The short answer: flush coco coir when the drain EC has climbed well above the feed EC and stays there, or when a root-zone test lands in the "extreme" band. Run two to three times the container's volume of water through it, in several passes rather than one flood, using a light feed at a quarter to half strength and a pH around 5.8 rather than plain tap water. Then go back to full feed with 15 to 30% drain so it doesn't happen again. Here's the detail.
Key facts
- Salts accumulate when plants take up water faster than nutrients and the leftover concentrates in the pore water. Leaching only happens when water actually drains out.
- A drain EC 0.2 to 0.3 mS/cm above feed is a warning. A gap of around 0.5 and rising, or a root-zone test above 7.8 mS/cm by pour-through, means flush now.
- For pots, two to three times the pot volume of water removes most of the free salts. In trials on composted substrates, eight container volumes took out 84 to 96% of soluble salts.
- Routine drain targets: 10% of applied water when your source water is below 1.0 mS/cm, 20% at about 1.5, 30% above 2.0.
- Water with alkalinity above 100 to 120 ppm CaCO3, sodium above 50 ppm or chloride above 70 ppm will keep refilling the coir with salts no matter how often you flush.
What salt build-up in coco coir actually is
Every fertigation delivers dissolved salts. The plant takes up water and some of the nutrients, and the rest stays in the pore water, concentrating as the slab dries between irrigations. If little or none of it drains, the EC around the roots ratchets up a bit each day.
Three things make coir prone to it:
- Low drain percentage. On dull days, or when volume is cut to save fertiliser, drain falls below 15% and root-zone EC climbs. Drain is the only way salts leave the container.
- Deep drybacks. Past about 40% moisture loss from saturation, the EC in the remaining pore water spikes sharply. The salts haven't increased, they've just concentrated.
- Hard or salty source water. Bicarbonate, sodium and chloride aren't taken up by the crop in any quantity. They sit in the coir and accumulate.
Coir adds its own twist. Its exchange sites hold potassium and sodium from the husk, and when you feed calcium and magnesium the coir swaps them, pulling Ca and Mg out of your feed and releasing K and Na. Our coco peat is washed with water to bring the EC down and isn't treated with calcium nitrate, so this exchange happens in the first weeks of the crop. See /blog/buffered-vs-unbuffered-coco-peat/ for what that means for your first feeds.
Symptoms
| What you see | What it usually means |
|---|---|
| White or yellowish crust on the surface or around drain holes | Salts wicking up and drying at the surface. Scrape it off before flushing or it dissolves straight back in. |
| Brown leaf tips and margins, older leaves first | High-EC injury. Marginal burn and wilting appear in the "very high" band and above. |
| Wilting while the coir is wet | Osmotic stress. Roots can't pull water against the salt concentration. |
| Drain EC rising week on week at constant feed EC | Salts accumulating faster than they're leached. |
| Lower leaves dropping, small new growth | Chronic high salts, often with root tip damage. |
A symptom alone isn't enough to flush. Calcium deficiency, root disease and heat stress all mimic tip burn. Confirm with a drain reading and a root-zone test first.
How to decide it's time
Compare the EC of what goes in with the EC of what comes out, measured the same way each time. /blog/coco-coir-ph-and-ec-management/ covers the pour-through and 1:1.5 methods and the target bands, so I won't repeat them here.
| Drain EC relative to feed EC | Action |
|---|---|
| Within 0.2 to 0.3 mS/cm either way | Healthy. Keep going. |
| 0.2 to 0.3 above feed | Coir is starting to hold salts. Increase irrigation volume, not strength, until drain reaches 15 to 20%. |
| 0.5 or more above feed, and rising over several days | Flush. Some growers wait until the gap reaches about 1.0, but by then symptoms are usually visible. |
| Root-zone test in the "extreme" band (above 7.8 mS/cm pour-through, above 2.25 by 1:2) | Flush immediately, whatever the drain reading says. |
Two cautions. Drain EC is not root-zone EC, because coir retains and releases ions, so a single high reading on a hot afternoon can mislead. Measure at the same time of day, an hour after irrigation, on several containers. And drain EC below feed EC in the first weeks of a new unbuffered slab is normal, not a reason to feed harder.
