Chapter 127 Quercus sp.
127.1 Oaks
An exceptional example of oak bonsai, beginning to change from green to fall colors. Link to original image
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Oaks are not part of traditional Japanese bonsai because there are no species native to Japan with leaves small enough for bonsai. The tradition of using oaks for bonsai actually originated in California in the 1950’s.
Several North American and Eurasian oaks have been adapted into bonsai. There are both evergreen and deciduous oaks. I have grouped several appropriate species based on whether they are adapted to drier soils, to fertile deeper soils, to wet or coastal sites, plus the species that are adaptable.
| Group | Generalist Species | Common Name | Leaves | Native Range | Notes |
|---|---|---|---|---|---|
| Generalist | Quercus alba | White oak | Deciduous | Eastern and central North America | In the wild, trees are usually as tall as they are wide. Slow-growing. Soil-tolerant. |
| Dry Sites | Quercus coccinea | Scarlet oak | Leaves | Eastern North America | Prefers dry, sandy, usually acidic soils |
| Dry Sites | Quercus douglasii | Blue oak | Deciduous, leathery | California | Drought tolerant |
| Dry Sites | Quercus ilex | Holly or holm oak | Evergreen | Southern Europe, northwestern Africa | Does best in sandy soils surrounding the Mediterranean basin. Does not tolerate freezing. |
| Dry Sites | Quercus hemisphaerica | Darlington oak | Evergreen to semi-evergreen | Southeastern US coastal region | Best in sandy soils in coastal sandhills |
| Dry Sites | Quercus myrtifolia | Myrtle oak | Evergreen | Southeastern North American coast | Prefers sandy soil. |
| Fertile Sites | Quercus phellos | Willow oak | Deciduous | Eastern North America | Prefers moister lowland areas. Dislikes dry sites. |
| Fertile Sites | Quercus robur | English oak | Deciduous | Europe, West Asia | Prefers fertile and well-watered soils |
| Wet Sites | Quercus laurifolia | Laurel oak | Slowly deciduous | Southeastern US coastal region | Often confused with Darlington oak. Often grows in sandy soil near rivers and along the edges of swamps if not too frequently flooded. |
| Wet Sites | Quercus nigra | Water oak | Deciduous | Southeastern North America | Notes |
| Wet Sites | Quercus palustris | Pin oak | Deciduous | Eastern North America | Adapted to poorly drained, alluvial floodplain and river-bottom soils with high clay content. Found on sites that flood intermittently in winter. Fast growing. Naturally develops a fibrous root system rather than a taproot. Only native red oak with leaves small enough for bonsai. |
| Coastal | Quercus chapmanii | Chapman oak | Slowly deciduous | Coastal SC to FL panhandle | Adapted to sandy, well-drained soils of scrublands and sand dunes. |
| Coastal | Quercus geminata | Sand live oak | Evergreen | Southeastern North America | Adapted to sandy coastal soils |
| Coastal | Quercus virginiana | Live oak | Semi-evergreen | Southeastern North America | Previous year’s leaves fall when new leaves emerge in spring. Best growth with ample water on well-drained sandy soil. |
127.2 Soil: General Deciduous / Rich Conifer Mix
Ratio: 3 parts bark, 1 part grit
Add-ins: dolomitic limestone (~5 lbs/yd), Tree-Tone (15 lbs/yd)
pH: ~6.5 (slightly below neutral)
Adjustments: the basic mix should be suitable for fertile site and adaptable sites. Add 1 part sphagnum peat for species that prefer wet sites. For coastal and dry-adapted species, add 1 part of coarse sand or chicken grit to ensure good drainage.
Moisture: Average. Water the oak thoroughly when the soil gets dry, but avoid constant soil wetness. Water less in winter but never let the rootball dry out completely.
127.3 Light
During development, oaks like to be kept in a mostly sunny location. Oak bonsai need about four hours of direct sun per day, but should spend the rest of the day in partial shade.
If sited in full shade, oak bonsai can suffer diebacks or develop extremely large leaves. Shaded branches on bonsai have a tendency to die-back, so trees should be rotated regularly to ensure all branches get good light.
