Complete guide to oak trees

As well as their importance in our history, English and sessile oaks are special for the rich web of biodiversity they support. Around 2,300 species are associated with native oaks and more than 300 of these rely solely on the oak for their survival. From nuthatches and wood warblers, to barbastelle bats and rare lichens, oak trees provide crucial habitats for wildlife across the UK.

Identifying oak

Oak tree leaves and acorns are useful in the identification of English oak and related species:

• English oak is often known as pedunculate oak, which refers to the long peduncle (or stalk) on each acorn cup. It has green, lobed leaves with barely any leaf stalk
• Sessile oak (meaning without a stalk) has clusters of acorns in stalkless cups. The leaves of sessile oak have 10-16mm stalks and are hairier and less deeply lobed than those of English oak

Both English and sessile oak produce slender male catkins and inconspicuous female flowers in spring. The foliage turns yellowy brown in autumn. Sometimes identification of oaks can be complicated where both species grow near each other and hybridise.

In winter, buds and bark are useful features to help distinguish oaks from other trees. Both English oak and sessile oak have several plump, orange-brown buds clustered at the end of twigs. The bark is dark grey-brown with vertical fissures.

Several other oak species are planted in the UK as ornamental trees, including holm oak, red oak and Turkey oak. Holm oak (Quercus ilex), also known as evergreen oak, is a native of the Mediterranean region. It has dark-green leaves with a pale underside and a rounded, domed habit. It is suitable for hedging, clipping and growing as a specimen tree. Red oak (Quercus rubra), a North American species, grows into a large tree and has fiery autumn colour.

Turkey oak (Quercus cerris) was first planted in the UK in the 18th century as an ornamental tree. Native to southern Europe and parts of Asia, it has the classic leaf shape with deep lobes, but they often taper to short points. It is easier to identify from the acorns, which are held in whiskery cups. Turkey oak is a fast-growing tree able to set seed here, so it has become naturalised across the UK.

How long do oak trees live?

Oaks are some of the longest-lived trees in the UK, with a potential lifespan of over 1,000 years. They reach maturity at around 40 years old, by which time many will be producing acorns. When trees reach 150-300 years old, they are usually classed as veteran trees. From 400 years old they are known as ancient oaks. Even when an oak tree dies, the dead wood continues to provide habitats for wildlife including beetles, birds and many species of fungi.

The location of the UK’s oldest oak tree is a much-debated subject. There are several notable contenders whose longevity is truly astonishing. The Marton Oak in Cheshire is thought to be over 1,000 years old and has a girth of over 14m. The wonderfully-named Big Belly Oak in Wiltshire is an ancient oak with a girth of more than 11m, with origins that are said to reach back to the time of William the Conqueror.

Another tree believed to be over 1,000 years old is the Bowthorpe Oak in Lincolnshire. The Major Oak in Sherwood Forest, under which Robin Hood and his Merry Men supposedly took shelter, might only be a mere 800-1,000 years old, but it is one of the UK’s largest oak trees.

The oak is a national symbol of strength. There are many tales linking oak trees to the monarchy, from the Royal Oak in Boscobel Wood in which the future King Charles II was said to have hidden from the Roundheads in 1651, to Queen Elizabeth’s Oak in Greenwich Park. According to legend, this ancient oak grew near the site where Queen Elizabeth I picnicked. Although the tree died over a century ago, it can still be seen lying on its side in the park.

Size, height and spread

English and sessile oaks can grow 20-40m in height, with a spread of 20m or more. A mature holm oak can reach 25m by 20m, and Turkey oaks can grow to around 30m. When grown as a hedge, oak can be clipped to an appropriate size.

Value to wildlife

Oak trees play a key role in supporting important ecosystems. Acorns provide food for mammals such as red squirrels, badgers and wood mice. Birds like jays and nuthatches also enjoy the autumn acorn feast. Over 1,000 invertebrate species have been recorded on oak trees. Caterpillars, spiders and other invertebrates in the canopy attract insectivorous birds such as wood warblers and blue tits.

Aphids feeding on fresh leaf growth produce honeydew, the primary food of wood ants. Many moth species, such as the pale tussock and black arches, have caterpillars that feed on oak leaves. The purple hairstreak butterfly relies on sessile and English oak as its sole larval food plants, and the adults feed in the canopy on the honeydew produced by aphids.

Many species of gall wasp lay their eggs on oaks, causing the tree to form abnormal growths called galls. Marble galls are hard, brown, spherical growths that harbour the larvae of the Andricus kollari gall wasp. Spangle galls and silk button galls can be found on the underside of oak leaves in autumn. Oak apple galls, which look like tiny apples, are formed in early summer by the Biorhiza pallida wasp. Galls are usually harmless, so they should not be detrimental to the health of the tree.

Oak woods have the greatest biodiversity of any of our native woodlands. They support a varied ground flora that can include bluebells, wood sorrel, honeysuckle and ferns, alongside a diverse range of lichens, mosses and liverworts. Numerous fungi are associated with oaks, including bracket fungi and many mycorrhizal fungi, which form mutually beneficial relationships with the trees.

Although introduced species of oak support fewer species, especially invertebrates, than sessile and English oak, their catkins attract pollinators, their acorns are eaten by birds and small mammals, and their canopies provide excellent places for birds to nest and roost. In addition to their value to wildlife, long-lived oaks contribute to a healthy environment by absorbing carbon dioxide, producing oxygen and creating a cooling effect.

