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Tree 36: Sessile Oak

Latin name: Quercus petraea
Family: Fagaceae
Other names: Welsh Oak, Cornish Oak, Irish Oak
Origin: Native all across Europe and into Asia Minor and Iran
Height: up to 40 metres
Lifespan: 500+ years
Autumn leaf loss: Deciduous
Gender Type: Monoecious (Male and female flowers on same tree)

The Sessile Oak is the national tree of Ireland. Its acorn cups are stalkless which is what ‘sessile’ means. The scientific name petraea refers to the rocky ground on which this species can thrive, extending its roots into crevices in the underlying bedrock.

In Wales there are ancient rainforests where the steep gradients and rocky terrain have saved the vegetation from clearance and over grazing. Here sessile oaks dominate in a verdant, moist atmosphere with mosses, liverworts, lichens and ferns .

The Sessile Oak, like its cousin the native English Oak, is a deciduous tree. It has a more upright trunk and straighter branches than its cousin. Its leaves are the same size, 7-11 cm long but flatter. Sessile Oak leaves have 5-8 pairs of forward pointed lobes, the English oak 4-6 more rounded lobes, deeper cut towards the midrib.

It is less commonly planted than the English Oak because its wood is softer, so has not had the same commercial value. However, as with its cousin, the Sessile Oak is extremely valuable for wildlife of all sorts, including 248 insect species and, of course, their predators (bats, birds etc.). Their acorns provide useful food for a wide range of birds and small mammals, and even their leaf mould supports specialist fungi.

Although our native oaks may produce acorns at 5 years of age, they need to be 40 years or more to produce significant amounts of seed and very few of these grow into young oak trees. Indeed there is poor natural regeneration of oak in Britain, with acorns being such an important food source for a variety of wildlife. The decline of mature oaks and the mysteriously lost ability of oaks to grow easily from seed in existing woods is a cause for concern. Oak mildew has been implicated in this barrenness. Fortunately it remains easy to grow an oak from an acorn in a pot or garden. Try it!