Poison oak - Toxicodendron diversilobum

In brief

Toxicodendron diversilobum, Anacardiaceae.

- Deciduous shrub, ground creeper or climbing vine
- Native to western North America
- Flowers March to June
- Fruit a whitish drupe, held into winter

Where it grows

Everywhere on this land, from the creek to the ridge, and through most of western North America west of the Sierra crest.

When it blooms

Flowers are small, greenish-white and easy to miss, from March into June. The plant is far more conspicuous in October, when it turns scarlet, and in winter, when the bare stems still carry white berries.

Seed and fruit

A small whitish drupe in loose clusters. Birds eat them and are unaffected; over sixty species have been recorded taking them.

Here

The reason people get caught is not that poison oak is hard to see. It is that it does not look like one thing.

As a climbing vine

It runs up trunks with aerial rootlets, holding flat against the bark like ivy, and can reach well into a canopy. The one photographed on May 14, 2023 had gone up a fire-blackened trunk.

And the many colors

Bronze-red when the leaves open in spring. Glossy mid-green through summer. Scarlet and orange in October, often the brightest thing on the hillside. Bare grey stems in winter, still carrying white berries.

A bare winter stem is not safe. The urushiol is in the wood and the roots as well as the leaves, and it stays active on dead material for years.

As a shrub, and along the ground

The form most people picture: an open, waist-high bush in the understory, leaves glossy and lobed.

It also spreads along the ground in shade, ankle-high, where it is easiest of all to walk into.

An open waist-high poison oak bush in dry understory, leaves glossy and lobed

The open shrub, April 23, 2021.

Poison oak standing on a single woody stem, shaped like a small tree

On one stem, May 18, 2025.

As a small tree

Given a clear patch and some years it will stand up on a single woody stem and read, at a glance, as a young tree.

Hidden inside another shrub

Its worst habit: growing up through deer brush or buckbrush so that the visible leaves belong to a harmless plant and the ones an arm goes through do not.

Red poison oak leaves coming up through the green foliage of a deer brush

Coming up through deer brush, August 28, 2026.

Poison oak leaves in threes, the middle leaflet on a longer stalk, with clusters of white berries

Leaves of three, and the white berries, June 7, 2023.

The rule that survives every disguise

Leaves of three, let it be.

Three leaflets, with the middle one on a longer stalk than the two beside it. That holds whether it is a vine, a tree, a shrub or a mat, and whether the leaves are red, green or absent.

Blackberry also has leaves of three here, and is thorny rather than poisonous. The advice is the same either way.

If you do walk through it: wash with a specific product for the purpose rather than plain soap, and wash anything the plant touched. Urushiol transfers from clothing, tools and dogs days later, and it is the second-hand contact that catches people out.

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