Bonsai at the National Bonsai & Penjing Museum at the United States National Arboretum

By 1940, about 300 bonsai dealers worked in Tokyo. Some 150 species of trees were being cultivated, and thousands of specimens were shipped annually to Europe and America. The first bonsai nurseries and clubs in the Americas were started by first and second-generation Japanese immigrants. Though this progress to international markets and enthusiasts was interrupted by the war, by the 1940s bonsai had become an art form of international interest and involvement.

Sample of a Pomegranate trained as bonsai, and shown after a late spring partial defoliation. This specimen was collected in the wild in California, the tree is estimated to be 125 to 175 years old. Note the sections of trunk deadwood which give character and reflect the old age of the tree.

All bonsai start with a specimen of source material, a plant that the grower wishes to train into bonsai form. Bonsai practice is an unusual form of plant cultivation in that growth from seeds is rarely used to obtain source material. To display the characteristic aged appearance of a bonsai within a reasonable time, the source plant is often mature or at least partially grown when the bonsai creator begins work. Sources of bonsai material include:

This juniper makes extensive use of both jin (deadwood branches) and shari (trunk deadwood).

The practice of bonsai development incorporates a number of techniques either unique to bonsai or, if used in other forms of cultivation, applied in unusual ways that are particularly suitable to the bonsai domain. These techniques include:

Over centuries of practice, the Japanese bonsai aesthetic has encoded some important techniques and design guidelines. Like the aesthetic rules that govern, for example, Western common practice period music, bonsai's guidelines help practitioners work within an established tradition with some assurance of success. Simply following the guidelines alone will not guarantee a successful result. Nevertheless, these design rules can rarely be broken without reducing the impact of the bonsai specimen. Some key principles in bonsai aesthetics include:

Bonsai displayed on an outdoor bench - note automated watering apparatus

A variety of informal containers may house the bonsai during its development, and even trees that have been formally planted in a bonsai pot may be returned to growing boxes from time to time. A large growing box can house several bonsai and provide a great volume of soil per tree to encourage root growth. A training box will have a single specimen, and a smaller volume of soil that helps condition the bonsai to the eventual size and shape of the formal bonsai container. There are no aesthetic guidelines for these development containers, and they may be of any material, size, and shape that suit the grower.

Completed trees are grown in formal bonsai containers. These containers are usually ceramic pots, which come in a variety of shapes and colors and may be glazed or unglazed. Unlike many common plant containers, bonsai pots have drainage holes in the bottom surface to complement fast-draining bonsai soil, allowing excess water to escape the pot. Growers cover the holes with a screening to prevent soil from falling out and to hinder pests from entering the pots from below. Pots usually have vertical sides, so that the tree's root mass can easily be removed for inspection, pruning, and replanting, although this is a practical consideration and other container shapes are acceptable.

This is a list of some notable bonsai artists. It is by no means exhaustive.

Japanese bonsai exhibitions and catalogs frequently refer to the size of individual bonsai specimens by assigning them to size classes (see table below). Not all sources agree on the exact sizes or names for these size ranges, but the concept of the ranges is well-established and useful to both the cultivation and the aesthetic understanding of the trees. A photograph of a bonsai may not give the viewer an accurate impression of the tree's real size, so printed documents may complement a photograph by naming the bonsai's size class. The size class implies the height and weight of the tree in its container.