Poisonous to Touch Weed Series: Poison Oak (Toxicodendron diversilobum) and (Toxicodendron pubescens)

Pacific poison-oak
Toxicodendron diversilobum Photo credit: Joseph M. DiTomaso, University of California – Davis, Bugwood.org

Pacific Poison Oak (Toxicodendron diversilobum) and Atlantic Poison Oak (Toxicodendron Toxicodendron pubescens) are the only two poisonous to touch plants in this series that do not grow in Canada except for the very south/western tip of British Columbia. They are not nearly as prevalent as poison ivy with Pacific poison oak occurring natively along the pacific coast of the U.S. and Atlantic poison oak occurring natively in the south/eastern portion of the U.S. These plants like poison ivy and poison sumac contain urushiol oil and touching any part of this plant can cause a painful itchy rash with red swollen skin and painful blisters. The skins reaction to contact with poison oak is actually an allergic reaction to the urushiol oil contained in the plant. This oil is so potent that only one nanogram (billionth of a gram) is needed to cause a rash (Gov. of Canada 2016). How severe a reaction you have will depend upon how much of the oil you got on yourself, how thick the skin is on the area that came in contact with the oil and how sensitive you are to the oil. Poison oak is said to be more dangerous than poison ivy.

Identification of Poison Oak

Overall Growth Habit:

  • Pacific Poison Oak (Toxicodendron diversilobum): Grows as a woody vine in shady areas and as a shrub in sunny areas. As a vine it can grow up to 15 meters (45 feet) in height; attaching it’s self to trees, rocks, fences or buildings through the use of fine aerial roots. The vines can also sprawl across the ground creating a carpet of poison oak. In a sunny location the plant will grow as a 1 meter tall dense shrub. “Poison oak leaves strongly resemble western poison ivy but they tend to have more rounded and irregular lobes than poison ivy leaves, which have pointed tips and are not distinctly lobed” (U.S. National Park Service rev. May 2020).
  • Atlantic Poison Oak (Toxicodendron ): Grow only as an upright shrub. The average height of these shrubs are about 60cm (2′ ) to 90cm (3′) but they can reach much higher heights. “Atlantic poison oak is frequently confused with Eastern poison ivy as it has three grouped leaves that are structurally similar in appearance. A unique feature useful in identification is the presence of clustered small green fuzzy berries on the plant” (Lofgran T. and Mahabal G. rev. Feb. 2021).
Atlantic poison-oak (Toxicodendron pubescens) Photo credit: James H. Miller, USDA Forest Service, Bugwood.org
Pacific poison-oak
T(oxicodendron diversilobum) Photo credit: Joseph M. DiTomaso, University of California – Davis, B

Leaves:

  • Pacific Poison Oak: leaves grow alternately up the vines or stems. Their compound leaves typically have sets of three lobed leaflets but unlike other toxicodendrons they can also have 5, sometimes even 7 or 9. The center leaflets stalk is longer than the 2 side leaflets. The leaves can measure up to 10 cm (4″) long and often appear thicker and more waxy when grown in full sun. Leaves growing in the shade on climbing vines are often thinner and duller. Leaf edges may appear scalloped, toothed, or lobed. The leaves look a lot like white oak leaves. New leaves are shiny and reddish. In the autumn leaves turn pink or a brilliant red, or yellow. 
  • Atlantic Poison Oak:  leaves grow alternately up the stems. Their compound leaves can measure up to 15 cm (6″) long and always have 3 lobed leaflets. The center leaflets stalk is longer than the 2 side leaflets. The leaves are glossy with small velvet bristles on them. Leaf edges may appear scalloped, toothed, or lobed. The leaves look a lot like white oak leaves. In the autumn leaves turn a brilliant red, or yellow.
Atlantic poison-oak
(Toxicodendron pubescens) Photo credit: Karan A. Rawlins, University of Georgia, Bugwood.org
Atlantic poison-oak (Toxicodendron pubescens) Photo credit: John D. Byrd, Mississippi State University, Bugwood.org
Pacific poison-oak
T(oxicodendron diversilobum). Photo credit: David Stephens, Bugwood.org

Twigs, Vines and Stems:

Another useful identification tool is looking for black markings on winter twigs, vines and stems. Urushiol turns black and hardens when it comes in contact with the air.

