Since bonsai is a set of techniques, rather than an individual species, two questions I commonly get asked are "Which species of trees work best for bonsai?" and "Can any tree be a bonsai?"
The short answer to the second question is yes, any tree can be turned into a bonsai. Actually, any plant with a permanent wood-like structure, like a cactus or shrub, is a candidate for bonsai. The full answer is longer, as it includes a few if-then's and maybe's and relates to the answer to the first question.
You can determine the suitability of a species for bonsai by answering the following questions:
- Does the plant grow well when the roots are confined to a limited space, like in a container?
- Does the plant tolerate the process of root pruning?
- Does the plant have trunk, stem, foliar, floral, and/or fruiting characteristics that you find desirable aesthetically?
- Does the structure of the plant reduce in size and react positively when pruned or otherwise trained?
- Is the plant suitable to your local climate? (indoor or outdoor)
A Good Example
First, I'll answer these questions for the popular Japanese Grey Bark Elm, Zelkova serrata. It grows well even in tiny containers; the roots will sometimes even push the tree up out of the container to make more room during the growing season. Zelkova are highly tolerant of root work, with up to 80% of the roots being pruned in some repottings without adverse effects. The natural characteristics that many people find desirable are small dark green leaves, smooth bark, wonderful fall foliage, and beautiful winter silhouette. The characteristic that is most-loved is the reduction in size of the twigs to get incredibly small, and form a dense crown of twigs under good training. Zelkova are not only aesthetically desirable, but also generally easy to grow, which makes it very suitable species for bonsai.
A Difficult Example
Due to the iconic form of old trees in the wild, Oaks capture the imagination of many people looking to create bonsai. The Valley Oak, Quercus lobata, is a deciduous tree native to California with rough bark. It grows quickly, reacts well to pruning and training, and otherwise satisfies the five key questions listed above. However, because even with careful training the leaves only reduce to 2-3" in size, and are more commonly 3-5" in size, Valley Oaks are a more difficult subject for bonsai. The leaves will never be in proper proportion to create a good image of a tree for small size bonsai. The tree pictured here is about two feet tall and wide; at this size the leaf characteristics are more suitable. Only if you're interested in developing and maintaining a larger bonsai will Valley Oaks be a good bonsai for you.
Suitability versus Bonsai Tradition and Availability
Just like exotic plant collectors are still discovering new species of flowers in the mountains of northern Myanmar, bonsai growers are still exploring and discovering which species work for bonsai. In some cases the use of species has been driven more by tradition, familiarity, and availability than it has by suitability. Starting with the established care for commonly used Asian species, bonsai growers in Europe and North America have identified similar species that are native and also suitable.
North American Species
Ulmus minor - field elm
Pinus sylvestris, Pinus Nigra
Suitability to Your Climate
Different species of trees will thrive across different climates. If you live in California you may find it difficult to grow Japanese maples due to the long dry summers, while a friend in New York may find them to be a highly desirable species due to their suitability to that climate. Your success with bonsai may differ greatly across different climates - this includes indoor and outdoor environments.
When in Doubt, Give it a Try
If you're unsure about a particular species, the best way to find out is to test the species. Obtain 5-20 seedlings, rooted cuttings, or otherwise similar plants, treat them all as a batch and use bonsai techniques to determine if they will do what you want.
Positive responses are growth and normal cyclical patterns in the habit of the tree. If a Zelkova is leafing out in early spring it is reacting to the local conditions appropriately. If it is going dormant in July, most likely you have a summer that is too cold or another problem. Watch the growth of your plants carefully and evaluate the quality and timing.
If your tree reacts positively and you like the way it looks, then you've found a suitable species for bonsai!