Coir compost: what it is and how to use it

Coir compost is made from the fibrous husk of the coconut, dried and compressed into a block that expands when you add water. It is the most widely used peat-free growing medium in UK gardening, and it behaves differently enough from peat that it is worth understanding before you use it.

The short version: it holds water well, rehydrates evenly, sits close to neutral on pH, and contains almost no nutrients. That last point is the one that catches people out.

• What coir compost is
• Where it comes from, and the salt question
• Coir compared with peat
• Can you use it on its own?
• Mixes that work
• Using it in borders
• Frequently asked questions

What coir compost is

A coconut has a thick fibrous layer between the outer skin and the hard inner shell. That layer is the husk, and coir is what it is made of.

When husks are processed, the long fibres are separated off for rope, matting and brushes. What is left is the shorter fibre and the fine dust between it, known as coir pith, and that is what becomes horticultural coir. It was treated as a waste product for a long time, which is a large part of why it is now such a sensible material to use.

Where it comes from, and the salt question

Most horticultural coir comes from India, Sri Lanka, Indonesia and the Philippines, as a by-product of coconuts already being harvested for food and oil. Coconuts crop year-round, so the supply is continuous rather than extracted from a finite deposit.

Raw coir carries natural salts, mainly sodium and potassium, which is a problem for plants at high levels. Good quality coir is washed to reduce them, and better quality coir is also buffered.

Buffering is worth understanding, because it is usually explained badly. It does not simply rinse salts away. The coir is soaked in a calcium and magnesium solution, and those ions displace the sodium and potassium clinging to the fibre, so they can then be washed out. The practical result is a medium that will not release a slug of sodium into your compost later, and that has calcium and magnesium already sitting where plants can reach them.

Coir compared with peat

Three differences matter in use.

• Rehydration. Peat that has dried out turns water-repellent, and you can water a dry pot of peat compost and watch it run straight down the sides. Coir takes water back up evenly. For anyone who forgets to water, this is the single biggest practical advantage.
• pH. Peat is acidic, typically around 3.5 to 4.5, and needs lime added for general use. Coir usually sits around 5.5 to 6.8, near enough neutral to suit most plants without amendment.
• Nutrients. Neither is fertile, but coir is particularly low, so anything growing in it for more than a few weeks needs feeding.

Then there is the reason peat-free matters at all. Peat extraction drains and destroys bog habitats that took thousands of years to form, and releases carbon those bogs had locked away. Coir is a by-product of something already being grown for another purpose.

Can you use it on its own?

For some jobs, yes.

Seed sowing and propagation. Pure coir is excellent here. It is fine-textured, holds moisture consistently, and is effectively sterile, which matters when damping off is the main risk. Seeds carry their own food reserves for the first few weeks, so the low nutrient content is not a problem at this stage.

Hydroponics. Coir is a standard medium, precisely because it is inert. You are supplying all the nutrition through the feed, so a medium that contributes nothing of its own is an advantage.

Anything longer term. No. A plant left in pure coir will run out of nutrients and stall, and you will be feeding it continuously to compensate. For potting on and growing on, mix it.

Mixes that work

Coir supplies moisture retention and aeration. Loam supplies structure, minerals and weight. Compost supplies fertility. A useful mix has all three.

General purpose: 1 part coir, 1 part loam, 1 part garden compost or well-rotted leafmould.

For seed sowing

• 2 parts coir
• 1 part loam
• A handful of perlite or vermiculite

Light and open, which is what fine roots want at the start.

For container plants

• 1 part coir
• 1 part loam
• 1 part garden compost
• Grit or perlite added where drainage needs improving

The loam is doing more work than it looks in a container mix. It gives the pot enough weight to stay upright in wind, and holds nutrients where a coir-only mix would let them wash straight through.

Using it in borders

Coir is a good soil improver because it breaks down slowly, so the benefit lasts several seasons rather than disappearing in one.

Spread a layer over the surface and fork it in, or include it in a mulch mix. On heavy clay it opens the structure up. On sandy soil it does the opposite job, holding moisture that would otherwise drain past the roots. Both improvements come from the same property.

What it will not do is feed the soil, so use it alongside compost or manure rather than instead of them.

Coir comes in more than one form. Compressed blocks and bricks are the most common, because removing the water makes them light to transport and easy to store. Loose coir compost is ready to use straight from the bag. Coir chips are chunkier and used for orchids and for aeration. Coir discs are pre-measured for seed trays.

The block we stock is the Vital Grow 650g, which expands to around 10 litres once soaked. It is entirely coconut husk fibre, peat-free, and sits at a pH that suits most plants without adjustment. It lives in Garden Supplies, and if you are using it for propagation, the timings and temperatures are in our guide to sowing tomatoes, chillies and peppers.

Shop Garden Supplies

Frequently asked questions

What is coir compost?

Coir compost is made from the fibrous husk of the coconut. When husks are processed, the long fibres go to rope and matting, and the shorter fibre and fine dust left behind, called coir pith, becomes the growing medium. It is usually sold dried and compressed into a block that expands when soaked. It holds moisture well, rehydrates evenly rather than turning water-repellent, sits close to neutral on pH, and contains almost no nutrients, so anything growing in it long term needs feeding.

Is coir better than peat?

For most garden uses, yes. Coir rehydrates evenly, where dried-out peat becomes water-repellent and sheds water down the sides of a pot. Coir sits around 5.5 to 6.8 on pH, near enough neutral for most plants, while peat is acidic at roughly 3.5 to 4.5 and needs lime for general use. And coir is a by-product of coconuts already grown for food and oil, whereas peat extraction destroys bog habitats that took thousands of years to form and releases the carbon stored in them.

Can you grow plants in coir on its own?

For seed sowing and propagation, yes, and it is very good at it, being fine-textured, moisture-retentive and effectively sterile. Seeds carry their own food reserves for the first weeks, so the lack of nutrients does not matter. Coir is also a standard hydroponic medium, precisely because it is inert and you supply everything through the feed. For anything longer term in a pot, mix it with loam and compost, or the plant will simply run out of food and stall.

What pH is coir?

Typically between 5.5 and 6.8, which is slightly acidic to neutral and suits the great majority of garden and container plants without any adjustment. That is one of its main advantages over peat, which sits around 3.5 to 4.5 and needs lime added before it is usable for general growing. If you are growing genuine acid-lovers such as blueberries or rhododendrons, coir on its own will not be acidic enough, and you will want an ericaceous mix instead.

How much compost does a coir block make?

A 650g compressed block expands to around 10 litres once soaked, which is enough for a good number of seed trays or several medium pots. Break the block into a bucket or trug, add warm water gradually, and let it take up what it needs rather than flooding it. It expands considerably, so use a bigger container than you think you need. Any surplus can be stored damp in a sealed bag, or dried out again without harm.