The English are masters of gardening and garden writing, but they tend towards dramatic understatement. The Royal Horticultural Society, for example, described perennial polemonium as a plant that “often seeds itself around rather freely.” Generally a statement like that means that the plant is prolific to the point of invasiveness and is best planted by people who don’t mind a few—or a few hundred—“volunteer” offspring.
It would be a shame if such a statement put people off polemonium, sometimes known as “Jacob’s ladder”, “sky pilot”, “American great valerian” and “charity”. Most gardeners and plant vendors refer to it as “Jacob’s ladder” and I refer to it as a godsend for shady areas in need of brightness. In my experience, the self seeding is not a problem and the plant’s value in the landscape outweighs the inconvenience that might be posed by its offspring.
Polemonium is distinguished by the fact that it lends its name to an entire plant family, Polemoniaceae. Other members of the clan include good growers like garden phlox and Cobaea scandans, better known as “cup and saucer vine.” DNA analysis shows that the Polemonium family is closely related to the primrose or Primulaceae family as well. In both groups the flowers tend to have five petals, joined at the base into a longer or shorter tube. Most of the common Jacob’s ladders feature ample clusters of small blue flowers that grow at the tops or ends of the stalks.
Named by the great plant taxonomist, Linnaeus, the genus Polemonium contains at least 25 species of annuals, biennials and perennials, native to various parts of the northern hemisphere and Chile. The plants generally feature pinnate or divided leaves, with each stem bearing 12 pairs of small leaflets. This ladder-like arrangement gave rise to the Jacob’s ladder nickname. The leaves are attractive at all times during the growing season, but in the late spring or early summer, small buds open out into flowers that are either a clear sky-blue or a somewhat deeper shade of blue-violet, depending on the species and variety. Among American and European gardeners, the most popular Jacob’s ladders are perennial species.
Many of the best Jacob’s ladders are varieties of Polemonium caeruleum. The species name means “blue” and the flowers are lovely. Caeruleum varieties are erect plants, rising between 18 and 24 inches. One of the best is ‘Brise d’Anjou’, an older variety with variegated leaves and blue-purple flowers. The variegated leaves are especially useful in the partial shade that the caeruleums love best. Too much sun, in fact, will burn the leaves, so save the brighter garden spots for your roses or tomatoes.
If you must put a Jacob’s ladder in a somewhat sunnier situation, choose the light-tolerant Polemonium caeruleum ‘Snow and Sapphires’, with variegated leaves and darker blue-purple flowers. The “ladders” on these plants also rise a bit taller than those of other polemoniums, topping out at about 30 inches.
If you are in need of a flowering ground cover for partly shaded corners, choose varieties of creeping Jacob’s ladder or Polemonium reptans. These North American natives are mounding plants that rarely rise more than 12 inches, with relatively weak, sprawling stems. ‘Stairway to Heaven’, features green leaflets edged in a cream color that becomes rose-pink in cooler weather. ‘Touch of Class’, is an offspring of ‘Stairway to Heaven’, with crisper white edges. The flower buds are pinkish before opening to pale blue.
Many Jacob’s ladders are at least somewhat fragrant, as well as deer resistant. They can also be grown successfully under black walnut trees, a situation that is toxic to many plants. The one non-negotiable requirement is consistent moisture, so if you are beset with dry shade, use soaker hoses or other forms of slow irrigation to keep the plants happy through hot summer weather.
Jacob’s ladder lifts the landscape successfully in shaded rock gardens or low borders. The variegated types might also succeed in mixed container arrangements, provided their fellow travelers are also water lovers. Nurseries and garden centers generally carry at least one variety, but you can also check the selection at Sooner Plant Farm, 25976 S. 524 Rd., Park Hill, OK 74451, (918)453-0771, www.soonerplantfarm.com.