Field maple (Acer campestre) is a popular species for bonsai, particularly in the UK and elsewhere in Europe. Its standout characteristic in bonsai cultivation is its vigorous growth and ability to thicken. This can help to build strong, girthy trunks, but can also offer some challenges in refining a bonsai.
Here's what we'll look at in this field maple bonsai tree care guide:
- Acer campestre
- Pests and diseases
- Propagating field maples
Also known as hedge maple as well as field maple, these trees have dark green leaves with 3 lobes. These can grow very large in trees in the ground but can be reduced quite easily on a tree planted in a pot.
Field maple bark is a light grey/brown color. It is relatively smooth on young trees but can develop interesting cracks and fissures as the tree ages. New growth also has the potential to have cork-like bark.
Acer campestre bonsai can add a nice range of colors to your garden throughout the year. In spring, their new growth has a pinkish-red color. This turns to a deep green as the leaves harden off. Their autumn colors are beautiful yellows and oranges.
Field maples are a very hardy species that generally enjoy full sun through the growing season. If the tree has weak roots or you live in a particularly sunny area, you may want to move the tree to dappled shade.
Sun is particularly important for hedge maple bonsai to reduce internode length when the tree is in refinement. Trees grown in the shade will naturally elongate their branches to try and grow past the surrounding canopy to reach the sun. This means leggy growth, which is harder to build a nicely scaled bonsai with.
Over winter most Acer campestre bonsai will not need any protection unless temperatures drop below 23°F (-5°C). Even then, they may only need healing in or moved to a cold frame as they are such a frost-tolerant species.
Acer campestre bonsai prefer to be watered slightly more than some other trees, but should still be allowed to dry out slightly between waterings to avoid overwatering. When you are applying water, make sure you thoroughly soak the tree to ensure the entire root ball is hydrated.
Hedge maple bonsai are not really sensitive to any particular water pH range.
Pruning Acer campestre bonsai
Major design decisions and structural pruning are usually best carried out just as leaves are dropping in autumn.
This is an ideal time because the tree is busy absorbing resources from its leaves and moving them around the vascular tissues to prepare for dormancy. This movement of resources will allow the tree to compartmentalize wounds and prevent large areas of dieback from occurring. Styling at this time also has the advantage of the tree being leafless, which helps with the visibility of the branches and makes wiring easier.
Large branches should be removed with a saw to create a very clean wound that will heal well. A small stump should be left to prevent dieback along the trunk of the bonsai. This can be tidied up the following year.
During the growing season, new growth should be regularly pruned back to 2 buds after leaves have hardened off. You can tell a leaf has hardened off by the deepening of the green color and the formation of a waxy cuticle.
In an established field maple bonsai, smaller leaf sizes and more refined growth can be achieved through the use of partial defoliation.
Partial defoliation involves removing around 70-80% of the tree's foliage mass. The loss of leaves will stimulate the tree to grow a second time, but since it has already used lots of its stored energy on the first flush of growth the second will be finer, have tighter internodes, and smaller leaves.
Some practitioners advocate going a step further and fully defoliating their bonsai, e.g. removing all of the leaves. This can certainly achieve good results but be warned that by removing the whole canopy you risk the tree dying if it is unable to grow back its leaves.
The benefit of partial defoliation is you can guarantee the tree still has something left to build from because it always has some leaves left to generate energy. If you keep the leaves at the ends of branches you can also help the flow of water and resources through the vascular tissue, which can help stimulate growth.
To perform partial defoliation, start by removing half of the leaves. Acer campestre grows leaves in pairs, so cut off 1 of each pair. Do this alternating left/right to help with light penetration and to promote new branches to grow on alternate sides.
Next cut the lobes off the remaining leaves with some sharp pruning shears. This step is often not required on bonsai with smaller leaves such as birch or cotoneaster, but on a large-leafed species such as field maple, it is essential to reduce the overall foliage mass of the tree.
Give a dose of fertilizer immediately after partially defoliating to stimulate some new growth, and be careful with watering. The tree will use less water now that it has fewer leaves, and it may need some protection from the sun for a short period as it may be unable to cool itself effectively without its leaves.
Partial defoliation can be performed multiple times in a year depending on how vigorously your bonsai is growing.
Wiring field maple bonsai
Wiring can be performed at any time from the onset of spring the around 2 weeks after leaves have dropped in autumn.
Being such a vigorous species you should be checking wire regularly once it is applied, potentially once or twice a week if you have a very vigorous tree.
Wire can leave marks on field maples despite lots of care being taken during the application, however, Acer campestre will heal wire marks very well and severe, permanent scarring will only occur when a wire has severely cut into a branch.
Care must be taken when bending branches because they can be fairly brittle. Try bending a pruned branch first to see how far it can be bent. Rotating the branch while bending will also help to increase the degree of bending before the branch breaks because it distributes the force of the bend across more tissue.
If you are propagating your own material, it is a good idea to wire your bonsai as young as possible to get movement into the trunk before it becomes too thick to bend.
Field maple styles
Acer campestre can be grown into many styles of bonsai, but the most common are medium-sized informal upright will a very stout, thick trunk. Field maple is very well suited to hollow trunk styles which can add a lot of interest.
Hedge maples can be propagated quite easily so hedge maple bonsai forests are not uncommon.
A final style that Acer campestre is well suited to is root over rock. Their vigorous growth habit speeds the development of thick roots embedded in rock, helping to create rugged compositions.
Repotting hedge maple bonsai
It is relatively easy to cultivate a fine and well-ramified root system on a hedge maple bonsai. They tolerate repotting extremely well when performed at the correct time of year and can handle heavy root pruning and structural root removal.
Repotting should be performed in early spring, just as buds are starting to break. The aim of each repot should be to:
- Remove congested soil on the top. bottom and sides of the root ball
- Extract any rotten soil or organic/compost type soil and replace it with an aggregate mix
- Encourage the lateral growth of roots to improve the nebari
Field maple bonsai are generally not found with the symmetrical flare of Japanese maple. They are more suited to a rugged, natural aesthetic, but should still be cultivated in a shallow pot to promote lateral root growth.
Acer campestre bonsai generally need to be repotted every 2-5 years, depending on the health of the tree and the amount of fertilization that occurs. If you are struggling to water your bonsai because it can't penetrate the rootball, or if the soil has decomposed and is holding too much water, then you should think about repotting your tree the following year.
If you have a tree in early development, then heavy fertilization with a liquid organic fertilizer such as a seaweed mix can help create a lot of growth and make your tree progress quickly.
With more established trees, you will usually find that field maples need less fertilization than other deciduous bonsai given they are such strong growers. If you fertilize a field maple in refinement too much, you risk creating very coarse and elongated growth that can ruin any progress you've made in creating a refined tree.
Pests and diseases
Field maple bonsai can be affected by many pests and diseases, but will generally tolerate them well.
They can be infected by aphids, mites, caterpillars, and scale, which can usually be treated with physical removal combined with horticultural oil or an insecticide if needed.
Like other broadleaf trees, Acer campestre bonsai can be affected by fungal infections. To keep these to a minimum, avoid watering the leaves and target only the rootball, as water will spread fungal infection.
Propagating field maples
Field maples can be easily propagated for bonsai. They can be grown from seed, which can be planted outdoors in autumn for growth in spring or stratified indoors in a refridgerator at other times of the year.
Softwood cuttings root very easily from hedge maples and this is a good way to propagate material.
Finally, field maples can be airlayered to create material from branches of larger trees or to improve the nebari of your bonsai.