| King Devil | Hieracium caespitosum |
| Kiss-me-over-the-garden-gate | Persicaria orientalis |
| Knapweed, Bachelor's Button | Centaurea cyanus |
| Knapweed, Spotted | Centaurea stoebe subsp. micranthos |
| Knawel, Annual | Scleranthus annuus |
| Knight's-spur, Doubtful | Consolida ajacis |
| Knotweed, Japanese | Fallopia japonica |
| Knotweed, Slender | Polygonum tenue |
| Korean-clover, Stipuled | Kummerowia stipulacea |