Greenhouse tomatoes grown in substrate such as coir typically need 15 to 22 litres of water per kilo of fruit, against roughly 60 litres for well-managed drip-irrigated field tomatoes and 100 to 300 litres for open field production in general. Most of the saving comes from much higher yield per square metre and from catching the drain instead of losing it below the root zone. Recirculating that drain takes another third or so off. Per hectare, though, a substrate greenhouse often uses more water than a field. It just turns that water into far more fruit.
Litres of water per kilo, by crop and system
These figures are irrigation water per kilo of fresh marketable produce. They come from different climates and years, so read them as ranges and directions, not exact benchmarks for your site.
| Crop | Open field | Greenhouse, drain-to-waste or soil | Greenhouse substrate with drain re-use |
|---|---|---|---|
| Tomato | 100–300 in general, about 60 with good drip irrigation in a dry climate | 27–40 in unheated plastic houses, about 22 in heated glass with CO2 | About 15, and around 4 estimated in a fully closed greenhouse that recovers condensate |
| Sweet pepper | About 300 | Not reliably published | About 23 |
| Cucumber | About 600 in a hot, dry climate | Not reliably published | About 14 |
| Lettuce | About 250 in a desert field | Not reliably published | About 20 in a hydroponic greenhouse |
| Strawberry | No solid per-kilo pair published | See below | See below |
A soil-versus-substrate comparison inside the same hot, arid greenhouse put tomatoes at about 65 litres per kilo in soil and about 41 in a soilless open system with a little drain re-use. That's half the irrigation water. The soil plots still yielded more fruit and made more money, which is a useful reminder that water per kilo isn't the only number on the farm.
The one coir-specific side-by-side worth knowing is from cut roses on a commercial farm in Kenya. Over six months, beds in cocopeat with drain re-use used 411 litres per square metre against 1,098 in soil, a 65% cut, and harvested 24% more stems per square metre.
Strawberries are where the story gets honest. For drain-to-waste coir in a cool climate, water and fertigation inputs have been estimated at more than double a field crop, because the field crop gets free rainfall and the bag doesn't. In a Spanish trial, on-demand irrigation of substrate strawberries used 763 cubic metres per hectare against 1,071 for a conventional soil crop, a 28% saving. So the system decides it. A bag that runs 30% drain to the ground can lose to soil. A bag on a demand-driven schedule with collected drain usually won't. For crop-specific setup see coir substrate for soft fruit.
Where the savings actually come from
Water per kilo is water divided by yield. Substrate culture improves both halves.
| Source of saving | What happens in soil | What happens in a coir slab or bag |
|---|---|---|
| Yield per m² | Field tomatoes give about 3–8 kg/m² per year | Greenhouse tomatoes give 50–80 kg/m² per year |
| Deep percolation | Water below the root zone is gone, often with nitrate | Everything that drains out of the slab can be measured and collected |
| Transpiration | Full sun and dry air drive it up | Lower radiation and higher humidity under cover cut it |
| Shot control | Big, infrequent irrigations to wet a large soil volume | Many small shots sized to what the plant used since the last one |
| Recirculation | Rarely possible | Drain goes back into the mix tank after disinfection |
Transpiration is around 90% of what a crop takes up, so anything that lowers it per kilo moves the number. Coir's part in this is practical. It holds enough water to buffer between shots and enough air that you can run it on a measured drain fraction rather than flooding it. That balance is covered in water holding and air porosity in coir, and shot sizes and drain targets are in irrigating coco coir slabs.
Drain-to-waste vs recirculating
Free-drainage substrate systems usually let 5 to 50% of what they apply run out of the slab. That drain carries a full-strength nutrient solution. Dumping it wastes water and fertiliser, and it's the main environmental load from substrate growing.
| Drain-to-waste (open) | Recirculating (closed or semi-closed) | |
|---|---|---|
| Tomato water per kg | About 22 L | About 15 L |
| Water use difference | Baseline | 10–50% less, around 30% is a common figure |
| Fertiliser use | Baseline | Often more than 50% less |
| Extra kit | Collection gutters at most | Drain tank, disinfection (UV, ozone, heat or slow sand), water analysis |
| Main risk | Nutrient discharge | Sodium and chloride building up, root disease spreading through the water |
On roses, simple EC-controlled drain recycling has saved about 42% of water and 55% of nutrients with no loss of flower quality. Automated recycling on tomato has cut fertigation water by about 31% and nitrogen, phosphorus and potassium by roughly 30% each.
The sodium ceiling
Recirculation stops being free when sodium builds up. Plants take up very little sodium or chloride, so whatever comes in with the source water stays in the loop and climbs with every top-up. Once it reaches the crop's limit, you have to bleed off part of the solution or dump the tank.
Older Dutch rules allowed discharge only once sodium in the drain reached 3 to 8 mmol per litre, depending on crop. Roughly 4 mmol/L applied to roses and 8 mmol/L to tomato, which tells you tomato tolerates about twice what roses do. Even in zero-emission trials of sweet pepper on coir, sodium was still the main reason water had to leave the system.
The fix is at the intake. Roof rainwater, reverse osmosis water or a low-sodium well lets you recirculate far longer. In hot regions with salty groundwater the limit arrives much sooner, which is covered in coir in hot climate greenhouses. Unbuffered coir can also release some sodium and potassium into the first drains of a new slab, so check drain analysis in the first weeks before you send it back to the tank.
