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How To Fertilize Bonsai Trees (Deep Dive)

How To Fertilize Bonsai Trees (Deep Dive)
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Fertilization is a really important aspect of bonsai but is something that is often overlooked. Appropriate fertilization brings health and vigor to your bonsai, but can also stimulate coarse growth that can ruin a nicely refined tree.

Knowing how to fertilize bonsai trees correctly is an essential skill for those hoping to level up their bonsai game. Read on while we dig deep into bonsai fertilization.

Do bonsai trees need fertilizer?

When people fertilize bonsai trees or house plants, they often say they are 'feeding' them. This is one of the first myths I want to dispel because it's vital to understand what fertilizer is to know how to use it well.

Fertilizer is not food for plants. Your bonsai trees will generate their energy stores via photosynthesis, where they convert carbon dioxide and water into sugars. They do this without any help from fertilizer and will continue to do so quite happily on their own.

Understanding this tells us that trees do not need fertilizer to survive, as they can generate their own food. Bonsai trees in your collection will likely be absolutely fine if they are never fertilized (and indeed for highly refined trees this is often a desired approach).

While bonsai trees do not require fertilizers as a food source, they may sometimes be lacking in certain trace elements due to the confined nature of their rootball. Without the natural decay of other plants and animals, combined with regular watering that can wash away valuable salts, the soil in your bonsai container may be lacking in certain nutrients the tree needs for healthy growth and fighting diseases.

This is where fertilizer for refined bonsai helps, as it replaces these nutrients in your soil system.

When it comes to developing younger pieces of material fertilizer acts as a growth stimulant. It is not required, but it will help to speed up root growth, trunk and branch thickening as well as help with back-budding in many species.

Components of fertilizer

Although there are different options to choose from when selecting a bonsai fertilizer, at their core almost all fertilizers are based on differing proportions of three main elements: Nitrogen (N), Phosphorus (P), and Potassium (K). This is known as the NPK ratio.

As well as these three main components, bonsai fertilizer also contains differing concentrations of micronutrients such as:

  • Zinc
  • Iron
  • Copper
  • Boron
  • Manganese
  • Molybdenum

These different elements can stimulate different types of growth on your tree, so many bonsai professionals will use fertilizers with different NPK ratios at different times of the year or with trees at different stages of development.

Before we look at each of these elements and how they impact bonsai growth, I want to make it clear that for most hobbyists the best option is usually to pick a balanced NPK ratio (such as a 6:6:6) and use this year-round. This will give you balanced growth and healthy trees and is overall far easier to manage than switching fertilizers based on the time of year.

With that said, here is a breakdown of each of the main components of bonsai fertilizer and how they can help develop your trees.

NitrogenIncreases the growth of green stems and leaves or 'above ground' growth.
PhosphorusContributes to healthy root growth. Stimulates the growth of flowers and fruits.
PotassiumStimulates faster plant growth and increases disease resistance.

Types of fertilizer for bonsai

There are a wide variety of fertilizers available that you can use on your bonsai, and it can sometimes be daunting to know which to use and why. There are liquid fertilizers, solid fertilizers, organic or synthetic fertilizers, as well as other treatments that fall loosely into this same category.

There are pros and cons to each of these fertilizer types, and the best option will depend on your trees as well as your budget.

Liquid fertilizer

Liquid fertilizers are soluble solutions that are usually diluted in water before being applied to your plants. The nutrition in liquid fertilizer is freely available to your bonsai and will be rapidly absorbed as it does not need to be broken down.

The fertilizer binds loosely to the soil and will be quickly washed away with watering and rain. This means more regular applications are needed with liquid fertilizer.

Solid fertilizer

Solid fertilizers are applied to the top of your soil and slowly release fertilizer over weeks or months. When dealing with organic fertilizer, this process is aided by healthy fungi and bacteria in the soil that also benefit the health of your trees.

Solid fertilizer is often easier to use than liquid when you are tailoring the fertilizer dose to the development stage of the tree (a practice I would highly recommend). You can apply varying weights of fertilizer to each tree depending on whether it is in early development or refinement, and this only needs to be done at the most once every 4 weeks.

Synthetic fertilizer

Synthetic fertilizers are man-made chemicals that contain the elements in very simple forms, that can very easily be taken up by your tree.

These fertilizers can produce extremely vigorous growth but have the disadvantage of decreasing the presence of helpful microorganisms in the rootball and can potentially burn roots if over-applied.

Synthetic fertilizers can be useful for trees in very early development, particularly those growing in the ground. They are much cheaper than organics and will result in very fast growth. I wouldn't recommend their use in more refined bonsai, as they can detriment the health of a root system in a bonsai pot and cause excessive thickening in an established tree.

Solid synthetic bonsai fertilizer

Organic fertilizer

Organic fertilizers are made from natural sources such as seaweed, rapeseed, or manure. They generally contain a wider balance of micronutrients when compared to synthetic fertilizers and will have a greater impact on the health of your trees.

As mentioned above, solid organic fertilizers are broken down with the help of the ecosystem in the bonsai pot, and their use can be a big boost to the health of your tree.

Unfortunately, solid organic fertilizers are of particular interest to animals such as dogs or squirrels. You may find wildlife steals your fertilizer from time to time, but to me, that's all part of the fun of bonsai!

How often to fertilize bonsai trees

How often you apply fertilizer to your bonsai trees should depend on the stage of growth they are at. Trees in early development should be fertilized heavily to stimulate vigorous growth, while refined trees should be fertilized only to supplement nutrients and maintain health.

If you are not interested in tailoring your fertilization to each tree, then a twice-yearly application of solid organic fertilizer can be used to maintain balanced growth in your trees. The first application should be in late spring after the first flush of growth has hardened off. The second should be in July or August, depending on the heat of your summers (fertilizing in extreme heat should be avoided).

