Coco coir vs. peat moss: how each behaves in your pots

Where each comes from, and the carbon question

Sphagnum peat is partly decomposed moss harvested from bogs, where waterlogged, acidic conditions have preserved it over thousands of years. It has been the backbone of professional potting mixes for decades because it is uniform, light and holds a huge amount of water, about 10–20 times its dry weight. Its cost is environmental: peatlands are among the planet’s largest land carbon stores, and draining and cutting them releases that carbon and destroys specialized habitat. Several countries, including the UK, are phasing peat out of retail compost.

Coir, or coco peat, is the spongy pith left over when fiber is stripped from coconut husks, a by-product of the coconut industry in countries such as Sri Lanka and India. It is renewable, though it is shipped long distances and needs careful processing before it is fit for plants.

Chemistry: pH, salts and buffering

Raw peat is strongly acidic, pH 3.5–4.5, so commercial mixes add ground limestone to bring it to about 5.5–6.5, the range where most houseplants take up nutrients best. Coir starts much closer to that range, at roughly pH 5.5–6.8, so it needs little or no lime.

Coir’s weakness is salt. Husks are often soaked in brackish water during processing, and unwashed coir can carry high levels of sodium and potassium. Its exchange sites also hold on to potassium and sodium and grab calcium and magnesium from the feed, which can leave plants short of both. Good horticultural coir is therefore washed to remove salts and “buffered”, pre-treated with calcium nitrate so the exchange sites are already loaded with calcium. Look for “washed” and “buffered” on the label.

Water, air and rewetting

Both hold plenty of water, but they hold it differently. Peat is fine and spongy; it stays wet for a long time and can collapse and compact as it ages over 2–3 years. Coir pith generally drains a little more freely and holds slightly more air at the same moisture level, and it resists breakdown longer. That makes a coir-based mix slightly more forgiving for heavy-handed waterers, although fine, dusty coir can compact as badly as peat.

Both turn water-repellent when bone dry, peat more stubbornly than coir. If water runs off the surface and down the pot sides, soak the pot in a basin for 10–30 minutes until the surface darkens, then drain it. Some mixes include a wetting agent to reduce this problem.

Preparing coir bricks

Compressed bricks are the cheapest way to buy coir and expand 5–7 times in volume when wet. A 650 g brick typically makes around 8–9 litres.

• Use a bucket that holds at least seven times the brick’s volume and add warm water gradually.
• Break up the expanding block by hand once it softens, after about 15–30 minutes.
• If the label does not say washed or low-EC, rinse it by filling and draining the bucket two or three times.
• If it is not buffered, feed with a fertilizer that contains calcium and magnesium from the first watering.
• Squeeze a handful: a few drops should appear, not a stream. Too wet? Spread it out to dry a little before mixing.

Which to use for which plant

For most houseplants either works as the base of the mix; what matters more is the coarse material you add, such as bark and perlite or pumice, and the pot size. Peat-based mixes are forgiving for ferns, peace lilies and calatheas that like steady moisture. Coir suits growers avoiding peat; like any organic base, it discourages fungus gnats best when the top 2–5 cm is allowed to dry between waterings. Per the manufacturer, Miracle-Gro Indoor Potting Mix (US) is one widely sold coir-containing indoor mix.

When you switch from one to the other, expect different drying times and check moisture by feel for the first few weeks. With coir, watch for magnesium deficiency, yellowing between the veins of older leaves, and choose a fertilizer that lists Ca and Mg in its guaranteed analysis.