Bonsai, as they are just a small version of regular trees or plants, need food and water just like any other plant. Because they live in a man-made environment rather than in nature, where they would get all the nutrients and water they need through the ground and rain, their living quarters need to be kept in the right condition.
The purpose of fertilizer is to get the appropriate blend of nutrients into your tree. Fertilizer is made up of N (Nitrogen), P (Phosphorous), and (K) Potassium. The ratios are expressed as 0:10:10 or 12:6:6, in the order NPK. For outdoor bonsai, feed your plant with a higher nitrogen content in the spring, to encourage growth – a 12:6:6 or so. During summertime use a balanced fertilizer like 20:20:20 (which is the same as 10:10:10 and 5:5:5). As autumn comes you will want to begin hardening your tree off for winter with a lower nitrogen level, like 0:10:10.
Each tree is different and it may take a while to adjust to the correct fertilizer level. Signs of over-fertilizing include browning or yellowing leaves, fertilizer accumulating at the top of the soil, and roots that are going black and limp (root burn).
Under-fertilizing can cause equally concerning issues like leaves shriveling up and dieback of branches.
You may have heard of foliar feeding – this is a hotly contested topic, so do plenty of research from a variety of a sources before deciding whether foliar feeding is right for your bonsai.
Watering your bonsai can seem confusing, but the most important thing is to observe your particular plant – each species is a little different and your climate, the amount of sun, and soil mixture will all influence how quickly or slowly the water is absorbed. The other major factor in watering is drainage – bonsai must have good drainage, in fact some experts even feel that it’s impossible to over-water a tree that has a good drainage system.
When you’re first adjusting to how much water your tree needs, do not set a watering schedule. Instead check the plant every day; when the surface starts to become dry, water thoroughly. Don’t allow the soil to become completely dry. The entire root system needs to be soaked – a good rule of thumb is to water the plant until water begins draining out of the drainage system. Use a fine-hole watering can and water from above the plant. If you can collect rainwater this is even healthier for your bonsai as it does not contain the chemicals that tap water sometimes can.
Under-watering causes leaves and the tips of the branches to begin drying up. The effects of under-watering a bonsai are evident very quickly. Damage from over-watering, on the other hand, can take much longer to show itself. Leaves yellow and fall off the tree and eventually root rot sets in, which may not be discovered until repotting time in the spring.
Pay close attention to your bonsai and follow instructions closely and you will soon get a feel for the healthiest feeding and watering routine for your specific plant.