Switching a greenhouse to coir is mostly an irrigation and feed-recipe job, not a construction job. Your gutters, drippers and fertigation unit will almost certainly stay. What changes is how often you water, how big each shot is, what you feed in the first month, and how you read your slab sensors. Do it in one trial bay for a full season before you convert the whole house, and the changeover is low-risk.
This article is for the manager planning that changeover. Crop-specific EC and pH targets are in the tomato, cucumber and pepper and strawberry guides, so they aren't repeated here. If you're still deciding whether to switch at all, read coco coir vs rockwool first.
What actually changes when you move to coir
Rockwool is inert. It holds what you put in and gives it back. Coir is an organic fibre with exchange sites, so it holds water longer, releases it more slowly, and swaps ions with your feed for the first few weeks. Peat is organic too, but it comes limed and buffered from the factory, which coir usually doesn't. Everything below follows from those two facts.
| Operation | Rockwool habit | Peat habit | What coir needs |
|---|---|---|---|
| Irrigation frequency | Small shots, 10 to 20 a day | Water when surface looks dry | Fewer shots than rockwool, roughly 1 to 12 a day by slab volume, never by surface look |
| Shot size | Smallest possible | Large, infrequent | Moderate. Pepper trials held the same slab moisture with 112 ml per shot in coir against 150 ml in rockwool |
| Drain target | 15 to 30% | 10 to 15% in bags | 10 to 25% commonly quoted, about 20% in trials |
| Start of irrigation (slab water content) | 70 to 80% | n/a | Lower, around 65% in pepper trials. Use a coir-calibrated sensor |
| Drain EC vs feed | Close to feed | Within 5 to 10% of feed | Wider band. Up to 1.5 times feed is a normal ceiling |
| Start-up feed | Standard recipe | Standard recipe | Extra calcium and magnesium, less potassium, for 2 to 4 weeks |
| Lime | None | Built into the mix | None. Don't carry over a peat liming habit |
| Dry-back steering | Fast and strong | Weak | Slower. Steer with EC and blend choice more than with water content |
Irrigation frequency and shot size
The most common first-season error, by a wide margin, is overwatering. Growers keep a rockwool schedule and drown the slab, or they keep a peat habit and water when the top looks dry while the bottom is saturated. Coir rewets easily and holds water well, so a bag that looks dry on top is often wet underneath.
Start by lifting a bag. Then set shots from measured slab volume, drain percentage and crop demand, not from the old programme. Coir usually wants fewer, slightly larger shots than rockwool. A coarser pith-and-chip blend drains faster and takes more frequent pulses than a fine pith slab. Specify the blend around your irrigation system, not the other way around. Our grow bags and open-top slabs are made to order in pith-to-chip ratios for exactly this reason.
Drain targets and drain EC
Rockwool growers are used to 15 to 30% drain. In coir, trials typically landed around 20% and commercial figures run 10 to 25%. Lower drain is possible because the substrate buffers EC. The catch is that drain EC in coir runs higher above the feed than you're used to, because the fibre adsorbs and releases ions. Rockwool rules like "drain within a few tenths of feed" will have you flushing a healthy slab. A drain EC up to about 1.5 times your input is a reasonable ceiling. Above that, raise frequency or drain, as you would in any substrate.
Start-up EC
Ask for the EC of the batch you're buying and the test method it was measured by, because figures from different methods aren't comparable. Washed coir at low EC shouldn't need leaching on the farm. Hydrate with water or dilute solution, let the slab stand a day, cut drain slits low in the side, then charge it with nutrient solution at the EC you'll feed the young crop before planting. One Belgian tomato grower who moved 13 hectares from rockwool to coir dripped about four litres of water per bag before planting and lost no time in the schedule. Roots had filled the bags in about three and a half weeks.
Feed recipe: calcium, magnesium, potassium
Washed, unbuffered coir arrives with potassium and sodium on its exchange sites. The first feed swaps them: calcium and magnesium are pulled from solution and potassium is released. For the first two to four weeks you raise calcium and magnesium above your normal recipe and trim potassium, then return to standard. Fail to do this and the crop shows calcium or magnesium deficiency that growers often blame on the coir rather than the recipe. Buffered coir has had this exchange done at the factory, but needs magnesium supplied and isn't accepted in organic production. The trade-off is explained in buffered vs unbuffered coco peat.
Two more recipe points. Some organic raw materials tie up nitrogen in the first weeks, so ask your supplier whether the blend does and add nitrogen early if so. And drop any lime that was in your peat recipe. Coir's own pH is already in the 5.5 to 6.5 band, and lime pushes it high enough to lock out iron, manganese and zinc.
