Golly, Sweet Rocket (aka Dame's Rocket; Hesperis matronalis) sure is pretty, blooming now in big patches here and there in roadside ditches. It looks nice and, especially in the evening, smells good. And it will bloom for a long time.
I spotted this big patch, a uniform deep pink, just east of Derby and stopped to take a closer look. I know a spot down in Wayne County, along a curving road that leads to a South Chariton crossing, where the full range of its colors --- white, variegated and pink--- is evident.
Trouble is, this is an invasive plant, not native to Iowa --- or to North America for that matter. It belong in flower gardens --- not spreading across the countryside, especially not to prairie remnants and other areas of native growth.
A member of the mustard family, Sweet Rocket is biennial (or a short-lived perennial), but self-seeds aggressively (unlike less invasive plants from old flower gardens) and grows spectacularly well, tall and sturdy.
Some firms that market packets of "wild flower mix" seeds include Sweet Rocket. They shouldn't, so check the packet for content next time you're tempted to buy. And if you see Sweet Rocket edging in on the prairie somewhere, pull it up and dispose of it.
Sweet Rocket looks a lot like phlox on steroids, but if you look at the individual flowers you'll see five petals on phlox; four petals on Sweet Rocket.