What is peat moss? What it does, and how it compares with coir

Peat and peat moss are not the same thing

The two words get used interchangeably, and the difference is worth holding onto because it explains what you are actually buying.

Peat is any accumulation of partly decayed vegetation formed in waterlogged ground. Slow drainage and cool temperatures starve the soil of oxygen, bacterial activity nearly stops, and plant matter piles up faster than it breaks down. What builds up over centuries is peat.

Peat moss is peat made mostly of sphagnum moss. What grows in a peatland depends on how much dissolved mineral the water carries:

• Bogs are fed only by rainfall and are nutrient poor. They suit sphagnum mosses, carnivorous plants, cranberries and ericaceous plants such as blueberries, heathers and rhododendrons.
• Fens take in groundwater carrying dissolved minerals, so they support a wider range: cattails, sedges, shrubs and the occasional tree.

Where sphagnum dominates, the peat that forms is what the trade sells as peat moss or sphagnum peat moss. Sphagnum is unusual in that it acidifies the water around it, which is a large part of why so little decomposes.

One more distinction that catches people out at the garden centre. A bale labelled sphagnum peat moss is raw, unlimed and acidic. A bag of peat-based potting mix has already had lime added to bring it up to something most plants will grow in. They are not interchangeable, and most of the bad advice about peat comes from treating them as if they were.

Harvesting in a peat bog.

How peat moss is harvested

The process is closer to open-cast mining than to farming, and it takes years to bring a site into production.

1. Ditches are cut to drain the site. A main channel runs around the perimeter, with shallower parallel ditches inside it, bringing the water content of the peat down to around 85%.
2. The living surface vegetation is stripped off to expose the peat beneath. That living sphagnum is kept, either for restoring worked-out sites or for making hanging basket liners.
3. The fields are levelled and slightly crowned so surface water runs off.
4. The exposed surface is milled and harrowed to break it up and speed drying.
5. Once the top layer is down to about 50% moisture, large vacuum harvesters collect it.

Each pass takes only the top few centimetres, so a field is harvested repeatedly across a season and across years.

What peat moss does in a mix

• It holds water and air at the same time. Peat substrates run to around 93% total porosity, holding roughly 58% of their volume as water while still leaving about 35% as air-filled pore space.^[1] That combination is the reason it became the industry default.
• It resists compaction. Dried peat is springy. It keeps its structure through a season in a pot rather than slumping into an airless block.
• It drains freely. Despite how much water it holds, excess moves through and out rather than sitting.
• It is clean. Peat forms in cold, acidic, oxygen-poor conditions that very little survives, so it arrives without weed seeds and with almost nothing in the way of root pathogens. This is not the same as sterile, and it is worth being precise about: peat is not biologically active, which is exactly why it is reliable for seed starting where damping off is the main risk. It is also why peat brings no disease suppression of its own, unlike coir.
• It holds nutrients where roots can reach them. Peat carries very little mineral content of its own, but it has a high cation exchange capacity, so the feed you add stays in the root zone instead of washing straight through.
• It is light and clean to handle. No dust cloud like compost, no static drift like perlite.

A limed peat-based mix is hard to beat for starting seeds.

Where peat falls short

• It is acidic, and usually too acidic. Raw sphagnum peat typically sits between pH 3.5 and 4.5. Only a narrow group of plants wants anything like that. Everything else needs the mix limed first.
• It brings no nutrition. Bog peat forms in nutrient-poor water, so the material is nutrient-poor too. Anything grown in it lives on what you feed it.
• It repels water once it dries out. This is peat’s most annoying practical failing. A pot that has gone properly dry will shed water straight down the sides and out the bottom, and rewetting it takes patient bottom-watering rather than a quick pour.
• It is not renewable in any useful sense. Covered below.
• It is not cheap. Pure peat costs more than most amendments, because getting it out of the ground is slow work.

When peat is the right choice, and when it is not

Peat’s acidity is the whole question, and it is where most advice about it goes wrong.

Genuinely acid-loving plants

These want a growing medium below pH 6, and unlimed peat suits them:

• Blueberries, cranberries and lingonberries.
• Azaleas, rhododendrons, camellias and heathers.
• Strawberries, which prefer around 5.5 to 6.5.

