- Name also: Greek Valerian, Jacob’s-ladder
- Family: Jacob’s Ladder Family – Polemoniaceae
- Growing form: Perennial herb.
- Height: 25–50 cm (10–20 in.). Stem unbranched, lower part glabrous, upper part densely short-haired.
- Flower: Corolla wheel-shaped, blue (occasionally white), 12–15 mm (0.48–0.6 in.) broad, fused, deeply 5-lobed. Lobes ovate, with quite blunt tips, throat hairy. Calyx 5-lobed. Stamens 5. Gynoecium composed of 3 fused carpels. Inflorescence many-flowered, corymbose, flowers fragrant.
- Leaves: Alternate, lowest long-stalked, upper almost stalkless, stalk edged, grooved, edges hairy. Blade pinnate, with terminal leaflet, lowest 8–12-paired, upper 5–6-paired. Leaflets narrowly elliptic–lanceolate, with tapered tips, with entire margins, glabrous, underside greyish. Leaflets often touching each others’ edges.
- Fruit: Roundish, brown, capsule with 3 compartments.
- Habitat: Broad-leaved forests, rich meadows, mountain birch woodland. Also ornamental and commonly an escape and leftover from cultivation around yards, banks and hedgerows.
- Flowering time: July–August.
Jacob’s ladder is a very popular and successful ornamental in Finnish gardens. It clings on tenaciously in old gardens and often thrives so well that it can spread form gardens to roadsides and wasteland, and sometimes even into broad-leaved forests with other wild plants, from where it can also be easily transplanted back into yards and gardens. Finnish botanists have long pondered if Jacob’s ladder is a native wild Finnish plant, or has it only gone wild from being cultivated as an ornamental. Stands in western Finland are to some extent originally from gardens, but the origin of Jacob’s ladder in eastern Häme and North Karelia has had botanists scratching their heads for a long time. The puzzle seemed to at last have been solved when pollen from the plant was found in the layer that corresponded to the warm period that followed the Ice Age. This seemed to infer that Jacob’s ladder had spread to Finland before people – unless the pollen was actually from another species, tall Jacob’s ladder (P. acutiflorum), because the pollen of the two species is indistinguishable.
Differences between the species are otherwise very small and a long time ago tall Jacob’s ladder was thought to be a mere internal mutation of Jacob’s ladder. Tall Jacob’s ladder has campanulate and slightly broader flowers. Jacob’s ladder’s lower leaves’ leaflets partly overlap while on Tall Jacob’s ladder they don’t touch, and are otherwise fewer with a maximum of 8 pairs. Tall Jacob’s ladder has also been planted in the past as an ornamental in gardens, just like Jacob’s ladder. Jacob’s ladder family also includes genus Phlox, whose best known representative is probably moss phlox (P. subulata), which spreads into the wild from around inhabited areas.