The short answer: fine, washed, low-EC coco peat is a good seed-starting medium on its own or blended with 20–30 % perlite and vermiculite. It holds a lot of water, stays open enough for air, and is soft enough for radicles and shoots to push through. Germination rates in coir are usually the same as in peat; the differences show up later, in seedling size, and they come down to salts and nutrition. Keep EC low, start feeding at the first true leaves, and keep trays warm and clean, and coir will produce plugs as good as any peat-based mix.
Key facts
- Coir dust holds "750 to 1100 %" of its dry weight in water and has 9.5–12.6 % air-filled pore space, with total porosity of 85.5–89.5 % (Evans, Konduru and Stamps, 1996).
- In a 16-medium trial, "medium composition did not affect tomato seed emergence", but transplants grown in more than 50 % coir were smaller (Arenas et al., 2002).
- Six ornamental species sown in a 70:20:10 coir–perlite–vermiculite mix germinated as well as in peat, and the share of marketable plugs was unchanged (Ayala-Sierra and Valdez-Aguilar, 2008).
- Seedlings and cuttings tolerate about half the EC that finished crops do (University of Kentucky Extension); most plug guides put medium pH at 5.4–6.2.
- No fertiliser until the cotyledons open, then 25–50 ppm N, rising to 50–100 ppm N until transplant (UMass Extension).
Why fine, low-EC coco peat suits germination
A plug cell holds only a few millilitres of medium, so whatever fills it has to hold water between irrigations while still letting air reach the seed. Coir dust does both. Evans, Konduru and Stamps measured five coir dust sources and found water-filled pore space of 73–80 % by volume and air-filled pore space of 9.5–12.6 %, with a bulk density of only 0.04–0.08 g/cm³. A University of Florida comparison published in Greenhouse Grower put one commercial coir at 26 % air-filled porosity and 67 % water-holding capacity, against 23 % and 69 % for a peat-based mix. The two materials are close; coir tends to be slightly more open.
Two other properties matter at the germination stage. Coir wets easily and rewets after drying out, where dry sphagnum peat becomes water-repellent. And coir dust usually sits between pH 5.6 and 6.9 (Evans et al.), inside or near the 5.4–6.2 window most plug guides use, so it normally needs no lime.
The qualifier is low-EC. Raw coir pith carries potassium, sodium and chloride from the husk and from the water it was processed in. Evans et al. found EC from 0.3 to 2.9 mS/cm across twelve sources, with chloride from 26 to 1,636 mg/L and potassium from 19 to 948 mg/L. Abad and colleagues measured saturated-extract salinity from about 0.4 to 6 dS/m. A germinating seed sits in a few millilitres of that solution, so the spread between a washed and an unwashed batch is the difference between a full tray and a patchy one. Washing removes most of the loose salt; buffering with calcium goes further and swaps out the potassium and sodium held on the fibre. The trade-offs are covered in buffered vs unbuffered coco peat. For most seed-starting work, well-washed unbuffered coir is sufficient, provided you test it.
Age matters too. A 2021 study in Horticulture, Environment, and Biotechnology tracked coir dust as it aged: non-aged material measured pH 5.04 and EC 9.19 dS/m in that study, the EC fell to about 1.8 dS/m by week four, and fresh weights of radish, Korean cabbage, tomato and red pepper plug seedlings increased with longer aging. Coir that has been washed and stored for some months is a safer bet for plugs than freshly milled pith.
What EC and pH seedlings tolerate
Seedlings are the most salt-sensitive stage of any crop. Numbers from different sources use different extraction methods, so the method is listed with each figure.
| Guideline | Value | Method | Source |
|---|---|---|---|
| Medium pH for most plugs | 5.4–6.2 | – | Master Plant-Prod, plug production guide (2025) |
| EC for plugs and seedlings | 0.25–0.75 mS/cm (press-extracted plugs read higher, about 0.8–2.3 mS/cm equivalent) | Dilution methods; press method noted separately | North Carolina State University, Press extraction notes |
| Young seedlings and cuttings | about half the EC of established crops | – | University of Kentucky Extension, HO-112 |
| EC causing injury in young plants | around 3.0 dS/m | SME | University of Georgia Extension |
| Nursery-grade coir, before blending | below 1 mS/cm | 1:1.5 dilution | Fernland, Australian nursery supplier note |
| Commercial 100 % coir medium spec | below 0.5 mmhos/cm; pH 5.5–7.0 | – | Premier Tech PRO-MIX CX datasheet |
The practical reading: a coir that tests below about 0.5–0.7 mS/cm by a 1:1.5 or 1:2 method is ready for seed trays as it is. Between 0.7 and 1.0 mS/cm, use it for blending or larger-seeded crops, or flush it once. Above 1 mS/cm, flush before sowing. Always ask which extraction method a figure comes from; the same batch reads higher by saturated extract than by 1:5 dilution, and press-extracted plugs read higher again.