Almost everything else, including herbs

Most herbs want a pH between 6.0 and 7.5, with about 6.5 as the target, and that includes the Mediterranean group people most often assume prefers acid: rosemary, thyme, oregano, marjoram, sage and lavender. Basil, mint and lemon balm want the same range. Straight peat at pH 4 is well below all of it.

That does not rule peat out. It means using a peat-based mix that has been limed, or adding lime yourself, rather than potting into raw peat and wondering why nothing thrives. If you are unsure where your mix sits, our guide to testing pH covers how to check.

Where to skip peat entirely

• As a mulch. Peat crusts and cracks as it dries, and then sheds water rather than letting it through. Bark or compost does the job better.
• When you have compost. Home compost delivers similar structural benefits, brings nutrition peat does not, and costs nothing.
• In the open garden as a general soil conditioner. Peat breaks down and the effect is temporary, which is an expensive way to improve a bed.

How to use peat moss

1. In a potting mix. The standard formula is an organic fraction for bulk and water holding, peat or coir, plus perlite or vermiculite for aeration, plus compost for nutrition, and lime to correct the pH. This is peat’s main job and where it is hardest to beat.
2. As a soil amendment. It raises water holding in sandy soil and improves drainage and compaction resistance in clay. It will also pull an alkaline soil down towards neutral, which is one of the few situations where the acidity is doing useful work on its own.
3. For seed starting. The combination of high moisture retention and no pathogens is close to ideal, and it is the reason nurseries have used peat plugs for decades.
4. In hydroponics. Water holding, free drainage, high cation exchange capacity and biological inertness all count in its favour.

Peat moss versus coconut coir

This is the comparison most people arrive wanting, and it has a clearer answer than the argument usually suggests.

When the two were tested head to head in bedding plant trials, coir-based substrates held more water than comparable peat-based ones, with no meaningful difference in bulk density or total pore space. Geraniums grown in coir put on more root, and marigolds and petunias grew taller with more shoot weight than the same plants in peat.^[2] Coir is not a compromise substitute.

The practical differences that matter day to day:

• Rewetting. Peat that has dried out repels water. Coir takes it straight back up. If you are inconsistent with watering, this alone is a reason to prefer coir.
• pH. Coir arrives near neutral at 5.5 to 6.8 and needs no correction. Peat needs liming for most plants.
• Chemistry. Coir arrives loaded with potassium and sodium and short of calcium and magnesium, so it needs buffering before use or your plants will show a deficiency you did not cause. Peat has no equivalent trap. This is the one place peat is genuinely simpler.
• Longevity. Coir breaks down more slowly and holds its structure longer.
• Cost. Peat is usually cheaper in North America; coir competes better in the UK and Europe.

Our coconut coir guide covers the buffering step and the rest of coir’s chemistry in detail. For how both sit against the other options, see our growing media guide.

The sustainability question

Peat accumulates at roughly 0.5 to 1 mm a year. A bag of it represents centuries of accumulation, and no restoration effort puts that back on a timescale that means anything to a gardener.

The stakes are larger than horticulture. Peatlands cover at least 3% of the world’s land surface but hold more than 600 gigatonnes of carbon, up to 44% of all soil carbon, more than is stored in every other type of vegetation on earth including all its forests. Drained peatlands release an estimated 1.9 gigatonnes of CO₂ equivalent a year, about 5% of global emissions from human activity.^[3]

Where you are changes what that means in practice.

In North America, most horticultural peat comes from Canada, where the industry’s total footprint runs to about 36,000 hectares against roughly 119 million hectares of Canadian peatland, under 0.03% of it. Producers restore worked-out sites using a moss layer transfer technique developed since the 1990s, and around a fifth of the industry’s footprint has been restored so far.^[4] The harvesting is a small fraction of a very large resource, and it is better managed than the debate usually allows.

