# Growing in coco coir

> Coco coir is a hydroponic substrate that looks like soil: inert-ish, airy, and fed every irrigation with a complete mineral solution at pH 5.8–6.2 and EC 1.2–2.4 mS/cm with 10–20 % runoff. Use pre-washed, buffered coco or buffer it yourself with a calcium/magnesium solution, because raw coir holds potassium and sodium and grabs calcium and magnesium.

- Canonical: https://genetixbay.com/knowledge/systems/coco.html
- Category: Systems · Level: Intermediate
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Any stage · Systems: Coco
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**What size pot for coco?** Coco pot size is a trade-off against how often you can irrigate. Bigger pots generally give bigger plants, as a meta-analysis of rooting volume across crops found, and they dry slower and forgive more. Small pots can still grow big plants with [high-frequency fertigation](https://genetixbay.com/knowledge/glossary/high-frequency-fertigation.md), which coco suits best, but need more irrigations per day (2–6 in flower) and leave less margin for error.

**Do I need to buffer coco coir?** Coco coir needs buffering unless it is sold pre-washed and buffered. Raw coir's exchange sites are loaded with potassium and sodium and will pull calcium and magnesium from your feed, causing early deficiency spots. Rinse until runoff EC is below about 0.5 mS/cm, soak 8–24 hours in a calcium-magnesium solution, then rinse again. Some substrate experts argue well-processed coir needs no buffering.

**What pH for coco?** The best pH for coco is 5.8–6.2 at the input, or 5.8–6.0 for seedlings and clones. That sits inside the accepted 5.5–6.5 soilless range, and gentle drift within it is normal. Don't use the pH 6.5 figures from soil guides, and check runoff pH a few times a week alongside runoff EC to see what the root zone is actually doing.

**How often should I water coco?** Coco should be watered when the pot has lost about 30–50% of its available water, always to 10–20% runoff, with full nutrients every time. In practice that is about once a day for seedlings, 1–3 irrigations a day in veg and 2–6 a day in flower, with smaller pots needing more frequent shots. Never let coco dry out completely.

**Why do my coco plants get calcium deficiency?** Coco plants usually show calcium deficiency because raw, unbuffered coir swaps potassium and sodium off its exchange sites and holds on to calcium and magnesium from the feed. Rust spots on newer leaves in weeks 1–3 are the classic sign. [RO water](https://genetixbay.com/knowledge/glossary/ro-water.md), low root-zone pH and cultivar demand also drive it; blaming LED lighting is a myth. Check runoff before adding more calcium-magnesium.

**Can coco dry out completely?** Coco should never dry out completely. Fully dry coir becomes hydrophobic and hard to re-wet, and plants can crash after a single hot day. Letting it dry out 'to make roots search' is a common mistake. Irrigate when about 30–50% of the available water is gone, and use a moisture probe if you can't check pots often enough.

**Coco vs soil for beginners?** For beginners, coco is less forgiving than soil because it behaves like hydroponics in a pot and has no starter nutrient charge. Coco needs a complete mineral feed at EC 1.2–2.4 mS/cm every irrigation with 10–20% runoff, and can harm plants within 12–36 hours if watering stops. Feeding coco like soil, with plain-water days and low EC, is a common beginner error.

**Short answer.** Treat coco as **hydroponics in a pot**. Feed a complete mineral solution at **pH 5.8–6.2**, **EC 1.2–2.4 mS/cm** (600–1,200 ppm500 / 840–1,680 ppm700) depending on stage, **every irrigation**, with **10–20 % runoff**, and never let it dry to dust. Use coco that is washed and **buffered** (exchange sites pre-loaded with calcium/magnesium), or buffer it yourself, because raw coir carries potassium and sodium and will take calcium and magnesium out of your feed [1][2].

## How it works

Coir pith holds a lot of water while staying airy, so roots get both. It has a modest [cation exchange capacity](https://genetixbay.com/knowledge/glossary/cation-exchange-capacity.md) that, when raw, is loaded with K⁺ and Na⁺ from the coconut husk [1]. Feed it calcium and it swaps calcium onto the exchange sites and releases potassium (and sodium) into the root zone, which is why fresh, unbuffered coco produces early calcium/[magnesium deficiency](https://genetixbay.com/knowledge/glossary/magnesium.md) symptoms. Buffering soaks the coir in a calcium (often [calcium nitrate](https://genetixbay.com/knowledge/glossary/calcium-nitrate.md)) or calcium-magnesium solution, then rinses [2].

