Chorus frogs are peeping, the trillium blooms have turned from white to purple, and leaves of all the deciduous plants are bursting forth in an orchestrated unfolding, filling, and spreading. And among the most beautiful displays is poison oak. Its buds open to crimson or red leaves that uncrinkle and spread. They stop at a fierce and stubby size in the sun, but continue to an impossibly wide, thin sheet in the deepest shade. And months from now, those leaves will stand guard, tough under the beating sun, or some feet away in the shade, will drape as delicately as do the leaves of any shaded woodland plant. In the sun, clusters of fruit hang like vegetable pearls, each orb enlaced with black lines, as delicate as jewelry. Then before fall fogs have sunken into the valleys, the leaves hint at, and finally take on a bronze or red hue. And when the wind and rain threaten to batter, leaves fall to the ground. Winter comes. Now the poison oak still pulls my view: the young winter twigs look reddish and rubbery. They are felted with fine hairs. There are small buds that I know, late in spring, will explode again with uncrinkling, spreading leaves.
Yes, I’m a fanatic.
It’s likely that a person never gets over the associations one has with an organism–or place, or person, or climate–in which, or with which, one has spent much time. I studied poison oak for my PhD dissertation many decades ago, which is where I learned the fine points of its style, but having grown up interested in plants and in poison oak’s native range, I already knew a lot.
As a kid, I knew that my poison oak grew as a shrub in the sun, and as vines or floppy shrubs in the shade. I knew that deer ate it, and that white sap oozed out of it if I snapped a stem or leaf. I knew that sap turned black. I knew that sometimes I got a rash quickly and sometimes slowly, sometimes the rash was intense but did not last long, and other times it gave a slowly increasing burn. I knew that some friends got much worse reactions and that friends claimed they didn’t react.
And I knew its scientific name was Rhus; I didn’t know that off and on since 1700, it had been called Toxicodendron, which is what it goes by today. I knew that sumac was also a Rhus. I knew that on the West Coast we had only one species, and we called it poison oak. But I didn’t know that there were three other species in the US, some of which are called poison oak or poison ivy differently in different regions. I also didn’t know that the most widespread of those species has nine subspecies that range over the eastern third of the contiguous US, Mexico part of the Caribbean, Japan, western and central China, and Taiwan.
A friend reminds me to warn, “Don’t try this at home.” Truly, don’t mess with poison oak or poison ivy unless you are willing to practice extremely careful hygiene with it. Someone close to you may react to one one-millionth the concentration that it takes for you to have a reaction. And you, or that person, may not react for two or even three weeks.
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What, you may ask, are the most astonishing facts about poison oak? Or at least, astonishing facts to me?
- The plant is important for wildlife. It provides shelter for rodents and perches for birds. Deer and other browsers eat the shoots, and in one study, at least 55 species of birds feed on its seeds.
- Poison oak keeps people from walking off of cleared right-of-ways: it discourages trespassing better than a fence.
- Poison oak is attractive to the eye for its foliage and its form. It was even sold in British nurseries as early as 1640.
- Although the foliage varies from clone to clone and from sun to shade environment, the leaves are usually attractive with their shiny, rounded leaflets and their array of colors, from eraser-red to honest summer green to bronze, red, and orange. Over much of its range, poison oak contributes the most conspicuous fall color of all the native vegetation.
- Its many forms include 1) the stiff, dense shrub in the sun, 2) the looser sprawling shrub in the shade, 3) the climbing vine (liana), usually on trees, and that usually has its foliage within the tree’s canopy rather than beyond it, 4) the lawn, often less than a foot high, maintained by deer browsing or mowing, 5) the rarer treelet form which has fewer but more vertical stems than the shrubs. 6) The last form, which I call basket form is on wind- salt-, and sand-scoured bluffs near the ocean. The plants have dense upward growing points that die back, thereby sheltering the plant lower in its own canopy.
