"The substrate looks fine" — and the next irrigation fires on the timer. But one coco block at the same irrigation rate has VWC 75% while the neighbouring one has 45%, because the dripper on the second is partially clogged. The plant in the second block is under constant water stress, but "looks fine" until it starts falling 3–4 days behind in growth. Judging substrate moisture "by appearance" or "by feel" is an assessment of the surface and a subjective impression. A VWC sensor shows the volumetric percentage of water in the substrate in real time — and reveals the difference between blocks before it becomes visible.
Quick glossary: VWC (Volumetric Water Content) — the fraction of substrate volume occupied by water, expressed as a percentage; at VWC 60%, 60% of the substrate volume is water. Field capacity — the maximum VWC retained by the substrate after excess drains away; for coco ≈ 70–80%, for rockwool ≈ 65–80% depending on density. Wilting point — the minimum VWC at which the plant can still absorb water; for coco ≈ 20–25%, for rockwool ≈ 15–20%.
Why Subjective Assessment Does Not Scale
With a single pot on a windowsill, "heavy or light" or "dark or light-coloured" coco gives acceptable accuracy. With 50 plants in a production greenhouse, manually checking every block every day is impossible. With 200 — even more so.
Subjective moisture assessment has a second fundamental problem: it evaluates the surface or the point the hand can reach. A 15×15×10 cm coco block may have dry top 2 cm (surface dried by airflow) while VWC at the centre is 65%. Or the reverse: a wet surface from condensation while the lower half has dried out. "Wet" and "dry" by touch, and actual VWC in the zone of the main root hair mass, are often two different things.
The second dimension that matters alongside VWC is substrate EC. Some sensors measure VWC and substrate EC simultaneously (pore water EC or bulk EC). At correct VWC but accumulated EC — the root zone is under osmotic stress regardless of moisture level.
VWC in Numbers: Target Ranges
Coco substrate:
- At irrigation: VWC rises to 70–75% (field capacity)
- Between irrigations: drops to 55–65% — optimal working zone
- Lower threshold between irrigations: 45–50% — signal that the next irrigation is needed
- Critical lower limit: 35% and below — water stress
Rockwool:
- Field capacity after irrigation: 70–80%
- Optimal working zone between irrigations: 60–70%
- Lower threshold: 45–50%
Overnight with a correct irrigation schedule: VWC gradually declines from the evening level to the morning level. A drop of 5–15% overnight is normal for a mature plant at 20–24°C. A drop of 25–30% overnight means the plant is consuming more than the daytime irrigations replenish — or daytime irrigation frequency is insufficient.
VWC Sensors: Types and How to Choose
Capacitive sensors — the most common for agricultural use. They measure the dielectric permittivity of the substrate, which changes with moisture content. Accuracy: 2–5% with calibration for a specific substrate. Most commercial sensors (Bluelab Pulse, Growlink, GrowDirector, Grodan sensors) are capacitive.
TDR (Time Domain Reflectometry) — more accurate, more expensive; used in scientific and premium commercial systems.
What to look for when choosing:
- Substrate compatibility: some sensors are designed for soil and give significant error in coco or rockwool. Verify whether a calibration exists for the specific substrate.
- Simultaneous EC measurement — sensors like Bluelab Pulse or equivalent give both VWC and EC at one point in a single reading.
- Integration with the automation system: if automated irrigation is in place, using the VWC sensor as the irrigation trigger (irrigation starts when VWC drops below a threshold) is significantly more effective than a fixed timer.
VWC as an Irrigation Trigger: Practice
Irrigating on VWC rather than a timer is a more flexible and accurate approach:
Principle: set a lower VWC threshold (e.g. 55% for coco). The controller starts irrigation when VWC drops to the threshold. Plants are irrigated simultaneously or by zone.
Advantages over a timer: automatic adaptation to weather (on a cloudy day transpiration is lower — fewer irrigations), adaptation to plant stage (a young plant absorbs less), compensation for differences between blocks (a block with poorer drainage structure dries more slowly).
Where to place the sensor: in the active root zone — 5–8 cm from the block surface at the depth where the main root hair mass is located. Not at the surface where drying is fastest, and not at the drainage point where it is always wetter.
Three Mistakes That Cost the Most
Using one sensor for the entire zone and assuming it is representative. One sensor reads one block. With different dripper flow rates, different block structures, or different plant sizes, VWC between blocks can differ by 15–25%. Spot-check 3–5 blocks across the zone once a week, or zone the sensors.
Not calibrating the sensor for the specific substrate. Generic capacitive calibration gives a 10–15% error in coco and rockwool compared to soil — which is what most inexpensive sensors are calibrated for. "VWC 50%" on the display may be actual 40% or 60% without calibration. Look for a manufacturer that provides substrate-specific calibration for coco or rockwool.
Responding only to VWC without accounting for substrate EC. VWC 65% at substrate EC 5.0 mS/cm — the plant is in a moist but osmotically stressful environment. Combined monitoring of VWC and drain EC or substrate EC gives the complete picture of root zone conditions.
How to Know Moisture Monitoring Is Set Up Correctly
VWC between irrigations stays within the optimal range — without excessive drying and without overnight substrate saturation. The difference in VWC between blocks on spot checks is less than 10%. The irrigation timer or VWC trigger is adapted to season and growth stage — not identical year-round.
For deeper understanding: Irrigation Regime in Substrate Systems: Frequency, Volume, and the Irrigation Window — explains how VWC sensor data or manual readings translate into a specific decision: when to start irrigation, how much to deliver, and when to stop.