Growing oak trees

Quercus robur thrives in deep, fertile soils in full sun or dappled shade. Planting an oak tree creates a legacy for future generations to enjoy.

Where to plant oak trees

Oak trees can be planted in UK gardens, but unless you have a very large space for a specimen tree that can reach 25m x 25m or more in size, oaks are best grown as a hedge. They do best in a spot in full sun or partial shade, and can be grown in most moist but well-drained soils. Avoid permanently waterlogged areas, especially for sessile oak. It is worth noting that oak is harmful to pets if eaten.

How to plant oak trees

The best time to plant an oak tree is during the dormant period in winter. This allows time for the roots to become established before the growing season begins.

• Soak bare-root oak tree saplings for about an hour before planting and water containerised plants thoroughly
• Ensure the area for planting is free of perennial weeds and grass
• Dig a square hole the same depth as the roots and at least two times as wide
• Loosen the soil in the sides of the planting hole with a garden fork if it is compacted
• Spread out the roots, ensuring the soil mark on the stem sits level with the soil, and backfill the hole, firming gently around the roots
• Add a stake to prevent root rock and fix the sapling with a tree tie. Water in well and finish by adding a thick mulch to conserve moisture and stop weeds
• Check tree ties every spring and autumn. Keeping the stake in place for two growing seasons should provide the tree with enough stability to manage without staking in future years

How to grow oak trees from acorns

Growing oak trees from acorns is a great autumnal activity for both adults and children. In order to have the best chance of germination, choose plump, brown, undamaged acorns and plant them as soon as possible after they fall, so they don’t dry out. If the acorn comes out of the cup easily, it is ripe and ready to sow.

• Choose a pot that is at least 15cm high to accommodate the long oak taproot. Make sure it has drainage holes
• Fill the pot about three-quarters of the way with peat-free compost, with a little added grit for better drainage
• Place an acorn on its side and cover with 2-3cm of compost
• Water and place the pot in a very sheltered place out of the rain and with plenty of light, such as a cold frame or unheated greenhouse
• Protect acorns from birds and other animals
• Keep compost moist once germination takes place in spring
• Repot your oak seedling when it reaches a height of 25cm

How long do oak trees take to grow from acorns?

After the first year, you can plant your oak seedling into the ground, but they can be grown on in larger pots for another year, if necessary. In ideal conditions, English oaks can grow around 50cm a year and they take about 40 years to start producing acorns.

How to care for an oak tree

Water oak saplings in dry weather and keep the area around the base free of weeds and grass for the first few growing seasons. After this initial period, oak trees require very little attention.

How to prune oak

These low maintenance trees do not need pruning other than to remove any damaged or crossing branches in late autumn or winter. The RSPB advises not to prune in summer, so oak hedges can be trimmed in September to keep them at the desired height. Small hedges could be trimmed in summer, provided you have checked very thoroughly to ensure nothing is nesting inside.

Pests and diseases

Oak trees are susceptible to several fungal diseases and the foliage may be eaten by a wide range of insects.

Powdery mildew is a fungal disease that affects a wide range of plants. Oak mildew causes a white, powdery coating on leaves, which is particularly noticeable on shoots with fresh foliage produced by oaks (and some other tree species) in late summer, known as Lammas growth.

Honey fungus can affect oak trees, causing symptoms that include dieback of roots and branches, and the eventual death of the tree. Clusters of honey-coloured fruiting bodies can appear in autumn, but they are not always evident. Honey fungus is thought to be a possible contributory factor of oak decline.

Most people plant oak trees to attract wildlife, and invertebrates are a vital part of the ecosystem supported by oak. Insects such as aphids and caterpillars feed on oak leaves, but most do not pose a threat to the overall health of the tree.

Winter moth larvae are particularly important as they attract birds such as blue tits, which rely on the emergence of the caterpillars to feed their young. A brood of blue tit chicks can consume over 1,000 caterpillars every day and they favour winter moths above other species.

Types of oak tree to grow

Quercus robur – the classic English oak, often dominating woodland. Flowers in April and May, and produces stalked acorns in autumn. Green leaves have around five pairs of deep lobes, and foliage turns golden brown in autumn. Long-lived tree with a height of up to 40m.

• Buy Quercus robur from Thompson & Morgan

Quercus rubra – red oak is a North American species with large, lobed leaves. It has been widely planted in parks and gardens over the last century, largely for its spectacular scarlet autumn colour. Prefers acidic soils. Produces acorns in shallow cups.

• Buy Quercus rubra from Crocus

Quercus ilex – holm oak is an evergreen species introduced to Britain from the Mediterranean region around 500 years ago. It is often grown in coastal locations and makes an excellent hedging plant due to its tolerance of fairly exposed sites and its suitability for clipping. Low branches and suckers can produce leaves with short, spiny teeth like holly leaves. Mature leaves lack spines and are very variable in shape, from slender elliptical to more rounded ovals.

• Buy Quercus ilex from Van Meuwen

Quercus palustris Green Pillar ('Pringreen') – like red oak, pin oak originates from North America and has similar leaves, though the lobes are more deeply cut. Green Pillar is a beautiful, columnar oak which reaches an eventual 12m in height with only a 3m spread, making it a good choice for avenues and smaller gardens. It has superb red autumn foliage and produces its best colour when planted in fairly acidic soils.

• Buy Quercus palustris Green Pillar from Crocus