Most Coffee leaves are massive, compound leaves which will destroy the visual effect. Arabica cultivars are the most commonly found varieties of coffee trees and do not make for ideal Bonsai. Varieties such as American Coffee Berry produce smaller leaves and are better suited. California coffee berry and several closely related cultivars such as Hoary coffee berry.
Hawaiian Coffee also known as Kona Coffee plants also make good subjects for Bonsai. Kentucky coffee tree is sometimes used but it is not a true coffee plant and produces toxic fruit, although the beans within the fruit are roasted and pass for coffee.
New Plants can be started from a softwood cutting . Softwood is green, flexible fibrous parts of the plan from a new section that has not hardened. Softwood cuttings are best taken in late spring or early summer as the new growth approaches maturity. See - New plants from Cuttings
Starting Coffee Plants from Seed
Many coffee plants are harder to locate than are their seeds, the coffee bean. If you are unable to locate the plant you want, starting from seed is a viable option.
Ideally, you should be starting with a ripe coffee fruit, which unfortunately is unobtainable for many. The germination rate is substantially lower with green coffee beans, worse assuming they have been hulled, and impossible if they've been roasted.
Fresh green seeds are easily obtained online. Be advised however, that coffee seeds are notorious for inconsistent seed germination, so plant more than you will need. They are suitable for planting year round indoors. Planting outdoors in semi, sub and tropical regions as well. In cooler regions the best time to sow them outdoors is spring to mid summer.
Germination is tediously slow, at best 2 months from good seed under optimal conditions. They have also been know to take 4 to 6 months to germinate from older seeds. Soaking the seeds in luke warm water for a day will help soften the shell a tad and slightly expedite germination.
Once you have germination and the plants have developed beyond their initial embryonic leaves [Cotyledon] they should be placed in a porous high nitrogen acidic soil. Soil pH below 7.0 is essential, a pH in the 6.0 to 6.5 range is optimal. See Tracking and Adjusting Soil pH.
Coffee plants need warmth not heat, just warmth. They survive temperatures below 60 F , but will not produce. Temperatures above 75 F will inhibit photosynthesis.
Water and Humidity
Humidity is another condition that coffee trees need to thrive. You can use a humidifier, or a simple humidity tray. Trays under potted plants catch the runoff water , fill a tray with pebbles and leave the runoff water there. As the water evaporates, it increases the humidity level.
During the spring and summer, water generously. Water the planter, allow the water to run off and then water again. Self Watering Containers when properly maintained are suitable. The soil should be consistently moist, not saturated just moist. Curtail the watering come winter, every other day is best.
Coffee fruit [cherries] will not appear for the first 2 - 5 years.
Coffee trees are self fertile so you will only need one tree for pollination, however it is always best to have several for genetic diversity, particularly if you plan on seed saving. To increase production and ensure adequate pollination, whether from multiple plants or a single specimen, hand pollination is advisable. See - Hand Pollination
Mature coffee plants easily reach 6 feet in height. This is bonsai we are talking about, so pruning training and more pruning will be needed to maintain the miniature nature you desire.
When training, if you decide to use wiring to attain a desired shape it should be done while the tree is relatively young. Once the stem matures it becomes woody, hard and at times brittle. It can easily snap under the bending stress. Pruning also becomes a tad more bothersome on a mature tree for the same reason.
Indoor Fruit Trees Coffee Trees Fruit Bearing Bonsai
Grow Lights Unusual Fruits and Berries Container Gardens