Optimizing Multi-Layer Forest Garden Irrigation: Calculating VPD with ESP32 and Home Assistant

The Challenge of Water Management in a Multi-Layer Ecosystem

In a forest garden designed according to permaculture principles, vegetation is structured in several vertical layers: from the canopy to ground covers, including fruit shrubs. Automating the irrigation of this type of ecosystem with simple soil moisture sensors poses a major problem: microclimate disparity. The intermediate shrub layer undergoes a different water stress compared to the ground cover layer, which is often protected by thick mulch.

To solve this problem with ultra-precision, we will abandon capacitive soil probes (which oxidize and drift over time) to focus on a much more reliable biophysical metric: Vapor Pressure Deficit (VPD). VPD precisely measures the atmospheric pull on the water contained in plant leaves, indicating their actual transpiration rate.

The IoT Architecture: Environmental Sensors and ESP32 under ESPHome

Our architecture is based on the deployment of ESP32 modules housed in 3D-printed waterproof enclosures (IP65), positioned at different heights corresponding to the forest garden strata:

• Layer 1 (Ground cover): DHT22 sensor placed 10 cm above the ground, under the mulch.
• Layer 2 (Shrubs): BME280 sensor placed 1.5 m high, in the heart of the berry bushes.

These sensors measure ambient temperature and relative humidity. Data is transmitted via Wi-Fi using the MQTT protocol to our Home Assistant home automation server.

Real-Time VPD Calculation in Home Assistant

VPD is expressed in kiloPascals (kPa). To calculate it dynamically, we use template integration in Home Assistant. Here is the mathematical formula implemented to calculate the saturated vapor pressure (VPsat) and the actual vapor pressure (VPact) to get the VPD:

template:
  - sensor:
      - name: "Shrub Layer VPD"
        unit_of_measurement: "kPa"
        state: >-
          {% set T = states('sensor.bme280_temperature') | float %}
          {% set RH = states('sensor.bme280_humidity') | float %}
          {% set VPsat = 0.61078 * e ** ((17.27 * T) / (T + 237.3)) %}
          {% set VPact = VPsat * (RH / 100.0) %}
          {{ (VPsat - VPact) | round(2) }}

Smart and Resilient Irrigation Automation

Using this VPD value, we program a localized, water-saving irrigation rule. If the VPD of the shrub layer exceeds 1.2 kPa (moderate water stress zone for young shrubs) while the soil is not saturated, a low-power Shelly Pro 1 solenoid valve triggers a targeted drip system for 15 minutes.

This approach respects permaculture design by preventing water waste while keeping plants in their transpiration comfort zone, thus limiting the development of fungal diseases favored by stagnant ground moisture.