Glaucous

glaucous [ GLAW-kuhs ] adjective: of a surface, having a bloom—a whitish or bluish powdery or waxy coating that tends to rub off

Native Juniperus virginiana (eastern redcedar) lance-oval, scale-like, crowded foliage with berry-like cones in August. — *Juniperus virginiana* (eastern redcedar).
Photo © Mary Free

Vaccinium corymbosum (Northern Highbush Blueberry) fruit in June. — *Vaccinium corymbosum* (northern highbush blueberry). Photo © Mary Free

Two words often seen in the description of plant parts, especially stems, leaves, and fruits, are glabrous and glaucous. Glabrous surfaces are smooth and lack hairs, bristles, and glands. Glaucous surfaces have a whitish, gray, or bluish-green coating, sometimes called a bloom. Most conspicuous on fruits, like blueberries and eastern redcedar, blooms are the waxy or powdery coating that tends to rub off. This coating is the outer surface of epidermal cells called the cuticle.

The origin of the word glaucous can be traced back to Latin, glaucus, Greek, glaukos, and Middle English, glauk. All had meanings that referred to colors such as dull bluish-green, greenish, blue, or gray. The first use of glaucous as a color was in 1671.

Records show that almost two hundred years later, the glaucousness of plants piqued the curiosity of Charles Darwin. Off and on for 19 years, up until twelve days before his death, he tested the bloom on a variety of species by “dripping, sponging, spraying, syringing, squirting, wetting, dipping, waving in liquids of varying temperatures, dusting with chalk or pumice and shaking the fruit and leaves to determine the degree of absorption, adherence and repellence and the degree of damage the action might cause.” (John van Wyhe, 2002) Darwin’s notes on these experiments are available here. In his 1881 introductory draft for a book he was writing about the protection of leaves by bloom, he concluded “if repelling water was one of the uses or effects of bloom, there were probably many other [sic] & whole subject seem [sic] worth investigating.”

Darwin died on April 19, 1882 and his book was never published, but he was correct that the glaucous trait in plants does much more than repel water. Modern research has shown that this cuticular wax layer protects against environmental stresses and mechanical damage and helps to limit moisture loss, insect and disease incursion, and ultraviolet radiation. (Wenjing et al. 2017)

Glaucousness in *Agave parryi* (Parry’s Agave) minimizes transpiration, keeps leaf surfaces cooler during the day, and protects from UV radiation that is more intense in the higher-elevation desert or semi-arid areas it inhabits.
Photo © Elaine Mills

For example, glaucousness on the top of a leaf reflects UV radiation, which can damage cells, as well as allows water droplets to more easily roll off of the surface, minimizing water accumulation and the potential for fungal growth. On the underside of a leaf, glaucousness reduces water loss and protects the vulnerable stomata—the pores that regulate temperature and the release of gases and water—as well as repels dirt and deters insects. The glaucous coating on a stem may make it more difficult for an insect to climb. In a study of wheat productivity, genotypes that exhibited glaucousness on flag leaf, stem, and spikes produced a higher yield than non-glaucous plants under drought and heat stressors. In some plants, glaucousness may change with age, appearing only when the plant is young and disappearing as it ages or vice versa.

The Latin names of some plants include or have included glauca or some variation as an epithet to indicate their distinctly glaucous nature. Most plant names, though, give no indication that a species has a glaucous part or parts, and in fact, botanists may not agree on whether it does or not. Plant descriptions can vary greatly, as it depends on the specimens observed—the season, the location, and their age as well as the observer’s perspective and judgment. Even the same plant can display variations. Additionally, less technical writings may use glaucous to denote a color rather than the display of a bloom. Below are some examples of (potentially) glaucous native plants that you may recognize or grow.