Discharge rules, in general terms
The Netherlands has required substrate growers to recirculate since around 2000. Since then the rules have tightened in steps:
- Crop-specific nitrogen emission norms that fall over time, with the sector aiming for near-zero emission.
- Yearly reporting of water use, discharge and fertiliser use.
- Any drain water discharged to sewer or surface water must first pass through purification that removes at least 95% of crop protection products, unless the grower can show they don't discharge at all.
Other European countries treat greenhouse leachate in nitrate vulnerable zones much like an industrial discharge, and several growing regions outside Europe now set limits on nitrate in run-off. The details and dates change, so check your local water authority before you design the drain side of a new house. The direction everywhere is the same: collect the drain, re-use it, and only discharge what sodium forces out.
The water used to make the coir
Coir isn't water-free before it reaches you. Raw coco pith carries soluble salts, mostly potassium, sodium and chloride, and it's washed with plain water to bring the EC down. Washing is the main water use in processing, and it happens in coconut-growing regions that mostly get a long monsoon. The salty wash water is usually settled, treated or applied to coconut groves, which tolerate salt.
We couldn't find a published per-tonne washing figure solid enough to put in a table, and the real volume depends on how salty the raw pith is and how low the EC target sits. What we can say is that the washing water is spread over every crop the slab grows. A slab used for two or three seasons carries a third of that burden per crop, and spent coir can go on as a soil improver afterwards (see what to do with spent coco coir). For the full footprint picture, including transport and land use, see the environmental impact of coco coir.
Worked example: one hectare of greenhouse tomatoes
Assume a modest 50 kg/m² yield, which is 500 tonnes per hectare per year.
| System | Water per kg | Water per hectare per year | Water per m² |
|---|---|---|---|
| Coir slabs, drain-to-waste | 22 L | 11,000 m³ | 1,100 L |
| Coir slabs, recirculating | 15 L | 7,500 m³ | 750 L |
| Difference | 7 L | 3,500 m³ | 350 L |
| Same 500 t grown in the field at 60 L/kg | 60 L | 30,000 m³ spread over about 6–17 ha | About 300 L per season |
Three things stand out. Recirculating saves about 3,500 m³ a hectare a year, which is roughly the drain the open system was throwing away. Per square metre, the greenhouse uses more water than a field season does, because it grows all year. And to grow the same 500 tonnes outdoors you'd need several times the water and somewhere between 6 and 17 hectares of land.
If your source water is salty and you bleed 10% of the drain to manage sodium, expect to land between the two greenhouse figures rather than at 15 L/kg.
About our coir
CVM Fiber and Industries makes coco peat blocks, briquettes, grow bags and open-top slabs, husk chips and custom blends in Kanyakumari, Tamil Nadu, India. Our coco peat is washed with water to reduce EC. We don't use calcium nitrate or other chemicals, and it isn't buffered. If you're planning a recirculating coir setup and want to talk through slab or bag formats, see our products or get in touch.
FAQ
Does coco coir use less water than soil?
Per kilo of produce, usually yes, often by a factor of three or more for greenhouse vegetables. Per hectare, a year-round substrate greenhouse can use more water than a field season. The saving comes from much higher yield and from collecting drain that soil would lose below the roots.
How many litres of water does it take to grow 1 kg of tomatoes?
Open field figures run from about 60 litres with good drip irrigation to 100–300 litres in general. Heated greenhouse substrate culture uses about 22 litres with drain-to-waste and about 15 with drain re-use. Fully closed greenhouses that recover condensate have been estimated at around 4 litres.
How much water does recirculating the drain save?
Closed systems use 10–50% less water than open ones, and around 30% is a common figure, with fertiliser savings often above 50%. On greenhouse tomato that is roughly 7 litres saved per kilo of fruit.
Why can't I recirculate forever?
Sodium and chloride from the source water build up in the loop because plants take up little of either. When sodium reaches the crop's limit, roughly 4 mmol/L for roses and 8 mmol/L for tomato under older Dutch rules, you have to bleed or dump. Low-sodium source water such as rainwater lets you recirculate much longer.
Is coir better than rockwool for water use?
Water per kilo is set mainly by climate, yield and whether you recirculate, not by the slab material. In zero-emission sweet pepper trials both substrates ran fully recirculated, with yields of 26.5 kg/m² on coir and 28.1 kg/m² on rockwool. Management of the drain matters more than the substrate.
Do strawberries in coir bags use more water than field strawberries?
They can. Drain-to-waste coir in a cool climate has been estimated to need more than double the water and fertigation of a rain-fed field crop. With demand-driven irrigation the picture flips, and one Spanish trial used 28% less water in substrate than in soil.
How much water is used to wash coco peat?
Coco peat is washed with plain water to lower its salt content, and that is the main water use in processing. Reliable published per-tonne figures are scarce, and the volume depends on the raw material and the EC target. Ask your supplier how they wash and where the wash water goes.
Can I discharge drain water from a coir system?
It depends on where you grow. In the Netherlands, discharged greenhouse water must first pass through purification that removes at least 95% of crop protection products, and nitrogen emissions are capped by crop. Elsewhere rules vary, so check with your local water authority.