Traditionally, in Japan, the flowers were harvested with their stems, and hung upside down for 24 to 48 hours in water before drying. This process slightly increases pyrethrin levels. The drying method affects pyrethrin concentrations very little, according to Gnadinger et al.(1933). Commercially, the flowers are dried in aerated buildings, in the sun or on drying racks, until moisture content is about 10%.
The fineness of the crushed flowers influences the pyrethrum’s efficiency as well as its longevity. The finer the powder the more efficient the pyrethrum will be against insects, but the more rapidly it will deteriorate. Part of the stems can be crushed with the flowers without really reducing the concentration of pyrethrins.
Pyrethrins are highly unstable components. They are quickly altered by light and heat and decompose in air in a matter of days. They decompose almost immediately in alkaline mediums. Pyrethrin concentrations can be maintained for at least six months by keeping the crushed flowers in a freezer, at temperatures between -2oC and -5oC. At room temperature, freshly harvested flowers will keep for about 10 days in a hermetically sealed container. Dried flowers kept in hermetically sealed containers away from light will keep for several months. Submerging pyrethrum in mineral oil or sesame oil is another form of preservation.eap.mcgill Home production of pyrethrum