Plant of the Week

Polemonium occidentale

Western Polemonium (Polemonium occidentale)

By Charmaine Delmatier (2016)

A taller herbaceous perennial forb, native to North America, western polemonium (Polemonium occidentale) is distributed from British Columbia to Colorado to California. Although it can tolerate sporadic dry hilly slopes, it is usually found in moist areas such as lakeshores, streams, meadows, wet open woodlands, forested areas with bogs, fens, and other associated wetlands. Reaching a height up to 40 inches, the long usually solitary stem is sparsely covered with sticky glands. Arranged along the stem are its namesake, the leaves. Each leaf is divided into a series of two individual leaflets placed opposite each other along a central axis, much like the rungs of a ladder. These rungs (oppositely arranged leaflets) usually number 9 to 17. This pinnately compound arrangement of leaflets is the reason for its other common name, western Jacob’s ladder; depicting Jacob’s biblical dream of a staircase to Heaven. The leaves are lance-shaped and glabrous (without hairs) above, with a waxy blue powder below.

Western polemonium is a member of the Phlox family (or Jacob’s-ladder family), Polemoniaceae. There are close to 25 genera and 400 species worldwide, both annuals and perennials, including herbs, shrubs, small trees, and vines. Much of its diversity occurs in California. Some of the key family features include five sepals, five fused petals which often form a tube (corolla), and five stamens which alternate between each corolla lobe. In most species, the stamens are also fused into the corolla tube, placed symmetrically between each petal lobe. Another distinguishing family feature is that the ovary’s three carpels are usually fused while retaining three individual chambers.

The vibrant blue flowers of western polemonium are bell-shaped with top portion of the five petal lobes free, and the remaining portion of the flowering petals fused below forming a tube. The inner color of the tube is white to a light yellow. Unlike other members of the Phlox family, these stamens are free and elongated beyond the petal lobes. The anthers are quite visible from a distance and bright yellow. The free petal lobes slightly overlap and are twice as long as the tube. The total combined number of flowers are arranged in a fairly open terminal cluster. The contrast of bright blue flowers with extended bright yellow anthers make this a true display that lasts most of the summer from lower to mid-elevations.

Little is known about the biology of western polemonium. According to L.J. Schmidt with the U.S. Forest Service, it can reproduce sexually by seed or vegetatively from short unbranched underground rhizomes (underground stems). The seeds are borne in capsules and are released once the capsules dry and ultimately open, allowing the seeds to spread into their surrounding environment.

The tubular morphology and flower structure in the Phlox family has been a popular study for pollination biology and evolutionary patterns. There are several correlations between tubular length, width, and shape with corresponding pollinator vectors such as bats, hummingbirds, beetles, butterflies, and moths. Assessing associated variations over time have generated insightful correlations to plant speciation. (Galen 2000; Mayfield et al. 2001; Campbell et al. 2003; Lendvai and Levin 2003).

For More Information

• PLANTS Profile - Polemonium occidentale, Western Polemonium
• Galen, C. 2000. High and dry: Drought stress, sex-allocation trade-offs, and selection on flower size in the alpine wildflower Polemonium viscosum (Polemoniaceae). Am. Nat. 156:72-83.
• Mayfield, M. M., N. M. Waser, and M.V. Price. 2001. Exploring the 'most effective pollinator principle' with complex flowers: Bumblebees and Ipomopsis aggregata. Ann. Bot-London 88:591-596.
• Campbell, D.R., R. Alarcon, and C. A. Wu. 2003. Reproductive isolation and hybrid pollen disadvantage in Ipomopsis. J. Evolution. Biol. 16:536-540.
• Lendvai, G., and D. A. Levin. 2003. Rapid response to artificial selection on flower size in Phlox. Heredity 90:336-342.