<em>Polemonium</em>: to plant, to grow, to care for

Description and botany

Les Polemonium are perennials or annuals in family Polemoniaceae and include 38 species. They are distributed across temperate regions of the Northern Hemisphere and the Arctic.

Polemonium caeruleum, called Greek valerian, is the only species found in the wild in France, where it is protected. This montane species occupies woods and damp pastures of high mountains up to 2,000 m altitude in Jura, Savoie, Haute-Loire, Puy-de-Dôme, Cantal and the central Pyrenees and extends to central and northern Europe, the Caucasus, Siberia and northern America. Its origins explain its very good adaptation to cold, damp climates. However, it dislikes humid heat. This species forms a rosette of dense leaves topped by several stiff bluish-violet flowering stems measuring 30 to 80 cm, sometimes reaching 120 cm in height. It has given rise to various cultivars with white flowers, large flowers up to 2 cm wide such as ‘Lambrook Mauve’, or with cream-margined leaves such as ‘Brise d’Anjou’…

The characteristic fern-like foliage of Polemoniums consists of leaves bearing dozens of small pointed leaflets inserted perpendicular to the central vein, like the rungs of a ladder—hence the common name Jacob’s ladder—and the leaves are odd-pinnate.

Polemonium caeruleum – botanical illustration

Alternate leaves with a petiole form a lush mass at the base while they dress the flowering stems more sparsely and discreetly.

Flower stems are hollow and furrowed, bearing at their tip a panicle of delicate bluish-violet flowers from spring to late summer that contrast with the deep green colour of the foliage. The corolla, surrounded by a green calyx with five villous, glandulous lobes, has a short tube ending in five obtuse oval lobes forming a cup 0.8 to 1.5 cm wide. The flower centre, tinged with golden yellow, is crowned by five pale-yellow stamens becoming golden at ripeness and by a long white pistil ending in three points. The genus offers a rather astonishing diversity of flowerings, such as the tubular orange corollas of P. pauciflorum, the large pale-blue bells of Polemonium ‘Stairway to Heaven’ or the intense blue of P. caeruleum ‘Sapphire’, the soft-pink funnel-shaped flowers of Polemonium carneum ‘Apricot Delight’…

Slightly scented flowers attract bees and butterflies, eager for their pollen.

Fruits are three-angled capsules containing four to six seeds within each of the three chambers.

Cut flowers of Greek valerian keep well in a vase; variety ‘Sonia’s Bluebell’ is particularly suited to this use as it flowers from May to August.

Formerly, in eastern United States, the dried roots of Polemonium reptans were used in herbal medicine. They were harvested in autumn, then dried for later use.

Some Polemoniums: P. caeruleum ‘Bressingham Purple’, P. reptans ‘Blue Pearl’, Polemonium ‘Lambrook Mauve’, P. carneum ‘Apricot Delight’ and the pretty variegated foliage of P. reptans ‘Touch of Class’

Main Polemonium varieties

Favourite varieties with spreading habit

Favourite varieties with erect habit

Polemonium caeruleum Lambrook Mauve

This horticultural creation results from a cross between Polemonium carneum and Polemonium reptans, two species native to North America. This herbaceous perennial forms a dense tuft 40–45 cm across with finely divided, somewhat olive-dark green foliage topped by mauve-lavender campanulate flowers borne on velvety purple stems.

• Flowering time July to September
• Height at maturity 40 cm

See the product sheet

Polemonium reptans Touch of Class

Prostrate, ground-covering perennial with very soft grey-green foliage finely marginate with white, renewing throughout the growing season. Loose clusters of small, pink-budded flowers open into mauve corollas in late spring. Ideal for edging a path or a rock garden in sun that is not scorching or in partial shade.

• Flowering time June, July
• Height at maturity 40 cm

See the product sheet

Polemonium carneum Apricot Delight

Very attractive, robust perennial for fresh, well-drained soil, with pink flowers shaded pale apricot at the centre. Pairs well with bellflowers, yarrows or dwarf roses.

• Flowering time June to September
• Height at maturity 40 cm

See the product sheet

Polemonium Sonia's Bluebell

Attractive rhizomatous perennial about 50 cm across, robust, with a bushy habit prized for its pale mauve flowers for cutting that renew throughout spring. Its divided foliage is purple at first, turning dark green at maturity.

• Flowering time June to August
• Height at maturity 50 cm

See the product sheet

Polemonium yezoense Purple Rain

One of the best recent introductions, this Polemonium forms an impressive tuft of lacy foliage in bronze to deep purple, especially in spring and autumn, from which dark erect stems arise bearing blue-violet flowers. Fabulous, very hardy and vigorous, this plant performs best in full sun or partial shade in fresh, damp, humus-bearing soil.

• Flowering time July to October
• Height at maturity 50 cm

See the product sheet

Polemonium caeruleum Brise d'Anjon

Variety of Greek valerian with dark green leaves marginate with cream, flowering later than the type species in bright blue-mauve tones.

• Flowering time June to September
• Height at maturity 50 cm

See the product sheet

Planting

Where to plant Polemonium?

