How NDVI and NDRE, measured using drones, can be powerful tools for monitoring apple orchard health throughout the growing season and guiding maintenance actions.

As of today, April 17, 2025, we are nearing the start of the active growing season in many apple-producing regions, making this a relevant time to plan monitoring strategies.

1. Understanding NDVI and NDRE

These are vegetation indices calculated from light reflected by plants, captured by multispectral sensors often mounted on drones.

• NDVI (Normalized Difference Vegetation Index):

  • Calculation: Uses Near-Infrared (NIR) light, which healthy leaves reflect strongly, and Red light, which chlorophyll absorbs for photosynthesis. The formula is roughly (NIR+Red)(NIR−Red)​.

  • What it Shows: General plant vigor, biomass density, and canopy cover. Higher NDVI values generally indicate denser, healthier vegetation.

  • Use in Orchards: Good for assessing overall growth, identifying areas with sparse canopy early in the season, or detecting severe stress that causes leaves to lose chlorophyll or wilt.

  • Limitation: NDVI can become "saturated" in dense, healthy canopies (like a mature apple tree canopy mid-season). This means it reaches a maximum value and can't easily distinguish between "healthy" and "very healthy" or detect subtle stress within that dense canopy.

• NDRE (Normalized Difference Red Edge Index):

  • Calculation: Uses NIR light and "Red Edge" light. Red Edge is a narrow band between red and NIR where reflectance changes rapidly. This band is particularly sensitive to chlorophyll concentration within the leaves, not just canopy density. The formula is roughly (NIR+RedEdge)(NIR−RedEdge)​.

  • What it Shows: More directly related to chlorophyll content and nitrogen status within the leaves. It's less affected by canopy structure and saturation than NDVI.

  • Use in Orchards: Excellent for detecting subtle stress before it's visible to the naked eye, particularly stress related to nutrient deficiencies (like nitrogen), water stress affecting chlorophyll function, or early stages of certain diseases. It's more effective than NDVI in dense canopies later in the season.

2. Using Drones for Data Acquisition

• Platform: DJI Mavic 3 Multispectral : capable of capturing specific light bands (Red, Red Edge, NIR).

• Process:

  • Flight Planning: Automated flight paths are planned over the orchard blocks to ensure consistent overlap and coverage.

  • Data Collection: The drone flies the planned route, capturing images. Flights should ideally occur under consistent lighting conditions (e.g., sunny, near solar noon) for better comparability between dates.

  • Frequency: Flights should occur at key phenological stages during the growing season:

    • Early Season (Post-Bloom/Leaf-Out): Establish a baseline, assess initial growth uniformity (NDVI useful here).

    • Mid-Season (Fruit Development): Monitor for developing stress, nutrient needs (NDRE becomes more critical). Multiple flights might be needed depending on conditions.

    • Late Season (Pre-Harvest): Assess late-season stress that could impact fruit quality or tree reserves for winter (NDRE is key).

• Data Processing: Specialized software stitches the individual images together, corrects for lighting variations, and calculates the NDVI and NDRE values for every point in the orchard, creating detailed maps.

3. Translating Data into Suggested Maintenance

The real power comes from interpreting the NDVI/NDRE maps and using them to guide specific actions:

• Identify Low-Index Zones/Trees (Potential Stress):

  • Low NDVI/NDRE: Indicates areas lagging in growth or experiencing significant stress.

  • Low NDRE (but maybe okay NDVI initially): Can be an early warning of nitrogen deficiency, water stress, or disease impacting chlorophyll before the canopy thins noticeably.

  • Suggested Maintenance:

    • Ground-Truthing: Always visit these specific low-index areas identified on the map. Visually inspect trees, check soil moisture, look for pests or disease symptoms.

    • Targeted Irrigation Checks: Ensure emitters/sprinklers in these zones are functioning correctly and delivering adequate water. Adjust schedules for specific zones if needed.

    • Variable Rate Nutrition: If ground-truthing and possibly leaf tissue analysis confirm nutrient deficiency (especially nitrogen indicated by low NDRE), apply fertilizer specifically to these low-performing zones rather than a uniform application across the entire block. This saves cost and reduces environmental impact.

    • Directed Pest/Disease Scouting: Send scouts specifically to these flagged areas to look for early signs of infestation or infection, allowing for targeted treatment before it spreads.

    • Soil Sampling: Take targeted soil samples from consistently underperforming zones to check for pH issues, compaction, or localized nutrient imbalances.

• Identify High-Index Zones/Trees (High Vigor):

  • High NDVI/NDRE: Generally indicates healthy, vigorous growth.

  • Suggested Maintenance:

    • Monitor Vigor Balance: While good, excessively high vigor can sometimes lead to poor fruit color/quality due to shading, or require more intensive pruning later. Note these areas for potential adjustments in pruning or perhaps slight reductions in nitrogen in future seasons.

    • Confirm Crop Load: Ensure these vigorous areas have an adequate fruit set; sometimes high vegetative vigor can compete with fruit development.

• Monitor Trends Over Time:

  • Comparing Maps: Look at how NDVI/NDRE values change in specific zones between flights. Are stressed areas recovering after intervention? Are new areas developing stress? This temporal analysis is crucial for evaluating the effectiveness of management practices.

  • Suggested Maintenance: Adjust ongoing strategies based on trends. If a zone repeatedly shows stress despite interventions, a more fundamental issue (e.g., soil problem, chronic disease) might need investigation.

In Summary:

Using drones to capture NDVI and NDRE data throughout the growing season provides apple growers with high-resolution spatial and temporal insights into orchard health. NDVI gives a general overview of vigor, especially early on, while NDRE offers a more sensitive indicator of chlorophyll content and subtle stress, particularly in dense canopies later in the season. By linking these data maps to targeted ground-truthing and specific maintenance actions like variable rate fertilization, precise irrigation adjustments, and directed scouting, growers can practice precision agriculture – optimizing inputs, potentially improving yield and fruit quality, detecting problems earlier, and managing their orchards more efficiently and sustainably.