The short answer: coco coir works as a plug substrate for petunia, pansy, begonia, impatiens and geranium, and in trials bedding plants in coir usually grow as big as or bigger than plants in peat. The changes are in how you run it. Coir carries potassium instead of lime and has a weaker pH buffer than peat, so it needs a calcium-rich, nitrate-led feed and no dolomite. It rewets more easily than peat, and plants in it tend to stretch a little more, so growth regulator rates may need nudging up. Get the grade and fill right for a shallow cell and the rest is feed management.
Key facts
- A 1:1 peat and vermiculite mix holds 2.8 % air by volume in a 288 cell and 0.5 % in a 648 cell. The same mix holds 13 % in a 4-inch pot. Cell height, not the mix alone, sets plug air space.
- Across 12 coir sources, pH ran 5.6–6.9, EC 0.3–2.9 mS/cm, potassium 19–948 mg/L and chloride 26–1,636 mg/L.
- Coir has a much smaller pH buffer than average peat, so ammonium-heavy feeds pull pH down faster.
Why the plug cell is the hard part
The difficulty with plugs is the cell, not the substrate. A short column of wet medium barely drains, and the saturated layer at the base (the perched water table) takes up most of a shallow cell.
The numbers are stark. Four commercial plug mixes tested in a standard 273 cell about 1 inch tall held 1–3 % air by volume after drainage. The same mixes in a cell twice as tall held 5–10 %. Cell shape matters too. A square 288 cell holds 6.18 ml against 4.66 ml for a round one, about a third more substrate and water at the same height.
Coir's own figures look generous next to that. In standard lab tests, coir pith measures 9.5–12.6 % air-filled pore space, 73–80 % water-filled pore space and 85.5–89.5 % total pore space. Those values come from a taller test column. In a 512 cell you'll see far less air than the lab sheet suggests, whatever the material.
What that means for coir in 288 and 512 trays
| Factor | Effect in a shallow cell | What to do with coir |
|---|---|---|
| Fines (the dust fraction) | Fill small pores, hold water, cut what little air there is | Ask for a screened fine grade with the dust taken out, not unscreened pith |
| Long fibre and chunks | Bridge in small cells and give uneven fill | Keep fibre out of 512 and 406 mixes. Short fibre is fine in 128 and 72 |
| Compaction at filling | Can halve air space or remove it entirely | Fill lightly, brush off the excess, don't press, don't stack filled trays |
| Moisture before filling | Dry particles nest together and pack tighter | Pre-wet. For peat plug mixes, about 200 % water by dry weight is the usual rule, against about 100 % for cell packs |
| Aggregates | Fine (#3) vermiculite flows well into cells but adds little air and compacts easily | Don't count on fine vermiculite for air. Pick the aggregate for drainage, not just flow |
Our guide to coco peat particle size grades covers sieve fractions and how to check fines on a sample.
Wetting up and the seed line
Peat gets harder to rewet the drier it gets. Coir doesn't behave that way. It takes water back even after drying, and in side-by-side tests peat captured less water than coir at every starting moisture content. Blending as little as 15 % coir into peat kept a mix at around 55 % water by volume through a wet-dry cycle. One caveat applies to both: when the starting moisture was cut to 30 %, peat and coir each captured less than half their maximum on the first watering. A coir plug mix that's been left to dry in the hopper still needs proper pre-wetting.
At the seed line, peat-free blends have run through filling machines without trouble in grower trials, but flow rates needed adjusting and moister media filled more slowly. Then there's the surface. Coir surfaces dry faster than peat, so the top few millimetres where the seed sits can go pale while the cell below is still wet. Germinating under cover or in a chamber at saturated moisture avoids most of that, and so does misting little and often after emergence rather than flooding the tray.
EC, potassium and the first feeds
Seed raisers aim low at sowing. Typical plug targets on a 1:2 extract:
| Crop | Media at sowing | During growth |
|---|---|---|
| Petunia | pH 5.5–6.0, EC about 0.75 mS/cm | pH 5.8–6.2, EC 1.0–1.5 mS/cm |
| Pansy and viola | pH 5.5–5.8, EC about 0.75 mS/cm | pH 5.5–5.8, EC 1.0–1.5 mS/cm |
| Seed geranium | pH 6.0–6.2, EC under 0.75 mS/cm | pH 6.0–6.5, EC under 1.5 mS/cm, 100–150 ppm N |
Coir's EC varies by source and by how well it was washed, which is why the 0.3–2.9 mS/cm spread above matters. Most of that EC is potassium, sodium and chloride, not useful nitrogen. Coir brings almost no nitrogen, calcium or magnesium, and its exchange sites hold onto calcium and magnesium you add while giving up potassium. Unbuffered coir that has only been washed will take calcium out of your first feeds. Treated coir, washed with a calcium and magnesium solution to strip potassium and sodium, has grown begonia and chrysanthemum as well as peat, and kalanchoe more vigorously. Untreated coir, by contrast, held back growth in some greenhouse crops.
