Pacific Poison Oak

What is Pacific poison oak?

Pacific poison oak is a native shrub and vine of the Pacific states, and UC IPM ranks it as the most hazardous plant in California when measured by lost work hours. Accurate Pacific poison oak identification matters because the plant changes its look to fit its setting: a low shrub in a hill pasture, a dense thicket in open sun, or a vine reaching high into the canopy in a shaded forest. It is related to poison ivy and poison sumac and carries the same irritating oil.

That oil, urushiol, is in the leaves, stems, roots and berries. Brush clearing, string trimming, pruning or trampling crushes the plant and puts the oil onto skin, gloves, tools and pets, and from there onto anything they touch. Because the plant is native and feeds deer, livestock and birds, the goal in most places is to keep it out of yards, play areas, trail edges and work zones rather than to wipe it out.

Pacific Poison Oak identification: what to look for

Look from a distance and never handle the plant to get a better view. Leaflet shape varies even on one stem, so rely on the combination: leaves in threes, a longer stalk on the middle leaflet, whitish berries, and a shrub or vine habit. In winter, treat bare stems in a known patch as if they were in full leaf.

• Leaves: Compound leaves alternate along the stem, usually with three leaflets, though five, seven or occasionally nine appear. Leaflets are about 1 to 4 inches long (Oregon State reports up to nearly 6), with scalloped, lobed or toothed edges and rounded tips. The center leaflet has a noticeably longer stalk.
• Color: New spring growth is green or reddish, summer leaves are glossy to dull green, and fall leaves turn orange and brilliant red before dropping.
• Growth form: A dense shrub, usually 1 to 6 feet, in open sun. In shade it climbs trees with aerial rootlets and can top 75 feet. It also trails along the ground as a low vine.
• Stems: Woody and thornless. Climbing stems are lined with short aerial roots where they grip bark.
• Flowers: Tiny greenish-white flowers in clusters at the leaf bases in spring, easy to overlook.
• Berries: Smooth, round, whitish-green to white berries in late summer that often persist into winter. Oregon State notes they are slightly larger than western poison ivy berries.

Where it grows in the United States

Pacific poison oak is a West Coast native, ranging from British Columbia to the Baja California peninsula. In California it grows across the whole state from sea level to about 5,000 feet, and Oregon State Extension describes it as common in western Oregon and western Washington. East of the Cascades it gives way to its relative, western poison ivy, which runs through eastern Oregon and Washington, Idaho and farther east.

Within that range it handles coastal areas, oak woodlands and savannas, conifer forests, brushland, rangeland and hill pastures, stream banks, wetlands, rocky canyons and recently logged ground. It also moves into roadsides, fencerows, abandoned land, parks and the back corners of suburban yards. Birds eat the berries, and seeds that pass through them germinate better, which helps explain how new plants keep appearing in landscapes.

Pacific Poison Oak look-alikes

A few plants get mixed up with it:

• Western poison ivy (Toxicodendron rydbergii): Also toxic and also three-leaflet, but its leaflets end in sharp points rather than the rounded, irregular tips of Pacific poison oak, and its berries are a bit smaller. It grows mainly east of the Cascades.
• Skunkbush sumac (Rhus trilobata): A harmless shrub with three leaflets that looks a lot like poison oak. The giveaways: its fruit is red to orange and hairy instead of whitish and smooth, and the middle leaflet has no long stalk of its own.
• Boxelder seedlings (Acer negundo): Young boxelder can carry three to five toothed leaflets, but its leaves grow in opposite pairs along the stem, while poison oak leaves alternate.

How to get rid of Pacific poison oak

Pulling, digging and mowing: For a few plants near a house or path, dig them out with as much root as you can get, but skip hand work if you react strongly. Work in early spring or late fall when the soil is wet; in hard, dry ground the roots snap and resprout. Wear washable cotton gloves over plastic gloves, long sleeves and long pants, then wash tools, clothes and boots. Seedlings can be mowed out only in their first couple of months. Mowing or string trimming established plants is not recommended because it throws oily debris and the plant regrows, and one pass with a tiller just spreads the rhizomes. Cut vines on trees at the base rather than tearing them off. Never burn the debris; dried stems still cause rashes, so bury it, stack it out of the way, or haul it to a disposal site.

Cultural control matters most, because the right conditions keep it from coming back.

• Keep desirable plantings thick and healthy so seedlings have less open ground to move into.
• Walk the property each spring and pull or dig new seedlings while they are small and before their roots establish.
• Goats and sheep will browse poison oak down in small areas, but grazing only clears it when it is intensive for several years.
• After clearing brush with heavy equipment, watch the bare soil for new seedlings and follow up quickly.
• Keep poison oak out of burn piles; the smoke carries the oil.

Herbicides: UC IPM and Oregon State Extension both point to glyphosate and triclopyr as the most practical options, with triclopyr sometimes paired with 2,4-D. Oregon State also lists dicamba, metsulfuron, aminopyralid and aminocyclopyrachlor, and UC IPM adds imazapyr. Glyphosate and imazapyr are nonselective, and imazapyr can stay active in soil; triclopyr spares grasses but harms broadleaf plants. Spray foliage in late spring or early summer once leaves are fully out, or in late summer after flowering, and treat regrowth in fall. Cut-stump treatments go on right after cutting in spring or fall, and basal-bark treatments work on bare stems in winter. Avoid windy or hot days, since some triclopyr forms can vaporize in warm weather, and plan on repeat treatments. Use only a product labeled for your site, including the specific formulations allowed near water. The label is the law. If you hire someone to spray, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Keeping it from coming back

Learn the plant in every season and make sure kids, crews and anyone working the property can spot it. Clear brush in long sleeves, long pants, gloves and loose coveralls; CDC also suggests a barrier cream beforehand. Afterward, wash work clothes separately, clean tools with soap and water or rubbing alcohol, and bathe pets that ran through a patch.

If you touch it, wash right away. UC IPM found that scrubbing within about 10 minutes removes roughly half the oil, while after about 30 minutes most has soaked in. Use a damp washcloth, dish soap and running water, rinse several times, and clean under your nails. Cool compresses, calamine and over-the-counter hydrocortisone ease the itch. See a doctor for a rash on the face, eyes or genitals, a widespread rash, or signs of infection, and get emergency help for swelling or trouble breathing.

Questions we get about Pacific poison oak

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