A changeover plan that works
Convert one bay, run it a whole season, then decide. A season is the minimum because the behaviours that catch people out happen at different times: ion exchange in the first month, salt build-up by mid-season, and dry-back behaviour under high summer light.
Before the trial
- Pick a bay on its own irrigation valve, so you can run a different programme without affecting the rest of the house.
- Order a representative sample and hydrate it the way you'll hydrate the real delivery. Measure hydrated height, volume and uniformity. Check EC and pH of the drain from the test slab.
- Decide the blend (fine pith, or pith with chips) and bag volume with your supplier against your dripper count, plant density and gutter width.
- Fit a slab sensor with a coir calibration in the trial bay, next to one with your existing calibration in a rockwool bay, so you can read both side by side.
What to measure, daily
| Measurement | Why |
|---|---|
| Drain volume as % of feed | Sets shot size and frequency |
| Drain EC and pH against feed | Tells you when the exchange sites have charged and when salts build |
| Slab water content curve (sensor) | Shows start time, overnight dry-back and how fast coir releases water |
| Uptake per unit of radiation | Lets you compare water and fertiliser use with the rockwool bays |
| Drain sodium | Rising sodium means increase drain, not feed EC |
What to record, weekly or at season end
- Plant observations: rooting speed, leaf colour in weeks 2 to 6, truss set under low light.
- Yield and grade against a matched rockwool or peat bay.
- Labour hours for slab laying, planting, clean-out and disposal.
- Water and fertiliser use per m².
- Disposal cost. Spent coir can be composted or spread. Rockwool has to be collected or landfilled.
If the trial bay matches or beats the control on yield and costs less to dispose of, you have your business case. If it's behind, the data tells you whether the gap is irrigation (drain percentage and water content), feed (drain EC and early deficiency), or blend (dry-back too fast or too slow).
Equipment compatibility
Drippers and stakes. Your existing pressure-compensating drippers and stakes carry over. Place the dripper away from the drain slit so solution travels through the bag before it leaves. One dripper per plant is normal. If you use two plants per bag, fit two.
Gutters. Coir slabs ship compressed and expand on hydration, mostly in height. Check the hydrated height and width against your gutter and plant-block clearance before ordering a full house. Standard slab footprints match common gutter widths, but confirm it on the sample.
Slab sensors. This is the one place rockwool kit doesn't transfer directly. Capacitance sensors read differently in coir than in stone wool, and generic coir calibrations still show meaningful differences between coir mixes. Most sensor vendors offer a coir profile. Use it, then check it with one gravimetric sample from a slab at field capacity and correct the offset. Compare bays on trends, not absolute percentages, until you've done that.
Recirculation and disinfection. Coir drain water carries humic acids, tannins and other organics, especially early in the season. They add load to UV disinfection and to carbon and sediment filters. Expect to check filter intervals and UV transmission in the first weeks, and allow for more frequent filter cleaning than with rockwool drain.
Fertigation unit. No change in hardware. You'll be editing recipes, not replacing equipment.
Staff training points
Keep the briefing short and concrete:
- Don't water by eye. The top of a coir bag dries before the middle. Lift the bag or read the sensor.
- Drain EC will sit higher above feed than in rockwool. That's normal. Flush only when it crosses the ceiling you've set.
- Early yellowing in weeks 2 to 4 is usually the calcium and magnesium exchange, not bad coir. Check the recipe first.
- Cut drain slits low, after hydration, not before.
- Sensors in coir bays read on a different scale from rockwool bays until recalibrated. Don't compare numbers across bay types.
- Handle bags by the body, not the film. Torn film means uneven wetting.
Storage and handling of bags on site
Compressed slabs and bags are dry and light until hydrated. Keep them off the floor, out of rain and sun, in the sealed pallet wrap until the day they go onto the gutter. A bag that gets wet in storage will expand, lose its shape and can grow mould before it's ever planted. Rotate stock so the oldest pallet goes out first. Full detail, including humidity and shelf life, is in how to store coco coir.
On laying day, place bags, cut dripper holes in the top film, set drippers and hydrate. Don't cut drain slits until the bag is fully expanded, or the slab won't hold the reservoir that gives coir its forgiving nature.
