By Larry Hodgson
Following the article published on June 30, 2021, about the origin of the insecticide pyrethrum, I received several questions about the possibility of growing Dalmatian pyrethrum (Tanacetum cinerariifolium) and using it to produce a homemade insecticide.
And the answer is yes, you can grow this plant and therefore eventually produce your own insecticide … but there are certain obstacles to overcome.
First, you need to find “true pyrethrum,” that is Dalmatian pyrethrum (Tanacetum cinerariifolium). If you ask for pyrethrum at most nurseries, they’ll most likely sell you instead a close relative, painted daisy (T. roseum), also called pyrethrum, but whose insecticidal effect is inferior. Make sure you come home with the real thing!
And that can be difficult, as Dalmatian pyrethrum plants are not widely available. You’ll probably have to order them by mail. That’s possible in the USA and Australia, possibly in Europe as well, but I don’t know of any sources in Canada.
Seeds of Dalmatian pyrethrum, on the other hand, are quite widely available, at least on the Internet. If possible, use seeds from African sources, especially Kenya or Tanzania, where cultivated varieties are especially rich in pyrethrum compounds, but those are hard to locate.
You can sow pyrethrum seeds on the surface of the soil indoors or out, but without covering them, as they need light to germinate. No other special treatment is necessary: the same techniques you’d use to start vegetables and annuals from seed are quite adequate. Just keep the soil somewhat damp during germination. Seeds germinate rapidly at about 70˚F (20˚C).
Grow pyrethrum in full sun in well-drained soil. Even poor or dry soils are fine.
The first year, you’ll find yourself growing a rather pretty plant with a rosette of deeply cut, somewhat silvery foliage.
In fairly mild climates (USDA zones 6 to 9, AgCan zones 7 to 9), you can grow Dalmatian pyrethrum outdoors all year as long as you can ensure perfect drainage. This is the ideal situation.
Gardeners in colder climates will need to bring their plants into a sheltered space (basement, garage, etc.) for the winter, a spot where temperatures rarely if ever dip below freezing. Cool conditions and intense light will be needed. Replant outdoors in mid spring.
The plant will flower the second summer, giving you a first harvest of pyrethrin-rich flower heads.
Dalmatian pyrethrum is a short-lived perennial. You’ll probably find you need to renew it from seed every 3 or 4 years.
Preparing Pyrethrin Insecticide
Harvest and dry the flowers, then grind them into powder.
The powder is very unstable and deteriorates in just a few days in light, heat and open air. It can be stored for a few months in an airtight container under cool conditions, protected from light, and even longer in the freezer.
To use, steep the powder in water for 48 hours. The solution should be used immediately due to its rapid biodegradability.
Finally, remember that even though that pyrethrum insecticide is considered organic, it’s still a insecticide with toxic properties. It is always important to use it with the greatest care.