**Native* Plants with Glauca (or variation) as Epithets**

Needles of *Picea glauca* (white spruce) are distinctly glaucous, especially when young. [*In the Mid-Atlantic, scattered populations occur throughout PA where it is native farther north.] Photo © Tim Rains CC BY 2.0

The pale green or blue-green leaves of Sedum glaucophyllum (cliff stonecrop) are usually glaucous. Darwin noted the bloom on sedum leaves caused the water “drop to roll off beautifully.” Photo © Kurt Stüber CC BY-SA 3.0 — The leaves of Sedum glaucophyllum (cliff stonecrop) are pale green or blue-green and usually glaucous. Darwin noted the bloom on sedum leaves caused the water “drop to roll off beautifully.” Photo © Kurt Stüber CC BY-SA 3.0

*Vernonia glauca* (upland or broad-leaf ironweed) has glabrous, glaucous stems and leaves usually with glaucous or whitish undersides.
Photo © Ashwin Srinivasan CC BY 4.0 [Rotated]

Native Plants with Glaucous Stems or Culms

Glabrous, glaucous stem and glabrous calyxes with short-ciliate margins of native Baptisia australis (blue wild indigo) — The stems of *Baptisia australis (blue wild indigo)* are glabrous and sometimes glaucous. Photo © Elaine Mills

The stems of *Eutrochium fistulosum* (hollow Joe-pye-weed) are glabrous and glaucous, especially when young. Photo © Elaine Mills

The culms of Panicum virgatum (switchgrass) are sometimes slightly glaucous. — The glabrous culms of *Panicum virgatum* (switchgrass) are sometimes slightly glaucous.
Photo © Elaine Mills

Greenish to purplish clumps of Schizachyrium scoparium (little bluestem) are sometimes glaucous. Sheaths are usually glabrous, but sometimes glaucous. — Greenish to purplish clumps of *Schizachyrium scoparium* (little bluestem) are sometimes glaucous. Photo © Mary Free

Solidago caesia</em> (blue-stemmed goldenrod) flower buds in August — Glabrous when young, the stems of *Solidago caesia (blue-stemmed goldenrod)* can turn glaucous with age. Photo © Elaine Mills

Native Plants with Glaucous Leaves

Acer rubrum (red maple) palmate leaves — The abaxial (lower) leaf surface of *Acer rubrum* (red maple) varies from glaucous and pubescent to green and glabrous to persistently tomentose (at least on the veins).
Photo © Mary Free

Leaves of *Aquilegia canadensis* (wild columbine) are glabrous and glaucous (with a slightly whitish, waxy bloom) above and glaucous and glabrous to pubescent below. Photo © Mary Free

The glaucous leaves of *Eryngium yuccifolium* (button eryngo, northern rattlesnake-master) have bristles spaced about a half-inch apart along the leaf margins. Photo © Mary Free

The usually green to bluish-green leaves of Fragaria virginiana (wild strawberry) can sometimes be bright green, glaucous, or slightly leathery. — The usually green to bluish-green leaves of *Fragaria virginiana* (wild strawberry) can variously be bright green, glaucous, or slightly leathery. Photo © Elaine Mills

The blue-green, sometimes perfoliate, leaves of the *Lonicera sempervirens* (trumpet or coral honeysuckle) vine are glaucous, especially below.
Photo © Mary Free

References

Thippeswamy H, Krishna H, Sinha N. et al. 2022. Assessing the role of glaucousness in imparting tolerance to moisture and heat stress in wheat. Biologia 77: 3279–3289. Doi: 10.1007/s11756-022-01177-6.

van Wyhe J. ed. 2002-. “On The Protection of Leaves from Water.” An introduction by Christine Chua & John van Wyhe. RECORD: Darwin, C. R. 1881.04.19. The Complete Work of Charles Darwin Online.

Weakley AS, Ludwig JC, Townsend JF. 2012. Flora of Virginia.

Wenjing Chu, Haiyan Gao, Shifeng Cao, Xiangjun Fang, Hangjun Chen, Shangyue Xiao. July 2018. “Composition and morphology of cuticular wax in blueberry (Vaccinium spp.) fruits.” Food Chemistry. 219: 436-442. ISSN 0308-8146. doi: 10.1016/j.foodchem.2016.09.186.