What is coconut coir? How it is made, what it does, and how to use it

Coir turns up in potting mix ingredient lists, in hydroponic setups and in the compressed bricks sold as a peat replacement. It is genuinely good at the physical half of a growing medium’s job and genuinely bad at the chemical half, and almost every problem people have with it comes from not knowing which half is which.

Where coir comes from and how it is made

Coir is a waste product of the coconut harvest, mostly in India and Sri Lanka. The husk and inner shell used to be discarded.

The husks are first soaked in fresh or tidal water to soften them. Husks retted in tidal water take up salt, which has to be flushed out later, and whether a manufacturer bothers is one of the main things separating good coir from bad.

The softened coir is then stripped from the husks and laid out to dry, which can take up to a year depending on the weather and the thickness of the material. Longer ageing gives better material. It is then baled and milled into the format being sold.

Some manufacturers sterilize the product before packing and some deliberately do not. That choice matters more than it sounds, for reasons covered below.

The three forms it is sold in

Milling decides what you get, and the three grades behave differently enough that buying the wrong one is a common first mistake.

1. Pith, also sold as coco peat, is the fine fraction and looks like ground peat moss. It is the version used in potting mixes and as a soil conditioner. On its own it is too fine and too absorbent for most container plants, and it will hold enough water to suffocate roots.
2. Fiber is the long strands, dried and otherwise unprocessed. This is what hanging basket liners are made from. It holds far less water than pith and its job in a mix is the opposite one: porosity and drainage.
3. Chips sit between the two and behave like an organic version of expanded clay pellets. Large enough to hold open air pockets, absorbent enough to carry moisture.

Most bagged coir and most bricks are pith, or pith with some chips blended in.

What coir is good at

The case for coir is physical, and it is a strong one.

• Water and air at the same time. Coir dust holds seven to eleven times its dry weight in water and still runs to 85 to 90 percent total pore space.^[2] Very few media give you that much reservoir without going anaerobic.
• It stays put. Coir breaks down more slowly than most organic components, so a mix built on it holds its structure through more of the season and needs replacing less often.
• It rewets. Peat that has dried out repels water and takes real effort to bring back. Coir is hydrophilic and takes water straight back up, which forgives a missed watering.
• A workable pH. Usually 5.5 to 6.8, close enough to neutral that you are not fighting it for nutrient availability the way you do with raw peat.
• It ships compressed. A brick is about a fifth of its working volume, which is why coir travels economically from the other side of the world and still competes on price.
• It suppresses some root pathogens, if it is alive. Coir has been shown to inhibit Phytophthora, Pythium and Fusarium in the lab, which is where its reputation for preventing damping off comes from. The important detail is the mechanism: sterilized and autoclaved coir showed no effect at all, so the suppression comes from the microorganisms living in the coir rather than the coir itself.^[3] If this is why you are buying it, buy unsterilized.

What coir will not do for you

This is the half that catches people out, and it is chemistry rather than physics.

There is almost nothing in it to eat. Coir carries very little available nitrogen. Anything growing in it long term is living entirely on what you feed it.

Its high cation exchange capacity is already spoken for. Coir has a high CEC, measured between 31.7 and 95.4 cmol/kg across sources.^[2] A high cation exchange capacity is normally a good thing, because it means the medium holds nutrients near the roots instead of letting them wash through. The catch is what is sitting on those exchange sites when the bag reaches you. Coir arrives loaded with potassium and sodium and short of calcium and magnesium.^[2] Feed it, and the calcium and magnesium you add get grabbed onto the exchange sites and swapped for the potassium already there, so the plant sees neither for the first few weeks.

That produces the single most common coir failure: a calcium and magnesium deficiency in a plant that is being fed perfectly well. Yellowing between the veins of new leaves, brown spotting, weak growth.

The fix is to fill those sites before you plant, which the trade calls buffering. Buy coir sold as buffered or pre-charged and it has been done for you. Otherwise soak the rehydrated coir in a calcium and magnesium solution, let it sit for several hours, and rinse. Then keep feeding calcium and magnesium as part of your normal routine rather than treating it as a one-off.

Salt content varies enormously. Salinity across coir sources has been measured spanning more than a tenfold range,^[2] which is the gap between coir that is ready to use and coir that will scorch a seedling. Rinse any coir you are unsure of with fresh water until it runs clear, and be more careful with cheap unbranded bricks than with named horticultural grades.

It cannot hold a plant up. All that porosity means very little structure. Anything tall grown in straight coir needs staking or a heavier component blended in.

How to rehydrate a brick

Coir is sold compressed and the packaging is often vague about what a brick actually makes.

Work on roughly four to five litres of water per kilogram of dry brick, and expect five to seven times the compressed volume back. A standard 650 g brick takes about three to four litres of warm water and opens up to around two gallons of usable medium, which is more than most people expect and worth putting in a bucket rather than a mixing bowl.

Use warm water, give it twenty to thirty minutes, and break it up with your hands as it swells. If it is unbuffered coir, this is the moment to buffer it, while it is wet and before it goes anywhere near a pot.

Coir comes as a dried brick and has to be rehydrated before it is any use.

