What a smart greenhouse actually measures

A greenhouse can be fitted with dozens of sensors, but more data doesn't automatically mean better crops. The readings that pay off are the ones that change a decision: when to water, when to vent, when to shade, when to feed.

Here are the measurements we install most often in smart greenhouse projects, why each one matters, what kind of sensor we use and how often it's worth taking a reading.

Air temperature

Temperature drives almost everything a plant does: how fast it grows, when it flowers and how much water it uses. In a closed greenhouse under strong sun, temperature can rise very quickly, so this is usually the first reading we automate.

• Decisions it drives: opening vents, running fans or pad-and-fan cooling, deploying shade nets.
• Sensor: a digital temperature sensor inside a ventilated radiation shield. Without a shield, direct sun makes the sensor read several degrees too high.
• How often: every 1 to 5 minutes, because conditions change fast.

Relative humidity

Humidity affects how much water plants lose through their leaves and how likely fungal diseases are. Too high, and diseases such as botrytis and mildew thrive. Too low, and plants close their pores and slow down.

• Decisions it drives: ventilation, misting or fogging, and the timing of irrigation.
• Sensor: usually combined with temperature in one shielded unit.
• How often: every 1 to 5 minutes, alongside temperature.

Vapour pressure deficit (VPD)

VPD isn't measured directly. It's calculated from temperature and humidity, and it's one of the most useful numbers a grower can watch. It describes how strongly the air "pulls" water out of the leaves, which combines the effects of heat and humidity into one figure.

• When VPD is too low, the air is nearly saturated, plants transpire slowly and disease risk rises.
• When VPD is too high, plants lose water faster than roots can supply it and become stressed.

Many crops grow well somewhere around 0.8 to 1.2 kPa, but the ideal range depends on the crop and its growth stage, so we set targets with the grower rather than using one number for everything.

Soil or substrate moisture

This is the reading that decides when to irrigate. Watering on a fixed timetable ignores the weather; measuring moisture means plants get water when they need it, which usually saves water and reduces root diseases from overwatering.

• Decisions it drives: starting and stopping irrigation for each zone.
• Sensor: we prefer capacitive soil moisture sensors. Cheap resistive probes corrode quickly in wet soil and drift out of calibration.
• How often: every 10 to 30 minutes. Soil moisture changes slowly except during irrigation.

Placement matters: install probes in the root zone, in more than one spot per zone, and away from drippers so a single wet spot doesn't fool the system.

Light

Light is the energy plants grow on. Measuring it tells you whether shade nets are needed in harsh midday sun and helps explain changes in growth over the season.

• Decisions it drives: shading, supplementary lighting where used, and irrigation, since plants use more water on bright days.
• Sensor: a basic lux sensor is fine for shading decisions. For research-grade work, a PAR sensor measures only the light plants use for photosynthesis.
• How often: every 1 to 5 minutes.

CO₂

Plants use carbon dioxide to grow. In a closed greenhouse on a sunny morning, plants can draw CO₂ down below outside levels, slowing growth. Some high-value growers add CO₂ to boost yields.

• Decisions it drives: ventilation timing, and CO₂ dosing where installed.
• Sensor: an NDIR CO₂ sensor, which is accurate and stable over time.
• How often: every 5 to 10 minutes.

For many naturally ventilated greenhouses, CO₂ is a nice-to-have. We usually add it once the essentials are working well.

Nutrient solution: EC and pH

For hydroponic and fertigation systems, the nutrient solution is as important as the air. EC (electrical conductivity) shows how concentrated the nutrients are. pH affects whether plants can actually absorb them.

• Decisions it drives: dosing nutrients, adjusting pH, and topping up or changing the solution.
• Sensor: EC and pH probes in the tank or return line. pH probes need regular calibration, typically every few weeks.
• How often: every 5 to 15 minutes.

At a glance

From readings to decisions

Sensors only pay off when their readings lead to action. A few principles we follow when setting up automation and alerts:

• Automate the routine. Irrigation by moisture and venting by temperature can run on their own, with the grower setting the targets.
• Alert on what needs a person. A sensor that stops reporting, a pump that runs without moisture rising, or temperature still climbing with vents fully open all need human attention.
• Avoid alert fatigue. If a phone buzzes all day, people stop looking. Alerts should be rare, specific and say what to check.
• Keep the history. A season of data shows what actually works for your crops and helps refine the targets each year.

This is the approach behind Viva: a small set of reliable sensors, sensible automatic controls and a dashboard that shows at a glance whether anything needs attention.