Working with trees can present a range of hazards and challenges but one in particular can leave you with a sore head.

UMBELLULARIA californica is a large hardwood tree native to the coastal forests and the Sierra foothills of California, extending northwards into Oregon.

This evergreen grows up to 30 m tall with a trunk up to 80 cm dbh. The largest recorded tree was in Mendocino County, in central California, standing at 108 feet (33 m) with a 119 feet (36 m) canopy spread.

Common names for this species include the balm of heaven, Oregon myrtle, pepperwood and Californian bay, laurel, olive tree, sassafras tree and spice tree, plus various tribal names given by the native peoples.

Botanically speaking, it belongs to the Lauraceae (laurel) family and is the sole member of the genus Umbellularia.

The small, yellow or yellowish-green flowers are produced in small umbels (hence the scientific name Umbellularia, ‘little umbel’). Unlike other ‘bay laurels’ of the genus Laurus, Umbellularia has ‘perfect’ flowers, with male and female parts in the same flower.

Also known as the California bay nut, the fruit is a round and green berry 2–2.5 cm long and 2 cm wide, lightly spotted with yellow, maturing to purple. Under the thin, leathery skin, it consists of an oily, fleshy covering over a single hard, thin-shelled pit and resembles a mini avocado.

Forestry Journal: he fruit are small avocado lookalikes.he fruit are small avocado lookalikes.

Umbellularia is in fact closely related to the avocado’s genus Persea, within the Lauraceae family. The fruit ripens around October–November in the native range.

The fragrant leaves are smooth-edged and lance-shaped, 3–10 cm long and 1.5–3.0 cm wide, similar to the related bay laurel, though usually narrower, and without the crinkled margin of that species.

And it is the leaves that can cause problems in some people. One whiff of this plant can spur intense, excruciating brain pain: hence, the headache tree. 
Crushed leaves release a volatile chemical compound dubbed umbellolone. When breathed in, scientists have discovered that this ingredient sets off a chain of reactions that eventually ramps up blood flow to the brain’s outer membrane causing intense pain and nausea.

Other headache triggers, such as chlorine, cigarette smoke and formaldehyde, interact with some of the same cellular pathways, suggesting they all work via the same pain-inducing mechanism.

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However, in small doses U. californica leaves were used in native herbal medicines and sometimes as an alternative to Mediterranean bay leaves in cooking.

Not to be confused with the leaves of a tropical shrub, Premna serratifolia, widely occurring in coastal Asia and the Pacific Islands, which were traditionally employed to treat headaches. Also sometimes dubbed the headache tree it was used to cure rather than cause headaches!


Phytophthora ramorum is a highly destructive, algae-like organism colloquially termed a water mould. It causes extensive damage and death to more than 150 plant species, including forest trees.

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U. californica is a host of P. ramorum, the pathogen that causes the disease sudden oak death (SOD) in the USA, decimating native oaks across natural woodlands in California and Oregon from around 1995 onwards.

The endemic headache tree is important in this sense because it is one of two tree species – tanoak (Notholithocarpus densiflorus) is the other –  on which this pathogen readily produces spores.

Forestry Journal: Headache tree (credit: Christine Reid).Headache tree (credit: Christine Reid).

Forest Research UK reminds us that across the Atlantic, it is different genetic forms of the P. ramorum organism from those now present in the UK that have resulted in catastrophic damage and mortality to species of native North American oaks (Quercus sp.) and the tanoaks (Notholithocarpus sp.).

However, the genetic forms of the organism now on the loose in the UK have so far had little effect on our two native oak species – the pedunculate or English oak (Quercus robur) and sessile oak (Q. petraea).

Among forest trees here, larch (Larix sp.), that are or were widely grown in the UK for the timber market, have proved particularly susceptible though to P. ramorum disease and vast numbers have been affected and felled. Since 2015, P. ramorum has also been detected on European sweet chestnut trees (Castanea sativa) at a number of locations in southern and central England.

In the UK, assorted arboreta and gardens house specimens of the semi-hardy headache tree; and at least two specialist nurseries market it. Maybe it should come with a health warning!