How to use coir

• In a soilless potting mix, blended with perlite or vermiculite for drainage and compost for nutrition. This is the main use and the one most bagged mixes are doing.
• As a soil amendment, opening up the drainage of a clay soil or raising the water holding capacity of a sandy one.
• For seed starting and cuttings, where its combination of steady moisture and high porosity is close to ideal, and where the disease suppression above matters most. See our growing media guide for how it compares with the alternatives.
• As a hydroponic medium, where the near-neutral pH means no correction before use and the high CEC keeps feed in the root zone, provided it has been buffered first.
• For orchids, ferns, bromeliads and anthuriums, which want roots damp but never waterlogged and are not heavy feeders.
• As a mulch, in containers and beds.

The one thing not to do is grow a container plant in straight coir. Blend it with compost and something coarse: perlite, vermiculite, sand or pine bark.

Coir is also sold as compressed bricks rather than loose, which is cheaper per litre and easier to store. Coco Bliss is an OMRI-listed option that comes as 650 g bricks, the size the rehydration figures above are based on.

Coir versus peat moss

Most of coir’s popularity is really an argument about peat. Peat is mined from bogs that took thousands of years to form and store a large share of the world’s soil carbon, so it is not renewable on any timescale that matters and harvesting it releases carbon that was locked away. Coir is a byproduct of a crop that was being harvested anyway.

They are close enough in the pot that swapping one for the other is realistic. Coir holds its structure longer, rewets far more easily and starts nearer neutral pH; peat is cheaper in most of North America and more acidic, which suits acid-loving plants without correction.

Our peat moss guide covers that comparison properly, including where peat still makes more sense.

Should you use it

Use coir when you want a medium that holds water without going airless, that will not collapse halfway through the season, and that forgives you letting a pot dry out. Seed starting, cuttings, hydroponics and any mix you are blending yourself are where it earns its place.

Treat it as inert. Buy it buffered or buffer it yourself, feed it properly, and pay particular attention to calcium and magnesium. Do that and it is one of the best growing media available. Skip it and you will spend a season diagnosing a deficiency that came out of the bag.

Questions & answers

Does coconut coir need fertilizer?

Yes, always. Coir has very little available nitrogen and is low in calcium and magnesium, so it contributes almost nothing nutritionally. Anything growing in it depends entirely on what you feed. Treat it as an inert medium that happens to hold water well.

What is buffered coco coir, and do I need it?

Buffered coir has had its cation exchange sites pre-loaded with calcium, so it will not strip calcium and magnesium out of your first few feeds. If you are growing anything for more than a few weeks, especially hydroponically, buy it buffered or buffer it yourself. For short-term seed starting it matters much less.

Can I use coconut coir on its own?

For seed starting, rooting cuttings, hydroponics and epiphytes like orchids, yes. For ordinary container plants, no. Straight coir holds too much water and gives roots nothing to anchor against. Blend it with compost and a coarse component such as perlite, vermiculite or pine bark.

Do I need to rinse coconut coir before using it?

Rinse anything you are unsure of. Husks retted in tidal water absorb salt, and good manufacturers flush it out while cheap ones may not. Salinity varies more than tenfold between sources. Run fresh water through it until it runs clear, and be more cautious with unbranded bricks than with named horticultural grades.

How much does a coco coir brick make?

Roughly five to seven times its compressed volume. A 650 g brick takes about three to four litres of warm water and yields around two gallons of medium. Use a bucket, not a bowl.

Is coconut coir better than peat moss?

Physically it is more forgiving: it rewets easily, breaks down more slowly and starts closer to neutral pH. Peat is cheaper in North America and its acidity suits acid-loving plants. The real difference is that peat is mined from bogs and coir is a byproduct. Our peat moss guide covers the trade-off in full.

Does coconut coir prevent damping off?

Unsterilized coir does suppress Phytophthora, Pythium and Fusarium in laboratory tests. Sterilized and autoclaved coir showed no effect, because the suppression comes from microorganisms living in the coir rather than the material itself.^[3] If disease suppression is why you are buying it, check that it has not been sterilized.

Why are my plants in coir yellowing between the veins?

Almost certainly a calcium or magnesium deficiency caused by unbuffered coir binding those nutrients before the plant can reach them. It looks like an underfeeding problem and is not one. Buffer the medium and add a calcium and magnesium supplement to your regular feed.

References

1. Sengupta, S. and Basu, G. (2016). Properties of Coconut Fiber. In Reference Module in Materials Science and Materials Engineering. https://doi.org/10.1016/B978-0-12-803581-8.04122-9 ↩︎

2. Abad, M., Noguera, P., Puchades, R., Maquieira, A. and Noguera, V. (2002). Physico-chemical and chemical properties of some coconut coir dusts for use as a peat substitute for containerised ornamental plants. Bioresource Technology, 82(3), 241–245. https://doi.org/10.1016/S0960-8524(01)00189-4 ↩︎ ↩︎ ↩︎ ↩︎ ↩︎

3. Hyder, N., Sims, J. J. and Wegulo, S. N. (2009). In Vitro Suppression of Soilborne Plant Pathogens by Coir. HortTechnology, 19(1), 96–100. https://doi.org/10.21273/HORTTECH.19.1.96 ↩︎ ↩︎