Robots and Bonsai

Gardening Robots

Gardening robots have appeared from time to time in science fiction, and there are already (as of this writing in 1996) grass cutting robots. The gardening application is a fairly natural one for robot technology. The robot gardener doesn't need to move quickly, so it doesn't need to have a high power level. Taking time pressure off task planning greatly reduces programming difficulty and system costs. Being an outdoor robot, it needs to be weather-proofed, but it has access to solar power in the daytime. At night, it just rests and plans the next day's activities using battery power.

Gardening Tasks

Gardening tasks include watering, fertilizing, cutting, and cleaning. Watering is easy to automate using "fixed" (non-robotic) automation, such as with recessed or overhead sprinkling systems. Fertilizing can be done by adding soluable fertilizer to the water (there are automatic systems that can do this) or by sprinkling solid fertilizer pellets. Robotic gardening tasks also include moving and repotting potted plants.

It's the cutting, cleaning, moving, and repotting tasks that form a reasonable agenda for robot programming. The cutting task includes any clipping, trimming, or pruning of living or dead plant material. Cleaning tasks include sweeping and picking up trash and dead leaves or other improperly placed plant material. It might also be reasonable to task a robot with capturing and destroying snails, slugs, and other slow-moving pests. Local application of insecticide to take care of scale or aphid infestations might also be within the range of robot capabilities.

Bonsai Tasks

Bonsai tasks can be considered a special case of the set of gardening tasks. However, the practice of bonsai is also an art form so the focus is on the routine maintenance aspects of bonsai plus those artistic actions that can be adequately defined by rules.

Let's assume that the bonsaist robot will not be asked to create bonsai from raw materials, but will merely do routine maintenance. Maintaining a bonsai includes unwiring (unwrapping stabilizing wire from the branches), pruning, rewiring, and repotting. Repotting is not done very often (every 3 to 5 years, generally) and selecting the right pot requires artistic judgment, so we can restrict the task list to just wiring tasks and pruning tasks.


Remove the thinnest wires first, alternately unwrapping each end of a wire and meeting in the middle. Continue with the heavier wires until all are removed from the tree.


"Clean" the bottoms of the branches: remove anything hanging straight down from a branch. Cut the tips of the secondary branches, starting at the tip of the primary branch, so that each secondary branch closer to the trunk is longer than the one outboard of it. Proportionally cut the tips of the tertiary branches.


Fully wire the tree (not including the trunk), starting with the first (lowest) branch. Select a wire size approximately 1/3 the diameter of the base of the branch being wired. Cut the wire to a length sufficient to wire the current branch, the length of trunk to the next branch (usually skipping a branch), and the next branch. Repeat until all primary branches are wired. Bend branches at base as necessary to achieve a uniform vertical angle for all branches. Wire all secondary and tertiary branches.

Crossing wires is not allowed (for both aesthetic and practical reasons). Any tree can be fully wired without crossing any wires (there are always at least two solutions to this problem). Proof.


A robot that just sits in one place and has bonsai brought to it by a human won't be much good. A robot that can move around and gather the materials it needs will also serve well as a general gardener. Hence, mobility is essential to a bonsaist robot. It's reasonable to expect a robot to complete a set of bonsai tasks for a single tree in a day. To retrieve the bonsai specimen, collect the tools it needs, and to replace the finished bonsai on its garden shelf might cover 200 feet of ground, in a typical residential situation. If we assume that we want the robot to be working on bonsai (not just moving and carrying) at least 4/5 of the time, and if we further assume that there are 10 good daylight hours in the day, then the robot needs an ambulatory speed of 100 feet per hour (= 1.67 ft/min). I suggest a six legged robot for quasi-static stability in walking.

Sensing and Control

The robot will share tools with the human owner so it must be able to find and recognize them. It must also be able to find the trees it needs to work on. Hence, a sophisticated vision system will be required. Force feedback control will also be necessary, particularly in the manipulators, so proper amounts of force can be applied when cutting and bending branches. Four manipulators, two heavy, and two for fine work, will provide capability for most gardening tasks. The robot should be built low enough that it won't easily be knocked over and so it can also reach the ground easily to pick up trash or dropped items.


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This page created May 31, 1996, last updated October 7, 2000.