127.4 Cultivation Notes
Oak Trees experience a quick spurt of growth in the spring, followed by slower spurts of growth throughout the summer and fall seasons. This growth spurt guides some of the care decisions.
127.4.1 Potting & Root Pruning
Oaks should be repotted and root pruned in EARLY spring, before the buds open. Re-pot every year in early spring. Roots should only be pruned in the spring before new growth emerges, and should be pruned only minimally. It is imperative that the roots are not pruned at any other time of the year. The pot should be a little large (by bonsai standards) due to the root sensitivity issue.
English oaks are an exception. An English grower found that:
”…it is possible that repotting after the first leaves have opened or in early Autumn is better for Quercus robur/English Oak…[those] damaged during dormancy would die back. But Oaks that were damaged during the growing season while they were still in leaf recovered. It didn’t seem to matter how dry the ground was or how badly damaged the root system appeared to be, the Oak that was in leaf would inevitably recover.”
Their recommendation is to wait until English oaks are in “red leaf” stage to root prune. This is when the leaves are in the process of unfurling or just opening. When air temperatures get above 75oF or so, it is probably too late.
127.4.1.1 Top & Branch Management
Heavy pruning: best carried out in early spring when the tree bursts into vigorous growth, sending out rather elongated shoots. Unwanted/undesired shoots should be cut away immediately.
After emerging from dormancy, a bonsai oak will typically throw long shoots, with as many as seven or eight nodes. Any shoots not being used as future branches should be pruned back to two or three nodes. Strong terminal buds can also be removed then.
Remove dead or wilted branches by cutting them flush with the trunk. Don’t leave stubs.
Fine pruning: cut back to two or three nodes any new branches you wish to keep. Leave about a third of the new growth to create a balanced appearance.
Prune out a few interior branches, pruning back to the trunk to increase air circulation and light penetration and to prevent fungal diseases.
Trim the upper parts of the crown attentively because they grow stronger than the lower branches. Pinch out new buds from the top of the tree to encourage horizontal branching and produce smaller leaves.
Wiring: most oak trees are highly responsive to wiring, especially the pin oak. Any wiring that is undertaken must be done gently so as not to cause damage to the bark, however. When oaks are wired be careful to remove the wire before it bites into the bark. Wire marks will be visible for a very long time. Guy wires can be a good choice instead.
Leaf pruning: definitely a no-no; oaks do not respond well to pruning techniques to reduce leaf-size. Complete defoliation can result in larger leaves. Defoliation also weakens the tree too much. Instead remove the apical/terminal buds before they extend.
An example of an oak that is still in training. The trunk line is exceptional, but some branches could be moved to create a cleaner outline. That said, this is well on its way to being a show piece tree. Link to original image.
127.4.2 Feeding
Apply fertilizer in the early spring around the time the initial spring growth spurt starts. When feeding oaks, use slow-release fertilizers with a nutrient ratio of 12-6-6 or 12-4-8 (3-1-2). Good options:
- Slow release, prilled shrub and tree fertilizer, applied at half the recommended rate.
- Organics such as Espoma Tree-Tone (6-3-2).
- Inorganic, water soluble fertilizers at 1/2 to 1/4 strength rather than full strength.
Don’t feed with very high nitrogen which would cause large leaves, long internodes and increased susceptibility to insects and mildew.
127.4.3 Pests & Problems
Oaks are susceptible to mildew, so should be placed so they have good air circulation. Oaks can be attacked by oak worms (caterpillars of several moth species) that skeletonize the leaves. They destroy leaves very quickly, so do not wait to spray. Their eggs hatch over several days; check sprayed trees regularly for 2-3 weeks for new damage.
127.4.4 Winter Storage
Protect from hard frost below -5°C.
127.5 Propagation Notes
Oaks can be grown from acorns, but expect to wait several years to start training them. Oak seedlings may remain less than a foot tall for 3+ years while the root system develops. Starting with larger container stock is the better option.
127.6 Artistic & Styling Notes
They look nice in upright and slanting styles, multi-trunk styles, and group planting in medium to large sizes. Use glazed pots in light and textured, earthy colors.