In the UK and Europe, where most lowland bog was destroyed long ago, the pressure is real. A retail ban was announced in 2022 for the end of 2024 and a professional ban in 2023 for the end of 2026, and neither has been enacted; the legislation stalled with a change of government and there is still no statutory ban. The industry has moved anyway. The RHS sells only plants grown peat-free or in peat already in the production cycle before the end of 2025 across its garden centres and online plant sales from January 2026.^[5]

Should you use peat moss

Use it when the acidity is the point: blueberries, cranberries, azaleas, rhododendrons and camellias. Use it, limed, when you want the most predictable seed-starting medium available, because nothing beats a clean, pathogen-free substrate when damping off is what stands between you and a tray of seedlings.

Reach for coir instead when you want the same physical performance without the liming step, when you water inconsistently and the rewetting problem would bite, or when the sustainability question matters to you and you would rather buy a byproduct than something dug out of a bog. The trials say you are not giving anything up.

And if you have a compost heap, use that first. It does most of what peat does, brings nutrition peat cannot, and it is already yours.

Questions & answers

Is peat moss acidic, and does that matter?

Raw sphagnum peat runs about pH 3.5 to 4.5, which is too acidic for most plants. Bagged peat-based potting mixes have already been limed to correct this; bales of straight peat moss have not. Check which one you are buying, because it is the single most common mistake with peat.

Can I grow herbs in peat moss?

Not in straight peat. Most herbs want pH 6.0 to 7.5, including rosemary, thyme, oregano, sage and lavender, which are often wrongly assumed to prefer acidic conditions. Use a limed peat-based potting mix, or add lime yourself.

Why does my peat-based mix repel water?

Peat becomes hydrophobic once it dries out completely, so water runs down the sides of the pot and out the bottom without wetting the middle. Stand the pot in a tray of water and let it draw moisture up from below, which takes an hour or two. Avoid it by never letting the pot dry out fully. Coir does not have this problem.

Is peat moss sterile?

Not technically, but close enough for practical purposes. It forms in cold, acidic, oxygen-poor conditions that very little survives, so it carries no weed seeds and almost no root pathogens. The trade-off is that it is biologically inert and offers no disease suppression of its own.

Is peat moss banned?

Not in the United States or Canada. In the UK, retail and professional bans have been announced twice and neither has been enacted, so there is still no statutory ban, though much of the industry has moved to peat-free voluntarily and the RHS went to no-new-peat plants in January 2026.

Is coconut coir better than peat moss?

In controlled bedding plant trials coir held more water and produced more growth than comparable peat substrates.^[2] It also rewets easily and needs no liming. Against that, coir must be buffered before use or it will lock up calcium and magnesium. Peat is simpler chemically; coir is more forgiving physically.

How much peat should I add to my potting mix?

Peat or coir usually makes up half to two thirds of a soilless mix by volume, with perlite or vermiculite for aeration and compost for nutrition. Add lime if you are starting from raw peat rather than a pre-mixed product.

What can I use instead of peat moss?

Coconut coir is the closest substitute and performs at least as well. Composted bark, wood fibre, leaf mould and garden compost all work as part of a mix. Compost is the one to reach for first if you already make it.

References

1. Londra, P., Paraskevopoulou, A. and Psychogiou, M. (2018). Hydrological Behavior of Peat- and Coir-Based Substrates and Their Effect on Begonia Growth. Water, 10(6), 722. https://doi.org/10.3390/w10060722 ↩︎

2. Evans, M. R. and Stamps, R. H. (1996). Growth of Bedding Plants in Sphagnum Peat and Coir Dust-Based Substrates. Journal of Environmental Horticulture, 14(4), 187–190. https://doi.org/10.24266/0738-2898-14.4.187 ↩︎ ↩︎

3. IUCN (2021). Peatlands and climate change. Issues brief. https://iucn.org/resources/issues-brief/peatlands-and-climate-change ↩︎

4. Canadian Sphagnum Peat Moss Association. Responsible peat extraction and Canadian peatland restoration. https://peatmoss.com/en/sustainability/responsible-peat-extraction/ ↩︎

5. Royal Horticultural Society. All RHS retail to sell only “no new peat” plants from January 2026. https://www.rhs.org.uk/advice/peat/rhs-retail-peat-free-from-2026 ↩︎