Peat moss
Also known as Sphagnum peat, Sphagnum peat moss, Peat
In short
Peat moss is the partly decomposed remains of bog plants, mainly Sphagnum moss, dug from peatlands. It has been the base of most commercial potting mixes because it holds a lot of water yet stays light, but its extraction damages carbon-rich peat bogs and it is being phased out of retail compost in some countries.
How it looks
Brown to dark brown, fibrous and spongy material, sold dry in compressed bales or bags. Lightly decomposed (fibric) peat still shows moss fibres; more decomposed (sapric) peat is finer and darker. It feels light when dry and swells and darkens as it takes up water.
Signs
Dry peat becomes water-repellent: water runs straight through or pools on top and the root ball shrinks away from the pot wall. A pot of dry peat-based mix feels unusually light. In the other direction, peat-heavy mixes that are kept constantly wet in large pots stay soggy for a long time.
What favours it
Peat forms in waterlogged, acidic bogs where lack of oxygen slows decay, and it builds up only about 1 mm a year. It is naturally acidic, so bagged mixes usually have limestone added to raise the pH; commercial soilless peat-based mixes are typically adjusted to about pH 6.2.
Prevention
- Moisten dry peat or peat-based mix before potting, and do not let finished pots dry out completely.
- Mix peat with an open material such as perlite, bark or grit so the pot does not stay waterlogged.
- For carnivorous plants, choose pure sphagnum peat with no added fertiliser, lime or wetting agent.
- Consider peat-free alternatives such as coir, composted bark, wood fibre or green compost.
Treatment
Start with the first step and move down only if the problem persists.
- Rewet a dried-out pot by standing it in a container of water until the mix has soaked up water, then let it drain fully.
- Water slowly in several small passes rather than one large pour, so the water soaks in instead of running down the sides.
Pros and cons
Pros
- Holds many times its own weight in water while staying light.
- Uniform and largely free of weeds, pests and diseases, which is why it became the standard base for soilless mixes.
- Naturally acidic, suiting acid-loving plants and carnivorous plants when used unlimed.
- Sphagnum peat contains fungistatic substances that help limit damping-off.
Cons
- Peatlands store around 42% of all soil carbon while covering about 3% of land; extraction releases that carbon and destroys bog habitat.
- Effectively non-renewable on human timescales because it accumulates only about 1 mm per year.
- Becomes hydrophobic and hard to rewet once dry.
- Needs lime to be suitable for most plants, and bagged products may contain fertilisers or wetting agents that carnivorous plants cannot tolerate.
- Retail sales to amateur gardeners in England were announced to be banned, and many gardening bodies discourage its use.
Look-alikes
- Sphagnum moss (long-fibre)
- Long-fibre sphagnum is the dried living moss, sold as long strands; peat is the decomposed, compacted material from deeper in the bog.
- Coco coir
- Coir is reddish-brown coconut husk fibre and dust, usually sold as compressed bricks; it rewets more easily than peat and is not dug from bogs.
- Compost
- Compost is decomposed garden or food waste, darker and more crumbly, with more nutrients and more variability.
Typical uses
Base of many commercial potting and seed mixes; unlimed, unfertilised peat is the classic medium for Venus flytraps, Sarracenia, sundews and other bog carnivorous plants, and for acid-loving plants.
Related
Sources
- Peat — Wikipedia
- Potting soil — Wikipedia
- Growing Media (Potting Soil) for Containers — University of Maryland Extension
- Growing Media — Texas A&M AgriLife Extension, Ornamental Production
- What are the alternatives to using peat-based compost? — RHS
- Sale of horticultural peat to be banned in move to protect England’s precious peatlands — GOV.UK
- Peat, Sand, Perlite — International Carnivorous Plant Society
Written in our own words from these sources. Check a product label before using any treatment.