It's worth noting that for all trees, you should only be fertilizing during the growing season. Fertilizing in winter will not stimulate growth and will just be a waste of money.

Fertilizing bonsai in early development

Bonsai trees in early development should be fertilized regularly to stimulate the growth of both green growth and roots. The approach is pretty much the same for both deciduous and coniferous trees.

If you are using solid organic fertilizer, it should be applied every 4-6 weeks, depending on how quickly the fertilizer breaks down on top of the soil. If the soil surface is still covered with old fertilizer then there is no need to apply more.

For those using liquid fertilizers, then applications every 10-14 days should stimulate strong growth. Be sure to read the label of your chosen fertilizer.

Secondary-stage bonsai fertilization

If your tree's trunk is thick enough then you will need to continue building out secondary branches.

For deciduous trees, this usually means partial defoliation throughout the growing season. For conifers, we need to prune to balance strength, holding back stronger areas and encouraging the growth of weaker areas.

For both of these things, we still need to drive growth, although not as much as we did in primary development.

We start fertilizing in late spring after the first flush of growth of hardens. This is to avoid creating long internodes in deciduous trees or excessively long needles in conifers.

When using solid fertilizer, it should be applied every 8-12 weeks. Liquid fertilizers can be applied every month or 6 weeks.

Refined bonsai fertilizer

Fertilizing bonsai trees in refinement

When you have a bonsai ready for refinement, it's time to start holding back on fertilization. Excessive fertilization at this stage of development will result in coarse growth and long internodes, which isn't what we want when refining a tree.

With deciduous trees, how much we fertilize depends on the techniques we are using and how the tree responds. It's common to partially defoliation on alternate years to continue the development of ramification. (See details on the partial defoliation techinique in our Birch bonsai care guide).

In the years you are defoliating you will need to fertilize a bit more to encourage growth after the pruning.

If you are not defoliating a deciduous bonsai, then it will probably require very little fertilization. When using solid organic fertilizer, a dose after growth has hardened in late spring and perhaps a second in midsummer may be all that is required. If your tree is developing excessively long internodes even with this level of fertilization then don't be afraid to fertilize only once a year or every other year.

If you are using a liquid fertilizer then you may only need to apply it every 3 months or so, increasing or decreasing the frequency based on the response of your tree.

Conifers in refinement can be fertilized with solids two or three times in the year to maintain health and improve foliage quality. Certain species, such as Japanese Black Pines, can be fertilized in specific ways to encourage multiple flushes of growth to increase ramification (although that's a topic for an entire blog of its own).

What is the best fertilizer for bonsai trees?

In my opinion, the best fertilizer for bonsai trees is a solid organic fertilizer with a balanced NPK ratio. Here are my reasons:

  • They are easier to apply than liquid fertilizer and save you time if you have lots of trees
  • They improve the health of your bonsai by encouraging microbial growth in the rootball
  • They provide a wide range of micronutrients that help your tree grow and maintain a healthy appearance

The gold standard that is widely used in Japan and elsewhere in the world is Biogold. This is a solid organic fertilizer that provides a great balance of nutrients to your trees. It breaks down naturally and encourages the growth of healthy microorganisms in the soil, so it's a perfect option for those who want a simple but very effective fertilization regime.

Where to buy bonsai fertilizer

Synthetic fertilizers are widely available from garden centers, nurseries, or online. When using liquid synthetic fertilizer to encourage rapid growth in early-stage material, any main brand fertilizer will do the job. I tend to opt for Miracle-Gro as a proven and effective brand. Be sure to follow the labeling when using synthetic fertilizers as too much can do more harm than good.

Organic liquid fertilizers are also available in most major retailers and I have tried a few brands which have all produced similar results. This liquid seaweed based fertillizer is now my go-to thanks to its low price and good results.

Finally, when sourcing solid organic fertilizer it can often be more helpful to go directly to a bonsai specialist, either online or in-store. Biogold is available on amazon, but with these more specialized fertilizers, it can help to get input from a known vendor.

When to fertilize bonsai after repotting

When you have repotted a bonsai there can be a real temptation to start fertilizing straight away to help the tree build up new roots. Unfortunately, this can potentially harm the tree and care must be taken with newly repotted material.

During the repotting process, roots are torn and cut. The resulting wounds in the root system need time to heal and produce callus. Good aftercare is critical here to help encourage roots to heal and grow rather than die and rot.

Strict and accurate watering makes a big difference when helping roots to recover from a repot, but withholding fertilizer also plays a role.

Fertilizers contain a high concentration of salts which can be toxic to exposed wounds in roots or freshly growing root tips. So-called fertilizer burn can be very damaging to a bonsai that relies on its root system for survival, so for that reason, it is strongly advised you don't apply any fertilizer in the first 2-3 months after repotting a bonsai. Once the tree has overcome the repot and is actively growing, you can then think about slowly introducing fertilizer to meet the stage of development the tree is in.

Remember this important point - a tree will never die from not being fertilized, but a newly repotted bonsai can die as a result of premature fertilization.

Do you fertilize bonsai seeds?

Although they can take decades to become established bonsai, planting seeds from bonsai seed kits is a common route into the hobby for many beginners. How to care for bonsai seedlings and whether to fertilize them is a common area for questions.

Tree seeds contain a store of resources to help the seed grow and establish its first roots and leaves. During this period, the seedling will be extremely sensitive and vulnerable to its surroundings. The seedling does not need fertilizer to establish itself as a little plant, and I would strongly advise against fertilizing a new seedling.

Once it is around 6 months old you can consider adding some fertilizer to help drive its growth and development.

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