Here's another guilty planting...
Years ago I started adding Verbena bonariensis to grassy plantings on roof tops. I liked the way it looked, held high and lightly above the cascading grass blades and later, their seed heads. It fixed the wildness in a way, gave it anchor and form. But it self seeds vigorously and is a pest. In my tiny space, it is easy to control by dead-heading before it sets seed. But I am suspicious of it en masse. You cannot control it on that scale.
It's a cliche now, this verbena, and tends to date a garden's design, in the way that lavender used to. I almost like lavender again. Actually, I've always liked it, but couldn't resist rolling my eyes and snorting a little if it was designed into yet another lavender and rose or lavender and boxwood combination.
I lean towards agastache now, for this tall blue effect. Though more substantial as a plant, it still carries tall flowers spikes above the fray, lasting long and late into the season. Its leaves are fragrant and edible and bees adore it.