A correct irrigation regime is not "when it dries out" and not "on a fixed schedule." It is the balance between sufficient moisture for transpiration and sufficient aeration for root respiration — a balance that shifts daily depending on plant stage, light levels, and temperature.
Quick Glossary
Irrigation window — the period of the day during which irrigation is permitted: from the start to the end of the light period; watering 1–2 hours before and after lights-off is not recommended.
Drainage — the portion of the nutrient solution that exits the substrate from below; drainage percentage (10–30% of irrigation volume) indicates whether the substrate is being flushed adequately.
Substrate field capacity — the state after irrigation where excess has drained and capillary moisture remains; the optimal point for root respiration and nutrient uptake.
Why "Water When Dry" Does Not Work in Coco and Rockwool
Coco substrate in a small pot or slab has a small volume and low buffering capacity. By "letting it dry before watering," the plant has already spent 2–4 hours under limited water supply. Repeated wet-dry cycles create wave-like stress that produces uneven growth, increased risk of blossom end rot, and osmotic fluctuations.
Irrigation Window Rules
First irrigation — 1–2 hours after lights on. The plant is already active but has not yet consumed large quantities of water.
Last irrigation — 1.5–2 hours before lights off. The plant still transpires after this final watering. Irrigating at or after lights-off leaves the substrate saturated overnight.
Night irrigation — only in exceptional cases. For most crops, a night without irrigation is a preventive measure against anaerobic root zone conditions.
Frequency and Volume: How to Calibrate
Small pot + young plant + cool day: 1–2 irrigations per day may be sufficient.
Large coco slab + mature tomato + hot day: 6–12 short irrigations per day via drip is standard in commercial greenhouses.
The "small but frequent" principle outperforms "infrequent but large" for most substrate-based crops.
Control via drainage percentage: with a correct regime, total daily drainage equals 10–30% of total daily irrigation volume.
Irrigation Regime for Drip Systems
Dripper flow rate: 0.5–4 L/hr depending on substrate block size and crop.
Single irrigation duration: 1–5 minutes for small pots (300–800 mL), 3–8 minutes for large slabs (1–2 L per event).
Adapting frequency to growth stage and weather: on overcast days and with young plants — reduce irrigation frequency. On hot days and with high-biomass plants — increase it.
Three Mistakes That Cost the Most
The same irrigation schedule on cloudy and sunny days. On overcast days, transpiration drops by 30–50% — the plant absorbs significantly less water. Adjust frequency or cancel individual irrigations during cloudy weather.
Not checking drain EC. As salts accumulate, drain EC rises above feed EC — and roots sitting in substrate at EC 4–5 mS/cm are under osmotic stress.
Watering in the evening or at night "so the plant doesn't run out of water." A night with a saturated substrate and no transpiration leads to root rot within 3–5 nights in warm conditions.
How to Know the Irrigation Regime Is Correct
Drainage appears after every irrigation and amounts to 10–25% of the irrigation volume. Drain EC is no more than 0.5–0.8 mS/cm above feed EC. Substrate between irrigations feels slightly moist but not wet when squeezed. Plants show no daytime wilting under normal solution parameters.