In a greenhouse, a coir slab with a fruiting crop on it gets watered somewhere between 5 and 15 times a day in small shots, not once or twice in big ones. You start an hour or two after sunrise, stop one to two hours before sunset, and set the number of shots so that 10 to 30 percent of what you put on runs out of the slab. In low light that's a handful of shots and almost no drain. On a bright summer day it can be 15 or more with a third draining away. Everything below is about how to land those numbers for your own crop.
How coir holds and releases water
Most irrigation mistakes on coir come from running it like rockwool or like peat.
Rockwool holds a lot of water but holds it loosely. Most of it is easily available, the slab drains fast and dries back fast, so rockwool schedules use many very small shots and tolerate a 20 to 30 percent overnight dry-back.
Coir pith holds a similar total volume (roughly 40 to 50 percent water by volume at container capacity, with air-filled porosity around 13 to 28 percent) but releases it more gradually as the plant pulls harder. Water also moves sideways through coir slowly, so a slab wetted at one end stays wet at that end. Coir forgives a missed shot and punishes a dripper that isn't where the roots are.
Peat holds water tightly, repels it once dry, and is rarely run as a drip-fed slab for that reason. Coir rewets willingly, which is why you can run a real overnight dry-back on it.
Two more things change your schedule. Coir buffers nutrients, so drain EC sits a little higher than you'd expect from the drip EC and climbs quickly if drain falls. And pure pith drinks and dries more slowly than a pith-and-chip blend, so a 70:30 pith-to-chip mix (common in hot climates) and a chip-heavy mix (common in northern Europe, where growers want more dry-back) need different shot counts under the same plants.
Shot size: small and often
Across published advice for tomato, cucumber and pepper on slabs, the recommended shot is 3 to 5 percent of the substrate volume. On a 12-litre slab that's roughly 350 to 600 ml per shot, or about 100 to 150 ml per dripper with four plants on the slab. Commercial trials on coir slabs with 2 L/h drippers have used shots of around 100 ml per plant.
For pots and bags watered by hand or by timer, the rule of thumb is bigger, 10 to 15 percent of the container volume per event, because you're doing far fewer events. Either way, give the roots what they take between events plus enough extra to push some drain.
Why not one or two big waterings? A big shot saturates the slab, drives air out and leaves roots too wet for hours. Because water moves sideways through coir so slowly, it also drains straight down under the dripper while the far side of the slab stays dry and collects salt. And yield in slab crops rises with irrigation frequency. The Dutch rule for tomato is the smallest possible shot at the highest possible frequency, more than ten times a day in full production.
How much water per day: let light decide
Light drives roughly 80 to 90 percent of a greenhouse crop's water uptake. Humidity deficit drives the rest. So the daily total is set by the radiation sum, not the calendar or the clock.
The working figure for a mature tomato crop is 2 to 3 ml of water per plant per J/cm² of outside radiation, with 2.8 to 3.2 ml/J a common climate-computer setting for fruiting crops. A full crop on a 2,000 J/cm² day therefore wants roughly 5 to 6 litres per plant. On a 300 J/cm² winter day it wants under a litre, and even two shots may be too many.
Most controllers run on this logic: a shot fires each time the radiation sum reaches a set value. On coir slabs, set points of 60 to 150 J/cm² between shots have been used in trials, lower in spring and summer, higher in winter. They gave 5 to 9 shots a day in January and 9 to 16 by May. Add a minimum gap of about 30 minutes and a maximum of about 3 hours so long dull spells still get water.
Indicative schedules by stage and light
These are starting points for a fruiting vegetable crop (tomato, cucumber, pepper) on a 10 to 15 litre coir slab with four plants. Dial them in using your own drain percentage and slab weight, as described further down.
