The short answer: coco coir does become water-repellent when it dries hard, but far less so than peat, and it recovers. In laboratory tests coir dried to 33% moisture by weight still took up water and reached its full container capacity after a few irrigations, while peat at the same moisture stayed hydrophobic (Michel, Jackson and Fonteno, Acta Horticulturae, 2021). The fix is not one big flood: it is several small applications of water, or a bottom soak, until the whole volume is dark and heavy again. Then keep it from dropping that low a second time.
Key facts
- Coir measured contact angles of 84° at 33% moisture and 70° at 50% moisture, both below the 90° hydrophobic threshold. Peat showed no capillary rise at all at 33% (Michel, Jackson and Fonteno, 2021).
- Coir at 45% initial moisture refilled to about 90% of its maximum in a single 200 ml irrigation; at 30% moisture it captured 29%, and at 15% only 21% (Fonteno, Fields and Jackson, Acta Horticulturae, 2013).
- Coir dried to 15% moisture without a wetting agent never reached the same maximum water content as wetter samples, even after ten irrigations (Fonteno, Fields and Jackson, 2013).
- A wetting agent raised coir's first-irrigation water capture from 0.20 to 0.30 cm³/cm³, but coir without one still reached full capacity (Michel, Jackson and Fonteno, 2021).
- Adding 15% coir by volume to peat kept the mix at about 55% volumetric water content through a wet-dry cycle (Bartley et al., HortScience, 2022).
Why dry growing media repel water
All organic growing media get harder to wet as they dry. Jean-Charles Michel's review (Vadose Zone Journal, 2015) describes two things happening at once: organic surfaces that attract water when moist expose water-repellent compounds (in peat, largely humic substances and waxes from decay) as the water film around them disappears, and the material shrinks, so the capillary pathways that would pull water in become fewer and narrower. The drier the particle, the higher its contact angle with water and the slower water enters.
The review ranks materials by how badly repellency develops on drying, from highly decomposed black peat (worst) through bark, white peat and wood fibre to coconut coir (least affected). The same group later defined four wettability classes: type 1 stays hydrophilic however dry it gets, while types 2 to 4 turn hydrophobic with increasing and partly irreversible loss of water retention (Michel, Jackson and Fonteno, Acta Horticulturae, 2017). Coir sits in or near the first class; white peat does not. Coir pith is lignin and cellulose rather than partly decomposed moss, with lignin at 20 to 40% of dry weight (Aileen Reid, The Passion Vine, 2021), and it carries little of the waxy coating that forms on dried peat. It is not immune, though: Fonteno, Fields and Jackson note in their 2013 paper that coir "took up little water when dry and without wetting agent".
How dry is too dry? The research numbers
Two studies from the same laboratory network give the clearest picture. Both express moisture as a percentage of total weight; for coir, 33% by weight is only 0.05 cm³/cm³ by volume and 50% is 0.10 cm³/cm³ (Michel, Jackson and Fonteno, 2021).
| Material and initial moisture (by weight) | Water held after 1st irrigation | Water held after 6 irrigations | Container capacity afterwards | Max capacity when never dried |
|---|---|---|---|---|
| Coir, 33% | 0.20 | 0.33 | 0.33 | 0.36 |
| Coir, 50% | 0.31 | 0.36 | 0.36 | 0.36 |
| White peat, 33% | 0.09 | 0.15 | 0.47 | 0.69 |
| White peat, 50% | 0.24 | 0.40 | 0.60 | 0.69 |
| Coir + wetting agent, 33% | 0.30 | 0.34 | 0.35 | 0.36 |
| White peat + wetting agent, 33% | 0.12 | 0.30 | 0.67 | 0.69 |
Values in cm³ water per cm³ substrate, from Michel, Jackson and Fonteno (Acta Horticulturae 1305, 2021), 200 ml water passed through a 200 cm³ column per event.
Even the drier coir got back to full capacity; drying only cost it extra irrigations. Peat lost a third of its capacity and did not recover it within the test.