- Poison oak is also often visible to the eye. It can grow in disturbed areas and along roadsides in billowy hedges. It is dispersed by birds, and so it becomes established beneath their perches–utility lines and fence posts. Lacking a bird, a researcher has to scarify the seeds with acids to get them to germinate.
- Poison oak’s allergenic resin, called urushiol, is inside the plant in leaves, and the bark of stems and roots. In an undamaged plant, urushiol does not sit on top of surfaces. Even extremely, uber-super-sensitive people cannot get poison oak from walking near it. A person who had a reaction had contact with urushiol, either from their touching a leaf or stem that had already broken or that they broke, or by their touching an object that had urushiol on it. That could have been a pet, a shoelace, or a rake. It could have been a hairbrush or a door handle.
- Urushiol does not volatilize in fires (or in drying ovens in the lab). When firefighters, campers, and others get poison oak from a fire, the trigger is the droplets of urushiol that are borne on ash.
- In stems and roots, urushiol is in ducts in the inner bark. Some very bad cases of poison oak and poison ivy have come from digging through soils that have poison oak’s fine roots in them. I did a wood ID once for a case in which two hospitalized contractors were suing a landowner because of their rash; they thought they had dug through toxic wastes. I identified the root samples from the soil pit as poison oak.
- Poison oak’s urushiol is very similar to the resin in a closely related species, also a Toxicodendron, in Japan. There, it has been used to make a black lacquer. Reportedly, many US GIs in Japan during World War II got rashes from lacquered surfaces that had not been cured enough for their sensitivity.
- When you nick the plant, the sap oozes out white, and then oxidizes to black. After a black stain is laundered, it maintains a lead-gray color. The Pomo Indians used it to color some of their materials for baskets. It stained the coveralls I wore for my research. Occasionally it made black plaques on my skin; these provoked bad reactions–more on that in the next post.
- I never saw a plant bloom in the shade. I frequently saw plants bloom in the sun.
- Separate plants have pistillate (female, seed-bearing) flowers and staminate (male, pollen-bearing) flowers. If you look closely, you see that the female flowers are a little smaller, and have pistils in the center; the male ones have stamens. Several of the 450 plants that I raised from cuttings had a different gender from the gender of the plant I took the cuttings from.
- It can grow into large clones. You can get a sense for the extent of individual clones when leaves senesce: patches the size of garages or tennis courts will differ in their timing and their color. You can also see it in the shape of the leaf margins if you compare plants with the same amount of sun.
- The fact that we can see these clonal patches is interesting. It was hard for me to get stem cuttings to make roots: in pilot experiments, I learned I had to use a very high concentration of rooting hormone. Also, I was surprised at how infrequently I observed plants “layering,” or producing new roots in places where the stems touched the ground. In many places, I traced stems twenty or even thirty feet along the ground–essentially, growing like vines on the ground even though the plant appeared as a shrub–and found no rooted nodes.
- The largest diameter stem I found was 6.5” (a shrub). The tallest vines are over 100 feet tall. I rarely cut the tall vines, but one that I cut had 40 growth rings.
- The largest leaf I found was in deep shade, and it was larger than my outstretched hand. The smallest fully-formed leaves are in the bright sun, the size of couple thumbnails put together.
- To draw a poison oak leaf accurately, you need to look at the stalks for each of the three leaflets. The central leaflet has a long stalk, and the lateral ones have very short or nonexistent ones.
- Aerial roots are short, thin, dark red outgrowths of the bark that grow in a line and adhere to the host. You probably know them from ivy. In poison oak, some individuals make aerial roots, even where they have nothing to attach to, and other individuals do not make aerial roots, even when they are touching a tree or a stake. The capacity to make aerial roots appears genetic, not plastic.
- In 1888, Charles Darwin described the four major mechanisms vines use to climb: they twine, they climb with hooks such as on the foliage, they climb with aerial roots, or they climb by scrambling, becoming wedged into crevices and looped over branches. Poison oak vines are almost entirely scramblers.
