Boxwood have something of a poor image as bonsai, mainly due to the proliferation of poorly designed trees being styled with pom-pom foliage. Boxwood have a naturally multi-stemmed growth habit that lends them to being styled like large spreading park trees and it is with this style that they look their best when grown as bonsai.
The two Buxus species that are commonly seen as bonsai are Buxus microphylla/Japanese Box and Buxus sempervirens/Common Box. Outwardly both of these species are very similar. The main difference between the two species is their vigour; Japanese Box being less vigorous and slower growing than Common Box.
Boxwood have many good characteristics that are useful for bonsai. Dark green leaves that reduce well, naturally short internodes and they can take hard pruning that prompts prolific back budding. Boxwood also have shallow, fibrous root systems that often produce powerful surface roots and nebari.
The bark of Boxwood looks mature at an early age but is thin and easily damaged so care must be taken when heavy pruning or wiring. As the bark becomes dirty very easily, accumulated dirt and algae can be cleaned using water and an old toothbrush.
Well-fed Boxwood are fast growers but are very slow to thicken. It is said that field grown Boxwood can have trunks of as little 3" after 20 or even 30 years. For this reason it is important to source older stock to use for bonsai. Old hedges and garden material are an excellent source of suitable material. Collect Boxwood in March and April removing all ground soil (bare-rooting). If in any doubt as to the vigour of a collected tree, one half of the rootsystem can be bare-rooted in the first year, the other half in the following year.
Boxwood can be airlayered successfully, and are best started in April. Cuttings can be taken from Autumn to early Spring; use cuttings of at least 4"/10cm length for greater success.
Small wounds on Buxus heal well but larger wounds, particularly on older parts of the tree, are very slow to heal and are better used as deadwood features. As Buxus wood is very hard, intricate carving and jinning can be carried out without fear of deterioriation.
Boxwood are naturally understorey trees and so it is often recommended that they are provided with dappled shade to avoid yellowing of the leaves.
However in the UK, give Boxwoods plenty of direct sunlight to encourage stronger growth and denser foliage.
Boxwood are hardy to only around -4°C, during temperatures below this, extra protection should be provided. Foliage can become yellow or bronzed after frosts during the Winter but it will green up again during the Spring.
Repot every other year and keep developed bonsai slightly rootbound on occasions; more frequent repotting can result in larger leaves as the tree becomes especially vigorous.
Pruning and trimming Boxwood
Boxwoods regularly need thinning of the foliage mass to allow light into the inner branches to stop them becoming bare and to prompt backbudding. Regular pruning helps to increase ramification and reduce leafsize as well. However, it is also important to allow some free growth to ensure the overall vigour of the bonsai is maintained. Free, unrestricted extension of the first flush of growth can be allowed in Spring (around April/May depending on your climate) to strengthen the tree, followed by strict pinching and pruning for the rest of the year to refine the foliage.
Promoting Ramification in Boxwood
Healthy Boxwood respond well to partial defoliation as new buds swell through the growing season.
Remove all but the last leaf pairs on a shoot as the new buds swell at the shoot tips. New shoots will then emerge from the leaf axils at the points where you have removed the leaves. (See above)
The result of the leaf thinning is that there will be a greater number of shoots, all with smaller leaves. The foliage can be thinned from early June onwards as new buds swell and begin to extend.
Once new buds have extended but before they start to harden, wire can be used to shape new branching while they are still pliable. Once older shoots harden they are impossible to wire properly without snapping as they become very brittle. Wire the new shoots loosely (air-wiring) and check often to make sure that the wire does not cut into the soft new bark.