| Stage | Light level | Shots per day | Shot size | Start / stop | Target drain (24 h) | Overnight dry-back |
|---|---|---|---|---|---|---|
| Just planted, roots in block | Any | 0 to 2 (water block only) | 80–100 ml per block | Morning only | None | Let block lose 40–50% of saturated weight before rooting into the slab |
| Rooting into slab (weeks 1–3) | Dull (< 500 J/cm²) | 2 to 4 | 100–150 ml per dripper | 3 h after sunrise / 3–4 h before sunset | 0 to 15% | 6 to 8% |
| Rooting into slab (weeks 1–3) | Bright (> 1,000 J/cm²) | 4 to 6 | 100–150 ml per dripper | 2 h after sunrise / 2 h before sunset | 15 to 25% | 5 to 6% |
| Vegetative growth / first trusses | Dull | 4 to 6 | 100–150 ml per dripper | 2–3 h after sunrise / 3 h before sunset | 10 to 20% | 8 to 9% |
| Vegetative growth / first trusses | Bright | 8 to 12 | 100–150 ml per dripper | 1–2 h after sunrise / 1–2 h before sunset | 15 to 25% | 7 to 8% |
| Full production | Dull (< 500 J/cm²) | 4 to 7 | 100–150 ml per dripper | 2–3 h after sunrise / 3 h before sunset | 0 to 15% | 8 to 10% |
| Full production | Medium (500–1,000 J/cm²) | 7 to 10 | 100–150 ml per dripper | 1–2 h after sunrise / 2 h before sunset | 10 to 20% | 7 to 9% |
| Full production | Bright (1,000–1,500 J/cm²) | 10 to 14 | 100–150 ml per dripper | 1–2 h after sunrise / 1–2 h before sunset | 20 to 30% | 7 to 8% |
| Full production | Very bright (> 1,500 J/cm²) | 14 to 18+ | 100–150 ml per dripper | 1 h after sunrise / 1–2 h before sunset | 30 to 40% | 7 to 8% |
| Last weeks before pull-out | Any | Reduce by a third | Same | Same | 10 to 15% | 8 to 12% |
A few notes on the table. The "dull" rows matter most. Running a bright-day schedule through three dull days in a row is the classic way to lose roots to Pythium. The overnight dry-back column is as a percentage of the slab's water content, not of its weight. And the drain column rises with light on purpose, which brings us to why you want drain at all.
Why 10 to 30 percent drain, and when to push it higher
Drain does two jobs. It tells you the slab is full, and it carries salt out.
Every shot leaves behind the ions the plant didn't take up. With no drain, root-zone EC climbs within days. In coir trials where drain fell below about 10 percent, drain EC more than doubled relative to drip EC within weeks and passed 6 mS/cm in the driest treatment. Letting 15 to 30 percent of the daily volume run through keeps EC roughly where you set it. Below 15 percent, raise the frequency. Above 30 percent on a normal day, lower it.
Drain is an outcome of EC control, not a target in itself, which is why it should move with light. Under 500 J/cm² a day, 0 to 10 percent. 500 to 1,000 J, 10 to 20 percent. 1,000 to 1,500 J, 20 to 30 percent. Above 1,500 J, 30 to 40 percent. Dull days need almost no drain because uptake is low and the slab is already wet. Bright days need plenty because far more solution is going through and far more salt is being left behind.
Measure it honestly. Collect drain from at least one whole slab, not from a gutter, and check one dripper's output in a cup over the same period. Steering EC and pH through the crop is covered in the pH and EC management article.
Start and stop times
The first shot goes on once the plant has started transpiring, not at dawn. For a fruiting crop that's 1 to 2 hours after sunrise, or when the radiation sum reaches 60 to 100 J/cm², or when slab water content has dropped 1 to 1.5 percent from its night level. On dull mornings push it to 2 to 3 hours after sunrise and wait for a bigger drop.
Aim for first drain late in the morning, around 400 J/cm² of accumulated light or 4 to 6 shots in. Drain from the first or second shot means the slab never dried back overnight.
The last shot goes on 1 to 2 hours before sunset on bright days, 3 to 4 hours before on dull ones, or when roughly 100 to 200 J/cm² of light remains. That leaves the plant time to draw the slab down before dark.
Overnight dry-back
Between the last shot and the first next morning, the slab should lose water. For coir slabs the working range is 7 to 10 percent of water content, with 10 to 15 percent from the day's maximum to its minimum often quoted as the daily target. Around 10 percent steers vegetative, nearer 15 percent generative. Pot growers using bigger, fewer shots often quote 15 to 25 percent because pots are smaller and drier by design.
Dry-back lets air back into the slab and gives you a daily signal of root activity. A slab that doesn't dry back is either over-watered in the afternoon or has roots that aren't drinking. Either way, cut afternoon shots first.
Night irrigation is rarely needed. Add one shot only if the overnight drop would otherwise exceed 9 or 10 percent, for instance under heavy pipe heat in summer.
Reading slab weight and moisture sensors
Weight is the oldest and most reliable method. Stand a representative slab on a load cell and log it. A rockwool rule that transfers well to coir is to irrigate once the slab has lost a set weight (500 g on a standard slab is a common trigger). The useful number is the difference between the day's maximum and minimum, not the absolute weight. A slab at 36.6 kg at its wettest and 33.0 kg at its driest has dried back 9.8 percent.
Dielectric moisture sensors read water content, EC and temperature in one probe. Two cautions with coir. The generic coir calibration on most sensors is an average over many blends, and the differences are real. Pure pith tends to read high above about 60 percent water content, and coarse pith-and-chip mixes can show up to 10 points wetter than they are at saturation. Do a one-point check against a weighed slab at full saturation and apply the offset. Then place the probe mid-depth, between plants and midway between drippers, in a slab that represents the average of the house, and don't move it.
