Polemonium, also known as Jacob's-ladder, Greek valerian or sky pilot, is a popular garden ornamental. There are about 25 species worldwide, found throughout the Northern Hemisphere and also the Andes of South America. They include both annuals (not so often grown) and perennials. The name Polemonium was coined by Dioscorides, possibly in honour of Polemon or from the Greek word ‘polemos' which means ‘war'. Those grown as garden plants typically have basal rosettes of pinnately compound leaves consisting of numerous small, rounded leaflets. Blue is the most popular floral colour, but white, lavender, pink and even yellow flowers do exist. Plant breeders have even selected forms with colourful foliage. Taller species are useful for the sunny border or partly-shaded woodland garden while the alpine species are ideal candidates for the rockery or alpine trough. Polemonium are often short-lived but many (especially P. caeruleum) self-seed with abandon. Most of the species grown by gardeners are quite hardy. Moist, yet well-drained soil with some added lime is ideal. As a word of note, many species, especially the alpine types, exude a skunk-like smell when the leaves are bruised.
Let's start with describing the taller, border types, those commonly called Jacob's-ladder or Greek valerian. Probably the most popular species is P. caeruleum, a species found throughout Europe. The wild form will grow 30 to 90 cm in height, blooming in early-mid summer although prompt dead-heading can extend the blooming into the fall. The open, bell-shaped flowers are produced in a somewhat rounded head. ‘Bambino Blue' is a popular blue cultivar. ‘Album' is a white-flowered version. In the 1990s came the variegated selection ‘Brise d'Anjou'. The white-edged leaflets with contrasting blue flowers was a knockout. However, this selection was temperamental and not as hardy as the wild type. Most recently has come ‘Snow and Sapphires', an equally beautiful variegated selection that is proving to be tougher than its predecessor. In 2002 Alan Bloom introduced a purple-leaved selection called ‘Bressingham Purple'. ‘Apricot Delight' is a shorter selection whose lavender-pink flowers have a distinctive apricot-yellow center. Some authorities consider this one to be a selection of P. carneum, a pink-flowered species from California. This one is not as hardy as the other border Polemonium.
Polemonium caeruleum and the cultivar 'Bressingham Purple' and 'Album'
Above are the cultivars 'Brise d'Anjou', 'Snow and Sapphires' and 'Apricot Delight'
A very similar species is P. yezoense, from Japan. Some authorities consider it a subspecies of P. caeruleum. Its claim to fame are its dark purple spring leaves that turn green as the season progresses. As recent as 2006, Jelitto Seed, from the Netherlands, released ‘Purple Rain', a selection whose leaves retain a purple tint all summer. It looks very much like ‘Bressingham Purple'.
Flower and foliage details of 'Purple Rain'
Another taller species is the eastern North American P. reptans, a species that looks similar to P. caeruleum but shorter and more delicate in appearance with lax, open flower-heads. A few of the named selections include ‘Blue Pearl' (25 cm; violet-blue), ‘Lambrook Manor' (45 cm; lilac-blue), ‘White Pearl' (white) and ‘Heaven Scent' (fragrant, deep blue-purple.) There is also a variegated version of this species called ‘Stairway to Heaven', discovered by William Cullina of the New England Wildflower Society. The white-edged leaves have touches of pink in cooler weather. This selection is much tougher than either ‘Brise d'Anjou' or ‘Snow and Sapphires'. Then, if this wasn't good enough, along came ‘Touch of Class', the newest variegated Polemonium with even more striking variegation. These last two have light blue flowers.
Polemonium reptans and the cultivars 'Stairway to Heaven' and 'Touch of Class'
For the mid to back border, you can grow the tallest species, P. foliosissimum, which hails from western US. Called the towering or leafy jacob's-ladder, it may reach 120 cm. Plants look like a taller, open, airy version of P. caeruleum. All of these tall species are rated hardy to zone 3.
Rather unusual among the Polemonium are the western US species P. brandegei and P. pauciflorum. Both of these have tubular flowers that are straw-yellow with a slight purple tint. Very unusual to say the least! The foliage however, is typical of Polemonium. These get to about 30 cm and can bloom the first year from seed if sown early. They are quite short-lived so save seed to keep these going. Hardiness information varies for these species. I've seen them rated as hardy to zone 4 while other sources say zone 7. I had P. pauciflorum survive in my zone 5b but it behaved as a biennial.
Above are the two very similar species P. brandegei and P. pauciflorum
The other popular Polemonium are shorter species (below 20 cm) more suited to the sunny rock garden or an alpine trough. My personal favourite among these is skypilot, P. pulcherrimum, a western North American species which extends from Alaska to California. In the wild, they range from the foothills to the alpine zone, often in rather dry, rocky locations. This zone 4 plant has stems 10-20 cm in height with flowers that range from lavender-blue, light blue to violet-blue. Polemonium delicatum is very similar (considered a subspecies of P. pulcherrimum by some) but hails from the western US. It is more tender, being rated for zone 6. Yet another look-alike is P. elegans, native to British Columbia and Washington State. This latter species has stamens that extend beyond the petals while those of P. pulcherrimum are shorter than the petals. Only a botanist can appreciate the difference!
Details of P. pulcherrimum
A choice rock garden plant is the western North American alpine species P. viscosum. This species reaches about 15-20 cm with small, succulent, upright leaves with very dense arrangement of leaflets. The leaves look almost fern-like. The flowers are held in a dense head and are deep violet-blue. Contrary to most Polemonium, this species prefers acidic soil. It has the strongest skunk-like smell of all the Polemonium (only when handled) and is commonly called skunkweed. It is rated hardy to zone 5. Closely related to this species is P. confertum, an endemic to Colorado. The foliage is not quite as dense as P. viscosum, but the plant is every bit as exquisite and desirable as a rock garden or alpine trough subject. It is rated for zone 3.
Details of P. confertum and P. viscosum
The last Polemonium of note is the northern Jacob's-ladder, P. boreale. This species has the widest range of any Polemonium, being circumpolar. It is also the hardiest species (zone 2), ranging high into the Arctic regions. In the northernmost regions they grow in full sun but further south, they prefer dappled shade. Most of the selections offered on the market, such as ‘Iceberg Point' and ‘Heavenly Habit' are from plants that originated from the southern part of the plant's range. This species will grow to about 30 cm and has medium violet-blue flowers. The habit is such that this one can cross borders between the front of the perennial border, woodland garden or rock garden.
Close-up of P. boreale
If you are a lover of blue flowers, then Polemonium come highly recommended since their diversity allows them to be used in just about any garden situation you may have to offer.
There are many people which need to be thanked for the use of their pictures. These include dravencat ('Snow and Sapphires'), poppysue ('Brise d'Anjou'), carolann ('Apricot Beauty'), echoes ('Purple Rain' foliage, P. pauciflorum), aimlyn19 ('Stairway to Heaven' flower), fairy1004 ('Stariway to Heaven' foliage), victorgardener ('Bressingham Purple'), altagardener (P. confertum), and baa (P. brandegei),