up around them, and after twenty years, a spot that once had full sun may now be shaded by trees that have grown up, and the plants are starving in the shadows. They need sun for photosynthesis to manufacture their food in the green leaves, transforming light into energy. Move them in the spring, and watch them take off.
