Thalia dealbataPowdery Alligator Flag

Thalia dealbata, the Powdery Alligator Flag, is the tallest marginal aquatic in common use: big, upright, lance-shaped blue-green leaves on long stalks, dusted with a fine white bloom, standing well over head height on a mature clump. Arching stems carry loose panicles of violet-purple flowers above the foliage in summer.

It is specified where a water body needs a strong vertical element rather than a soft one, and it dies back cleanly in winter to reshoot from the rhizome.

Specifications

• Height1.5 to 2.5 metres
• Width0.8 to 1.2 metres
• Native toThe south-eastern United States
• FoliageLarge, upright, lance-shaped blue-green leaves on long stalks with a fine powdery white bloom; herbaceous, dying back in a cold winter
• Growth rateFast

Conditions

• SoilPlanted in heavy loam in pond mud or aquatic baskets, in saturated ground or water to about 0.5 metres deep
• LightFull sun to partial shade
• WaterGrown permanently in shallow standing water or saturated soil
• ClimateThrives in warm temperate to subtropical regions; frost hardy, dying back and reshooting from the rhizome

Additional Features

• FlowersArching stems carrying loose panicles of small violet-purple flowers well above the foliage in summer
• FormStrong upright architectural clump, the tallest of the commonly used marginals
• UsesIdeal for pond and lake margins, formal water features, constructed wetlands and vertical accent in water
• SpacingPlant 0.8 to 1.2 metres apart along a margin
• Low MaintenanceCut spent foliage back in late winter; divide the clump every three years
• Pest ResistanceGenerally resilient
• Environmental BenefitsTakes up nutrients in treatment wetlands and shades the water surface

Powdery Alligator Flag is the marginal to specify when a water feature needs height rather than more softness.