Polemoniums are easy to grow and are planted in any soil that stays cool in summer but drains well. They appreciate humus-rich ground. Avoid very acidic or very calcareous soils.
Choose a position not exposed to intense sun or in partial shade. Planting in shade produces few flowers but remains interesting for the aesthetic of the dark glossy, finely divided foliage, variegated with cream or purple.
Avoid the combination of high heat and humidity in regions with hot summers. Foliage is sensitive to scorching sun rays, so this plant is best suited to regions with cool summers.
Polemoniums are very hardy and can withstand temperatures of -20 to -30°C depending on species.

When to plant?

Polemoniums are preferably planted in autumn to encourage good establishment before summer.

How to plant?

This plant is easy to grow.

• Soak the buckets in a bucket of water.
• Work the plot with a digging fork to loosen the soil deeply.
• Position young plants about 50 cm apart.
• Dig a hole the size of the rootball with a small trowel.
• If soil is very clayey, add a shovelful of compost to each hole or plant on a raised mound to ensure good drainage.
• Remove the bucket and tease out any circling roots if necessary.
• Plant the rootball taking care not to bury the collar and firm lightly.
• Water then mulch.

Polemoniums can be grown in large pots of at least 18 cm diameter. Fill the container, perforated at the base, with a mixture of potting compost and coarse sand.

Caring for Polemonium

Care is simple and can be summarised in a few steps :

• Ensure soil stays cool even in summer.
• To encourage flowering and improve foliage retention, cut back some spent inflorescences. Leave some seedheads to ensure renewal of young plants, which have a fairly short lifespan.
• If plant tends to spread excessively, remove seedheads.
• Polemonium does not need to be divided and does not tolerate transplanting well.
• Fertilisation is unnecessary.

Multiplication

Easiest propagation is to divide clump in spring or autumn but Polemoniums can also be raised by propagation by cuttings from young spring shoots or by sowing in autumn or under cover in February–March.

Polemoniums are generous : they tend to self-sow abundantly yet never become invasive. They are then very easy to replant.

Clump division

Proceed when foliage is barely developed in late winter or in autumn before dying back :

• With a spade, separate rosettes with as many roots as possible.
• Replant immediately in light soil without burying the collar.

Propagation by cuttings

Prepare a deep pot by filling it with potting compost mixed with sand, or cultivate a patch of soil protected by a cold frame ;

• Take shoot tips about 10 cm long.
• Remove leaves located near base of cutting.
• Insert these to two-thirds of their length, leaving enough space between them for development until following spring.
• Firm soil gently around each to remove air pockets and ensure good contact between potting compost and cutting.
• Place them under a cold frame in shade.
• Next spring, separate rooted cuttings and plant them out directly.

Sowing

• Harvest Polemonium seeds in June or July by opening the capsules.
• If sowing in spring, place seeds in fridge for a few weeks before sowing.
• Sow in a seed tray filled with seed compost, covering seeds lightly.
• Water gently and place in a heated room. Germination occurs in 10 to 14 days.
• Pot up seedlings into buckets when they can be handled.
• Pinch stems when strong enough to encourage ramification.
• Plant out in autumn if clump has become well established or wait until following year.

Uses and associations

Greek polemonium is a bedding plant well suited to the mixed border to bring lightness alongside other perennials or to flower the base of roses. Stage it in a border in blue–pink–white tones or in contrast with a complementary shade such as orange. Its flowering coincides with that of late tulips, Oriental poppies, peonies, Potentilla fruticosa ‘Hopleys Orange’ or globe flowers, columbines, daylilies and geums, which also prefer fresh soils.

An idea for a part-shade combination: Polemonium caeruleum ‘Bressingham Purple’, Geranium phaeum ‘Samobor’, Brunnera macrophylla ‘Mr Morse’, Polystichum setiferum ‘Proliferum’, Aquilegia ‘Green Apples’ (‘White Barlow’ would also work), Allium aflatunense ‘Purple Sensation’

Play with foliage combinations by mixing the large leaves of Hostas, Brunnera macrophylla, the fronds of ferns and the dentate, richly coloured leaves of Heucheras, within a wooded garden or a partly-shaded rockery.

Define shaded path edges with a bed of Greek polemonium which, with its larkspur-like habit, provides attractive groundcover and a generous flowering.

You can also prominently place a specimen such as Polemonium yezoense ‘Purple Rain’, whose purple foliage remains decorative for a long time, in a deep container to serve as a focal point.

An example of a combination: Polemonium and knotweed (here, Polygonum alpinum) go well together (astilbes such as ‘Sprite’ or ‘Hennie Graafland’ would also do the trick) !

Within a rockery or on the edge, the spreading forms of Polemonium reptans can mingle with the bell-shaped blooms of pasque flowers richly tinted violet or carmine red or with white spring anemones (Anemone sylvestris…). The variety Touch of Class, with very soft green–grey foliage, finely marginate with white, prefers part shade to shade exposures such as a woodland, a courtyard or a shaded garden. It pairs wonderfully with blue corydalis, bleeding hearts, Geranium nodosum or Epimediums. It is also a good plant to dress the edges of water features and very useful in naturalistic gardens to attract pollinating insects.