Oak Basics - Quercus Species
Public Domain Species Information
Quercus is a genus of about 600 species of deciduous, semi-evergreen and evergreen monoecious trees and shrubs. Nearly all are slow-growing and long-lived species, some reaching heights of up to 40-meters.
Species suitable for Bonsai
- Quercus ilex, aka Holly Oak is native to Europe in the Mediterranean climates and England. It is hardy to zone 7. It is in leaf all year, in flower from May to June, and the seeds ripen from September to October. The flowers are monoecious (individual flowers are either male or female, but both sexes can be found on the same plant) and are pollinated by Wind. The plant prefers light (sandy), medium (loamy) and heavy (clay) soils and can grow in heavy clay soil. The plant prefers acid, neutral and basic (alkaline) soils and can grow in very alkaline soil. It can grow in semi-shade (light woodland) or no shade. It requires moist soil. The plant can tolerate maritime exposure.
- Quercus robur, aka English Oak - Slow-growing and long-lived deciduous tree. Lobed leaves are yellow-green in Spring turning to bronze in Autumn up to 5-15cm long with very small stalks. There is much hybridization between Q.Robur and Q.Petraea found growing in Europe. Height to 35m.
Bonsai cultivation notes
- Position - Full sun and good air circulation. Protect from hard frost below -5°C. Shaded branches on Oak bonsai have a tendency to die-back.
- Feeding - Every two weeks at half strength after first growth has hardened off in Spring.
- Repotting - Standard practice says that Oak should be repotted yearly as buds start to move in Spring until the tree is 10 years or older, then every 2-3 years. However, it is possible that repotting after the first leaves have opened or in early Autumn is better for Quercus robur/English Oak. Also see - Collecting and Repotting English Oak
- Pruning - Trim new shoots and pinch out apical buds unless extension is required.
- Propagation - Sow seed outside as soon as ripe. Air-layering with difficulty.
- Pests and Diseases - Very susceptible to mildew problems unless good air circulation is provided, also oak wilt, aphids and gall wasps.
- Styles - Upright and slanting styles, multi-trunk styles, group planting in medium to large sizes. Suitable for subtle deadwood features.
- Pots - Glazed in light and textured, earthy colors.
- Notes - Does not respond well to pruning techniques to reduce leaf-size, complete defoliation can result in larger leaves, infrequent repotting helps, as does removal of apical/terminal buds before they extend.
One of the most common trees that you will see driving the roads of California in rural areas are oaks. Many of these trees are hundreds of years old. They are the dominant plants in many habitats. The wildlife of California depend heavily upon oaks. If we break down the state into sections where different types of oaks live, you can see the species of oak that you may commonly encounter in your area.
- Quercus agrifolia/ Coast Live Oak - with shiny, evergreen prickly leaves, that are usually rolled under. This oak is very drought tolerant, looks nice all year, and grows fairly quickly for an oak. Coast Live Oak will grow up high on the hillsides in central California (in the draws where there is more moisture). Under stress (drought, insects, diseases, etc.) the leaves will roll under and even fall off. No stress and the leaves are flat (like Interior Live Oak, Quercus wislizenii, BUT, for all oaks a little stress is good! Flat- leaved Coast Live Oaks are more susceptible to SOD (sudden oak death) or other diseases.
- Quercus durata / Leather Oak is one of the many scrub oaks. The rolled leaf edges on a small tree, almost shrub, is the key. Use as a dwarf oak or as a 'tree' in a miniature forest.
- White Oak - Leaf: Alternate, simple, oblong to ovate in shape, 4 to 7 inches long; 7 to 10 rounded, finger-like lobes, sinus depth varies from deep to shallow, apex is rounded and the base is wedge-shaped, green to blue-green above and whitish below. Flower: Monoecious; male flowers are yellow-green, borne in naked, slender catkins, 2 to 4 inches long; female flowers are reddish green and appear as very small single spikes; appearing with the leaves in mid-spring. Fruit: Ovoid to oblong acorn, cap is warty and bowl-shaped, covers 1/4 of the fruit; cap always detaches at maturity; matures in one growing season in the early fall. Twig: Red-brown to somewhat gray, even a bit purple at times, hairless and often shiny; multiple terminal buds are red-brown, small, rounded (globose) and hairless. Bark: Whitish or ashy gray, varying from scaly on smaller stems to irregularly platy or blocky on large stems. On older trees smooth patches are not uncommon. Form: A very large tree; when open grown, white oaks have rugged, irregular crowns that are wide spreading, with a stocky bole. In the forest crowns are upright and oval with trees reaching up to 100 feet tall and several feet in diameter.
- Quercus suber/ Cork Oak - Originates from the Mediterranean region and is grown commercially for the thick cork bark which is harvested mainly for producing corks for wine bottles. Portugal is the world's major producer of cork products.
Cork Oaks are indigenous to the Mediterranean region where they occur in open woodlands on hills and lower slopes. They form a thick cork bark which is harvested mainly for the manufacture of wine bottle corks although the cork also makes a good heat and electrical insulator so is used for gaskets in engines and for isolative materials used in home interiors. By cutting off only the outer, dead corky bark, the tree is able to regenerate new cork tissue from the underlying live bark. In this way it is possible to cut off cork from a tree about every 10 years and the tree itself is able to live for about 150 years.
Cork Oaks are grown mainly in Portugal where there are over 60 square kilometers of cork orchards. Not surprisingly, cork products are Portugal's main export. Spain also grows cork commercially but to a much lesser extent than Portugal. There have been attempts to grow Cork Oaks commercially in other parts of the world but these other countries have not been able to compete with Portugal in terms of skillfulness and cheapness of labor. Cheap plastic stoppers for wine bottles are to some extent a threat to the cork industry but for the better wines, cork stoppers are still preferred.