Aster thomsonii - Thomson's aster

Aster thomsonii, or Thomson's aster, is a perennial of Himalayan origin with a long summer flowering period in a mauve-blue hue. This very hardy and reliable, perennial aster forms graceful, airy clumps in cottage garden borders or naturalistic gardens. It bridges the gap between early summer blooms and autumn asters, while providing nectar and pollen for bees and butterflies. This robust botanical species is less susceptible to powdery mildew, which affects some of its relatives.

Thomson's aster belongs to the Asteraceae family. It is now placed in the genus Cordiofontis under the name Cordiofontis flexuosa, with the main synonyms being Aster flexuosus and Kalimeris flexuosa. It is part of the "summer asters". It is a deciduous herbaceous perennial with a short, rhizomatous crown and fibrous roots, whose above-ground growth completely disappears in winter. In the wild, this species is found from western Pakistan to Nepal, on slopes and edges of cool forests, between 2,100 and 3,000 metres in altitude. Its species name pays tribute to the British botanist and surgeon Thomas Thomson, who explored the flora of the Himalayas in the 19th century. This wild aster also served as a parent for the well-known hybrid Aster × frikartii ‘Mönch’, valued in gardens for its prolonged flowering.
The plant forms a bushy clump, composed of numerous thin, dark stems, rough to the touch, upright then slightly arching, 50 to 90 cm tall depending on the soil and climate. The foliage consists of single, alternate, ovate to elliptical, 5 to 10 cm long leaves with a pointed tip and a coarsely toothed margin. They are semi-embracing at the base, of a fairly clear medium green, and slightly rough on the underside. Flowering begins in July and continues until September: at the ends of the stems, solitary or grouped heads develop, 3.5 to 5 cm in diameter, composed of numerous very fine ligules of a bright mauve-blue, surrounding a central golden-yellow disc. Pollinated flowers produce small achenes topped with a silky pappus. The rhizomatous crown allows the clump to widen gently without becoming invasive. In cultivation, one also encounters the dwarf form ‘Nanus’, which is much more compact (about 40 cm in all directions).

Thomson's aster is perfectly suited to country or mountain gardens and flowering meadows. It can be planted in the middle of a border, in small spaced clumps that link the different plants, or in a slightly tall border along a path. It pairs well with the vertical spikes of Agastache ‘Blue Fortune’, the frosted foliage of Carex comans ‘Frosted Curls’ and the very natural small pompons of the great burnet Sanguisorba officinalis ‘Pink Tanna’. Japanese anemone ‘Dreaming Swan’ will take over as autumn approaches.

The Aster genus is extremely rich, coming in a wide range of colours and sizes for all soil types. Depending on the cultivar, it is even possible to spread the flowering period from spring until the end of autumn. Gardening enthusiasts of these pretty, countryside-style daisies can enjoy them for a large part of the year, both in the garden and in cut flower arrangements.