Managing ‘Ulu Fruit Drop and Storm Damage Workshop

1. Biological Mechanisms of Fruit Drop

• Internal Carbohydrate Demands: Fruit drop is often a deliberate internal decision by the tree when it determines it cannot sustain the carbohydrate load required to mature all set fruit.
• Ethylene-Driven Abscission: This is the same internal process trees use to drop old leaves and ripe fruit. By shedding old, damaged, or photosynthetically inefficient leaves, or dropping excess fruit, the tree helps its remaining fruits grow larger.
• Hormonal Regulation
  • Auxin: Promotes growth and suppresses ethylene.
  • Ethylene: When auxin degrades, ethylene levels rise. This shift signals the fruit to transition into a sugary, soft phase and can cause both mature and premature fruit (as well as aging leaves) to drop from the branch.
• Stress Response: Environmental stressors (e.g., wind, temperature spikes, or water availability issues) trigger these hormonal changes, sometimes causing trees to shed fruit as a survival mechanism.
• Internal vs. External Causes: Internal causes are usually within a grower’s control, such as nutrient deficiencies, drought (to some extent), root disturbance, and pest damage. External causes are often out of our control, such as extreme temperatures, high winds, and waterlogged soil.
• State of the Research: Fruit drop is very well studied in other fruit like citrus, apples, and mangoes – but it has not yet been specifically studied in ʻulu. Most of what we know is borrowed from those crops and from growers’ own observations.

2. Nutrient and Soil Health Strategies

Nutrient management is considered the primary factor within a farmer’s direct control to reduce fruit drop. In Hāmākua orchards, the most common culprits are potassium and calcium.

Potassium (K)

• Breadfruit has high potassium requirements similar to bananas.
• High-potassium fertilizers—such as 10-5-20, 10-5-25, or 10-3-30 blends—with approximately a 3:1 ratio of potassium to nitrogen are recommended.
• Potassium leaches rapidly in well-drained soils subject to heavy rainfall.
• Trees demonstrate “luxury consumption,” meaning they absorb excess potassium prior to fruiting when it is available; timing fertilizer before or at fruit set is critical.
• Potassium sulfate is preferred over potassium chloride. Organic options include K-Mag (sulfate of potash-magnesia) and liquid kelp.
• Calcium (Ca):
• Strengthens the structural integrity of both the fruit stem and the tree’s branches against strong winds.
• Calcium leaches easily in high rainfall environments, but tends to get stuck in the top few inches of certain regional soils (such as Hilo/Hāmākua).
• Liming (e.g., calcium carbonate) addresses calcium needs while raising low soil pH to improve overall nutrient availability.
• Prilled calcium or soluble forms are faster acting once trees are established, whereas coarse dolomite can take years to break down unless applied pre-planting.

Boron (B):

• A crucial micronutrient that helps regulate hormone degradation (specifically managing the auxin-to-ethylene shift).
• Because it leaches easily, applying a light annual sprinkling of simple borax in the root zone is recommended if tissue tests show a deficiency. It’s an easy and inexpensive fix, since trees need only a small amount, but it often gets forgotten.

Nitrogen (N):

• Excess nitrogen promotes unwanted vegetative growth over fruit development.
• Avoid applying heavy nitrogen until after the harvest season has concluded.
• Nitrogen deficiency manifests as a general yellowing and loss of shine across the entire leaf, rather than along the margins.

Soil vs. Tissue Testing

• Soil Testing: Essential before planting to fix foundational deficiencies (especially calcium).
• Tissue Testing: For established trees, tissue testing is the preferred method to determine what nutrients the plant is actively absorbing. The co-op offers members tissue and soil testing during set sampling windows in the spring and fall. 
• Foliar Sprays: Functions as a fast-acting, temporary fix when trees display immediate nutrient deficiencies or face upcoming stress events. Requires more frequent applications to mitigate imbalances in the long-term.

3. Pruning Techniques, Wound Care, and Disease Prevention

Pruning helps manage tree height for harvesting, opens the canopy, and reduces wind resistance during severe weather.

• Timing: The best time to prune is immediately following the main fruit harvest. This aligns with the transition into the wet season, ensuring the tree has abundant water to rebound.
• Selective Shoot Pruning: Heavy pruning causes aggressive shoot development. Rather than hard-topping every year, growers can perform structural prunes every other year, selectively thinning smaller internal shoots after harvest.