The 2013 study went drier, testing coir at 45, 30 and 15% moisture by weight over ten irrigation events:
| Coir, initial moisture | Wetting agent | Share of maximum reached after 1 irrigation | After 3 | After 10 | Maximum water content |
|---|---|---|---|---|---|
| 45% | no | 0.89 | 0.99 | 0.99 | 69.6% vol |
| 30% | no | 0.29 | 0.59 | 0.88 | 71.8% vol |
| 15% | no | 0.21 | 0.46 | 0.85 | 64.3% vol |
| 45% | yes | 0.95 | 1.00 | 1.00 | 74.1% vol |
| 30% | yes | 0.43 | 0.82 | 0.98 | 71.6% vol |
| 15% | yes | 0.29 | 0.61 | 0.99 | 73.9% vol |
From Fonteno, Fields and Jackson (Acta Horticulturae 1013, 2013), wetting agent AquaGro-L at 187 ml/m³.
There is no single "critical" moisture figure for coir in the literature, but between 45% and 30% by weight coir stops wetting in one pass and starts needing several, and at 15% by weight, about as dry as a compressed block, untreated coir lost some of its maximum capacity within the test. Keep working coir well above that.
How to tell coir has gone hydrophobic
You rarely need a meter. The signs are the same in a 10 cm pot, a 1 m grow bag or an open bed.
- Beading. Drops sit on the surface or roll off instead of soaking in.
- Channelling. Water runs down the gap between the shrunken root ball and the pot wall, or through a few paths in a slab, and reaches the drain within seconds. Drain volume looks fine; the root zone stays dry.
- Light weight. A dry pot weighs a fraction of a watered one. Lifting a few pots per bench is still the fastest check there is.
- Shrinkage. Coir has pulled away from the container walls, or the slab has sunk and gone concave.
- Colour. Wet coir is dark brown; dry coir is pale tan. Dark on top and pale a finger's depth down means surface wetting only.
- Drain EC jumping. The same fertiliser salts sit in less water, so drain EC reads above the feed. Purdue University's substrate guidance notes that sampling dry pots also inflates EC readings.
A dried slab cut open usually shows a wet skin around a dry, dusty core: hydrophobic coir with water channelling around it.
Rewetting methods that work
The principle is the same every time: give water time to be pulled in by capillarity instead of forcing it through. Coir recovers with repeated hydration events, so the number of events matters more than the volume of any one.
Pots and trays: bottom soaking
- Stand the pots in a tray, trough or tank with a few centimetres of water, deeper for large pots, but never above the rim.
- Leave them until the surface darkens. Small pots take minutes; a large pot with a hard, shrunken root ball can take an hour or more.
- Lift, let drain, and check weight. If a pot is still light, repeat.
- Follow with a normal top irrigation once the coir is accepting water, so that salts pushed upward by the soak move back down.
The water front rises through the whole cross-section rather than finding one channel, which is why this is the method for plug trays and small liners.
Slabs and grow bags: pulsed irrigation
You cannot submerge a slab in a greenhouse, so use the dripper and time. Give short pulses, with a gap between them long enough for the previous pulse to soak in, and stop each pulse when drain appears. Each pulse adds a little, the coir swells and reopens pores, and the next pulse finds a more hydrophilic surface. A deeply dried slab can need several pulses over a day; the tables above show why. Weigh a slab or push a probe into the centre, not the dripper spot, to confirm the core has wetted. If the slab has shrunk away from the plastic, move the dripper stake towards the centre of the mass.
Loose coir and beds
Spread it thinly, wet with a fine rose, and turn it with a fork between passes, raking out pale pockets. A small volume applied five times beats the same volume applied once. Warm water has lower viscosity and surface tension than cold, so it penetrates faster; it is a cheap help for a batch of hard coir, but keep hot water away from established roots.
Wetting agents: when they are suitable
A wetting agent (surfactant) lowers the surface tension of water so it spreads over and into surfaces it would otherwise bead on. The tables above show the effect on coir: first-irrigation capture rose from 0.20 to 0.30 cm³/cm³ at 33% moisture, and at 15% moisture treated coir reached 99% of maximum after ten irrigations where untreated coir stopped at 85%. Fields, Fonteno and Jackson (HortScience, 2014) tested six components at 50% and 25% moisture with four surfactant rates and found that wetting agents improve hydration efficiency "but did not always overcome hydrophobicity".
Points to weigh before using one on coir:
- Type. Nearly all root-zone products are non-ionic (alcohol ethoxylates, block copolymers or organosilicone types). Anionic detergents carry a higher phytotoxicity risk; one hydroponic lettuce study found root damage from an anionic detergent at 250 ppm and above (summarised in Science in Hydroponics, 2025). Washing-up liquid is anionic and is not a substitute.
