The short answer: coco coir works well in greenhouse pot-plant mixes for poinsettia, potted chrysanthemum and kalanchoe, usually as 10–50 % of a peat-reduced blend rather than on its own. Washed and calcium-treated coir has grown begonia, chrysanthemum and schefflera as well as peat, and kalanchoe grew more strongly in it. What changes is the management. Coir brings potassium and sodium but little calcium or magnesium, plants in it tend to grow a bit taller, and it behaves differently on an ebb-and-flow bench. Plan for those three things and coir is a sound pot-plant component.
Key facts
- Poinsettia and chrysanthemum are heavy feeders, with a PourThru EC target of 2.6–4.6 mS/cm. Kalanchoe sits in the medium group at 2.0–3.5.
- Poinsettia pH targets run from 5.8–6.4 down to a tighter 5.8–6.2. At pH 6.5, new poinsettia leaves go yellow from iron deficiency.
- A low potassium to calcium ratio cuts poinsettia bract necrosis and stem breakage. High potassium and ammonium make both worse.
- Coir stays easy to rewet when it dries. Peat turns water-repellent, and at low starting moisture peat's container capacity drops by about 40 % by volume.
What a pot-plant substrate has to do
A 10–15 cm pot is short, so the saturated zone at the bottom after watering (the perched water table) takes up a fair share of the column. The mix has to drain enough air into the rest of the pot to keep roots healthy over a crop that can run 16 weeks or more.
General guidance for container substrates is 50–85 % total porosity, 10–30 % air space and 45–65 % container capacity, all by volume. For poinsettia, one substrate supplier's guidance asks for 20–30 % of the volume as large, air-filled pores. The same supplier's general and high-porosity mixes measure 20–25 % and 30–35 % air space.
| Property | General container range | Poinsettia guidance |
|---|---|---|
| Total porosity | 50–85 % | — |
| Air space | 10–30 % | 20–30 % |
| Container capacity | 45–65 % | — |
| pH | Nutrients broadly available at 5.4–6.2 | 5.8–6.4, or 5.8–6.2 for a tighter target |
Fine coir pith on its own holds a lot of water. In a 10 cm pot that can push air toward the bottom of the range. Coarser coir, husk chips or perlite fix that. For the long poinsettia crop, aim at the upper end of the air range, especially if you water overhead.
How much coir goes into the mix
There's no single right share. Published work and commercial practice cover a wide span.
| Crop and setting | Coir share | Result |
|---|---|---|
| Poinsettia, public-garden crop of 24 varieties | 40 % coir, 60 % peat | Whole crop grown in it after a smaller trial the year before. Peat use down 40 % |
| Potted chrysanthemum, substitute-substrate trial | 20 % coir in a 2:4:2:2 coir, moss peat, perlite and pine needle mix | Best of the substitutes tested |
| Peat and coir wettability tests | 10–30 % coir in peat | Coir improved rewetting of dried peat, though less than a wetting agent did |
| Begonia, chrysanthemum, schefflera, kalanchoe | Treated (washed, Ca and Mg exchanged) coir in place of peat | Equal growth to peat for the first three. Kalanchoe grew more strongly |
| Commercial poinsettia nursery, Germany | 50 % peat reduction, mixed alternatives | Kept after the project ended because growth, plant build and rooting were better |
Two lessons from the German work are worth knowing. Short crops of up to 10 weeks usually handled a 50 % peat reduction without trouble. Long crops like poinsettia were harder. In a later poinsettia test, both the peat-free and the 50 % peat mixes contained 30 % wood fibre, and both locked up nitrogen early (an incubation test showed 188 mg/L less N). Growth was delayed and branching cut, though roots grew better in the peat-free mix. If you blend coir with wood fibre, plan extra early nitrogen.
The same untreated-coir work is the clearest warning. Raw coir caused growth restrictions, and analysis showed calcium and magnesium deficiency driven by high potassium and sodium and by calcium and magnesium binding to the coir. Washing and exchanging the coir removed the problem. Whether you buy coir buffered or wash-only changes how you build the base fertiliser. Our buffered vs unbuffered coco peat guide covers the trade-off.
pH and EC targets by crop
Coir's own pH sits naturally between 6 and 7, higher than peat. Peat-free and peat-reduced mixes often run above the 5.5–6.0 peat target, so start checking pH in week one.
