Wild Ideas: Rise of the resilient polecat weed

What native plant now blooming produces heat well above the ambient temperature, has a bizarre bloom, can live for a century and stinks? The answer is skunk cabbage, whose amazing biology makes it an early bloomer and resilient survivor.

I had last looked for emerging skunk cabbage in the wetlands at the bottom of the mountain where I live on Feb. 3, during a warm spell. Returning to the patches last Friday (Feb. 15), when temps soared to near 70 degrees, I was not surprised to see dozens of plants in various stages of emerging out of the muck. I apparently had missed the first to emerge and found that they had paid a price for their timing.

While the hardy eastern skunk cabbage ( Symplocarpus foetidus ) has built-in adaptations for dealing with erratic weather cycles such as we’ve experienced this winter, this was the first time I’ve seen such damage to so many of the plants. It was probably due to the ice storm that had occurred in the interval between my trips to the patches rather than to the plunge in temperature. A skunk cabbage bloom at left was apparently damaged by the recent ice storm, while the bloom at right, on the same plant, escaped the ice, emerging during the warm spell that followed. In the Arum plant family (Araceae), eastern skunk cabbage grows in wet, shaded places. It is among the earliest native plants to bloom in our area, sometimes as early as January, depending on the weather. First to emerge is the spathe, a pointed, 4-6 inch leathery hood that wraps around and protects the plant’s spadix. The egg-shaped spadix holds a cluster of up to around 60 tiny, greenish-yellow, petalless flowers. Together, the spathe and spadix make up the plant’s inflorescence — the reproductive organ of seed-producing plants. This type of inflorescence is common in the Arum family, which includes jack-in-the-pulpit and the notorious titan arum, aka corpse flower.

The female flowers of the skunk cabbage are located below the males on the spadix, and the flowers bloom in sequence, from bottom to top, which prolongs the reproductive period. Although a spadix can be damaged or destroyed by ice, each skunk cabbage can produce up to four every year.

Scientists who have tested the human response to the skunk cabbage’s pungent odor found it likened to that of garlic, apples, turnips and carrion, according to a 1999 Journal of Biospheric Science (JBS) article (tinyurl.com/wi-jbs). All parts of the plant can carry the odor, which can vary from plant to plant and often is unnoticeable unless the plant is broken or bruised. Its fetid smell has also earned it other common names, including tabac-du-diable (Devil’s tobacco), chou puant (stinking cabbage) and polecat weed. Among the less-pejorative common names is bearweed. Rise of the Resilient Polecat Weed Slideshow

Although the skunk cabbage’s smell is likely an adaptation to ward off some herbivores, the leaves have an additional weapon: calcium oxalate, the crystals of oxalic acid, which are also common in kidney stones. One wildlife biologist, after tasting a leaf, said his tongue felt like it had “tiny needles” in it, according to an article on the Alaska Department of Fish and Game (ADFG) website. The effect, usually referred to as a “burn,” can last for hours.

Despite the plant’s offenses, some wildlife still eat it. Deer target the early spring leaf shoots and the spadix. The latter is rich in protein, which is “critical to hungry deer after the lean pickings of winter,” as the ADFG article points out. Deer and geese eat the leaves throughout the summer, leaving just the center stem. Bears dig up and eat the roots as well as the leaves and buds. Along with the nutrition in them, the fiber can help get digestive systems going for bears coming out of winter hibernation.

Beyond its bizarre look and funky smell, skunk cabbage stands out for another evolutionary adaptation that is rare in plants: it produces heat through its flowers’ metabolism (thermogenesis), enabling the plant to raise its temperature as high as 63 degrees above the ambient temperature. This gives the plant a respiration rate equivalent to that of a shrew or hummingbird. Some tropical arums also produce heat, indicating that this capability may have evolved not to help the plant heat its way up out of frozen ground, as some believe, but rather to draw in insect pollinators. The heat volatizes the chemicals that causes the plant’s distinctive odor, which attracts the insects.

The asymmetry of the spathe opening sets up a cyclonic vortex inside it, keeping some of the heat and smell moving around but releasing enough to draw pollinators, which may stay longer to take refuge from cold, thereby collecting more pollen. Flies, bees, springtails, beetles, sow bugs, true bugs, and the larvae of butterflies and moths have been discovered in the spathes, according to an article on website of the conservation nonprofit Northern Woodland . With few insects active as early as the skunk cabbage can bloom, the plant also spreads pollen through wind.

Once established, a skunk cabbage is hard to remove because its roots contract as they grow further down into the earth every year, pulling the plant down with them. If a skunk cabbage’s crown is damaged, lateral shoots on the root can grow into the inflorescence, for reproduction, and into leaves, for photosynthesis. According to an article on the Virginia Native Plant Society website, individual skunk cabbages “can be very long-lived, some having been estimated to be nearly a century in age.”

Skunk cabbage spathes are normally a light green mottled with maroon, but the ice storm had apparently turned many of them brown or black, with the flowers inside also black or rotted off. This was especially true of those on the open side of the spadix. Most of the plants had sent up only one or two spadixes so far so should be able to produce two or three more, continuing their reproductive cycle. And some plants were just barely getting started, with healthy-looking spathes just starting to poke up through the mud.

© 2019 Pam Owen


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