Native to Europe and common across the British Isles, the sessile oak (Quercus petraea) is the national tree of Ireland. Its name means “stemless,” in reference to its small green acorns that grow directly on the branches. Sessile oak feeds and nourishes dozens of animals but is a particular favorite for the caterpillars of purple hairstreak butterflies.
I. Appearance and Characteristics
Quercus petraea, commonly known as the sessile oak, Cornish oak, Irish Oak or durmast oak, is a species of oak tree native to most of Europe and into Anatolia and Iran. The sessile oak is the national tree of Ireland, and an unofficial emblem in Wales and Cornwall.
Quercus petraea was first described by Heinrich Gottfried von Mattuschka in 1777 as a variety of Quercus robur, Quercus robur var. petraea. It was raised to a full species by Franz Kaspar Lieblein in 1784.
The sessile oak is a large deciduous tree up to 40 metres (130 feet) tall, in the white oak section of the genus (Quercus sect. Quercus) and similar to the pedunculate oak (Q. robur), with which it overlaps extensively in range. The leaves are 7–14 centimetres (2+3⁄4–5+1⁄2 inches) long and 4–8 cm (1+1⁄2–3 in) broad, evenly lobed with five to six lobes on each side and a 1 cm-long (1⁄2 in) petiole. The male flowers are grouped into catkins, produced in the spring. The fruit is an acorn 2–3 cm (3⁄4–1+1⁄4 in) long and 1–2 cm (1⁄2–3⁄4 in) broad, which matures in about six months.
Comparison with pedunculate oak
Significant botanical differences from pedunculate oak (Q. robur) include the stalked leaves, and the stalkless (sessile) acorns from which one of its common names is derived. (With the pedunculate oak, it is the acorns which are pedunculate, i.e. on stalks, while the leaves are not.) It occurs in upland areas of altitudes over 300 m (984 ft) with higher rainfall and shallow, acidic, sandy soils. Its specific epithet petraea means “of rocky places”. Q. robur, on the other hand, prefers deeper, richer soils at lower altitude. Fertile hybrids with Quercus robur named Quercus × rosacea are found wherever the two parent species occur and share or are intermediate in characters between the parents.
Charles Darwin, in Chapter II of On the Origin of Species, noted that the sessile and pedunculate oaks had been described as both distinct species and mere varieties depending on the authority consulted.
II. How to Grow and Care
Sunlight
Sessile oak thrives under full sun exposure, benefitting from daylight’s intensity to foster robust growth and optimum health. These optimal lighting conditions ensure sessile oak’s photosynthesis processes are maximized, leading to good foliage and acorn production. While sessile oak is tolerant of partial sun, the deviation can result in reduced vitality and slower growth rates, although it maintains a degree of adaptability in lower light environments. Sessile oak exhibits no significant color change or adaptive traits concerning light exposure and is habitually cultivated outdoors, requiring open spaces that receive direct sunlight for the major part of the day.
Temperature
Sessile oak grows in a large range of temperatures. It is adaptable in hardiness zones 5-9.It prefers well drained soil with adequate ventilation in the tree canopy. Sessile oak will not do well in standing water. Also, leaf molds pose serious threats if the leaves cannot dry out in the wind. So make sure they are not stuck against buildings where rainwater may drain.
Watering
Adapted to the temperate climates where it thrives, sessile oak has evolved to manage with moderate moisture levels. This species is resilient and can withstand periods of dryness, indicative of its preference for a balance in soil water content. Regular watering is required only once every two weeks to maintain its health. As an outdoor plant often found in forests, sessile oak benefits from mulching which helps retain soil moisture and mimics the natural leaf litter found in its habitat.
Soil
Ideal soils are well-drained loam, sandy-loam, or sandy-clay soils. Some alluvial fan areas and silts harbor good oaks stands. The key in all of these is that the soil is well-drained. Standing water cannot be tolerated by oaks.
Fertilizing
For sessile oak, fertilize with balanced nutrition fertilizers in spring for healthy growth and high nitrogen fertilizers in late winter to support leaf development. Apply twice yearly, once at bud-break and again midsummer, adhering to product-specific guidelines for quantity. Benefits include vigorous growth and robust health. Special care: avoid over-fertilization to prevent root burn. Seasonal adjustments enhance efficacy, with lighter applications as dormancy approaches. Tip: incorporate into the topsoil around the tree’s drip line for optimal uptake.
Pruning
Branches should be pruned to avoid moist pockets or where heavy branches may fall on people or buildings. Avoid having branches that grow with leaves tight together or pressed against buildings. If rain collects in these pockets then molds and fungi can attack. Sessile oak love having their leaves dry out in a well ventilated breeze.
To ensure that no danger occurs from the brittle hardwood branches, make sure that heavy branches do not hang over walking paths or outbuildings. Also, do not let children play near oaks in a thunderstorm as they are susceptible to falling branches and lightning strikes.
Propagation
As a stout deciduous hardwood, sessile oak is primarily propagated through sowing. Gardeners should collect its acorns directly from trees, ensuring they are healthy and free from pests. These seeds then require a period of cold stratification, mimicking winter conditions to break dormancy and encourage germination. Planting in well-drained soil, at a depth where the top of the acorn is just below the soil surface, optimizes seedling development. With attentive care, proper watering, and protection from wildlife, seeds grow into robust saplings.
Transplanting
The optimal season to transplant sessile oak is during the S10-S2 period, which is late winter to early spring. This timespan ensures the plant settles before active growth. Make sure to choose a location with well-drained soil and adequate sunlight. When transplanting, remember to handle the root system gently to prevent damage, aiding sessile oak’s successful move.
III. Uses and Benefits
Sessile oak is one of the most important species in Europe both economically and ecologically. Oak timber is traditionally used for building, ships and furniture. Today the best woods are used for quality cabinetmaking, veneers and barrel staves. Rougher material is used for fence construction, roof beams and specialist building work. The wood also has antimicrobial properties. It is also a good fuel wood. During autumns with good acorn crops (the mast years), animals are traditionally grazed under the trees to fatten them.