Common first-season mistakes
| Mistake | What you see | Fix |
|---|---|---|
| Keeping the rockwool irrigation programme | Saturated slabs, slow rooting, Pythium | Reset frequency and shot size from drain % and sensor data |
| Watering on surface dryness (peat habit) | Same as above | Lift bags, trust the sensor |
| Standard feed from day one | Ca or Mg deficiency in weeks 2 to 4 | Raise Ca and Mg, trim K, for the first month |
| Adding lime from the peat recipe | pH climbs, iron and manganese lock-out | Drop the lime |
| Flushing on rockwool drain-EC rules | Unnecessary water and fertiliser use | Set a wider drain EC band, up to about 1.5 times feed |
| Using the rockwool sensor calibration | Water content readings make no sense | Load a coir profile and check against a gravimetric sample |
| Comparing EC from different test methods | Supplier and farm numbers disagree | Ask for the method, test the same way |
| Letting slabs get wet in storage | Lumpy, mouldy bags at planting | Keep pallets wrapped, dry and off the floor |
| Infrequent, heavy feeds | Salt build-up, falling drain pH | Little and often |
End-of-season handling
Coir doesn't have to be thrown away after one crop. Many growers run slabs for a second cycle, sometimes without steaming, after cutting roots out and flushing. Others pasteurise or take the coir out to compost or field spreading. The choice depends on disease pressure in the first crop and on how the slabs held their structure. The options, and how to prepare used slabs, are in reusing coco coir. Factor the disposal saving against rockwool into your trial-bay costing from the start.
How to brief your supplier
The more you specify, the fewer surprises on laying day. A greenhouse brief should include:
- Crop, plant density, planting date and expected season length.
- Blend: fine pith, or pith with chips, and the ratio you want. If you aren't sure, describe your irrigation system and let the supplier propose.
- Bag or slab dimensions, both compressed and hydrated, and expansion volume.
- Moisture, pH and EC limits, with the EC test method named.
- Washing and buffering status. Say plainly whether you want washed-only or buffered coir.
- A chemical and physical analysis for the batch: EC, pH, main ions and, if available, water uptake behaviour.
- Raw-material origin and whether anything in the blend ties up nitrogen.
- Packaging, pallet configuration and film type for your climate.
- A sample for a hydration test before you confirm the order.
Our coco peat is washed with water to lower EC. No calcium nitrate or other chemicals are used, so it isn't buffered. That suits growers who want to run their own calcium and magnesium start-up or who need an unbuffered product for organic programmes. Blends, bag sizes and film are made to order. Greenhouse formats are on the greenhouse applications page.
If you're planning a trial bay for next season, send us your crop, bag size and irrigation set-up and we'll propose a blend and send a sample to hydrate.
FAQ
Can I keep my rockwool irrigation schedule when I switch to coir?
No. Coir holds water longer and releases it more slowly, so a rockwool programme will oversaturate it. Expect fewer, slightly larger shots and a lower starting water content. Set the new programme from measured drain percentage and a coir-calibrated sensor, not from the old recipe.
How long should a coir trial run before converting the whole greenhouse?
One full season. The behaviours that matter show up at different times: ion exchange in the first month, salt accumulation by mid-season and dry-back under summer light. A six-week trial tells you about rooting and nothing about the rest.
Do I need new drippers or gutters for coir slabs?
Usually not. Pressure-compensating drippers, stakes and gutters carry over. Check hydrated slab height and width against your gutter and block clearance on a sample, because coir expands on wetting. The item that does need attention is the slab sensor calibration.
Why do my slab sensors give odd readings in coir?
Capacitance sensors are calibrated for a specific medium, and rockwool profiles don't read correctly in coir. Load your vendor's coir profile, then check it with one gravimetric sample from a slab at field capacity. Until then, compare trends within a bay rather than numbers between bay types.
Why are my plants showing calcium deficiency in the first weeks on coir?
Fresh, unbuffered coir pulls calcium and magnesium from the feed and releases potassium until its exchange sites have charged. Raise calcium and magnesium and trim potassium for the first two to four weeks, then return to your standard recipe. Buffered coir has had this done at the factory.
What drain percentage should I aim for in coir?
Trials typically ran around 20% and commercial practice quotes 10 to 25%. That's a little lower than rockwool. Accept a drain EC further above feed than you would in rockwool, up to roughly 1.5 times input, before increasing drain or frequency.
Can I reuse coir slabs for a second crop?
Many growers do, after cutting out roots and flushing, with or without steaming depending on disease pressure in the first crop. Check slab structure first. If it has slumped, the air space is gone and it should be composted instead.
Should I buy buffered or unbuffered coir for a greenhouse changeover?
Unbuffered, washed coir costs less and is accepted in organic programmes, but you manage the calcium and magnesium start-up yourself. Buffered coir skips that step but still needs magnesium and isn't organic-compliant. Either works if the irrigation and recipe are set for it.