- Rate. Trials reporting benefits without injury have generally used about 50 to 1,000 ppm in the irrigation water (Science in Hydroponics, 2025). Follow the label; start low.
- Drainage changes. In tomato on new and reused coco fibre, a weekly 0.1% nonylphenol ethoxylate surfactant through the drip increased drain percentage and reduced water uptake, while leaving drain EC and pH unchanged (Guillén, Sánchez, Urrestarazu and Mazuela, Acta Horticulturae, 2005). Adjust irrigation volume after treatment or you will overdrain.
- Is it needed? For coir that beads, one application during rewetting is useful. For routine use, the research shows coir rewets on its own given several irrigations, and growers supplying residue-conscious buyers often prefer to add nothing. Coir-specific products are marketed on faster initial wetting of compressed bags (Cocogreen, reported in HortiDaily, 2015 and 2016); those are manufacturer claims, not independent trials.
Coir is in fact added to peat so that mixes rewet without surfactants. Michel and colleagues found 10% coir by volume enough to bring a 50%-moisture peat mix back to full capacity (2021), and Bartley and colleagues found 15% coir kept a peat blend at about 55% volumetric water content through a wet-dry cycle (HortScience, 2022).
Preventing it: moisture floors, scheduling, shade
Rewetting costs labour and sets the crop back. Prevention is cheaper.
- Set a moisture floor. The problem zone for coir is roughly 30% moisture by weight and below, far under any normal night-time set point, so a slab that gets there was missed by the irrigation rather than steered too lean. Weigh reference pots or slabs at the same time each day and set a minimum weight.
- Check the edges, not the average. End-of-row bags, pots under a vent and trays beside a fan dry first and go hydrophobic first.
- Match dry-back to the container. Night-time dry-back is a legitimate steering tool in slab crops, but shallow plug trays and propagation beds dry in hours in summer; a short midday pulse is cheaper than rewetting a tray of plugs.
- Shade and cover. Bare coir loses water fast. Light shade or a thin layer of coarse husk chips slows surface drying in propagation. Keep the white film on grow bags intact and planting holes small, and do not leave hydrated bags unplanted for days in a warm house.
- Avoid dusty grades for exposed uses. Fine pith crusts and beads once dry; a blend with some fibre or chip keeps pore channels open. Fibre itself holds little water, so use short cut fibre sparingly to avoid dry pockets (Reid, 2021).
Compressed blocks: hydrate them properly the first time
A compressed block is coir at its driest. The first wetting sets the structure: flood it quickly and the outside swells and seals while the core stays dry, leaving a hard, pale centre that channels water for the rest of its life. Give the block time, add water in stages, break it apart and wet any pale core before use. The procedure, water volumes and expected expansion are in how to use coco peat blocks; the formats are on the blocks page.
What happens to EC after rewetting
When coir dries, the dissolved salts stay behind, concentrated in the remaining water or precipitated on particle surfaces. The first water through a rewetted root zone dissolves them and carries them to the drain, so expect the first drain readings to be high, sometimes well above the feed EC, and then to fall over the next irrigations. That spike is real salt reaching roots, not a meter error. Rewet with plain water or a low-EC feed, drain to waste for the first cycles, and re-check before returning to the normal programme. The wetting agent is not the cause: in the coir tomato trial above, surfactant did not change drain EC or pH (Guillén et al., 2005). Sampling and correction methods are in coco coir pH and EC management.
Storing opened coir so it does not go hydrophobic
- Dry compressed products: keep blocks, briquettes and unexpanded bags dry, off the floor and under cover. What harms them is repeated wetting and drying, which makes them wet unevenly later.
- Opened bags of loose, moist coir: close the bag and keep it out of sun and wind. Moist coir left open dries at the surface and forms a pale, repellent skin. If it has dried, rewet it in several passes before filling pots; do not fill dry and water afterwards.
- Hydrated coir you cannot use within days: keep it covered and moist, or let it dry fully and rewet later. Half-dry coir sitting for weeks is the worst case: hydrophobic at the dry edge, mouldy at the wet one.
- Part-used slabs from a previous crop: dry zones plus a salt load. Cut the plastic, soak, flush and test EC before reuse.
Across every study above, coir recovered its water-holding capacity after drying and peat did not. For greenhouse slab crops where dry-back steering is routine, that tolerance is one reason coir is used. If you want advice on which grade suits an exposed bed or a slab crop, contact us and tell us the crop and the container.
