Generalit Thalia dealbata
Thalia dealbata is a perennial, deciduous, rhizomatous aquatic plant native to the United States. It looks like a thick head of long pointed, oval leaves, which grow on rigid stems, even 100-150 cm long; they are bluish-green, reddish on the edge, and covered with a whitish pruinose patina. In summer, from July to September, it produces an ear of blue-violet flowers, at the apex of a long stem that overlooks the leaves, which are also very long (reach 40 cm) and have a lanceolate shape.
The inflorescences are very long, spike-shaped, and generally exceed and overhang the stems in height. They have purple flowers.
In autumn the flowers are replaced by small greenish-gray berries, containing numerous seeds. They have rapid growth and, in favorable climates, they can reach considerable dimensions, sometimes becoming too bulky for a modest sized water garden.
As for this genus, there are other plants appreciated from an ornamental point of view such as the beautiful Thalia geniculata, different from the dealbata due to the smaller and more compact shape and the inflorescences.
During the winter the aerial part of the plant dries and dies while the portion that remains underwater survives, protected from slime and mud from low temperatures.
Thalies prefer very bright places, in full sun or in partial shade, possibly sheltered from the wind; they fear the cold, therefore in winter it is advisable to place them in a conveniently deep pond, so that the roots do not come into contact with the frost. If grown in places with particularly cold winters, it is advisable to transfer them to containers to be transported indoors; in this case they must be grown as tropical plants, with abundant watering and frequent vaporization of distilled water.