Tanacetum cinerariifolium Seeds | Pyrethrum
LATIN NAME: Tanacetum cinerariifolium
COMMON NAME: Pyrethrum
Plant Profile
Pyrethrum is a clump-forming perennial daisy from the eastern Adriatic coast, where it grows on dry limestone slopes above the sea. It makes a rosette of deeply divided, silvery-green leaves and sends up wiry stems forty to sixty centimetres tall, each carrying a single white flower with a yellow centre that opens over a long season from late spring. The plant looks like a modest chrysanthemum and behaves like one in the border, forming a clump that widens slowly and flowers heavily in full sun. Its leaves stay low through winter in mild climates. The dried flower heads are the source of pyrethrum powder, the oldest botanical insecticide still in wide use, and the reason the plant was once a crop in Dalmatia, Kenya and Tasmania.
Science & Biotechnology
The insecticidal activity sits in six esters called pyrethrins, which form in the achenes and in the flower head tissue and which attack the sodium channels of insect nerve cells. Their advantage over older compounds is that mammals break them down quickly, so toxicity to people and pets is low, and they degrade in sunlight within days, leaving no residue. That same instability limits their use in the field and explains why synthetic lookalikes, the pyrethroids, were developed from them. The plant is also a study in chemotypic variation: populations from different parts of the Adriatic differ by a factor of three in pyrethrin content, and breeders have selected lines that concentrate the esters in the achenes so the seed itself can be milled. Genetic work suggests the pathway is controlled by a small number of loci, which makes selection rapid.
Uses & Benefits
The dried and powdered flowers are dusted onto crops, stored grain and livestock, and the extract is the active ingredient in many household sprays and pet shampoos. It is one of the few insecticides permitted in organic farming, though it is not selective and will kill bees if applied to open flowers. In the garden the plant is grown as much for ornament as for use: the white daisies are long-lasting in a border, the foliage is neat, and the flowers are excellent for cutting. It is a good companion plant in a mixed vegetable bed, though the effect is modest unless the powder is actually applied. The plant is toxic to fish, so never use the powder near ponds or watercourses.
Growing from Seed
Sow in late winter or early spring, surface-sowing because the seed needs light. Press it into a moist, free-draining compost and do not cover it, then keep the tray at fifteen to twenty degrees. Germination takes two to three weeks and is uneven, so leave the tray for a month before giving up. Prick out when the first true leaves appear and grow on in a cool, bright place, since warmth makes the seedlings leggy. Plant out after the last frost into a sunny bed with sharp drainage and thin, poor soil; rich ground produces leaf and few flowers. Divide the clumps every three years to keep them vigorous, and deadhead to prolong flowering.
Our Take
A garden daisy that also happens to be a crop. Grow it in poor, sunny ground and it will flower for months.
Related Reading
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