What to flush with
Plain tap water is the usual forum advice. In coir it's often the wrong call.
| Flush water | Pros | Cons |
|---|---|---|
| Plain tap water | Free, simple | Carries its own bicarbonate, sodium and chloride. Hard water can push substrate pH up while you're trying to fix EC. |
| Pure RO or rain water | Removes free salts fastest | Contains no calcium or magnesium, so coir's exchange sites strip them from your next feed. Repeated RO flushing is how growers end up with calcium deficiency after a "clean" flush. |
| Quarter to half-strength feed, pH about 5.8 | Keeps Ca and Mg on the exchange sites, no shock to roots, removes the excess | Slightly slower. You'll see anything from a quarter strength to half strength recommended. |
My recommendation: half-strength feed, adjusted to pH 5.8 with acid. For a severe crust with drain far above target, start with one pass of acidified low-EC water, then light feed for the remaining passes.
Step-by-step flush
Pots and containers
- Scrape off any surface crust and discard it.
- Measure the pot volume. A 10-litre pot needs 20 to 30 litres of flush water in total.
- Apply in passes of a third to half the pot volume, letting each drain fully before the next. One big flood channels down the sides and leaves the core untouched, especially if the coir has dried back hard. If it has, rewet it first using the method in /blog/rewetting-dry-coco-coir/.
- Measure drain EC after each pass. Stop when it's within about 0.3 of your flush solution, or at three pot volumes.
- Let the pot drain for an hour, then feed at normal strength with 20 to 30% drain.
Slabs and open-top slabs
Slabs are flushed through the drip system over a morning rather than by hand. The Dutch routine for bringing slab EC down works well in coir:
- Start irrigating early, before radiation reaches about 600 W/m² or 400 J/cm² cumulative.
- Make the first cycles large: 4 to 6% of the slab volume per cycle, so a 15-litre slab gets 600 to 900 ml per cycle.
- Aim to reach first drain within the first three or four cycles.
- Run the following cycles at 40 to 80% drain for the rest of the morning. Use your normal feed or a half-strength version, not plain water.
- Check drain EC and pH at midday. Repeat the next morning if the gap to feed EC is still above 0.5.
- Once drain EC settles, return to normal volumes with 15 to 20% drain per cycle.
For tomatoes on long-season slabs this is usually a four-day job, not a single morning. In coir tomato trials, root-zone EC came down simply by raising irrigation volume over four days. More on slab management in /applications/tomatoes/.
Grow bags
A grow bag is a smaller slab with two or three plants, so the method is the same, with less volume and a quicker response. Check that the drain slits are open and low on the bag. A bag with blocked or high slits holds a saline pool at the bottom that no amount of flushing from the top will clear. Cut fresh slits at the base if needed. See /products/grow-bags/ for the formats we make.
Re-establishing feed afterwards
Don't leave the coir sitting in low-EC water for days. Empty exchange sites will pull calcium and magnesium out of the first full-strength feed, so:
- Go back to full feed at the next irrigation, not a week later.
- Keep drain at 20 to 30% for a few days, then step down to 15 to 20% once drain EC tracks feed EC.
- Retest the root zone with the same method two or three days later, once the coir is back at normal moisture. A saturated slab reads low.
Preventing it coming back
Flushing is a reset. Prevention is drain management and water quality.
Drain percentage targets
Leaching fraction is drain volume divided by applied volume. The right number depends on your source water:
| Source water EC | Routine drain (leaching fraction) |
|---|---|
| Below 1.0 mS/cm | 10% |
| About 1.5 mS/cm | 20% |
| Above 2.0 mS/cm | 30% |
Greenhouse slab guidance is 15 to 30% drain overall, 15 to 20% per cycle once slab EC is in range, with 30% over 24 hours as the older checkpoint. Below 15%, root-zone EC rises. Measure drain on three or more containers per zone, on a clear day, an hour after the last cycle.
