草もの - Bonsai trees are sometimes traditionally displayed in a Tokonoma, consisting of a Bonsai tree, a scroll and an accent plant (representing men, heaven and earth respectively).
Adding a scroll, or in this case an accent plant to the composition is done to accentuate the tree on display and create a sense of harmony. An accent plant (or companion planting) can be a (flowering) plant, bamboo, grass, ferns, a moss variety or even some mushrooms. It depends on the season, style of Bonsai tree and many other factors what kind and size of accent plant fits the design.
Selecting the proper accent plants
As mentioned above, the accent plant is added to a display to accentuate the season, flow and style of the Bonsai. Let's discuss these separately:
Season: the accent plant should resemble the current season. This means the color of leafs and the presence of fruits or flowers is of great importance.
Style and size: Bonsai trees that resemble a struggle to survive (like a windswept or a growing in a rock styled tree) should be accompanied by a not too luxuriantly growing accent plant. Taller styles, like the literati, can be displayed with a tall grass variety. The accent needs to resemble the story the tree tells.
Contrast: Bonsai trees with flowers or fruits should be contrasted by non-flowering accent plants, and vice-versa.
Origins: when combining several plants to create one accent plant, make sure these come from similar origins as we want to resemble nature.
Pots: companion plantings can be placed in small glazed or unglazed pots, but also on a tile or slate. In the latter case, moss should be grown around the soil to create a mature looking plant.
A successful companion planting is beautiful in its own right, but should not become the dominant part of the three-pieced Tokonoma display.
Obviously these guidelines should not work to limit your personal taste; overall accent plants are rather easy and inexpensive to grow (and the options are endless). Make sure to water regularly, as the small pots and slates do not hold much soil.