Jacob's ladder
Polemonium caeruleum. Family Polemoniaceae. Perennial.
Also called Greek valerian.
Jacob's ladder is a clump-forming perennial grown for its blue, bell-shaped flowers and neatly divided, ladder-like leaves. It suits cool, partly shaded borders with moist soil, but often becomes short-lived in hot, humid summers or dry ground.
At a glance
- Sun
- Partial shade; full sun in cool climates if the soil stays moist. Protect from strong afternoon sun in warmer areas.
- Water
- Keep the soil evenly moist, especially during establishment and flowering; avoid waterlogging.
- Soil
- Humus-rich, moisture-retentive but well-drained soil; tolerates acidic, neutral and mildly alkaline conditions.
- Height
- 1.5-3 ft (0.45-0.9 m)
- Spread
- 1-2 ft (0.3-0.6 m)
- Flowers
- Late spring to early summer; usually blue to violet-blue flowers with conspicuous yellow stamens. White-flowered forms also occur.
- Foliage
- Green, pinnate leaves with paired leaflets arranged like ladder rungs; usually dies back in winter.
- Growth
- Moderate; usually forms a flowering clump within one or two growing seasons.
- Care needed
- Low
The year at a glance
- Pollinators
- Cold hardy
- Low allergen
| J | F | M | A | M | J | J | A | S | O | N | D | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Flowers | ||||||||||||
| Plant |
Northern hemisphere months. In Australia, shift them by six.
- Difficulty
- Easy
- Toughness
- Fairly tough
Will it suit your garden?
Pick your zone, country, light and whether you have pets.
Full sun is 6 or more hours of direct sun a day, part shade 3 to 6 hours, shade under 3.
How it grows
Planting day
1 ft (0.3 m)
After 1 year
1.4 ft (0.43 m)
After 2 years
1.8 ft (0.55 m)
After 3 years
2.2 ft (0.68 m)
full size
About 3 years to full size.
From a typical nursery size, at an average rate for this kind of plant. A rule of thumb, not a forecast.
Good to know
- Cats and dogs
- Safe
- Native to
- the UK
- Growth rate
- Moderate
Care
Jobs through the year
- WaterMar–Apr, Sep–Oct, every 14 days
- Water established clumps if rain is insufficient; keep young plants consistently moist.
- WaterMay–Aug, every 7 days
- Water deeply in dry weather, keeping roots evenly moist without waterlogging.
- MulchMar–Apr
- Apply a thin compost or leaf-mould mulch, keeping the crown clear.
- DivideMar–Apr
- Divide crowded or declining clumps every three to four years.
- DeadheadJun–Jul, every 7 days
- Cut spent stems to the leafy base; leave a few only if seedlings are wanted.
- ProtectMar–May, every 7 days
- Check emerging leaves for slugs; support leaning flower stems with twiggy stakes.
- PruneNov
- Remove collapsed dead foliage at ground level, leaving healthy green growth.
- Water deeply during dry spells so the root zone does not dry out.
- Apply a thin layer of compost in spring; rich soil rarely needs additional fertiliser.
- Mulch with leaf mould or compost to conserve moisture, keeping it clear of the crown.
- Divide crowded or declining clumps in spring every three to four years.
- Support tall flower stems with small twiggy stakes if they lean in exposed positions.
Pruning. Remove spent flower stems to the leafy base to limit self-seeding and sometimes encourage a smaller second flush. Cut dead foliage to ground level in late autumn or late winter, leaving any healthy green growth.
Through the year
- Early spring
- Plant or divide clumps once the soil is workable, and water them in well.
- Spring
- Apply compost and a moisture-conserving mulch; check young growth for slugs.
- Late spring
- Support leaning flower stems and water during dry weather.
- Early summer
- Deadhead after the main flowering flush, or leave a few stems if you want seedlings.
- Summer
- Keep roots moist and remove badly scorched or mildewed leaves.
- Late autumn
- Remove collapsed foliage and check that the crown is not sitting in waterlogged soil.
In the garden
- Plant in small groups through a partly shaded border for blue flowers above finely divided foliage.
- Combine with hostas and ferns along a woodland edge for contrasting leaf shapes.
- Use with hardy geraniums and columbines in a cottage border for overlapping late-spring flowers.
- Place beside a shaded path where the patterned leaves and yellow stamens are easy to see.
Garden styles it suits: Cottage garden, Woodland, English country, Romantic rose garden, Blues and silvers and Pollinator garden.
Plant it with: Hostas, Ferns, Columbines, Hardy geraniums, Lungwort and Astilbes.
Wildlife: Flowers provide pollen and nectar for bees and other visiting insects.
Problems to watch for
- Slugs and snails can damage emerging leaves; inspect after dark, remove them and use iron phosphate bait according to the label if needed.
- Powdery mildew may affect foliage, especially when roots are dry; water at soil level, improve spacing and remove badly affected leaves.
- Hot sun and dry soil cause brown leaf edges and early dieback; provide afternoon shade, mulch and water deeply.
- Waterlogged winter soil can rot the crown; improve drainage or move the plant to a better-drained position.
Safety
Not known to be toxic to cats, dogs, horses or people; no particular poisonous part is recognised. It is not a food plant, and eating garden foliage can still cause stomach upset.
Easily confused with
Creeping Jacob's ladder (Polemonium reptans)
Usually shorter, with looser, spreading stems and flower clusters rather than the taller, more upright flowering stems of Polemonium caeruleum; their height ranges can overlap.
Common valerian (Valeriana officinalis)
Has opposite leaves and broad clusters of tiny pinkish-white flowers, rather than alternate leaves and individual blue, bell-shaped flowers; it is also generally taller.
The name
The common name refers to the paired leaflets resembling ladder rungs, while caeruleum is Latin for sky-blue.
Propagation. Divide established clumps in spring. Sow seed fresh in autumn in pots outdoors; stored seed may benefit from a period of cold, moist conditions before germination. Seedlings may differ from named cultivars, so divide these to retain their characteristics.
Checked against open data
- Accepted botanical name
- Polemonium caeruleum
- In the US and Canada
- Introduced in Canada and the lower 48 states.
- Native range
- Altay, Austria, Baltic States, Belarus, Central European Rus, China South-Central, Czechoslovakia, East European Russia, Finland, France, Germany and Great Britain.
- Measured
- Soil pH 6.5 to 7. Average height 14 cm.
Sources: GBIF (CC BY 4.0), USDA PLANTS and Trefle (CC BY 4.0). The care and design notes are LandscapeGPT's own.
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