| Crop | Feeding group | PourThru EC (mS/cm) | SME EC (mS/cm) | pH notes |
|---|---|---|---|---|
| Poinsettia | Heavy | 2.6–4.6 | 2.0–3.5 | 5.8–6.2 tight target. Iron deficiency at 6.5. Low pH limits molybdenum |
| Potted chrysanthemum | Heavy | 2.6–4.6 | 2.0–3.5 | Most nutrients available at 5.4–6.2 |
| Kalanchoe | Medium | 2.0–3.5 | 1.5–3.0 | Most nutrients available at 5.4–6.2 |
Poinsettia is also named as one of the salt-sensitive species in peat-free demonstrations, along with antirrhinum, primula and rex begonia. Peat-free media there usually ran at higher EC than peat. Remember that coir's potassium and sodium count toward the reading, so a "normal" EC can hide a low calcium supply. For routine sampling methods, see managing pH and EC in coco coir.
Feeding around coir's potassium
Coir gives you potassium you didn't ask for and holds back calcium and magnesium. For pot plants, and poinsettia above all, that's the wrong direction. Bract necrosis and stem breakage drop when the potassium to calcium ratio is low and rise with high potassium and ammonium.
| Stage | Typical poinsettia programme | Coir adjustment |
|---|---|---|
| Growing on | 200–250 ppm N constant feed. Most varieties take up to 350 ppm | Use a nitrate-led, calcium-carrying feed and drop the potassium share |
| After the pinch | Halve the feed for 7–10 days until new shoots elongate | Same |
| Calcium | Alternate a calcium nitrate based feed, more often from October. Weekly calcium sprays of 200–400 ppm Ca from first bract colour if climate is hard to control | Start alternating earlier, from potting, with unbuffered coir |
| Magnesium | A typical poinsettia feed gives 33 ppm Mg at 300 ppm N | Test water and substrate to see if that's enough with unbuffered coir |
| Molybdenum | Keep leaf Mo at 1–5 ppm. Corrective drench at 50 ppm Mo (0.1 g/L sodium molybdate) | Keep pH out of the low range |
| Finishing | Last 2 weeks: 50–100 ppm N once a week | Avoid finishing on high-K feeds |
One more point from postharvest research. Bract edge burn doubled when nitrogen supply went from 200 to 400 ppm, and late ammonium feeding leads to bract burn and then botrytis. Coir gives you no reason to push the feed harder at the end.
Plant height and growth regulators
Plants often grow bigger in coir. Potted sunflowers grew taller in coir-based than peat-based substrates, and kalanchoe was more vigorous in treated coir than in peat. Dahlia height didn't change.
The good news is that coir doesn't weaken paclobutrazol drenches the way pine bark does. In potted sunflowers, drench activity at 2 mg per pot was the same in coir and peat. At 4 mg per pot it was greater in coir. The percentage of height control was similar in both, which means the extra growth in coir may need a slightly higher starting rate, not a different product. Bark is the one to watch. It has been shown to reduce drench activity in chrysanthemum, so a coir and bark blend can behave differently from coir and peat.
| Situation | What to expect | What to do |
|---|---|---|
| Coir replacing part of the peat | Slightly taller plants | Trial your usual drench rate on one bench first, then nudge up if needed |
| Coir plus pine bark | Bark can tie up drench activity | Rely more on sprays, or trial higher drench rates |
| Dry growing for height control | Poinsettia height can be held with no growth regulators by controlled drying | Coir rewets easily, which makes the dry-down safer than with peat |
Foliar sprays go in through the leaves, so the substrate matters less for them. Drenches and dry growing are where coir changes the picture.
Ebb-and-flow benches and capillary mats
Ebb-and-flow has long worked for poinsettias, kalanchoes, mini roses, Christmas cactus and chrysanthemums. It's been found to use about 40 % less water than hand watering. Coir does well on these systems, with two caveats.
First, how dry the mix is at the first flood matters a lot. In a wettability study, coir at 50 % starting moisture reached 51.6 % water by volume from below against 73.5 % from overhead. Peat did far worse. At 33 % and 50 % starting moisture, subirrigated peat stayed under 20 % water by volume. Coir captured the most water of the three materials tested. Pot into moist mix and give the first watering from above.
Second, salts move up. With subirrigation there's practically no leaching, so fertiliser salts collect in the top 1 cm or so of the pot. Roots are rarely there, so it doesn't usually hurt the crop, but it does skew any sample taken from the surface.
- Measure pot EC weekly by PourThru rather than relying on feed EC.
- The old rule of "half the overhead concentration" isn't always right. Trials on subirrigated begonia and petunia found it gave less than optimal growth, so adjust by measured EC.
