How and When to Fertilize Bonsai Trees

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We need to consider several variables to optimize fertilizer applications. Application timing must coincide with growth phases, soil temperatures, plant hardening needs, nutrient requirements, and local climates. Nutritional elements need to be specific in addressing bonsai development phases.

Timing is essential in love, life, and fertilizer applications. Wherever you live, we’ve got you covered on the fertilizer application front; you’ll have to work out the love and life parts yourself. Let’s start at the beginning and work through this essential element of growing Bonsai.

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Should You Fertilize Your Bonsai?

Trees in nature have extensive root systems to maximize water and nutrient acquisition, but this is not the case with Bonsai, which are confined to shallow pots and have compromised root systems.

The Bonsai artist’s essential role is to meet the plant’s nutritional needs.

Too much fertilizer can cause more harm than good, and a healthy approach is to focus on cultivating soil health rather than focusing on growth. The former precedes the latter.

Soil Testing

There’s an adage, “Measure what matters.” Healthy soil is the foundation for all your gardening dreams, so knowing its condition is vital. Soil tests inform you of your soil’s nutrient levels, pH, and ability to manage those nutrients – the Cation Exchange Capacity (CEC).

Soil pH

The test will tell you if your soil is sour (acidic) or sweet (alkaline) – it even tells you if lime is needed and how much. An iron deficiency is often caused by too alkaline soil – a high pH.

The test results will indicate the actual pH compared to the target range – based on soil type and what you intend to grow.

Generally, lime is added to increase the pH, and sulfur decreases the pH (increases the acidity). Lime Lifts, and Sulfur Suppresses it.

Understanding Soiltest Recommendations

The test’s recommendation will specify the quantity per thousand square feet and the NKP ratios. Let’s review the numbers:

• N – nitrogen – boosts growth, but not flower growth necessarily.
• K – phosphorus – essential for flowers, fruit, and roots.
• P – potassium – also called potash, boosts plant hardiness.

The test measures the phosphorus and potassium content – nitrogen is too volatile to measure accurately and is often the most limiting nutrient.

Nitrogen

• Promotes green, leafy growth
• Most commonly deficient
• It tends to leach into the soil, polluting surface and groundwater
• Insufficient nitrogen stunts growth and causes leaves to turn yellow – starting with the oldest leaves
• Too much nitrogen:
  • Burns plants
  • Can increase pest problems
  • Reduces flower yields
  • Reduces plant resilience to stress

Phosphorus

• Promotes root growth, flower, fruit, and seed production
• Held tightly by CEC action in soil–leaching rare
• Causes pollution when soil erodes
• It needs to be incorporated before planting – at least a month before
• Frequently fertilized soils often have too much phosphorus, which negatively affects nitrogen-fixating microorganisms
• Insufficient phosphorus causes:
  • Reduced growth
  • Plants dark green – Purple or reddish color to older leaves

Note: Phosphorus is not taken up well in cold or wet soils. Deficiency symptoms in winter are usually due to cold weather rather than a lack of nutrients in the soil

Potassium

• Increases drought tolerance and disease resistance and improves winter hardiness
• Deficiency is hard to observe, though levels are often low

The Essential Nutrient Element – Organic Matter (Compost)

Compost has several benefits only it can provide – improved soil health, microbe support, tilth improvement, pH buffering, and aggregate formation. It adds some nutrients, but its primary benefit is the soil’s improved water and nutrient-holding capacity – it enhances the efficacy of fertilizers.

Organic Fertilizer Alternatives

• Nitrogen – Blood meal is 12% nitrogen
• Phosphorus – Bonemeal is 10% phosphorus
• Potassium (Potash) – Greensand is 5% potassium and 1% phosphorus.

Cation Exchange Capacity (CEC)

Your report will provide you with a CEC measurement, the ability of your soil to hold and release water and nutrients to your plants – think of it as your soil’s nutrient magnetism.

There’s a whole science to CEC, but the answer is always organic matter – compost. Compost can boost your CEC beyond what any other substance can.

Fertilizer Timing for Bonsai Development Phases

Phase I: Growing Material

Our goal in the early stages of bonsai development should be to ensure that it grows as healthily as possible. At this point, the purpose of growth is to form a thick trunk.

The fertilizer required at this point should be balanced, although it may favor nitrogen because it is the key component for vegetative development.

It is important to ensure the plant receives a consistent supply; thus, slow-release fertilizer is perfect.

Phase II: Bonsai Branch Structure Development

Since the first stage of growth is somewhat frenzied, there is a chance that coarse branches will sprout that are out of proportion to the trunk.

Controlling branch development is the goal, and allowing branch development on trees with unusually robust trunks while still in the field will hasten the process.

Phase III: Ramification

After developing a basic branch structure, you should develop secondary branches to create volume in the foliage canopy or pads. Feeding control is essential in the ramification phase.

Excessive feeding will result in very long internodes, which is undesirable as it will limit the number of fine twigs you can grow or support in the region or zone of the outline or silhouette of the canopy.

