Tanacetum cinerariifolium - Pyrèthre de Dalmatie, Chrysanthème insecticide, Tanaisie à feuilles de cinéraire

Tanacetum cinerariifolium, the Dalmatian pyrethrum, is still often offered under its former name of Chrysanthemum cinerariifolium. This Mediterranean perennial forms a clump of finely divided grey-green foliage, topped in summer with white flowers with a yellow centre resembling small daisies. Ornamental for its foliage and natural flowering, it is best known as the main plant source of pyrethrins. Undemanding and hardy, it thrives in rock gardens and on dry banks. It is also grown in gardens dedicated to useful plants.

Dalmatian pyrethrum belongs to the Asteraceae family. It is a herbaceous perennial plant: its vegetation dies back at the end of the season, while the rootstock remains in the soil and produces new stems in spring. Completely deciduous in cold regions, it may partially remain visible in winter in mild climates. This botanical species is native to southeastern Europe, more precisely the eastern coast of the Adriatic and the western Balkan Peninsula. It grows in Croatia, Bosnia-Herzegovina, Montenegro and Albania, in dry grasslands, on rocky slopes and among stones on degraded limestone soils. This origin explains its preference for poor, stony, very well-drained soils, as well as its good drought tolerance once established. The plant has a rootstock with short rhizomes, from which emerge numerous deeply divided leaves. A fine hairiness softens their green colour and gives them a greyish to silvery hue. The clump of leaves remains quite low. The flower stems, erect and sparsely leafy in their upper part, rise to 40 or 60 cm in height. Flowering occurs from June to August. Each stem usually ends in a solitary head 4 cm in diameter. This consists of a domed disc of small yellow florets and a crown of white ligulate flowers, sometimes tinged with pink when opening. The heads are visited by various insects which ensure their pollination. They then produce small dry fruits, the achenes. Pyrethrins are present throughout the plant, but their concentration is significantly higher in the flower heads, especially around the developing seeds. After harvesting and processing, these substances act on the nervous system of insects; they are used in the composition of certain plant-based insecticides. However, cultivated Dalmatian pyrethrum does not directly protect plants growing nearby. It withstands temperatures close to –18 °C if its rootstock remains dry in winter.

Dalmatian pyrethrum finds its place in full sun in rather poor soils that do not retain water in winter. You can pair it with common sage 'Icterina', whose yellow-variegated leaves remain decorative for much of the year, as well as with the vermilion-red flowers of cottage pink 'Lady in Red'. Stachys 'Big Ears', with its large woolly leaves, thrives in the same type of soil, as does santolina 'Lemon Fizz' whose golden foliage highlights the white flowers of the pyrethrum. This perennial can also be grown in a deep pot, filled with a poor substrate and generously mixed with gravel or coarse sand.