The short answer: for shrubs and container trees that stand outdoors for one to three seasons, coir doesn't replace bark. It replaces the peat. Most outdoor nursery mixes are 80 to 100% bark by volume, and the 20 to 30% fine fraction that used to be peat is where washed coir pith goes. Coir chips can stand in for part of the bark if bark is short or expensive, but a pure coir pot is too wet and too soft for a two-year crop. The rest of this page is about the ratio, the feed and the watering once you've made that swap.
Why bark runs the show
Nursery stock is a long crop in a big pot outdoors. The substrate has to drain after a week of rain, hold its shape for two winters and come out as a root ball that survives handling. Aged or composted conifer bark does that cheaply.
| Property (fresh material) | Sphagnum peat | Coir pith | Aged pine bark | Wood fibre |
|---|---|---|---|---|
| Total porosity, % vol | 75 (89–94 quoted elsewhere) | 91 (92–94 quoted elsewhere) | 79–83 | 86–96 |
| Air space after drainage, % vol | 22 | 12–13 | 26–29 | 25–36 |
| Water held at container capacity, % vol | 53 | 78 | 50–57 | 60–61 |
| Dry bulk density, g/L | 100 | 60–70 | 180–280 | 50–150 |
| pH as supplied | 3.8–4.4 | 5.5–6.8 | acid, varies with age | near neutral |
Bark has roughly double the air of coir pith after drainage and weighs three to four times as much, which keeps 10-litre pots upright in wind. Coir holds far more water than bark or peat. Each of those is a strength or a weakness depending on how much you use. Published ranges and how to measure your own are in coco coir water holding and air porosity.
The two places coir fits
As the fine fraction. Peat was never in a nursery mix for air. It held water between irrigations, stopped the bark drying into a hydrophobic brick and gave young roots something fine to grow into. Coir pith does the same job, rewets more easily and sits at a pH where lime isn't needed. Adding 20 to 30% coir pith by volume to a bark-dominant mix is the standard starting point, and several professional peat-free nursery stock ranges are built the same way: bark plus wood fibre plus coir.
As chips replacing part of the bark. Coconut husk chips behave physically like a coarse bark grade: open, slow to break down and free of the pine-bark supply squeeze some regions face. They don't bring bark's weight, so pot stability drops as the share rises. Chip breakdown against bark is covered in coco coir mulch, and pith-to-chip ratios in coco pith to husk chip blend ratios.
Starting ratios by crop (% by volume)
| Crop | Bark 0–15 mm | Coir pith 0–6 mm | Wood fibre | Coir chips or coarse bark | Green compost |
|---|---|---|---|---|---|
| Liners, 9 cm to 2 L, one season | 50 | 30 | 20 | 0 | 0 |
| Shrubs, 3–10 L, one to two seasons | 60 | 20 | 10 | 10 | 0 |
| Container trees, 15–80 L, two to three seasons | 60 | 15 | 10 | 15 | 0 |
| Peat-free with compost | 55 | 20 | 10 | 0 | 15 |
| Bark-short region | 40 | 20 | 10 | 30 | 0 |
Green compost at 10 to 15% is how some long-established peat-free tree growers add weight and base nutrients. Trialled at a third of the mix across 29 nursery trials, it raised bulk density 30 to 50% over the peat control, so it's also the fix if pots blow over.
Stability over a one-to-three-year crop
A pot changes while the crop grows. In a 70-week outdoor trial with cotoneaster, aged pine bark lost 16 to 17% of its height, air space fell from 26% to about 20% and water held rose from 57% to 63%. It still finished inside the accepted ranges for container production (air 10 to 30%, water 45 to 65%). Any fine fraction starts that clock from a lower air figure, so what matters is how fast it decays.
Coir pith is more than 45% lignin with less cellulose than peat, so it breaks down slowly. Quoted working life runs from about two years for fresh material to four for aged, coarse grades, and the finer the pith the shorter the life. That's why the table drops the pith share and raises the chip share as the crop gets longer.
The failure mode is well documented by specimen tree growers who trialled coir-based peat-free mixes in 80-litre pots. Early on, water pooled at the base, roots formed a dense layer there and nowhere else, and root balls fell apart on lifting. After a 12-month cycle the surface turned powdery and hydrophobic. Reworking the mix, the irrigation cycles and the wetting agent dealt with most of it. Some genera stayed difficult in any peat-free mix, among them Stewartia, Cornus, Styrax and ericaceous subjects such as Camellia and Magnolia. Budget for those separately.