Too little fertilizer, however, will weaken the tree’s ability to support defoliation, a key technique needed when developing ramifications.

Final Phase: Refinement and Maintenance

It would help if you aimed for limited growth and maximum health in this final and extended phase, ensuring your tree is resilient against diseases and insect attacks.

Incorrect fertilizing can lead to branch die-back – where small twigs stop growing, particularly in spring. While fertilizer may be the cause, inadequate growing medium could also be the culprit.

A Season Bonsai Feeding Guide

Spring

Because everything grows in the spring, any nutrients in the growing media need replenishment. All young, developing trees, especially pines, should be fed from the start of the season to help the decandling process. (which involves the complete removal of given spring shoots)

However, it is best to wait a few weeks until the spring leaves harden off before fertilizing more developed or refined trees.

Summer

The heat in the summer might cause your trees’ growth to either slow down significantly or cease altogether, depending on where you are located and if your climate is tropical or not.

You can resume when the temperature falls once more. Your trees typically respond with another growth spurt, though it won’t be as strong as in the spring.

Autumn

It’s crucial to feed your Bonsai now, pre-dormancy since it guarantees your trees will accumulate a supply of nutrients to boost development the following spring.

If you have a Japanese white pine, you probably already know that, with a few exceptions, fertilization is only done in the fall as this is the way to control needle length (as these trees are not subjected to the same decandling techniques which double flush, two-needle pines are).

Some incorrect advice, to the effect that you should not stimulate development in your trees when they should be shutting down, is being marketed as common wisdom.

This advice fails to acknowledge that all plants require all three essential building blocks (NKP) for regular cell functions.

Thus, utilizing fertilizer that eliminates or provides nitrogen, phosphorus, or potassium at nearly minute levels will limit the plant’s ability to function properly.

This does not refer to foliage growth only but all plant functions. Instead, whenever fertilizing, use a modest, balanced fertilizer.

Winter

Trees enter a dormant state during the winter, with minimal or no development. Again, this will depend on your local climate, but if your trees go through a dormant season, fertilizer will not be necessary and will just be squandered if you do.

Another reason to avoid fertilizing during these times is minimal absorption via the fine root hairs in frigid climates.

Bonsai Fertilizing by Tree Species

The feeding quota varies according to plant species; just as a 7-foot grid-iron player needs more food than I do, some tree species need more nutrients than others.

Generally, this is the case with anything which thrives. Species like the Australian Brush Cherry or Syzygium and the Wisteria vine come to mind, but many others exist.

These plants have exceptionally rapid growth, so they can quickly exhaust the nutrients in their immediate surroundings, and this depletion will occur more rapidly in a small area.

Some plant care instructions mention that a particular species is a gross feeder or comparable; however, this is not always true.

So, in general, if a tree is frequently utilized as a hedge, like the previously mentioned Brush Cherry, privet, kei apple, firethorn, and others, it is likely a species that requires a lot of food.

What Can Go Wrong When You Feed Your Bonsai Too Close to Winter?

Plants need resilience to endure the winter freeze. Part of that resilience comes from having a healthy, established plant at the end of the growing season.

Fresh growth taxes the plant’s energy, even if supported by fertilizer. The stress of the growing phase makes the plant more susceptible to diseases and attractive to pests. Maturity has advantages, and boosting growth too close to the winter months could cause damage from which the plant cannot recover.

To avoid this, don’t add nutrients to your bonsai from 21-days before the first frost.

Annual Fertilizer Schedule For Bonsai

Your bonsai plants don’t need a lot of nutrients. Fertilizing every second year is enough, especially if the bonsai plants are well established.

The best fertilizer is organic, and the nutrients in compost are enough for an annual feed.

If you wish to boost your Bonsai leaves for a brighter display, do this about 42 days after the last frost when the plants have had an opportunity to acclimatize to the warmer weather and the second set of leaves has emerged.

If you’re repotting your Bonsai, do it at least a month before the first frost date. Consider aiding the reheating of your soil for an extended growing season with a seedling growth mat.

Frequently Asked Questions

When should I fertilize my Bonsai tree?

Bonsai trees should be fed from early spring until late summer with good quality bonsai feed. Feeding during the dormancy or winter periods is not required, and the bonsai trees should be allowed to rest.

How much fertilizer does a Bonsai need?

Apply feed weekly during the growing season. Follow the directions that come with your feed. Liquid feed uses one tablespoon per gallon or 4 liters of water. Fill a mesh feeding basket using solid feed and place 3 per tree.

Are coffee grounds good for Bonsai trees?

Coffee grounds are a good additive to add to your bonsai soil when growing Bonsai trees. It aids in aeration and avoids compaction.

In Closing

We learned that your Bonsai might have varied nutrient demands depending on its development phase and seasons. We also explored the effects of the different nutrients in NKP fertilizer, the importance of timing, and the need for a stable plant to cope with winter stressors.