How do I fertilize my bonsai tree?

Bonsai trees are grown in limited amounts of soil and often that soil may consist more of inorganic well-draining ingredients which will retain less nutrients for the tree to use for growth. This means adding the correct fertilizer, at the right time, and in the right amounts is important for healthy growth of your bonsai tree.

Conceptual rendering of fertilizing a bonsai with the tea bag method — You can add granular fertilizer to a tea bag and use a toothpick to keep it in place on the soil

Recommended bonsai fertilizer

Bonsai tree fertilizer should contain nitrogen, phosphoric acid, and potash often referred to as NPK on the label. Ones containing chelated iron are especially beneficial. A typical label ratio for NPK would read like 15-9-12. Both organic and inorganic fertilizer can meet that need. They both come in liquid and granule forms.

Recommended fertilizer list

Biogold - designed for bonsai application specifically
Osmocote Plus - inorganic granules
Superthrive - liquid organic, includes fish emulsion
Worm castings - worm 💩
Worm tea

Organic fertilizers tend to work better when temperatures are warmer and are less likely to burn your treee. Inorganic fertilizers are less affected by temperature and are less likely to smell. I often alternate depending on season, e.g. organic in summer inorganic in cooler months. This also helps to ensure your tree gets a varied mix of nutrition if one is lacking some sort of micronutrient.

How to fertilize your bonsai

It's recommended to use half the application amounts shown on the fertilizer instruction label. Make sure the bonsai tree isn't drought-stressed before adding fertilizer and water before applying the fertilizer. If your tree is having trouble with the heat it may be a good idea to skip the fertilizer--especially if synthetic to prevent burn.

With liquid fertilizer you simply dilute the fertilizer following the instructions on the label (use 1/2 the recommended rate) into water and soak it into the roots of the tree. This may need to be done more frequently depending on the soil your bonsai is potted in. Ones with more inorganic components like rocks won't retain the fertilizer as long and will drian out with waterings.

With slow release granules you apply the fertilizer to the top of the bonsai soil at half the rate of the label directions.

Tea bag method

Pelletized granule fertilizer are convenient because they can often last 6 months while slowly releasing nutrients versus liquid that needs to be applied more often. The downside is that it tends to slide or wash off the surface of the bonsai soil. In addition, the granules take away from some of the aesthetic. The tea bag method is a way of holding granule or powdered fertilizers in place and also allows for easy removal for exhibition.

Fill tea bags or fertilizer cups with fertilizer powder or granules
Secure tea bag to bonsai soil with toothpick

This is especially helpful for larger pellet fertilizers like Biogold since they take longer to break down. Fertilizer cups may work better than tea bags if birds are trying to eat your fertilizer.

When to fertilize your bonsai

In general, you should fertilize your bonsai during active growing seasons and before dormancy. This usually means toward the later part of spring in the April-May months and later toward fall before the trees go dormant so they can replenish after the growing season before entering dormancy.