- Family: Poppy Family – Papaveraceae
- Growing form: Annual herb. Latex-containing.
- Height: 30–60 cm (12–25 in.). Stem leaved, hairy.
- Flower: Regular, 3–7 cm (1.2–2.8 in.) across. Petals 4, red with dark base. Sepals 2, fall off when the flower opens. Stamens numerous. Gynoecium with several fused carpels. Stigma star-shaped. Flowers solitary terminating stems.
- Leaves: Alternate, stalked, hairy. Blade pinnately lobed with 1 mm long spine at ends. At least lowermost leaves with broad and blunt lobes.
- Fruit: Club-like, smooth, hairless capsule, 2–3 times as long as broad. Stigma-disc usually with 6–9 rays.
- Habitat: Cereal fields, ports, sometimes rubbish tips and roadsides. Also an escape.
- Flowering time: June–July.
Long-headed poppy is an annual that is associated with cultivated plants, especially rye. It is originally a species of semi-oceanic climate. It spread to Southwestern Finland with seed corn from the south. Long-headed poppy has declined following the change in agricultural practices like many other weed species. Earlier the cleaning of seed corn was imperfect and, hence, the seeds of the weeds were sown together with the corn. Many weed species that are week competitors have vanished from the fields due to modern technology, efficient herbicides and seed corn inspections.
Long-headed poppy and its look-alike, common poppy, also called field poppy and corn poppy (P. rhoeas), occur in similar habitats but the arts can be distinguished by the shape of their capsules. That of common poppy is almost spherical with 7–16 stigma-rays.
Also prickly poppy, also called pale poppy, pricklyhead poppy, may resemble long-headed poppy. However, its petals are narrower and with black spot at base and the capsule is hairy and its stigmatic disk has 4–6 rays.