Typical sensor targets for coir run around 50 to 65 percent water content, lower than rockwool's 60 to 80. Don't chase someone else's absolute number. Chase your own daily shape: a morning drop, a rise to a late-morning plateau with drain, a steady afternoon, a clean dry-back overnight.
Pots, slabs and open-top bags
The same physics applies, but geometry changes the schedule.
Slabs (10 to 20 litres of coir lying flat, usually 100 cm long, 4 plants) are shallow with a large footprint. Water spreads poorly sideways, so dripper placement and drain-slit position decide everything. Cut the slits low on the side, not the bottom, so a shallow reservoir remains.
Open-top bags sit taller on a smaller footprint, so water moves down rather than across, drain comes faster per shot, and the top layer dries out and collects salt if shots are too small to wet it through. Setup is covered in grow bags explained, and our open-top slabs are made for this drip set-up.
Pots, especially under 10 litres, need bigger shots (10 to 15 percent of volume), fewer per day (3 to 8) and 10 to 20 percent drain. One dripper placed off-centre beats one straight at the stem.
The mistakes that cost yield
Watering by the clock in low light. The schedule that was right in July drowns roots in November. If your controller can't run on radiation, halve the shot count on dull days and start later.
Too few, too small shots. Four shots a day that each wet the top five centimetres under the dripper leave the rest of the slab dry and salty, while drain percentage looks fine because water runs straight down a channel. Feel the far corner of the slab. If it's dry, increase shot size until the whole slab wets, then control volume with frequency.
No drain for days. Root-zone EC can double in a fortnight. By the time leaves show it you've lost a truss. Check drain EC at least twice a week.
Drain from the first shot. The slab went into the night too wet. Pull the last shot earlier.
Treating all coir alike. Fine pith and a 50:50 pith-and-chip slab hold different amounts of water and air. Re-tune when you change supplier or blend. If you're moving from rockwool, read the coir vs rockwool comparison first, because the start-up weeks are where the differences bite.
Skipping pre-saturation. A dry pressed slab needs full hydration and a flush before planting. Trials have run 40 litres per slab per day for a week beforehand. Unbuffered coir, including ours (washed with water, no calcium nitrate), will still swap some potassium and sodium for calcium in the first weeks. Extra drain then is cheaper than correcting the crop later.
Tomato-specific numbers (density, EC by stage, truss work) are in growing tomatoes in coco coir.
FAQ
How many times a day should I water coco coir?
For a drip-fed fruiting crop on slabs, 5 to 15 shots a day is normal, rising to 18 or more on very bright summer days and dropping to 2 to 4 on dark winter days. Hand-watered pots are usually 1 to 3 times a day. The number of shots should follow light, not the clock.
What drain percentage should I aim for in coco coir?
Between 10 and 30 percent of the daily volume over 24 hours for most of the season. Use the low end on dull days and up to 30 to 40 percent on very bright days. If drain EC climbs more than about 1 mS/cm above drip EC, you need more drain regardless of the percentage.
Can you overwater coco coir?
Yes. Coir holds water more tightly than rockwool, so a slab that is kept saturated with no overnight dry-back runs short of air and roots die back, especially in low light. The signs are no weight loss overnight, drain from the first shot of the day, and later a crop that wilts in sun despite a wet slab.
How much should coco coir dry back overnight?
For slabs, a 7 to 10 percent drop in water content between the last shot and the first shot the next morning, with 10 to 15 percent as the total daily range from wettest to driest. Pots can take 15 to 25 percent. Use a slab weight or a moisture sensor to measure it, not the look of the surface.
When should I start and stop irrigating each day?
Start 1 to 2 hours after sunrise on bright days and 2 to 3 hours after on dull ones, or once the slab has lost 1 to 1.5 percent water content. Stop 1 to 2 hours before sunset on bright days and 3 to 4 hours before on dull ones so the slab has time to dry back.
How big should each irrigation shot be?
About 3 to 5 percent of the substrate volume per shot for drip-fed slabs, which works out to roughly 100 to 150 ml per dripper on a 12 litre slab with four plants. Pots watered a few times a day need 10 to 15 percent of the container volume per event.
Do coco coir slabs need more or less water than rockwool?
Roughly the same daily total, since the plant decides that, but fewer and slightly larger shots. Coir drains and dries back more slowly, so growers generally run fewer events than on rockwool and target a smaller overnight dry-back. Drain EC also tends to sit a little higher on coir because it holds onto nutrients.
How do I know if my moisture sensor is reading coir correctly?
Saturate a slab, weigh it, let it drain, and compare the sensor's reading to the water content you calculate from the weight. Coir blends differ enough that the generic coir setting can be off by several points, particularly above 60 percent water content. Apply the offset and then track daily changes rather than absolute values.
If you're setting up a slab or grow-bag crop and want to talk through blend, bag size or drain layout, get in touch.