  • Pacific Poison Oak: Twigs are slender and largely unbranched with tendrils and short fuzzy hairs on them. Young bark is smooth and tan; older bark is gray-brown and rough.
  • Atlantic Poison Oak:  Have no vines and their stems are smooth, slender and may have short fuzzy hairs on them or be hairless. The color of the stems and trunk is a grayish brown.

Roots

  • Pacific Poison Oak:  roots form along the rhizomes (the under ground stems).
  • Atlantic Poison Oak: roots form along rhizomes.

Flowers

  • Pacific Poison Oak: the star-shaped flowers with 5 petals and are yellow/green maturing to a whitish in colour. They are very small, measuring between 1/2 to 1 cm. The flowers appear March to June in panicles 3-8 cm (roughly 1-3″) long at the base of stems.
  • Atlantic Poison Oak: Flower description is the same as Pacific poison oak with bloom time being April to June.

Berries and Seed

  • Pacific Poison Oak: The fruits appear as drupes in late summer. The berries are small and round with striped grooves like a pumpkin. The colour is yellow/green maturing to a baize white.
  • Atlantic Poison Oak: The berry description is the same as Atlantic poison oak.
Pacific poison-oak
Toxicodendron diversilobum) leaves and berries. Photo credit: Joseph M. DiTomaso, University of California – Davis, Bugwood.org

How Does Poison Oak Spread?

Poison oak can sprout vegetatively from it’s rhizomatous stems (stems just under the soil surface that can grow roots) or it can germinate from seed.

Poison Oak Look A-likes

  1. Eastern Poison Ivy, (Toxicodendron radicans) and Western Poison Ivy (Toxicodendron Rydbergii )
  2. Sunkbush Sumac (Anacardiaceae Rhus trilobata Nutt.)
  3. Fragrant Sumac (Anacardiaceae Rhus aromatica Aiton)

Where Does Poison Oak Grow?

  • Pacific Poison Oak: is native to western North America. It grows along the pacific coast from Mexico, through California all the up to the south/west tip of British Columbia.

The plants live in a range of habits from damp, semi-shady areas near running water and moist evergreen forest to dry scrubland and woodlands.

  • Atlantic Poison Oak: Is native to the southeast quarter of the United States.

These plants prefer drier habitats with plenty of sun. They can be found growing in open woodlands, sandy fields, sandy thickets and forests.

Pacific poison-oak (Toxicodendron diversilobum) Distribution map from the U.S. Department of Agriculture displayed here in accordance with their Policies).
Atlantic poison-oak (Toxicodendron pubescens) Distribution map from the U.S. Department of Agriculture displayed here in accordance with their Policies).

Symptoms Of A Poison Oak Reaction

As mentioned in the introduction, skin reactions to the urushiol oil found in poison oak is actually an allergic reaction known as contact dermatitis. Exposure to the oil, whether it be contact with the plant it’s self or contact with any surface that has any residue of urushiol on it such as pets fur, lawn mowers, shoes, shoe laces, clothing, golf clubs, etc. can cause a skin reaction. It does not take much Urushiol oil to have a reaction. It is so potent that only one nanogram (billionth of a gram) is needed to cause a rash (Gov. of Canada 2016)

Classic symptoms include:

  • Intense itchiness
  • Rash that may appear within 24 hours to seven days.
  • Swelling
  • Redness
  • Painful blisters
  • Difficulty breathing, if you inhale the smoke from burning poison oak.
  • Gastrointestinal inflammation if ingested.

The severity of your symptoms depend largely on how much urushiol you get on your skin and how sensitive the individual is to the oil. It is quite common for a person who was not sensitive to urushiol oil in the past to suddenly become sensitive to it.

Note: The puss from the oozing sores will not spread it to other parts of your body or to others.