So the first feeds in a coir plug should be calcium nitrate-based, carry magnesium, and go light on potassium. If you buy unbuffered coir, either buffer it before mixing or run a calcium and magnesium charge through the first irrigations. The trade-offs are set out in buffered vs unbuffered coco peat. The general seedling feed schedule by stage is already in our coco peat for seed germination guide, so it isn't repeated here.
pH and lime: usually none needed
Sphagnum peat is limed to reach plug range. Coir starts at 5.6–6.9 across sources, already inside or just above the 5.4–6.4 band that suits most greenhouse crops. Adding lime to coir pushes pH too high, and in plugs high pH means iron chlorosis, the classic yellow top on petunia and pansy.
Check the pH of each delivery before you decide. A lot at the top of the range is better corrected with an acid-reaction feed than with any amendment.
Ammonium vs nitrate in a weak buffer
Ammonium-heavy feeds lower root-zone pH and nitrate-heavy feeds raise it. Peat's humic acids blunt that swing. Coir, wood fibre and other peat alternatives have a much smaller pH buffer, and in a pelargonium trial the buffer of the medium and the feed's ammonium to nitrate ratio clearly interacted. In practice, a feed that held pH steady in peat can drift faster in a coir plug.
Ammonium also drives soft, stretched growth in plugs, so petunia programmes already call for nitrate-form fertiliser at stage 2 (under 100 ppm N, under 0.7 mS/cm) and only an occasional balanced ammonium and nitrate feed at stage 3 if growth is slow (100–175 ppm N, 0.7–1.2 mS/cm). In coir, follow that closely. Test pH weekly, and switch to a more acidic or more basic feed rather than reaching for lime or acid drenches.
Growth regulation in coir
Coir doesn't block growth regulators, but it grows bigger plants. Trial results:
| Crop and treatment | Result in coir vs peat |
|---|---|
| Petunia and marigold, bedding substrates | Taller plants with heavier shoots in several coir mixes. The largest petunias came from 80 % coir with 20 % perlite |
| Marigold | Flowered sooner in coir |
| Geranium 'Pink Elite' | Heavier roots in coir. No difference in height, shoot weight, branching or days to flower |
| Sunflower, paclobutrazol drench at 2 mg per pot | Similar percentage height reduction in coir and peat |
| Sunflower, paclobutrazol drench at 4 mg per pot | Greater height control in coir |
Start at your peat rate and increase slightly if plugs run long. Control stretch with temperature, light, low ammonium and drier cycles first. Daminozide and paclobutrazol spray rates for bedding plugs vary several-fold between North American and Northern European programmes, so trial any change on a few trays first.
Rooted cuttings in coir plugs
Vegetative geranium, impatiens, begonia and petunia cuttings want steady moisture at the base and air around the stem. Coir gives both if the grade is right. Poinsettia cuttings stuck in coir briquettes formed more roots than in peat pellets at the same moisture tension, a result put down to coir's better aeration. In pelargonium paperpot trials in the Netherlands, coir-filled pots showed good root penetration, while a peat-free compost mix scored slightly lower because of its physical structure.
For cuttings, use a slightly coarser coir or more perlite than you would for seed, keep EC low until roots form, and watch for drying under mist. Formed plugs (netted, bonded or paperpot) suit sticking lines better than loose fill, and the formats are compared in coco coir discs and propagation plugs.
Transplanting to packs and root-ball integrity
This is the question every plug grower asks. Loose coir pith has less of the long, binding fibre that holds a sphagnum plug together before roots fill the cell. In UK demonstration trials on peat-free bedding plugs (petunia, pansy, geranium, antirrhinum, verbena and others), plug integrity varied between the media tested. The practical advice was to hold plugs slightly longer so roots knit the cell before transplant, and not to keep the media too wet in propagation.
