Summary: Pick pyrethrum flowers when most of the disc florets have opened (about 1/2–3/4), dry them at moderate temperature (usually no higher than about 60 °C) or in shaded, well-ventilated air to around 10% moisture, then store them sealed, dry, dark and cool. Whole flower heads retain the active compounds better than powder; deliver to the extraction plant as early as possible.

The value of pyrethrum flowers lies in their pyrethrin content. These compounds are sensitive to heat, UV light, oxygen and moisture — so post-harvest handling determines most of a lot’s quality.

Table of Contents

1. Pick at the right stage

Pyrethrins accumulate mainly in the achenes; content is usually high when most disc florets in the head have opened.
Picking too early (buds) or too late (fading, shedding petals) both reduce active-ingredient yield.
Harvest on dry days, take only the flower heads and minimise stems and leaves.

2. Drying methods

Method	Advantages	Notes
Shade drying, good airflow	Low cost, suitable for smallholders	Avoid harsh direct sun; spread thinly, turn evenly; protect from rain
Hot-air drying	Weather-independent, uniform	Keep temperature moderate, usually no higher than about 60 °C; ventilate well
Direct sun drying	Fast	UV degrades actives; use only briefly

Target: crisp, well-coloured dry flowers at about 10% moisture (no more than 12%) to prevent mould.

3. Storage

Pack airtight (PE liner inside woven bags) to exclude air and moisture.
Store in a dark, cool, dry place, raised off the floor, away from fertilisers and chemicals.
Label each lot: plot, harvest date, drying date, weight.
Do not grind into powder until needed: powder has a larger exposed surface and loses actives faster than whole heads.
Pyrethrin content declines over storage time, so deliver to the extractor early and test each lot.

4. Quality control of dried-flower lots

Parameters extraction plants typically check: moisture, foreign matter (stems, leaves, soil), colour, odour and total pyrethrin content (%) determined by chromatography (HPLC). Take a representative sample per lot before handover.

FAQ

How long can dried pyrethrum flowers be stored?

Properly sealed, dark and cool storage slows losses, but pyrethrin content still declines over months. Plan deliveries within the season and test lots before use.

Is flower powder better than whole flowers?

For storage, no — whole heads keep actives longer. Grind only just before extraction or formulation.

Thao Duoc Xanh provides harvesting, drying and storage guidance to growers in its pyrethrum trial areas. See our products or contact us about growing partnerships. Background: pyrethrum in Vietnam.

Note: this article provides general knowledge. Thao Duoc Xanh’s pyrethrum products are currently raw/botanical materials and are not registered plant protection products; they must not be used as a substitute for registered pesticides. When using any insecticidal product, follow the label and applicable regulations.

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Drying and Storing Pyrethrum Flowers: Keeping Pyrethrins from Field to Factory