"The substrate has dried out a bit, so I water, soak it and that's that." In coco, though, "dried out a bit" means the plant has already spent several hours under water stress and some of its root hairs have dried out. Or it goes the other way: watering "morning and evening" a small plant on a cold, overcast day keeps the substrate permanently wet, DO in the root zone falls to zero, and root rot sets in. A sound irrigation regime follows neither the dryness of the pot nor a fixed schedule. It is a balance between enough moisture for transpiration and enough aeration for the roots to breathe, and that balance shifts every day with the plant's stage, the light and the temperature.
Quick glossary: The irrigation window is the part of the day in which irrigation is allowed, running from the start to the end of the lit period; watering in the final 1-2 hours before lights-off, or after it, is not recommended, because a plant without light does not transpire and the substrate stays too wet through the night. Drainage (runoff) is the share of the irrigation solution that leaves the substrate at the bottom; the drainage percentage (10-30% of the volume applied) shows whether the substrate is being flushed enough. Equilibrium substrate moisture is the state after irrigation in which the excess has drained away and the capillary water remains; it is the optimal point for root respiration and uptake.
Why "Water When It Is Dry" Fails in Coco and Rockwool
In open soil, "dried out" is a signal that enough water has gone since the last irrigation and it is time to top up. Soil has an enormous buffer volume, and even with the surface visibly dry there is still available water further down.
Coco substrate in a small pot or slab has a small volume and a low buffer capacity. By the time it has "dried out enough to water", the plant has spent 2 to 4 hours on a restricted water supply. Root hairs in contact with dried-out pores die. Repeat the wet and dry cycle regularly and you get wave-like stress that produces uneven growth, a higher risk of blossom end rot and osmotic swings.
Rockwool holds even less water when stood vertically. It dries faster, and the critical threshold arrives sooner.
Irrigation Window Rules: When to Water and When Not To
The first irrigation comes 1 to 2 hours after the lights come on. The plant is already active, with stomata open and transpiration under way, and has not yet used much water. Water earlier and the substrate is too wet at the start of the day.
The last irrigation comes 1.5 to 2 hours before the lights go off. The plant keeps transpiring and taking up water after that shot. Water at lights-off or later and the substrate stays saturated overnight. With no transpiration at night the water does not leave, and DO in the root zone falls.
Night irrigation only in exceptional cases. With very heavy fruiting and a large crop load, some schedules include 1 or 2 small night shots. For most crops and most scales, though, a night without irrigation is what prevents an anaerobic root zone.
Frequency and Volume: How to Set Them
There is no single correct irrigation frequency - only principles: pot size, plant age and weather set it, from 1-2 waterings a day for a small pot in a cold spell to 6-12 short drip shots a day for a mature tomato on a large slab in hot weather.
Small pot, young plant, cold day: 1 to 2 irrigations a day may be enough. The plant takes up little and the substrate dries slowly.
Large coco slab, mature tomato, hot day: 6 to 12 short drip irrigations a day is standard practice in commercial Dutch glasshouses. The aim is to hold substrate moisture in the 60-75% range with no peaks of waterlogging and no excessive drying.
The "small and often" principle works better than "large and rare" for most substrate crops: smaller swings in moisture and EC in the substrate, a more stable root zone, and better DO between irrigations.
Control through the drainage percentage: with a correct regime, total daily drainage equals 10-30% of total daily irrigation volume. Below 10% there is a risk of salts building up in the substrate, with substrate EC climbing. Above 40% means overwatering and fertilizer going to waste.
The First Irrigation of the Day: The Start and the EC
By the first irrigation in the morning after the night break, the substrate has partly dried and EC in the root zone has risen through concentration: the water evaporated or was taken up, and the salts stayed behind. The first irrigation of the day uses the normal working solution.
In a very hot season, though, with drainage EC trending upward, the first shot can use a solution 0.2-0.3 mS/cm below working EC for a flushing effect.
Irrigation Regime With Drip Irrigation
Automated drip irrigation on a timer is the most common setup for substrate crops: dripper flow is sized to the slab (0.5-4 L/h), shot duration to the substrate volume (1-8 minutes), and frequency is adjusted to growth stage and weather rather than fixed on a daily schedule.
Dripper flow: 0.5-4 L/h depending on the size of the substrate slab and the crop. Too fast a flow and the solution runs off the surface before it can wet the slab evenly.
Duration of one irrigation: 1 to 5 minutes for small pots (300-800 ml), 3 to 8 minutes for large slabs (1-2 liters per shot). Check drainage after the first irrigation: with a correctly calculated volume it should appear 1 to 3 minutes after the shot ends.
Adjusting frequency to growth stage and weather: on an overcast day and with a young plant, cut the number of irrigations. On a hot day and with a plant carrying a large leaf mass, add more. A timer with a variable schedule, or a substrate moisture sensor, gives far more flexibility than a fixed daily timer.
Three Mistakes That Cost the Most
The same irrigation regime on an overcast day and a sunny one. In cloudy weather transpiration drops by 30-50% and the plant takes up far less water. The same timer that is right on a sunny day overwaters on a cloudy one, and the root zone loses its oxygen. Adjust the frequency or skip individual shots in cloudy weather instead of leaving the timer on its usual setting.
Not checking drainage EC. "Irrigating every 2 hours and everything's fine", with no measurement of the EC coming out of the substrate. As salts build up, drainage EC climbs above feed EC, and a root sitting in substrate at EC 4-5 mS/cm is under osmotic stress even while you feed at EC 2.0. Measure drainage EC at least once a week.
Watering in the evening or at night "so the plant is not left without water". A night without water is normal for most crops. A night in waterlogged substrate with no transpiration brings root rot within 3 to 5 nights when the water is warm. The last irrigation 1.5 to 2 hours before the lights go off is an iron rule.
How to Tell the Irrigation Regime Is Set Up Right
Every irrigation produces drainage, amounting to 10-25% of the volume applied. Drainage EC runs no more than 0.5-0.8 mS/cm above feed EC, a sign that salts are not accumulating. Between irrigations the substrate feels slightly moist yet not wet when squeezed. And the plant shows no midday wilting with the solution parameters in range.
If you want to go deeper: Drainage EC: what it tells you about the state of the substrate explains how analyzing the EC leaving the substrate lets you catch salt build-up early and correct the irrigation regime before the plant shows any stress symptoms.