- In the sun, if I tied plants to stakes (because I didn’t have time to let them scramble), they grew as vines. If I didn’t tie them to stakes, they grew as shrubs. It didn’t matter whether the cuttings came from vines or from shrubs, from the woodlands, or from the coastal bluffs. Growth form was plastic, not genetic.
- In the shade, even the shrubs grew floppy. I believe this strategy was adaptive because it allowed shade shrubs to grow and flop, essentially foraging for support. When they find support, they can grow upward as vines, and if they get into enough light, they can flower.
- Vines do not have to provide their own mechanical support, but shrubs do, and not surprisingly, vine stems are much longer than shrub stems if they have the same diameter. Surprisingly, vines and shrubs of the age I studied didn’t differ in the amount of stem they made relative to leaf. The difference was that vine stems made a unit of long, thin stem and a leaf, whereas a shrub made a section of short, thick stem and a leaf.
- Stem form can be variable, even within one plant. A stem is thin where it winds through a chain-link fence (where it is supported, and thus a vine), but its diameter can even increase at the top of the fence or on a branch (where it supports itself and is a shrub).
- Because vines do not have to provide that mechanical support, they can devote more of their stem cross-section to water transport. Poison oak vines make wood that is twice as conductive (that is, twice as much water can move per cross-sectional area) as shrubs.
- Vines have twice as much leaf area per stem cross-sectional area as shrubs.
- The last two facts taken together mean that poison oak leaves will receive the same amount of water, regardless of if they are on vines or shrubs, which is a handy strategy physiologically.
I’ll close with some words written about poison oak’s relative, poison ivy. The first quotations are from Thomas Horsfield, who studied poison ivy for his 1798 doctorate in medicine. He wrote, “It appears to delight in ascending the lofty oak trees, which surround the fields of our farmers.” He also wrote, “It delights in fat soil, in low, marshy places. … Two swamps, which I several times visited … are the most desert and gloomy places, which are to found in the vicinity of Philadelphia. They are accessible with difficulty; and appear devoted by nature, to the habitation of injurious plants and venomous reptiles.”
The second quotation is from Captain John Smith, the English explorer who was also one of the leaders of the colony at Jamestown. In 1609 he wrote, “The poisonous weed, being in shape but little different from our English ivie; but being touched causeth redness, itchings, and lastly blysters, the which howsoever, after a while they pass away of themselves without further harme; yet becasue for the time they are somewhat painefull, and in aspect dangerous, it hath gotten itselfe an ill name, although questionless of noe very ill nature.”
Poison oak, like poison ivy, is “of noe very ill nature.” Apart from a few issues (that, admittedly, can be life-threatening), poison oak rocks.
Sources: Personal observations, or reported or cited in my PhD thesis papers or its appendix:
Gartner BL. 1991. Appendix: The logistics of working with poison oak. Appendix, Ph.D Dissertation, Consequences of the vine vs. shrub growth forms for biomechanics, growth, and hydraulic architecture of western poison oak, Toxicodendron diversilobum. Stanford University.
Gartner BL. 1991. Relative growth rates of vines and shrubs of western poison oak, Toxicodendron diversilobum (Anacardiaceae). American Journal of Botany 78: 1345-1353.
Gartner BL. 1991. Structural stability and architecture of vines vs. shrubs of poison oak, Toxicodendron diversilobum. Ecology 72: 2005-2015.
Gartner BL. 1991. Stem hydraulic properties of vines vs. shrubs of western poison oak, Toxicodendron diversilobum. Oecologia 97: 180-189.
Gartner BL. 1991. Is the climbing habit of poison oak ecotypic? Functional Ecology 5: 696-704.
Gartner BL, Wasser C, Rodriguez E, Epstein WL. 1993. Seasonal variation of urushiol content in poison oak leaves. American Journal of Contact Dermatitis 4: 33-36.