Nitrogen immobilisation: coir, bark and wood fibre differ
Microbes breaking down high-carbon material pull nitrogen out of the substrate solution. Wood and bark have carbon-to-nitrogen ratios of 300:1 or more, and tie-up starts to matter above about 30:1. Coir is high-carbon too, but far less active. A standard 21-day incubation of individual components gave these results.
| Component | Nitrogen lost from solution in 21 days |
|---|---|
| Washed coir pith | 0% |
| Sphagnum peat | 4% |
| Pine wood fibres (four types) | 2–4% |
| Aged pine bark | 9% |
Two caveats. The lab test only partly predicts the pot, and tie-up climbs with the wood share, becoming a real risk at 40 to 60% wood fibre. In the cotoneaster trial, substrate nitrate ran two to four times higher in pine bark than in a wood-based mix at the same fertiliser rate, and the wood mix fell below the 15 to 25 mg/L nitrate target by July at the lower rate.
In practice: swapping peat for coir pith doesn't change your nitrogen plan, since coir ties up less than peat. Swapping bark for wood fibre does, so add nitrogen or hold wood fibre at 10 to 20%. Aged bark ties up more than composted bark, and composting takes 5 to 7 weeks with nitrogen added (hardwood bark needs 1.2 to 1.8 kg of actual nitrogen per cubic metre).
Controlled-release fertiliser and lime
CRF. Woody crops in bark are typically fed at 4 to 8 kg/m³ of controlled-release fertiliser, with the higher end the usual recommendation for woody plants. A coir-for-peat swap doesn't need more. If part of the bark becomes wood fibre, go to the top of the range or top-dress in midsummer. Check with pour-through: 0.5 to 1.0 dS/m and 15 to 25 mg/L nitrate are the working targets outdoors. Coir has a lower cation exchange capacity than peat, so it buffers less and a missed feed shows sooner.
Lime. Peat-based mixes carry 3 to 6 kg/m³ of dolomitic lime to lift pH from about 4 to the 5.5 to 6.5 target. Coir arrives at 5.5 to 6.8 and needs none. In a bark-and-coir mix the lime is for the bark only, so cut the rate and check pH after a week of wetting. One trial saw pH slide to 5.0 when bark reached 60% of the mix, so don't drop lime to zero in bark-heavy recipes. Raw coir also brings 170 to 600 ppm potassium and some phosphorus, so the first weeks' feed can be lighter than peat habits suggest.
Pot sizes, watering and winter
Fully saturated coir pith holds 9 to 12% air against 19 to 24% for bark. At 20 to 30% inclusion that gap mostly closes, but the mix still holds more water than bark alone and dries differently. Peat-free pots look dry on top while the bottom half is wet. Lift pots to judge weight, or probe to the base.
| Pot size | What changes with 20–30% coir pith in the bark | Watering habit |
|---|---|---|
| 9 cm to 2 L liners | Larger water reserve, fewer missed-day losses | Water by weight, not surface colour |
| 3–10 L shrubs | Base stays wet after rain, surface dries fast and may shed water | Shorter, more frequent cycles in summer, wetting agent if the top layer repels water |
| 15–80 L trees | Risk of a saturated base with roots crowded above it | Raise the coarse share, check moisture at three depths, cut winter irrigation hard |
Across the 29 peat-free trials, growers irrigated slightly more often but used no more water in total. Keep runoff EC under about 1.5 times what you apply. Outdoors, rain does most of that for you.
In winter the pot carries more water into cold weather, so it's heavier and slower to freeze through and to thaw, and the risk of a waterlogged base rises during a wet thaw. Microbial activity drops to almost nothing, so nitrogen tie-up from bark or wood fibre stops being an issue until spring. Stop irrigating earlier in autumn than you would with bark alone, stand pots on free-draining ground so the bottom few centimetres never sit in water, and keep the pith share lower for anything spending two winters in the same pot.