What to do if You Come Into Contact With Poison Oak

If you are fortunate enough to spot the plant and realize you have made contact the first thing you should do is get out of the sun and heat and wash it off with cool water and soap. The sooner you was it off the less urushiol you will absorb into your system and cool water is recommended as hot water opens the pores and increases the spread. If you do not have soap you can use rubbing alcohol diluted with water (1/2 cup to 1/2 cup of water) or vinegar (2 tablespoons in 1 cup of water). Thoroughly clean any surface you may have contacted after touching the plant, using gloved hands. Wash the clothing you are wearing, shoes and shoe laces, gloves, door handles, etc.

If you are experiencing the symptoms but are unaware of where you came in contact with the plant or the urushiol oil then still follow the above steps but you will likely have to broaden your disinfecting area as you will have touched many more objects.

If your reaction is strong visit your doctor who will prescribe a topical corticosteroid cream to apply to the affected skin areas and an anti-inflammatory like prednisone to take orally.

Over the counter remedies include antihistamines like Benadryl or anti inflammatory pain relievers like Advil or Motrin or Aleve. In milder cases oatmeal products or calamine lotion may provide some relief from the itching as will after bite, apple cider vinegar or witch hazel. All in all the symptoms take between 2 and 4 weeks to go away, depending on how bad a case you got. If it is persisting chances are you are being re-infected.

The amazing thing about urushiol is that it can remain allergenic on clothing for up to 10 years! (Crosbie J. 2020)

If you are out in the woods hiking or camping with few resources at your disposal there are a few natural remedies that may offer some relief.

  • Burdock or cocklebur (Xanthium strumarium L.) crush the leaves and apply to infected area.
  • Jewelweed (Impatiens capensis) slice the stem, and then rub its juicy inside on exposed parts. This will promptly ease irritation and usually prevents breakout for most people.
  •  Common Plantain (Plantago major) shred the leaves and apply a poultice to the infected area. Helps heal poison oak rash and relieve virtually any skin irritation.

Other Plants That Contain Urushiol

Poison ivy and poison sumac are the two other most notable plants that contain urushiol. Surprisingly pistachios, cashews and mango also contain urushiol.

How Can You be Sure Your Reaction Is To Poison Oak And Not Poison Sumac Or Poison Ivy?

If you did not actually see the plant you will not be able to differentiate the specific plant you brushed up against by the rash it produces, other than perhaps the thin red lines caused by the sharp edges of poison ivy. The reaction and treatment are the same for all three of these plants and thus it really doesn’t matter.

How To Get Rid of Poison Oak Plants

Before attempting to remove poison oak plants be sure to wear the proper protective clothing; close-toed shoes or boots, long pants, long sleeved shirt, water proof gloves and ideally face and eye protection. Do not touch your face or any bare skin with your gloved hand. Also try not to touch the surface of anything other than the tools you are working with. When you have finished removing the poison oak wash your tools and any surface you may have touched. Wash your shoes and shoelaces off and change and wash your clothing.

Cultivation: Poison oak does not survive cultivation. It can however become a problem in uncultivated areas.

Pull or Dig Poison Ivy: In the spring after poison ivy has broken dormancy and while the ground is still very moist pull or dig the plants and their roots.

Cover Area With Black Plastic: Cover the effected area with some heavy black plastic weighted down with some mulch for a year or more.

Caution: Dead poison ivy plants can still cause rashes and must be handled with care, since the urushiol oil can stay active on any surface for up to 5 years. (Gov. of Canada 2016)

Disposal: Black bag poison ivy debris (do NOT compost or brown bag it) and label the bag poisonous plant material. Put it to the curb with your house hold waste. Alternately if you live in the country you can bury dead plants at least 30 cm deep. NEVER BURN POISON Oak!

Chemical Control: Use a systemic herbicide like Glyphosate. This is most effective when the plants are young. Multiple applications may be required due to the extensive underground stems.