Scoggins and colleagues at NC State found that coir plug extracts contained much more potassium and sodium than peat extracts. High potassium in solution competes with calcium and magnesium, so if you see tip burn or pale new leaves in coir plugs, check Ca and Mg in the feed before adding nitrogen.
Preparing briquettes and blocks for trays
Compressed coir arrives as 650 g briquettes or 5 kg blocks and has to be hydrated, loosened and, if necessary, flushed before it goes into a tray.
- Hydrate in a clean tub. Retail instructions for a 650 g briquette typically call for 4–5 litres of warm water and give an expanded volume of 8–9 litres, which lines up with the 750–1,100 % water-holding capacity measured by Evans et al. A 5 kg block needs proportionally more; hydrate it in a mixing bin, not in the bag. Use good irrigation water, not recycled drainage.
- Break it up fully. A briquette loosens in 5–15 minutes; a dense block takes longer and has a dry core if rushed. Crumble it so there are no lumps, which become dry pockets in a tray.
- Test EC. Mix 1 part coir with 1.5 or 2 parts distilled water, stand for 30 minutes, read, and note the method.
- Flush if needed. If EC is above your limit, drain and soak again in fresh water. One flush removes most of the loose salt. Drain to damp, not dripping: saturated medium is colder, which the RHS notes slows germination and encourages rot.
- Blend after hydration so volumes are right, then fill loosely. UConn Extension recommends pre-moistened medium to avoid compaction. Do not press it into cells or nest filled trays. Dibble shallow holes for seed.
Mixes commonly used
Straight coir works for many crops, but most nurseries blend it. Perlite raises air-filled porosity and drainage; vermiculite adds a little water and nutrient holding and is a common seed cover. The ratios below are from published trials and horticultural references, not from any one supplier.
| Mix (by volume) | Where it is used | Source |
|---|---|---|
| 100 % fine coir pith | Plug and seedling trays; cuttings | Fernland nursery note; Cornell AgriTech propagation trial |
| 70 % coir, 20 % perlite, 10 % vermiculite | Ornamental plug production (six species tested) | Ayala-Sierra and Valdez-Aguilar (2008) |
| About 60 % coir with vermiculite and perlite | General greenhouse mixes, used in place of peat | K. Moore, University of Florida, in Greenhouse Grower (2011) |
| Up to 50 % coir with peat, vermiculite or perlite | Tomato transplants; more than 50 % coir gave smaller transplants | Arenas et al. (2002) |
| Coir alone, or 1 part coir to 2 parts perlite | Conifer cuttings (Picea glauca 'Conica') | Nawrocka-Grześkowiak and Bielecka (2008) |
| 2 parts coir, 1 part perlite, 1 part vermiculite | Home and small-nursery seed starting | Common recipe in gardening references |
Match the perlite grade to the cell size: fine or medium grade for 288-cell trays, coarse for 50- or 72-cell trays and cutting beds. Do not add compost or soil to a germination mix; it brings in damping-off organisms and uncontrolled salts, and the main reason to use coir is that it is clean.
Sowing and watering practice
- Sow at the right depth. Small seeds (lettuce, petunia, basil) go on the surface or under a thin dusting of fine vermiculite or sieved coir; larger seeds to about twice their diameter. Deep sowing stresses seedlings (UConn Extension).
- Mist until emergence, then move to wet–dry cycles so roots chase water down the cell. Coir looks dry on the surface before it is dry underneath, so judge moisture by tray weight or a finger in the cell, not by colour.
- Keep the medium warm. Penn State and UConn Extension both link media below 20 °C (68 °F) before germination with damping-off; bottom heat at 21–24 °C (70–75 °F) gives quick, even emergence for most vegetable and bedding crops.
- Avoid standing water under trays, and remove domes or fleece as soon as the first shoots appear.
Herbs such as basil and coriander are a common first crop for growers switching to coir; see herbs for the production side.
Feeding young plants in coir
Coir has almost no plant-available nitrogen or phosphorus. Evans et al. measured nitrate-N at 0.4–7.0 mg/L and ammonium-N at 0.1–0.2 mg/L across twelve sources, with calcium of 1–24 mg/L. The seed carries enough reserves to reach the cotyledon stage, and after that the grower provides everything.
