Quercus suber, cork oak
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Broadleaf Evergreen Trees

Quercus suber

cork oak

Fagaceae

Western Mediterranean Basin — Portugal, Spain, southern France, Italy, Morocco, Algeria, Tunisia

At a Glance

TypeTree
HabitUpright
FoliageEvergreen
Height30-60 feet (9-18 m)
Width30-50 feet (9-15 m)
Maturity25 years

Growing Zones

USDA Hardiness Zones

8 - 10
Zone 8
Zone 9
Zone 10
These zones indicate the coldest temperatures this plant can typically survive.
Frost Tolerancehardy

Key Features

Deer Resistant
Drought Tolerant
Maintenancevery low

Overview

A medium to large broadleaf evergreen tree reaching 30-60 feet (9-18 m) tall and 30-50 feet (9-15 m) wide. In the Pacific Northwest, typically 25-40 feet (7.5-12 m). Growth rate is slow, adding 6-12 inches (15-30 cm) per year. Long-lived, with specimens exceeding 200-250 years documented in Mediterranean cork forests (montados). The defining feature is the thick, deeply fissured, spongy bark — commercial cork — which can be 2-4 inches (5-10 cm) thick on mature trees. The outer bark is gray-brown with deep fissures; when harvested (stripped), the inner bark is bright reddish-orange, gradually weathering to gray-brown as a new cork layer regenerates over 9-12 years. Leaves are alternate, ovate to oblong, 1-3 inches (2.5-8 cm) long and 0.5-1.5 inches (1.3-4 cm) wide, dark gray-green to blue-green above, pale gray-green with fine pubescence beneath. Margins are variable: entire to shallowly toothed or wavy. Leaves are smaller and more gray-green than those of Q. ilex. Flowers appear in May to June: male flowers in yellow-green pendulous catkins; female flowers inconspicuous. Wind-pollinated. Acorns approximately 0.75-1.25 inches (2-3 cm) long, oblong, enclosed one-third to one-half by a thick, warty cup. Acorns ripen in fall, some in the first year, others requiring two seasons. The trunk and major branches develop the distinctive thick cork bark beginning at approximately 15-20 years of age. Exceptionally drought-tolerant once established. Hardy to approximately 10-15°F (-12 to -9°C); marginally hardy in much of the Pacific Northwest.

Native Range

Native to the western Mediterranean Basin, from Portugal and Spain through southern France, Corsica, Sardinia, Sicily, Morocco, Algeria, and Tunisia. The dominant tree in the Portuguese montado and Spanish dehesa agroforestry systems. Found in open woodland, savanna-like landscapes, and maquis on acidic to neutral soils from sea level to approximately 3,000 feet (900 m).

Suggested Uses

Planted as a specimen tree where the extraordinary cork bark texture can be appreciated. The bark provides year-round architectural interest and is a unique conversation piece in garden settings. Suitable for large gardens, parks, and Mediterranean-style landscapes. The exceptional drought tolerance makes it increasingly valuable as Pacific Northwest summers become warmer and drier. Not suitable for small residential gardens due to eventual size. Acorns provide food for wildlife. In the Pacific Northwest, best suited to zone 8b-9 or sheltered microclimates in zone 8a.

How to Identify

Immediately identified by the thick, deeply fissured, spongy cork bark — distinctly different from any other tree bark. Mature bark is gray-brown with deep fissures; freshly exposed inner bark is bright reddish-orange. Leaves are small, ovate, dark gray-green to blue-green above with pale pubescent undersides. Distinguished from Quercus ilex by the thick cork bark (Q. ilex has thin, smooth to finely fissured bark) and the more gray-green (vs. darker green) leaf color. Acorns are oblong with thick, warty cups.

Appearance

Size & Dimensions

Height30' - 60'
Width/Spread30' - 50'

Reaches mature size in approximately 25 years

Colors

Flower Colors

yellow

Foliage Colors

gray green
blue green

Bloom Information

Bloom Period

~2 weeks
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SpringSummer
Flowers in May to June. Male flowers appear as yellow-green pendulous catkins; female flowers are inconspicuous. Wind-pollinated. Acorns 0.75-1.25 inches (2-3 cm) long ripen in fall, with some requiring two growing seasons.

Detailed Descriptions

Flower Description

Yellow-green (catkins)

Foliage Description

Dark gray-green to blue-green above; pale gray-green beneath

Growing Conditions

Sun Requirements

Full Sun
Requires 6-12 hours of direct sunlight daily
• Full Sun: 6+ hours of direct sunlight
• Partial Shade: 3-6 hours of direct sunlight
• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range5.0 - 7.0(Neutral)
357912
Soil Types
loamsand
Drainage
well drained

Water & Climate

Water Needs

Low

Frost Tolerance

hardy

Time to Maturity

15-25 years

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

Plant in full sun in well-drained soil, preferably acidic to neutral (pH 5.0-7.0). Avoid heavy clay and waterlogged conditions. Exceptionally drought-tolerant once established — requires no supplemental irrigation after the first 2-3 growing seasons in the Pacific Northwest. In zones 8a-8b, site in a sheltered, south- or west-facing position protected from cold winter winds, especially when young. Young trees are more frost-sensitive than established specimens. Mulch with 3-4 inches (8-10 cm) of organic material but keep mulch away from the trunk. No serious pest or disease problems in the Pacific Northwest. Do not attempt to harvest cork bark from landscape specimens — commercial cork harvesting requires specialized skill to avoid damaging the cambium.

Pruning

Prune in late winter to early spring to shape and remove dead or crossing branches. Minimal pruning is needed once the canopy structure is established. The slow growth rate means structural training when young is important — select a central leader and remove competing co-dominant stems. Do not damage the cork bark during pruning. Avoid heavy pruning, which can expose the inner bark to weather damage.

Pruning Schedule

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winterearly spring

Maintenance Level

very low

⚠️ Toxicity Warning

Non-toxic