Double flush pines are a relatively small and specialized group of pine species, capable of producing two flushes of growth within a single growing season. This trait enables accelerated development of branch structure and refinement, making these pines especially desirable for bonsai cultivation. However, maximizing their potential requires precise seasonal care. This guide outlines a year-round maintenance schedule to help you grow strong, healthy, and beautifully ramified double-flush pine bonsai.
What Are Double Flush Pines?
A double flush pine is one that can be decandled in summer to induce a second flush of growth. Only a few pine species consistently exhibit this behavior, such as:
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Japanese Black Pine (Pinus thunbergii),
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Japanese Red Pine (Pinus densiflora), and
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Loblolly Pine (Pinus taeda).
These species typically come from warm climates with long growing seasons, which gives bonsai growers the advantage of more styling opportunities within a single year, and are often subject to disturbances like windstorms or fires—conditions that favor a two-flush adaptation—in their native environment.
Figure 1: Japanese Black Pine No. 7
Spring: Building Strength
Repotting
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Timing: Early spring, just as buds swell and change color.
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Root Work: Limit root removal to 30–40% (20–30% for older trees). Excessive root pruning can significantly weaken double-flush pines, reducing their vigor and potentially compromising the strength needed to support a second flush of growth later in the summer. Remove 40–70% of old soil, preserving the root core.
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Soil Mix: Use a standard mix of:
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1 part akadama
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1 part pumice
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1 part lava rock
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For Young Trees: Peat and river sand or potting mix to encourage rapid growth.
Fertilization
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Early Development: Use synthetic high-nitrogen fertilizer (e.g., Osmocote Plus).
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Mid/Refined Trees: Use organic fertilizers like BioGold.
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Goal: Heavily fertilize to build energy for summer decandling.
Pests & Disease
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Pests: Watch for candle-boring caterpillars; treat preventatively.
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Disease: Apply fungicides preventively to prevent summer needle blights.
Styling
- Allow strong extension of first flush.
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Avoid pruning, needle plucking, or wiring in spring. These actions can compromise the tree's strength and reduce its ability to support a vigorous second flush of growth during the summer.
Summer: Decandling & Energy Balance
Decandling is the most crucial summer technique for double flush pines. Its timing is highly dependent on your specific climate and even microclimates within your yard, often requiring some experimentation to perfect. Weather variations year to year also mean that consistent results might not be achieved by decandling at the exact same time annually.
Figure 2: Before and after decandling
Timing
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Adjust based on climate and tree size:
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Large Trees: ~June 1–15
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Medium Trees: ~June 15–30
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Shohin Trees: ~June 30–July 15
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Adjust year to year to prevent an overly long second flush.
Decandling Techniques
There are two primary decandling techniques:
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Two-Step Technique (Best for Energy Balance)
- Identify Candle Strength: First, identify weak, medium, and strong candles across the entire tree. Figure 3 (below) highlights the varying candle strengths found on a single tree.
- Weak candles: Leave untouched.
- Medium Candles (First Step): Decandle medium-strength candles by cutting them back flush to last year's needles. This will immediately induce a secondary push of growth. At the same time, pluck last year's needles in these areas back to about 10-12 pairs around the cut site.
- Strong Candles (Second Step): Wait about 10-14 days, then decandle the strongest candles using the same technique. Pluck last year's needs in these areas back to 5-7 pairs of needles.
- Benefits: This two-step process gives medium-strength areas a two-week head start in producing secondary growth. It strategically weakens strong areas by reducing the number of needles to 5-7 pairs, while strengthening the weaker areas by leaving 10-12 needle pairs.
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One-Step Technique (Less Control)
- Identify Candle Strength: Again, identify weak, medium, and strong candles.
- Weak candles: Leave untouched.
- Medium Candles: Decandle medium-strength candles by cutting them back flush to last year's needles and plucking back to about 10-12 pairs.
- Strong Candles: At the same time, prune back the strong candles, but leave a small stub just above last year's needles. Pluck these areas back to 5-7 pairs of needles.
- Considerations: This method is not as controlled as the two-step process. However, it is a good option for collections with many pines, limited time, or for trees that are already well-balanced.
Figure 3: Japanese Black Pine Candles
Fertilization
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Developmental Trees: Continue synthetic fertilizers.
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Refinement Trees: Stop fertilizing ~4 weeks before decandling.
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Post-Decandling: No granular fertilizer. Use low-nitrogen liquid feeds like kelp/fish emulsions for health.
Styling
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Ideal time for detailed wiring—structure is fully visible after needle plucking and candle removal.
Pest & Disease Management
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Pests: Continue systemic protection.
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Diseases: Treat for needle cast/blight throughout July.
Fall: Refinement & Recovery
Candle Selection
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Cut clusters down to two similarly sized candles to prevent swelling.
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Orientation is secondary; wire as needed.
Needle Plucking
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Some older needles may naturally yellow and fall—this is normal.
Fertilization
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Resume organic fertilizers in September to build bud strength for spring.
Styling
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Heavy Bending: September–October is ideal for raffia-wrapped bends or wedge cuts.
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Protection: Heavily worked trees may need winter shelter.
Grafting
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Ideal time for fall scion grafting—if you can prevent frost exposure all winter.
Winter: Rest and Protection
Temperature Guidelines
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Down to 20°F (-6°C): Place on ground, possibly mulch (leave soil surface visible).
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Below 15°F (-9°C): Move into unheated garage or cold frame.
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Extreme Cold: Brief indoor protection may be necessary.
Disease & Pest Prevention
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Apply diluted lime sulfur before winter storage.
Styling
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Heavy Work: Avoid in deep winter.
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Detail Wiring: OK throughout dormancy.
Grafting
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Late Winter/Early Spring (Feb–Mar): Ideal time for scion or approach grafting, as this period aligns with early bud activity while also avoiding exposure to damaging frost.
Final Thoughts
Caring for a double flush pine bonsai is a seasonal dance of energy balance, timing, and observation. With attentive seasonal techniques, these trees offer unmatched opportunities for refined ramification and dynamic design. Follow this guide as your annual schedule, and adjust based on your region’s climate and your tree's individual responses.