UMass Extension's schedule for bedding plant seedlings is the clearest published guide:
| Stage | Feed |
|---|---|
| Sowing to cotyledons open | No fertiliser |
| First true leaves developing | 25–50 ppm N |
| True leaves to transplant | 50–100 ppm N |
| Compromise constant feed for mixed crops | 100–150 ppm N |
The RHS gives the same starting point for peat-free seed composts: begin at the first true leaves, roughly once a week, and increase if plants look pale. Three coir-specific adjustments:
- Use a feed with calcium and magnesium. Coir supplies potassium, not calcium. Rose and Haase found Douglas-fir seedlings in coir had lower foliar nitrogen and calcium than in peat and recommended fertiliser programmes tailored to coir rather than copied from peat.
- Watch nitrogen. Arenas et al. suggested nitrogen immobilisation and coir's high carbon-to-nitrogen ratio as the likely reason transplants in more than 50 % coir were smaller. Feed at the upper end of the ranges once true leaves are out, and do not skip feeds.
- Monitor EC weekly. Press a few plugs from one tray or run a 1:2 sample. If EC climbs past the plug ranges in the table above, water through with clear water.
Damping-off and hygiene
Damping-off is the main cause of lost seed trays and it is largely preventable. UConn and Penn State Extension name Pythium and Rhizoctonia as the usual culprits, with Fusarium, Phytophthora, Alternaria and Botrytis also able to cause it. These organisms do not come with washed coir; they arrive on dirty trays, hose ends, floors, hands and recycled water, and spread by splashing. Cold, wet medium before emergence and warm, humid, crowded trays after it favour them.
What the extension fact sheets recommend, applied to coir:
- Start with clean, washed coir and new or disinfected trays. UConn advises against reusing plug trays that held diseased plants, because organic matter in small cells stops disinfectants working.
- Keep clean trays off the greenhouse floor; use clean tools.
- Sow at the recommended density; germinate warm and keep humidity low once seedlings are up.
- Water early in the day; let the surface dry slightly between waterings after emergence.
- Discard entire infected flats. Neighbours that look healthy are often already infected.
- Treat recycled irrigation water before it reaches propagation.
Over-fertilising causes the same collapse, with shrivelled roots from salt injury, so check EC before reaching for a fungicide.
Coco peat for cuttings
The same qualities suit softwood and semi-ripe cuttings: constant moisture at the base, air around the stem, and a clean medium that a rooted cutting pulls out of easily. In a Cornell AgriTech trial on hemp clones, cuttings stuck in coir plugs had the highest share of successfully rooted cuttings by root-vigour score, ahead of rockwool and a peat-based potting mix; the same report notes that coir dries faster under mist, so the timing of the move from mist to transplant matters. For conifer cuttings, Nawrocka-Grześkowiak and Bielecka recommended coir alone or 1 part coir to 2 parts perlite.
Use coarser coir or more perlite for cuttings than for seed, because cuttings sit longer and need more air at the base; keep EC low, since wounded stems are as salt-sensitive as radicles; and do not feed until roots have formed.
Common mistakes
| Mistake | What happens | Fix |
|---|---|---|
| Using unwashed or untested coir | Patchy emergence, stunted or scorched seedlings | Test EC by a stated method; flush above 1 mS/cm (1:1.5) |
| Treating coir like peat and adding lime | pH drifts above 6.5, iron and manganese lock-out | Test pH first; coir usually needs no lime |
| Watering by surface colour | Over-watering, cold medium, damping-off | Judge by tray weight or finger test |
| Not feeding after cotyledons | Pale, slow seedlings; coir holds almost no N or Ca | Start 25–50 ppm N at first true leaves, with Ca and Mg |
| Copying a peat feed programme | Potassium excess, calcium and magnesium shortage | Choose a feed with Ca and Mg; lower K |
| Compacting cells or nesting trays | Poor rooting, uneven moisture | Fill loosely with pre-moistened coir, brush level |
| Reusing dirty trays | Damping-off | New trays, or disinfect; never reuse trays that held disease |
Coir vs peat for seedlings, in one paragraph
Germination is a draw: Arenas et al. found no effect of medium on tomato emergence across 16 peat–coir mixes, and Ayala-Sierra and Valdez-Aguilar found no difference in germination or marketable plugs across six ornamentals, although plant height was 13–30 % lower in coir for three of them. Seeds do not mind; young plants are smaller unless the feed is adjusted for coir's low nitrogen and calcium and high potassium. The broader comparison is at coir vs peat, and nursery-scale uses are on the nurseries page.
FAQ
Can I germinate seeds in 100 % coco peat?
Yes, if it is fine, washed and tests low in EC. Many nurseries use straight coir pith in plug trays. Blending in 20–30 % perlite and vermiculite improves drainage and consistency in small cells, and is the mix used in published plug trials, but it is not essential.
What EC should coco peat have for seedlings?
