We get a lot of questions on fertilizing bonsai trees. In the newsletters we mention on certain months what you should be fertilizing, we thought a more specific account on what and how much fertilizer to use would be helpful.
Fertilizers come in organic and inorganic forms, all have the same three main nutrients, those are the three numbers you see on the front of the package. The first number is nitrogen N, which goes mainly to vegetative growth, second number is phosphorus P and promotes flowers and root growth, third number is potassium K and is for root growth and helps the tree to overcome stress. Also contained in most fertilizers are secondary nutrients, micro nutrients and minors, all are needed. There are pros and cons in using only organic or inorganic. Organic fertilizers tend to break down and may eventually clog the soil. Inorganic fertilizers are bound by salt and may result in a salt build up in the soil. Best to use some of both to assure your trees are getting all the nutrients they require.
The two most common inorganic fertilizers used are: granular time release, such as Osmocote and water soluble fertilizer. Granular time release has a temperature soluble covering that releases fertilizer in proportion to the ambient temperature and gradually feeds the tree every time you water. Water soluble fertilizers provide a more immediate dose of nutrients. The nutrients do linger in the soil but are quickly flushed out with each watering.
Most organic fertilizers used are: cottonseed meal, bone meal, blood meal, fish emulsion and composted chicken manure. Generally these are used more for specific species at specific times of the year. They are mainly nitrogen and are good for promoting growth.
How much to fertilize is based on certain criteria, what stage of growth your tree is in. A younger tree needing to grow out will require much more fertilizer then a more finished tree in a bonsai pot. What time of year is it? Trees primarily feed throughout their active growing season, hence no need to fertilize a dormant tree. Main objective is to fertilize to maintain optimum health. Too much fertilizer will result in rapid growth, long internodes and large leaves. Too little will result in an unhealthy tree. Application rates should be full strength as per instructions listed for houseplants. Root feeding in general is best, foliar feeding, acts immediately and promotes more active leaf and root growth. If a tree is in need of nutrients quickly, then foliar feed.
Our fertilization program is on a much bigger scale, but it should still work for you, some tailoring for specific species may be needed. We start fertilizing all the tropicals and cold hardy evergreens in April with Osmocote 16-9-12 as soon as they start flushing out with new spring growth. Our deciduous trees we wait until late April to early May, when their spring leaves have hardened off, reached full size and are dark green. Mid-month May we will fertilize all the potted bonsai with a water soluble 20-20-20, all of our grow out trees we fertilize mid-monthly with composted chicken manure and water soluble 20-20-20. Mid-month fertilizing of nitrogen will help trees handle the heat better. Some species are heavy feeders and will get chlorotic if not fertilized enough, fertilize them mid-month with Ironite. Starting in June we mid-month fertilize all the potted bonsai with alternating composted chicken manure and fish emulsion, any that seem to be stressing a little we will fertilize with water soluble 20-20-20. We will continue this program until October, then we will cut back on feeding the tropicals and cold hardy evergreens to start them hardening for winter. November we fertilize the deciduous trees with potassium to promote better root nutrient storage. Over winter the Osmocote has diminished and we fertilize the tropicals monthly with water soluble 20-20-20, we donít want to promote growth, only maintain a healthy state over winter. It would be a good idea to keep a fertilization schedule. Keep up with the monthly newsletters and they will help also.