Summary

Poison oak does not affect animals or birds but it can produce severe reactions in humans. It grows abundantly in many different types of environments but is is especially fond of drier habitats with plenty of sun in the east and moist habitats or dry scrubland or woodlands in the west. It’s appearance varies from ground covers to bushes to massive vines that climb to the top of trees in the west and shrubs only in the east. Leaves can also vary appearing scalloped, toothed, or lobed but they are never pointed like poison ivy and usually have several lobes rather than just a few like poison ivy. The leaves typically have sets of 3 leaflets although western poison oak can sometime have 5,7,or 9. The center stalk is always longer than the 2 side leaflets and the leaves are always alternate up the stems. The stems never have thorns. Symptoms may include intense itchiness followed by a rash (often swollen and red). This is followed by painful blisters. If the poison ivy oils get in the lungs there may be difficulty breathing. The severity of one’s reaction to poison ivy can range from no reaction to mild, severe or even death. Learn how to recognize poison oak for your own safety, then teach your family and friends.

Poisonous to Touch Weeds Covered in This Series Include:

  1. Poison Ivy (Toxicodendron radicans and Toxicodendron Rydbergii)
  2. Giant Hogweed (Heracleum mantegazzianum)
  3. Cow Parsnip (Heracleum maximum)
  4. Wild Parsnip (Pastinaca sativa)
  5. Poison Sumac (Toxicodendron vernix)
  6. Poison Oak (Toxicodendron diversilobum) and (Toxicodendron pubescens)

Other Poisonous to Touch Plants Not Covered in This Series Include:

  1. Poke Weed (Phytolacca decandra): besides being mildly to highly toxic this plant can cause a rash in sensitive individuals.
  2. Stinging Nettle (Urtica dioica): the leaves and young stems of this herbaceous plant are fitted with stinging hairs tipped with formic acid and other irritants. If touched, these needle-like hairs inject the stinging acid into the skin, triggering a burning, tingling sensation and an itchy rash. Thankfully the symptoms usually do not persist longer than 24 hours.
  3. Monkshood (Aconitum napellus): This is a highly toxic plant. The neurotoxins, aconitine and mesaconitine can be absorbed through the skin and cause severe respiratory and cardiac problems. So do not pick or handle this plant without gloves, especially by the root.
  4. Oleander (Nerium oleander): This is a highly toxic plant. The gummy sap can cause skin irritation or possibly dermatitis in those who are sensitive. If you are growing this plant as an ornamental in your garden be sure to were gloves, long sleeves, long pants, closed shoes and protective eye wear before pruning this plant.
  5. Bloodroot (Sanguinaria canadensis): the flower of this plant produces a reddish toxic sap that looks very similar to blood and contains the toxin sanguinarine. If this sap comes in contact with bare skin it can cause burning. If you get a high concentration of sap on your skin it can even eat away the flesh. If ingested, it is deadly to animals.
  6. Wood Nettle (Laportea canadensis): like stinging nettle have stinging hairs along its stem.

References

Crosbie J. 2020 (updated) D.O.August 5, 2019, Des Moines University, Retrieved on Jan. 31, 2021 from: Everything you need to know about poison ivy | News | Des Moines University (dmu.edu)

Gov. of Canada 2016 (modified). Poison Ivy. Retrieved on Jan. 31, 2021 from: https://www.canada.ca/en/health-canada/services/home-garden-safety/poison-ivy.html

Lofgran T. and Mahabal G. rev. Feb. 2021. Toxicodendron Toxicity. Retrieved on Mar. 15, 2021 from: Toxicodendron Toxicity – StatPearls – NCBI Bookshelf (nih.gov)

U.S. National Park Service rev. May 2020 Pacific Poison Oak. Retrieved on Mar. 15, 2021 from: Pacific Poison Oak (U.S. National Park Service) (nps.gov)

Other Resources

Poison Ivy, Oak and Sumac – InterNACHI®

poison oak | Description, Distribution, & Poison | Britannica

Toxicodendron: Types, uses, and what to know (medicalnewstoday.com)

Toxicodendron pubescens (Atlantic Poison Oak, Eastern Poison Oak, Poison Oak) | North Carolina Extension Gardener Plant Toolbox (ncsu.edu)

Virginia Tech Dendrology Fact Sheet (vt.edu)

Virginia Tech Dendrology Fact Sheet (vt.edu)

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