Water treatment
Get your source water tested. Guidelines for greenhouse irrigation water:
| Parameter | Recommended | Treatment usually needed |
|---|---|---|
| EC | 0.3 to 1.0 mS/cm | above 2.2 not recommended for sensitive crops |
| Alkalinity (as CaCO3) | up to 100 ppm | above 200 ppm (problems often start around 120) |
| Sodium | up to 30 ppm | above 50 ppm |
| Chloride | up to 50 ppm | above 140 ppm, with foliar damage possible above 70 |
High alkalinity is the one that catches people out. Water pH itself matters little, but bicarbonate raises substrate pH week after week, and flushing with that water makes it worse. Acid injection to bring alkalinity down to around 40 to 80 ppm fixes it. Never neutralise to zero, because the resulting low pH damages roots. Sodium-softened water should not be used at all. For high sodium or chloride, RO blended back with raw water to a known EC is the practical route, with drain raised to match.
Common mistakes
- Flushing with RO or rain water repeatedly. Strips calcium and magnesium from the exchange sites. The next problem will look like calcium deficiency, and it is.
- Flushing with high-alkalinity tap water. You trade a salt problem for a pH problem. Acidify first.
- One huge flood. Water channels down the sides of a dry pot and the core stays salty. Several passes, and rewet first if the coir has dried hard.
- Acting on a single drain reading. Afternoon drain on a hot day is always higher. Compare like with like over several days.
- Flushing a healthy crop on a schedule. Weekly flushes sold as maintenance mostly wash fertiliser down the drain. If drain EC tracks feed EC, there's nothing to flush.
- Not fixing the cause. If drain is 5% and the water has 180 ppm alkalinity, the flush will hold for a fortnight.
If you're specifying coir for a fertigated crop and want washed, unbuffered material you can condition to your own water, /contact/ us with your water analysis and crop.
FAQ
How often should I flush coco coir?
Only when the numbers say so. If drain EC stays within about 0.3 mS/cm of feed EC and the plants look right, there's nothing to flush. Most growers with managed drain of 15 to 30 percent never need a full flush during the crop. A pre-plant rinse of new coir and a flush when drain EC drifts 0.5 or more above feed and keeps rising is the realistic pattern.
Should I flush coco with plain water or nutrient solution?
Use a light nutrient solution, around a quarter to half of your normal strength, adjusted to about pH 5.8. Plain water removes free salts but leaves coir's exchange sites empty, and they then strip calcium and magnesium from your next feed. Tap water also brings its own bicarbonate and sodium. Pure RO water once is fine for a severe crust, followed by light feed for the remaining passes.
How much water does it take to flush a coco pot?
Two to three times the pot volume in total, applied in several passes rather than one flood. A 10-litre pot needs 20 to 30 litres. Measure the drain EC after each pass and stop when it reads within about 0.3 of the flush solution. Trials on composted substrates needed about eight container volumes to remove 84 to 96 percent of soluble salts, so a stubborn pot can take more.
What causes a white crust on coco coir?
Salts from fertiliser and hard water wick up with the water as the surface dries and are left behind when it evaporates. It's most common where drain is low, where top-dripping wets only part of the surface, and where source water is high in bicarbonate, sodium or chloride. Scrape it off before flushing or the salts dissolve straight back into the coir.
My runoff EC is higher than my feed EC. Do I need to flush?
Not necessarily. A gap of 0.2 to 0.3 mS/cm means the coir is starting to hold salts, and the fix is more irrigation volume at the same strength until drain reaches 15 to 20 percent. Flush when the gap is 0.5 or more and rising over several days, or when a root-zone test lands in the extreme band. A single high reading on a hot afternoon is not enough on its own.
Can flushing coco coir cause calcium deficiency?
Yes, if you flush repeatedly with RO, rain or distilled water. Coir's exchange sites release potassium and sodium and bind calcium and magnesium. Low-EC water leaves those sites empty, so they pull calcium and magnesium out of the first feeds afterwards and the plant goes short. Flush with a light feed instead, and return to full feed at the next irrigation.
What drain percentage stops salt build-up in coco?
Match it to your water. About 10 percent of applied water when the source water is below 1.0 mS/cm, 20 percent at about 1.5, and 30 percent above 2.0. Greenhouse slab guidance is 15 to 30 percent overall and 15 to 20 percent per cycle when slab EC is in range. Below 15 percent the EC around the roots climbs. Measure it on several containers, an hour after irrigation, on a clear day.