What the trials say about growth against peat
| Trial | Crop and mix | Result |
|---|---|---|
| Two-season outdoor nursery trial | Goldenrod, lavender, chrysanthemum in pine bark + coir vs pine bark + sphagnum | No difference in growth or development |
| Container shrub trial | Viburnum and lilac in coir-based vs comparable peat-based mixes | Growth similar to or greater than peat |
| 29 trials, 25 nurseries, 267 subjects | Peat-free and peat-reduced container stock vs peat control | All plants marketable, internodes sometimes shorter, surface liverwort suppressed |
| Six-week bedding trial | Petunia in 70% peat + 30% coir vs 100% peat | No growth loss, while 30% wood fibre cut growth and flowering |
| Conifer seedlings, 21 weeks | Douglas-fir in 100% coir-based media vs peat | Seedlings smaller, lower foliar nitrogen and calcium |
| Specimen trees, 1,500 trees, 100+ genera | Coir-based peat-free vs 50/50 peat-reduced | Some varieties saleable sooner than expected, a group of genera rooted poorly, nutrition needed topping up |
The pattern holds. Where coir replaces the peat fraction in a bark mix, growth matches peat. Where coir is the whole pot and the feed hasn't changed, young plants run short of nitrogen and calcium. For your own comparison: at least six pots per block, your current mix as the control plus two peat-free candidates, identical irrigation and wetting agent on every treatment, and EC readings at the start, at intervals and at the end.
Specifying coir for a nursery buying in bulk
A nursery mixing its own needs a different coir from a hydroponic tomato grower. Put these on the purchase order.
- Grade: washed pith, 0 to 6 mm, for the peat fraction. Chips at a stated size band if you're replacing bark. Say whether loose fibre is acceptable, since it adds air but clumps in potting machines.
- EC: below 1 mS/cm in a 1:1.5 volume extract is the usual nursery threshold, with sodium and chloride reported alongside. Raw coir can carry 400 to 700 ppm chloride, tolerable at low EC and a problem above it.
- Washed, not buffered: for a bark mix fed with CRF outdoors, washed unbuffered coir is the normal choice. CVM Fiber and Industries, in Kanyakumari, Tamil Nadu, India, washes coco peat with water to lower EC and adds no calcium nitrate or other chemicals, so what arrives is unbuffered washed pith.
- Format: 5 kg blocks for a mixing line, or a custom pith-and-chip blend if you'd rather the ratio arrived fixed.
- Consistency: the same particle distribution batch to batch matters more to a potting machine than one perfect spec sheet. Sieve a sample from every delivery.
Tendering, sampling and shipment checks are in buying coco coir in bulk, and the nursery grades on offer are on the nurseries page.
If you're reworking a bark mix for next potting season and want pith or chip samples to trial against your current peat fraction, contact us with your target ratio and pot sizes.
FAQ
Can I grow nursery stock in 100% coco coir?
You can, but outdoors in large pots it's a poor choice. Fully saturated coir pith holds only 9 to 12% air, it weighs a fraction of bark so pots blow over, and the fine fraction breaks down over two seasons. Keep coir to the 20 to 30% peat fraction and let bark or chips carry the structure.
Does coir tie up nitrogen like bark and wood fibre?
Far less. In a 21-day incubation washed coir pith immobilised none of the nitrogen supplied, peat about 4%, wood fibres 2 to 4% and aged pine bark 9%. Replacing peat with coir doesn't change your nitrogen plan. Replacing bark with wood fibre does.
How much controlled-release fertiliser should I use in a bark and coir mix?
The same as your bark and peat mix, usually 4 to 8 kg/m³ with woody plants at the higher end. Only raise the rate if you've also brought in wood fibre. Check with pour-through, aiming for 0.5 to 1.0 dS/m and 15 to 25 mg/L nitrate.
Do I still need lime when coir replaces the peat?
Less. Coir sits at pH 5.5 to 6.8 and needs no lime itself, so the lime in the recipe is only there for the bark. Reduce the rate and test pH after the mix has been wet for a week rather than cutting lime entirely, since bark-heavy mixes can drift down to pH 5.
Can coir chips replace bark completely?
Physically they come close, since chips are open and break down slowly, but they don't bring bark's weight. Keep bark or another heavy component in the mix for pots that stand outdoors in wind, and trial any chip-heavy recipe in your largest pot size first.
Will plants in a coir and bark mix overwinter differently?
The pot carries more water into winter, so the main risk is a saturated base layer during wet thaws rather than cold. Stop irrigation earlier in autumn, stand pots on free-draining ground and keep the coir share lower for crops spending two winters in the pot.
What EC should I specify for nursery-grade coir?
Below 1 mS/cm in a 1:1.5 volume extract is the usual threshold, with sodium and chloride reported alongside. Chloride of 400 to 700 ppm is common in raw coir and is tolerable at low EC. Outdoors, rain leaches residual salts, so nursery stock is more forgiving than slab crops under glass.