FAQ
Why won't my coco coir absorb water?
It has dried below the point where its particles still attract water. Dry organic surfaces expose water-repellent compounds and the shrunken pores stop pulling water in, so water beads or runs around the edges. Coir is much less prone to this than peat and recovers if you rewet it slowly, in several passes, rather than flooding it once.
Is coco coir hydrophobic when dry?
Only when very dry. In laboratory tests coir dried to 33% moisture by weight still had a contact angle below 90 degrees and took up water, while peat at the same moisture was fully hydrophobic. At around 15% moisture coir without a wetting agent did lose some of its maximum water capacity, so do not let working coir get that dry.
How do I rewet dry coco coir in pots?
Stand the pots in a few centimetres of water and leave them until the surface turns dark, which takes minutes for small pots and an hour or more for large, shrunken root balls. Lift, drain and check the weight; repeat if still light. Then give a normal top watering to move salts back down.
How do I rewet a coco coir slab that has dried out?
Use short dripper pulses with a gap between them long enough for each to soak in, and stop each pulse when drain appears. Repeat until a probe or the slab weight shows the centre is wet, which can take most of a day for a deeply dried slab. If the coir has shrunk away from the plastic, move the dripper towards the centre of the mass.
Should I use a wetting agent on coco coir?
It helps when the coir is very dry: in trials a wetting agent raised coir's first-irrigation water capture by about half and let coir at 15% moisture reach full capacity. For normal use it is optional, because coir usually recovers with repeated irrigations. If you use one, choose a non-ionic product at label rate and expect drain volume to increase afterwards.
Can I use dish soap to rewet coco coir?
It is not a good idea. Household detergents are anionic surfactants, which carry a higher risk of root damage than the non-ionic products sold for horticulture, and dose control is poor. For a hobby pot, patient bottom soaking does the job without any additive.
Why is my drain EC high after the coir dried out?
The salts that were dissolved in the coir did not leave when the water did; they concentrated and partly crystallised. The first water through dissolves them and carries them to the drain, so the reading spikes and then falls over the next irrigations. Rewet with low-EC water, drain to waste for the first cycles, and retest before resuming normal feed.
Does coco coir dry out faster than peat?
Coir's water retention curve is similar to peat's, so a pot of either dries at a broadly similar rate under the same conditions. The difference is what happens next: dried peat turns hydrophobic and loses capacity, while dried coir rewets and recovers. Fine coir in shallow trays or exposed bags still dries fast in summer and should be shaded or pulsed.
Sources
- Michel, J.C., Jackson, B.E. and Fonteno, W.C.: The use of coir for reducing risks of peat-based substrate hydrophobicity, Acta Horticulturae 1305 (2021)
- Fonteno, W.C., Fields, J.S. and Jackson, B.E.: A pragmatic approach to wettability and hydration of horticultural substrates, Acta Horticulturae 1013 (2013)
- Fields, J.S., Fonteno, W.C. and Jackson, B.E.: Hydration efficiency of traditional and alternative greenhouse substrate components, HortScience 49(3) (2014)
- Michel, J.C.: Wettability of organic growing media used in horticulture: a review, Vadose Zone Journal 14(6) (2015)
- Michel, J.C., Jackson, B.E. and Fonteno, W.C.: Classification of organic substrates' wettability from contact angle measurements and hydration efficiency tests, Acta Horticulturae 1168 (2017)
- Bartley, P.C. et al.: Quantifying the sorptive behavior of traditional horticultural substrate components based on initial hydraulic conditioning, HortScience (2022); summarised by the American Society for Horticultural Science (2023)
- Guillén, C., Sánchez, F., Urrestarazu, M. and Mazuela, P.: Effect of wetting agent on fertigation parameters in tomato on new and reused coco fiber, Acta Horticulturae 697 (2005)
- Reid, A. (Strawberry Growers Association of Western Australia): Understanding Coir, The Passion Vine, Passionfruit Australia (Winter 2021)
- Science in Hydroponics: Exogenous root applications of wetting agents in soilless media (2025)
- HortiDaily: H2CoCo speeds up initial wetting, improves irrigation efficiency (2015); UK: H2CoCo wins UK Grower Award for best new product (2016)
- Purdue University Extension: Commercial greenhouse and nursery production, substrate EC and pH testing guidance