Quercus castaneifolia (chestnut-leaved oak)

Native to the mountains of the Caucasus and northern Iran.

Buds of Quercus castaneifolia (Image: James Morley, RBG Kew)

Chestnut-leaved oak is deciduous and for most of the year has shiny dark green leaves, which turn a deep bronze and then brown before falling. Specimens can grow up to 50m tall. Its horizontal branches grow from remarkably low on the trunk, giving it a good width to go with its not inconsiderable height. The young twigs are soft and hairy.

The acorns of the chestnut-leaved oak are flushed with orange at the base and although bitter, they are eaten by jays, squirrels and other wildlife. However, this tends to be a last resort, the acorns being eaten only when other food supplies have run dry. The acorn cups have a diameter of 1.5-2.5cm, and the acorns are 2-3.5cm long. These closely resemble the acorns of the Turkey oak, which have similarly ‘mossy’ cups. However, its leaves do not have the lobes that typify many oaks and are shaped more like the tip of a spear. They are in fact, more like that of the sweet chestnut (Castanea sativa). The leaves are thin-textured, narrowly elliptic, with a margin with 7-14 teeth. The young leaves have a grey-green lower surface with covering of stellate hairs.

Like that of many trees, the bark of Quercus castaneifolia changes with age. At first it is black and smooth and later turns a dark grey and gains a ridged appearance. The bark can split or fissure as the tree’s girth increases with age.

As long as conditions are favourable, oaks tend to grow quickly, perhaps more so than is commonly assumed for such large trees. The trunk of the oldest chestnut-leaved oak at Kew grew 3m in girth in 60 years, and younger trees grow at an even faster rate, around 30cm every four years.

The chestnut-leaved oak is not threatened.

Quercus castaneifolia is a forestry and ornamental species. The acorns are sometimes eaten by wildlife.

The acorns of chestnut-leaved oak are flattened at the top and half enclosed in a cup coated with reflexed, downy scales. The acorns ripen in the second year, putting chestnut-leaved oak into the ‘black/red oak group’. Acorns in this group usually show embryo dormancy and need either cold stratification or planting in the autumn to germinate.

To check for viability the acorns can be placed in water; those that float should be thrown away. Air pots are the best containers to grow the seedlings in, as these encourage lateral root growth and reduce spiralling roots in the pot, which are common as a result of the strong taproots. The taproot should be pruned after the first year. Grafting can also be carried out, with a rootstock chosen from the ‘black/red oak group’. Young trees should be planted out during the dormant season after the second year. The oldest chestnut-leaved oak tree at Kew is crown-thinned every five years.

Quercus castaneifolia leaves are eaten by caterpillars of the oak processionary moth (Thaumetopoea processionea); contact with, or inhalation of the hairs from these caterpillars can cause skin irritation and allergic reactions, and so they should not be disturbed by gardeners.

Quercus castaneifolia by the Orangery at Kew Gardens (Image: RBG Kew)

The chestnut-leaved oak was introduced to Britain from the Caucasus and Iran as seed in 1843. The specimen on the lawn behind the Waterlily House was the first introduction; it is thought to have been planted out by William Hooker in 1846, during the planting of the new 45 acres. Today, at over 30 metres tall and 30 metres spread, it is the biggest, finest and unrivalled specimen of its type in the world. It is the largest and fastest-growing tree in the arboretum and continues to grow at an alarming rate; it is also a TROBI (Tree Register of the British Isles) champion. This species is not widely planted in this country and is therefore particularly valuable.

In 1987, the year of the great hurricane, when many trees surrounding it on this lawn were blown over, the chestnut-leaved oak withstood the forces of nature without the loss of a single limb and now stands dominating the location.

Trunk of Quercus castaneifolia (Image: James Morley, RBG Kew)

The Quercus castaneifolia at Kew is the pride and joy of the arboretum collection – Kew’s largest tree. The chestnut-leaved oak is also one of Kew’s fastest-growing trees, and in order to keep it in such peak condition the arboretum team chose it as one of the first to receive a pioneering health boost. First, the team dealt with compaction of the soil around the tree’s roots. The soil at the base of many of Kew’s trees has solidified during many years of admiring visitors walking on it, but the arboretum team brought in a German power tool specially designed to tackle the problem. The device, looking a bit like a pneumatic drill, sends a blast of harmless nitrogen gas down into the compacted earth to give everything a good shakeup. This creates new routes for water, enabling rainfall to trickle down to the tiny microfilaments at the roots that absorb moisture.

In a second prong of attack, the team went on to treat the roots with mycorrhizae; microscopic friendly fungi. These fungi can help the tree form cooperative relationships, exchanging soil nutrients gathered by the mycorrhizae for those produced in the tree’s leaves by photosynthesis. Nowadays you can buy such mycorrhizae in your local garden centre, to give your garden trees the professional treatment. Kew’s Quercus castaneifolia has certainly never looked better.

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