The purpose of this post is a general introduction to fertilizing your bonsai. I will focus on the use of fertilizer during the seasons of the year as well as the stage of development for your tree. I will avoid discussing specific brands or commercially available products, as each practitioner has their own opinions on this. At our monthly meetings we frequently discuss fertilization, and you will hear the merits of various products.

All Bonsai should be fertilized. The frequency and amount vary by species, age, stage of development and health of the tree. Generally, the younger and less developed a tree is, the more fertilizer is applied. As a tree becomes more refined we pay closer attention to the amount and timing of fertilization.

Since we are using a porous soil mixture, we can control the rate of application and precise timing throughout the growing season. As a general rule, fertilize less in hot weather or winter months.

For trees still in the development stage (Trying to increase the trunk diameter and branch caliper prior to ramification) and where you are not concerned about internode length or leaf size, you can begin fertilizing in early spring as the temperatures warms.

For trees in refinement (maintaining the silhouette, growing fine branching), you can begin fertilizing after the first flush of leaves have hardened off when they become somewhat darker in color and firmer. The tree has stored energy from fall fertilization and does not need new fertilizer for its first Spring flush.

Wait approximately four weeks after repotting to fertilize, as cut roots are highly susceptible to salts found in most fertilizers. New growth on a newly repotted tree is a good signal that it’s safe to fertilize. Continue fertilizing through Fall, and stop when the leaves have turned to Autumn hues. Evergreens, such as boxwoods and azaleas, can be fertilized all through the growing season.

Method of Application

There are several options – placing pre-formed cakes on the surface, loading loose organic fertilizer into tea bags or spreading it directly on the soil. This latter method is a good starting place. Keep in mind that when applied directly to the soil, as it breaks down it will percolate into the soil and eventually clog the air spaces between your soil particles.

Distribute the fertilizer evenly over the soil surface every four weeks. As a point of reference, use one tablespoon on a 10” round pot as our basis for amounts. Add more or less for larger or smaller pots. When the pot is watered the first time the fertilizer may spread out and form a thin slick over most of the soil surface. When that happens, wait until the fertilizer top layer is pretty dry, then break it up. If you do not break it up it will remain a slick each time you water.

Amount to use

If you use teabags, put one teaspoon fertilizer in each bag. Again let’s assume we will use teabags on a 10” round pot. Place three teabags evenly around the pot. For smaller pots use fewer bags; larger pots use more bags. Generally we use a toothpick or nail to keep the bags in place.

The Process

Four weeks later, leave the first bags in place and position a new bag to the left/clockwise of the first one. Four weeks later repeat. You should now have nine bags on the soil surface. At the end of that third, four-week period, remove the first bag and replace it with a new bag. Continue this procedure for the duration of the fertilizing season. The reason for this rotation is that it only takes three to four weeks for all the nitrogen to be gone from the fertilizer, but it takes twelve weeks to break down the other nutrients it contains, including phosphorus and potassium.

The importance of the Fall season

The most important time to fertilize ALL trees, regardless of species or stage of development is in the Fall. That is when they are gathering and storing resources for the upcoming winter dormancy and eventual spring flush of growth. Begin after the heat of summer is gone, about September, and continue through to the first frost or leaf drop.

Winter Fertilizing

Starting in November (depending on the weather) your trees are entering full winter dormancy. They will continue in this state until sometime in February or March, again depending on the weather and the species. During these months many of us will use a fertilizer that contains no nitrogen, 0-10-10 as an example. This continues to provide nutrition to the roots of the tree without the “growth stimulant” of nitrogen.

To learn more about this important topic for the health of your bonsai, please join us at one of our monthly meetings.