Don't overshoot, though. A root-bound petunia plug slows or stops growing and can take 1–2 weeks to recover after transplanting. The window is narrower in coir. Pull test a few plugs a day or two before the planned transplant date, and if the media crumbles, give it a few more days. Binders, nets and paper sleeves remove the problem in the smallest cells.
What European peat-free trials report
- UK, 2021–2022 demonstrations: commercial-quality plugs and bedding plants grew in every peat-free blend tested. Surfaces dried faster, EC usually ran higher, and pH could exceed the 5.5–6.0 used for peat. Antirrhinum, primula, Rex begonia and poinsettia were the most sensitive.
- Netherlands, treated coir: begonia, chrysanthemum and schefflera grew as well as in peat, and kalanchoe grew more vigorously.
- Petunia in compost and wood fibre mixes: in a first round of 30 mixes, every peat-free variant gave chlorotic plants, and the best low-peat mix still held 30 % peat. High compost pH was the main cause. Coir wasn't tested.
- Switzerland, ten commercial nurseries: irrigation and nitrogen feeding were the two critical points with peat-reduced and peat-free bedding media.
Peat-free plugs work when irrigation and nutrition are adjusted, and fail on a peat programme.
Producer specification for a coir plug mix
| Item | What to ask for |
|---|---|
| Grade | Fine, screened pith with dust removed. Ask for a sieve analysis per lot |
| Fibre | None for 512 and 406. Short fibre only for 128 and larger |
| EC as delivered | Low enough that your finished mix is near 0.75 mS/cm on a 1:2 extract |
| pH | 5.5–6.5 on arrival, no lime in the mix |
| Potassium, sodium, chloride | Reported per lot, since these drive coir's EC |
| Buffered or washed only | Know which you have. Washed-only coir needs a calcium and magnesium charge |
| Aggregate | Agreed type and share, trialled in your own cell size |
| Wetting agent | Optional for coir. Useful if trays dry between filling and sowing |
| Moisture at filling | Pre-wetted, uniform, no dry pockets |
Where CVM fits
CVM Fiber and Industries produces coco peat in Kanyakumari, Tamil Nadu, India, from husk sourced across several districts. Our coco peat is washed with water to reduce EC. We don't add calcium nitrate or any other chemical, and it is not buffered, so plug growers should plan a calcium and magnesium charge or buffer it themselves. It ships as 5 kg blocks, 650 g briquettes and custom blends mixed to your grade. To discuss a plug mix trial, contact us.
FAQ
Can I use coco coir instead of peat in 512 plug trays?
Yes, if the grade is fine and screened to remove dust, and the trays are filled lightly with pre-wetted media. A 512 cell holds very little air with any substrate, so fines and compaction matter more than the material itself. Run a few trays alongside your peat mix before switching a whole crop.
Do I need to add lime to a coir plug mix?
Usually not. Coir typically tests between pH 5.6 and 6.9, which is already in or near the range plugs need. Adding lime can push pH high enough to cause iron chlorosis in petunia and pansy. Test each delivery and correct with feed choice if needed.
Why do my coir plugs go yellow when peat plugs didn't?
The two common causes are calcium and magnesium shortage in unbuffered coir and high pH. Coir holds onto calcium and magnesium while releasing potassium. A calcium nitrate-based feed with magnesium usually fixes the first, and checking pH fixes the second.
Do growth regulators work in coir?
Yes. Paclobutrazol drench activity was similar in coir and peat at a low rate and stronger in coir at a higher rate. Plants in coir often grow taller untreated, so you may need slightly higher rates to finish at the same height.
Should I use ammonium in a coir plug feed?
Keep it low. Ammonium lowers pH and coir has a weaker pH buffer than peat, so the drop comes faster. Ammonium also causes stretch in plugs. Use mainly nitrate-form feeds and add ammonium only occasionally if growth is slow.
Will coir plugs fall apart at transplant?
Loose coir plugs can be less cohesive than peat plugs until roots fill the cell. Holding plugs a few days longer before transplant usually solves it. Pull test a few plugs before the planned date, and use nets, binders or paper sleeves in the smallest cells if crumbling persists.
What EC should a coir plug mix have at sowing?
Plug programmes for petunia and pansy aim for about 0.75 mS/cm on a 1:2 extract at sowing, and geranium for under 0.75. Coir varies widely by source, so test every lot before mixing. If EC comes in high, leach before filling rather than after sowing.