Browning foliage is a sign that your bonsai is under distress. Poor irrigation practices, lack of sunlight, fungal diseases and insect infestations are just some of the factors that can distress bonsai, which can be most any potted plant pruned and trained into an artistic shape. Although your bonsai's recovery is not guaranteed, there are steps that might bring it back to help.
Inspect your ailing bonsai closely. Look for signs of insect infestation and disease, such as foliage spots, webs, mites and wilt, as well as browning leaves.
Pinch away brown and wilted leaves from the stems. Trim away dead and damaged stems and branches with sharp pruning shears. Sterilise the shears with alcohol before making your cuts and between each cut if you suspect a fungal or bacterial disease.
Treat your infected or infested bonsai with a gentle chemical treatment. Spray the infected bonsai with a fungicidal or insecticide spray designed to treat your bonsai's symptoms. Mist the foliage thoroughly so that the entire tree and all its remaining foliage is covered with the chemical.
Allow your bonsai to acclimate itself if you recently moved it to a new location; the browning foliage could be a sign of shock. Expect deciduous bonsai trees to drop their browning leaves in the fall as they prepare for dormancy.
Check the soil moisture levels of the bonsai. Stick your finger 1 or 2 inches into the soil. Irrigate your bonsai immediately if the soil feels dry -- browning leaves could be a sign of dehydration.
Water the plant with tepid water. Irrigate your bonsai slowly and evenly across its surface so all the roots receive water. Pour the water until it begins to flow from the drainage holes.
Place your bonsai in a warm location. Select a partially shaded location that receives full, indirect sunlight to aid its recovery. Be certain to choose a location with good air circulation.
Repot root-bound bonsai trees as the browning foliage could be a sign of lack of nutrients and space. Slide the bonsai from its potting container. Run a butter knife or wooden stick between the inner pot and the roots if the tree is stuck in the container.
Crumble the excess soil gently from the root mass. Comb about one- third of the root mass from the top and bottom of the root system with a sterile root hook. Substitute the root hook with a bent fork, if needed. Work slowly to avoid damaging the root system.
Inspect the root system closely for wilt and rot. Cut away wilted, broken and damaged roots. Clip these roots back to the healthiest area of the root or all the way back to the root mass, if necessary. Avoid removing more than one-third of the root system, if possible. Discard the bonsai if more than half of the root system is mushy and accompanied by a foul odour, as this is a sign that the bonsai has succumb to root rot.
Repot the bonsai in a clean container. Choose one with several drainage holes and line each hole with wire mesh. Plant the bonsai in a well-drained, water-etentive loam. Purchase a quality bonsai soil mixture or make your own. Mix equal parts of pine bark, coarse sand and nutrient rich potting soil.