Aim for below about 0.5–0.7 mS/cm measured by a 1:1.5 or 1:2 dilution, and no higher than 1 mS/cm. Extension guidance says seedlings and cuttings tolerate about half the salt level of established crops. Ask the supplier for the test method, because saturated-extract and press readings come out higher than dilution methods on the same material.
Do I need buffered coco peat for seed starting?
Usually not. Buffering replaces potassium and sodium on the fibre with calcium and is mainly chosen for long-term fertigated crops in slabs. For seed trays and cuttings, well-washed unbuffered coir with a low EC is sufficient, provided you supply calcium and magnesium in the feed once true leaves appear.
How much water does a coco peat briquette need?
A 650 g briquette expands to roughly 8–9 litres and needs about 4–5 litres of warm water; product labels vary slightly. Hydrate it in a clean tub, break up all lumps, test the EC and drain it to a damp, crumbly state before filling trays. Soaking wet coir is cold and slows germination.
When should I start feeding seedlings in coco peat?
Not until the cotyledons have opened. Begin at the first true leaves with 25–50 ppm nitrogen, rising to 50–100 ppm N until transplant, using a feed that includes calcium and magnesium. Coir contains almost no nitrogen, phosphorus or calcium, so skipping feeds shows quickly as pale, slow seedlings.
Does coco peat prevent damping-off?
Clean, washed coir does not carry damping-off organisms, but it does not prevent them either. They arrive on dirty trays, tools, floors and water. Prevention comes from sanitation, warm medium at germination, moderate watering and good airflow after emergence. Over-watered coir is as prone to damping-off as any other medium.
Is coco peat better than peat moss for germination?
Trials show germination and emergence are the same in both. Seedlings in coir can be smaller if they are fed the same way as in peat, because coir has less available nitrogen and calcium and more potassium. With the feed adjusted, plug quality is equal. Coir rewets more easily and does not need liming.
Can I reuse coco peat from seed trays?
Not for propagation. Spent medium carries roots, salts and possibly pathogens, and small plug cells cannot be cleaned well enough to disinfect. Use spent coir as a soil conditioner or in outdoor containers, and start each propagation batch with fresh, tested material.
Sources
- Evans, M.R., Konduru, S. and Stamps, R.H.: Source variation in physical and chemical properties of coconut coir dust, HortScience (1996)
- Abad, M., Noguera, P., Puchades, R., Maquieira, A. and Noguera, V.: Physico-chemical and chemical properties of some coconut coir dusts for use as a peat substitute for containerised ornamental plants, Bioresource Technology (2002)
- Arenas, M., Vavrina, C.S., Cornell, J.A., Hanlon, E.A. and Hochmuth, G.J.: Coir as an alternative to peat in media for tomato transplant production, HortScience 37(2) (2002)
- Ayala-Sierra, A. and Valdez-Aguilar, L.A.: Coir dust as an alternative substrate for ornamental transplant production, Revista Chapingo Serie Horticultura 14(2) (2008)
- Rose, R. and Haase, D.L.: The use of coir as a containerized growing medium for Douglas-fir seedlings, Native Plants Journal (2000)
- Horticulture, Environment, and Biotechnology 62: Aging effects on coir dust chemistry and plug seedling growth of four vegetables (2021)
- Scoggins, H.L., Bailey, D.A. and Nelson, P.V.: Development of the press extraction method for plug substrate analysis, North Carolina State University (2001–2002)
- North Carolina State University Extension: The Press extraction method for plug trays, interpretation notes
- University of Kentucky Cooperative Extension: Greenhouse soluble salts and pH, HO-112
- University of Georgia Extension: interpretation of soluble salts for greenhouse crops
- UMass Extension Greenhouse Crops and Floriculture: Fertilizing bedding plant seedlings
- UConn Extension IPM: Damping-off of ornamental and vegetable seedlings, P.S. Mercure, revised L. Pundt (2024)
- Penn State Extension: Damping off, G.W. Moorman (2023)
- Royal Horticultural Society: How to use peat-free compost for seeds and cuttings
- Moore, K., University of Florida, in Greenhouse Grower: Tips for best use of coir (2011)
- Cornell AgriTech, DeSario, A., Zingg, T. and Smart, L.: Evaluation of clonal propagation of 'TJ's CBD' in different media types (2020)
- Nawrocka-Grześkowiak, U. and Bielecka, M.: Influence of coconut fiber on rooting of Picea glauca 'Conica' cuttings, Nauka Przyroda Technologie (2008)
- Master Plant-Prod Inc.: Mastering plug production, techniques for high-quality seedlings (2025)
- Premier Tech Horticulture: PRO-MIX CX product specification
- Fernland: Why Australian nurseries are moving away from peat and what to use instead
Need washed, low-EC coco peat in briquettes or blocks for propagation? Contact us with your tray sizes and annual volume.