Purple flowering form of Ruellia at UT Gardens in Knoxville, TN

Oenothera speciosa held in check at gas station island

Some plants are incredibly aggressive. Herbicides like Roundup™ won’t phase them. They often escape and take over other areas of your garden or neighborhood.

Four notorious examples are ditch lilies (Hemerocallus fulva), swamp milkweed (Asclepias incarnata), Mexican petunia (Ruellia brittoniana), and pink evening primrose (Oenothera speciosa). You can grow these four weedy offenders in confinement. All will survive on totally inhospitable sites and their spectacular flower show lasts a month or more.

Ditch lilies grow in absolute worst conditions. They bloom for 2-3 weeks starting in late June (USDA zones 6 -8). Each bloom lasts one day, but the large floral scape holds many flowers. Orange is the common color, although other colors are found. Flowers are mostly sterile, but their rootsystems are aggressive.

Leaves of swamp milkweeds are favorite food source of monarch butterfly caterpillars (larvae). Their rootsystem spreads aggressively and they also seed in. Plants form clumps that survive over many years in wetland areas. Swamp milkweeds grow 3-5 feet tall on erect branched stems. Small, fragrant, pink to mauve flowers (1/4 inches wide) are common and a white flowering form is occasionally seen. Orange butterfly milkweed (A. tuberosa), also native, is far less aggressive.

Mexican petunia, aka Texas ruellia, is three-foot-tall evergreen shrub that bears many tubular, blue or purple, petunia-like flowers on dark stems through the summer. This fast grower may self-seed and roots are prolific. It is grown as an annual in cooler zones and will perennialize in zones 6-b where I live.

Pink evening primrose is another nightmarish perennial that blooms almost one month. It spreads aggressively by rhizomes. Plants die back after flowering in the summer heat.

All four are great choices for planting in confined spaces such as in parking lot islands or median strips along busy highways. Plant them in containers and keep blooms deadheaded to prevent seed formation.