Oak (Quercus) trees are not yet well known as bonsai. Their broad leaves make them undesirable when it comes to the art of cultivating miniature trees. These trees are best known for their characteristic seeds called acorns. Many animals rely on these acorns as a food source such as squirrels.
Oak trees are mostly deciduous, but you do get a few evergreen species.
There are several hundred oak species in the Quercus genus that can be found growing wild in Europe, North Africa, Southwest Asia as well as Central and North America.
The best Oak bonsai usually come from the Southern and Mediterranean Oaks. They are smaller than the frost-hardy species such as the European Oak (Quercus robur) and the American White Oak (Quercus alba).
The Southern and Mediterranean Oaks are also evergreen, unlike most Oak species.
European Oak (Quercus robur) and the American White Oak (Quercus alba) are very similar species. Both are deciduous, frost-hardy, can grow around 130 feet (40 meters) tall, and can live for around 800 years. Their spectacular trunks are usually around 47 inches (120 cm) in diameter with extraordinary characteristics.
The wood from oak trees is quite popular for anything that requires woodwork. It’s often used in the construction of tools, furniture, boats, and buildings. In ancient times it was also used for shipbuilding.
These trees look quite spectacular as bonsai with their heavy branches that bend outwards in a characteristic zigzagging line. As your tree ages, you will also notice that the crown gets quite wide.
If you have a frost-tolerant species as a bonsai, you will still need to offer it some protection in winter.
Frost hardy species aren’t that frost tolerant when kept in a pot.
Care Guidelines For Oak Tree Bonsai
Caring for an Oak tree bonsai might sound very simple, but it really isn’t that easy. You will need to build up your knowledge and exercise a lot of care to keep this little tree healthy.
Fortunately, it is very rewarding to look after an Oak tree bonsai which makes all the learning worth it.
During the growing season, the best place to keep your Oak bonsai is in an open, airy environment with full sun. If you’ll be keeping your tree indoors, the best place for it is a window that gets sun all day long.
If you don’t have that option, you might have to invest in some grow lights to supplement sunlight.
Some species like White Oak, European Oak, and other northern oaks are quite frost-hardy when planted in the ground. If kept in a bonsai pot, however, they do need protection against severe frost.
The southern and Mediterranean oaks on the other hand don’t handle frost too well and will need to be protected against it.
If you’re unsure of the type of oak you have, make sure to always protect it against frost. Oak trees require a cold environment in winter to become dormant so don’t place your oak bonsai in an area with artificial heating. The best place to keep your tree in winter is in a greenhouse, garden shed, or garage where it will be cold but protected against frost and cold drafts.
Oak bonsai needs to be watered regularly in the growing season. Make sure the soil never dries out completely, but allow some drying out between waterings. Oak trees don’t tolerate constantly soggy soil.
To make sure you don’t overwater your bonsai do a simple moisture test before watering. Simply stick your finger about 1 inch (2.5cm) into the soil. If the soil feels dry, water your oak tree thoroughly.
If the soil feels wet, you can wait another day before watering.
When watering your bonsai, make sure to water enough that the water drains out the bottom. Allowing the water to drain prevents a salt build-up in your soil from the fertilizer that may damage the roots of your plant.
In winter you can reduce watering. Do make sure, however, that the root ball never dries out completely.
Soil And Fertilizing
Oak bonsai needs to be fertilizer regularly to stay healthy.
Just like most bonsai, oak bonsai are grown in very little soil which means the soil gets depleted of nutrients quickly.
You can use a solid fertilizer once a month during the growing season to supplement your tree. Once winter comes around, you can stop fertilizing until spring comes again. Your tree will be dormant during the three months of winter and won’t need the nutrients as much.
If you want to interact with your tree more, you can also consider a liquid fertilizer. If you’re using a liquid fertilizer you’ll need to fertilize your tree weekly during the growing season. Once winter comes around, you can reduce fertilizing to once a month. This allows your dormant tree access to some nutrients to stay alive and survive winter.
Make sure to use a fertilizer low in nitrogen. Fertilizer with a high nitrogen content encourages the growth of large leaves and long internodes. It also increases your bonsai’s susceptibility to some pests and diseases like mildew and mites.
The best time to hard prune your oak bonsai is in early spring before the buds open. This is also the best time to remove strong terminal buds. You can allow your tree to grow before pruning the new shoots back. Make sure to always leave two leaves on the shoot your pruning to encourage health and vitality.
You can remove some of the undesirable large leaves on your tree as well, but make sure to not defoliate your tree completely.
Doing so will weaken your tree making it susceptible to pests and diseases.
You will also need to pay more attention to the crown of your tree. This area grows much stronger than the lower branches and will require more attention to stay on track with your design.
If you have a young tree, you will need to repot it every two years to accommodate its initial growth.
Once your tree gets a bit older, you can reduce repotting to every three to five years.
The best time to repot your oak bonsai is in spring before the buds open. This way you’ll be reducing the stress on your tree and give it time to recover before the burden of growth starts.
Repotting is also a good time to do root pruning. Root pruning keeps your tree small and healthy. It allows water and air to pass through and creates space for new roots to grow. Just make sure to never cut too deep into the root ball and don’t trim away more than ⅓ of the roots.
Oak trees aren’t that fussy about soil as long as your soil drains well while still retaining some moisture. Oak trees don’t like their root balls to dry out completely. Standard potting soil usually does the job.
If you want to increase moisture retention, you can add some bark or other organic material to the mix.
The best way to propagate oaks is by using seeds. You will need to be patient however since seeds can take a while to germinate. You will also need to carefully select your seeds for health and leaf size. Some oaks aren’t great bonsai specimens due to the size of their leaves.
Unfortunately taking cuttings doesn’t work that well on oak trees. Cuttings are very difficult to grow and rarely form roots. The same goes for air layering.
If you want an oak bonsai, the best course of action is to simply buy an oak sapling from your nearest nursery.
Potential Pests And Diseases
Oak bonsai are very susceptible to a variety of pests and diseases. One of the most common problems you will encounter on your oak is powdery mildew.
To a lesser degree, you also need to look out for oak leaf blister, bacterial leaf scorch, and pine-oak gall rust.
Mildew can be treated by mixing 10g baking soda with 10ml rapeseed oil, one drop of dishwashing detergent, and one liter of water. Spray this solution on the mildew several times over a period of a few days.
In some cases, you’ll need a specific pesticide to treat your oak bonsai. There are a few insects and mites that cause gall, but they don’t usually harm your tree. Other insects you need to look out for are aphids, leafminers, and oak worms. These pests can usually be manually removed, washed away with a stream of water, or killed with organic solutions created from neem oil or vinegar.
If your homemade solutions don’t seem to do the job, you can consider chemical pesticides from any local gardening store in your area. If your choice of action is to collect the pests, beware of the caterpillars of the oak processionary moth. The hairs on this caterpillar are toxic to humans.
Wiring Your Bonsai
If you’ll be wiring your tree, care must be taken not to damage the bark. Make sure to remove the wires in a timely fashion to prevent growth restriction and bark damage as a result. In most cases, it’s best to use guy wires to prevent bark damage. If the bark does get damages, the scars will be visible for a very long time.
When removing wires from your tree, make sure to cut them instead of unwinding them.
Cutting the wire prevents unnecessary damage to your tree.