14. Speedwell
Speedwell, or Veronica, is a large family of flowering perennials that grows in several useful habits including low groundcover and large, spikelike flowers reminiscent of salvia. While a few members of the genus are considered weeds, most can be easily controlled to grow within the desired area.
Check with your local gardening center to make sure the variety you want doesn’t expand too aggressively. Just like anything else, you sometimes can have too much of a good thing!