Please refer to our pruning guide and broken branches blog for further educational resources. 

4. Storm Damage Mitigation and Harvest Strategies

High wind events (such as Kona Lows or hurricanes) cause significant mechanical stress, leading to delayed fruit drop.

• Delayed Drop Phenomenon: Most fruit does not fall during the storm itself. Instead, internal stem twisting and micro-fractures cause a severe wave of fruit drop 3 to 5 days (and up to a week) post-storm, as stressed trees relinquish crop loads.
• Reading the Pattern: If storm-related fruit drop is concentrated around the edges of the orchard, wind stress is the likely cause. If it is spread evenly throughout, waterlogged soil stress may be to blame, depending on how much rain fell during the storm.

Pre-Storm Pruning & Foliar Sprays

• Trim long, extended branches prior to storm season to reduce weight and leverage, preventing main branch snapping under wind loads. Keeping trees pruned to a lower profile overall also helps them avoid harsh winds and makes harvest easier.
• Consider applying a fast-acting foliar spray rich in micronutrients immediately ahead of a known approaching storm to supply the tree with vital resources before the stress event.

Preemptive Harvesting Options

• If mature fruit is on the tree before a severe storm, harvest proactively to save the crop.
• Grade A fruit harvested slightly early can still be processed.
• Grade C fruit (full-sized but immature) can be sold to the co-op at $0.65/lb (about half the Grade A rate, only on Hawai’i Island), where it is used for flour production. The co-op cannot accept fruit that has already dropped and been sitting on the ground, so if trees are aborting fruit en masse or a storm is coming, harvesting full-size immature fruit early may be worthwhile, provided the price covers your harvest labor.
• Very small, spiky, immature dropped fruit can be salvaged for farm/home use by pickling (baby ‘ulu pickles set in 24 hours and keep for months). Check out the recipe here on our website.

Windbreak Design

• Species Choice: Clumping bamboo provides dense wind protection. Seed-grown Gliricidia is also effective because it develops a deep taproot, whereas Gliricidia grown from cuttings lacks a taproot and frequently gets knocked over in strong winds. Making incisions in the bark of cuttings prior to planting can help produce broader root development.
• Orchard Layout: Stagger tree rows rather than planting in a straight grid. Staggered layouts eliminate straight wind tunnels and force wind up and over the orchard canopy.
• Living Windbreaks: Trees on the outer edge of an orchard bear the brunt of the wind and protect the inner ones, so a border of ʻulu can act as its own windbreak (Tom uses ʻulu partially as a windbreak for his cacao). Separate windbreak species are harder to establish on slopes, which are common across Hāmākua.

5. Water Stress and Drought Planning

Water stress is harder to manage than nutrients, but growers have a few tools.

• Too Much Water: Waterlogging is difficult to control, but fortunately Hāmākua soils are very well drained.
• Mulching (Preventative): Mulch under trees supplies slowly available nutrients over time, protects the soil from drying out, and holds water longer during dry spells.
• Irrigation (Treatment): Irrigation can help once drought effects set in. Few farms in the  Hilo/Hāmākua area have irrigation installed since it’s so rarely needed.
• Planning for Extremes: Hawaiʻi has seen a historic drought immediately followed by a historic storm season, and this may be the new normal. About 70% of winters following an El Niño are drier than normal in Hawaiʻi, and after the 1877 El Niño, one of the strongest on record, drought dried up loʻi kalo and cane fields across the islands. Having a contingency plan for both drought and storms is worthwhile.

6. Variety Insights

Maʻafala

• Yield & Productivity: Highest overall annual yield on a weight/poundage basis.
• Stress Tolerance & Fruit Drop: Highly sensitive to stress, drought, rain spikes, and temperature changes. Drops a higher percentage of fruit during stress events.
• Ideal Elevation & Growth Form: Compact, stout build that resists branch breakage better. Prefers warmer, lower elevations; drops out rapidly above 1,500 feet.

Hawaiian / Otea / Other Varieties

• Yield & Productivity: Lower total fruit set count per tree, but produces larger individual fruits.
• Stress Tolerance & Fruit Drop: Lower percentage of fruit drop compared to ma’afala, though still susceptible in stressful conditions. 
• Ideal Elevation & Growth Form: Longer, stretched branches that are more prone to catching wind and snapping under heavy crop loads. Tolerates higher elevations somewhat better than Maʻafala, but grows poorly above 2,000 feet.