- In the same trials, a coir-based mix ran a lower EC than a peat mix of higher water-holding capacity under the same feed, while growth was much the same.
- Account for your water. If your water EC is 0.95 mS/cm rather than 0.15, the target range rises by about the difference.
Capillary mats run on the same physics as a flood bench. A coir mix that's already moist when potted will wick up evenly. A dried-out peat pot may not.
Shelf life and postharvest quality
Quality in the shop is mostly decided in the last few weeks of production. For poinsettia that means a low-salt root zone at shipping, a low potassium to calcium ratio, no late ammonium, and even watering. Water stress causes leaf drop on older leaves. This is where coir's easy rewetting helps. A coir-containing pot that dries out at retail takes water back more readily than a dry peat pot.
For succulent kalanchoe species, a 30-day post-production test found coconut fibre kept ornamental quality better than washed sand. That trial used three species, not the common flowering Kalanchoe blossfeldiana, so treat it as a pointer only.
A grower specification for pot-plant coir
Write the spec around what the crop needs, then check every delivery against it.
| Item | What to ask for | Why |
|---|---|---|
| Washing | Fully washed, low-EC coir pith. Washed coir should test below 1 mS/cm in a 1:1.5 volume extract | Unwashed coir can test 6 mS/cm or more, too salty for salt-sensitive poinsettia |
| Buffering | State whether you want it buffered or wash-only | Decides how much Ca and Mg goes into the base dressing |
| Grade | Screened pith with fines limited for 10–12 cm pots. Add chips or coarse fraction for 13–15 cm and long crops | Keeps air space in the 20–30 % range poinsettia likes |
| Share in mix | Start at 20–40 % in peat for the first season | Matches the commercial and trial results above |
| Batch testing | pH, EC, K, Na, Cl on each delivery, by your own lab | Coir varies by source and wash |
| Moisture at potting | Pre-moistened, never dusty-dry | Better first water capture, especially on flood benches |
If you're making the blend yourself, our notes on coco peat for potting mix producers cover handling at scale. For the coarse fraction, coconut husk chips open the mix in larger pots. Rooting cuttings is a separate stage, covered in coco coir for bedding plant plugs.
CVM Fiber and Industries makes coco peat blocks, husk chips and custom blends in Kanyakumari, Tamil Nadu, India. Our coco peat is washed with water to bring EC down, with no calcium nitrate or other chemicals, so it's unbuffered. If you'd like a sample to trial in your own pot-plant mix, get in touch.
FAQ
Can you grow poinsettias in 100 % coco coir?
It can be done, but most growers use coir as part of a peat-reduced mix. Poinsettia is a long, salt-sensitive crop that needs good calcium supply, and coir alone brings potassium and sodium while holding back calcium and magnesium. A 20–40 % coir share is an easier place to start.
What pH should poinsettia substrate be?
Targets run from 5.8–6.4, with many growers aiming for 5.8–6.2. At pH 6.5 new leaves can yellow from iron deficiency, and low pH limits molybdenum, which poinsettia needs more than most crops. Coir starts higher than peat, so check pH from the first week.
Do plants grow taller in coir?
Often a little. Potted sunflowers grew taller in coir-based substrates, and kalanchoe was more vigorous in treated coir than in peat. Paclobutrazol drenches work at least as well in coir as in peat, so a small rate increase usually covers the extra growth.
Does coir reduce growth regulator drench activity like bark does?
No. Paclobutrazol drench activity was the same in coir and peat at 2 mg per pot and greater in coir at 4 mg. Pine bark is the component known to reduce drench activity, so watch blends that combine coir with bark.
Is coco coir good for ebb-and-flow benches?
Yes. Coir captures water from below better than dried peat does, and it stays easy to rewet. Pot into moist mix, give the first water from above, and measure EC weekly because salts collect in the top layer of subirrigated pots.
How should I change my feed for coir in potted chrysanthemum?
Lower the potassium share and raise calcium and magnesium, using a nitrate-led feed. Chrysanthemum is a heavy feeder with a PourThru EC target of 2.6–4.6 mS/cm. Remember that coir's own potassium and sodium add to that reading.
Does coir affect poinsettia shelf life?
Not directly. Shelf life depends more on a low-salt root zone at shipping, a low potassium to calcium ratio and no late ammonium. Coir can help at retail because a dried-out pot rewets more easily than one with dry peat.
Should I buy buffered or unbuffered coir for pot plants?
Either can work. Buffered coir has already had potassium and sodium swapped for calcium and magnesium. With washed, unbuffered coir you build that calcium and magnesium into the base dressing and feed from the start.