Thalia dealbata - Powdery Alligator Flag, Pond Marginal Plant

Thalia dealbata, known as powdery alligator flag or water canna, is a tall marginal plant with long-stalked, blue-green leaves dusted with white powder. From June to September it carries sprays of purple flowers on tall, slender, arching stems.

Powdery alligator flag at a glance

Is Thalia dealbata hardy in the UK?

Not reliably. The RHS rates it H2, which means it needs protection from frost. UK aquatic nurseries sell it as an outdoor marginal and advise moving it into deeper water for winter so the roots do not freeze. In cold gardens, keep it somewhere frost-free over winter.

Where to plant it

• A sunny, sheltered pond shelf with up to 15 cm of water over the crown
• The back of a larger pond, where its height can be seen
• A planting basket of fertile loam

Is it good for wildlife?

Insects visit the flowers, but the RHS notes that the flowers can trap and kill small insects such as hoverflies and small bees while they feed.

How big does it grow?

Large. The RHS gives 1.5–2.5 m, and Wetland Plants says it can take up to 5 years to reach full height, so allow plenty of room.

Which size should I buy?

Bare-root plants (7 cm, 9 cm, 1 litre and 3 litre) arrive without a pot or compost and are the most economical way to plant a group. Potted plants (9 cm, 1 litre and 3 litre) arrive growing in our peat-free compost and give a faster display. Packs of 3 come in 7 cm, 9 cm, 1 litre and 3 litre bare-root sizes and cost less per plant. The 2x Pot & Compost option is two plants supplied with pots and compost.