Coco dries fast, so it rewards frequent small irrigations (high-frequency fertigation). Once fully dry it becomes hydrophobic and hard to re-wet.

## Numbers that matter

| Parameter | Seedling/clone | Veg | Flower | Notes |
|---|---|---|---|---|
| Input pH | 5.8–6.0 | 5.8–6.2 | 5.8–6.2 | Jardin coco band 5.8–6.2 [5] |
| [Input EC](https://genetixbay.com/knowledge/glossary/input-ec.md) (mS/cm) | 0.6–1.0 | 1.2–1.8 | 1.8–2.4 | ppm500 = EC × 500; ppm700 = EC × 700 |
| Runoff | 10–20 % per day | 10–20 % | 10–20 % | Less if [drain-to-waste](https://genetixbay.com/knowledge/glossary/drain-to-waste.md) is steered ([crop steering](https://genetixbay.com/knowledge/systems/drain-to-waste-crop-steering.md)) |
| [Runoff EC](https://genetixbay.com/knowledge/glossary/runoff-ec.md) vs input | ±0.3–0.5 | ±0.5 | ±0.5–1.0 | Higher = salts building, irrigate more volume |
| Irrigations/day | 1 | 1–3 | 2–6 | Smaller pots need more |
| Ca:Mg in early weeks | Slightly higher Ca/Mg | | | Jardin "buffer check" in week 1 [5] |
| Perlite | 0–30 % | | | Adds air; reduces water holding |

> **House position: pot size is a trade-off.** Bigger pots give bigger yields, within reason; across many crops, bigger rooting volume generally means bigger plants (Poorter et al. 2012 meta-analysis). But root volume isn't the whole story: the founder has grown a QP+ (4 oz+, ~115 g+) from a 16 oz (~0.5 L) cup. If you push water and nutrients hard enough (high-frequency fertigation, which coco suits best), big things come in small packages. Small pots need more frequent feeding and watering and leave less margin for error; big pots are more forgiving but dry slower. Match the pot to how often you can irrigate.

## How to run it

1. **Prep:** hydrate the brick, rinse until runoff EC is below ~0.5 mS/cm, then soak 8–24 h in a buffering solution and rinse again. Pre-buffered commercial coco can skip this.
2. **First feed:** full mineral nutrients from day one at the seedling EC. Coco has no starter charge.
3. **Irrigate** when the pot has lost ~30–50 % of its available water, to light runoff. Never let it dry out fully.
4. **Measure runoff** EC and pH a few times a week. Adjust volume, not just strength.
5. **Reuse?** Possible after rinsing and re-buffering; root debris increases water holding and disease risk.

## Failure modes

| Problem | Likely cause | First check |
|---|---|---|
| Rust spots on new-ish leaves, weeks 1–3 | Calcium/magnesium pulled by unbuffered coco | Runoff Ca/Mg or runoff EC; was it buffered? |
| Burnt tips, runoff EC far above input | Too little runoff, salts accumulate | [Root-zone flush](https://genetixbay.com/knowledge/glossary/root-zone-flush.md) with a light solution (EC ~0.4–0.8, with Ca/Mg), then more runoff volume |
| Droop with wet pots | Over-watering, low [air porosity](https://genetixbay.com/knowledge/glossary/air-filled-porosity.md) | Pot size vs plant size, perlite |
| Plants crash after a hot day | Coco dried out | Irrigation frequency, a VWC probe |

## Jardin notes

Jardin treats coco as a "hydro-behaving medium": dryback plus EC control from VWC, with the irrigation method chosen separately (recirculating drip, drain-to-waste with steering, or flood & drain). Time-to-harm on pump failure: 12–36 h. Automation level 3 [5].

## Common mistakes

- **[Flushing before harvest](https://genetixbay.com/knowledge/glossary/flushing.md).** Pre-harvest flushing is a *Myth* that costs you yield and resin: it starves the plant while it packs on its final weight, trichomes and resin. Feed through to harvest at stage strength. The only valid flush is a root-zone flush to clear [salt build-up](https://genetixbay.com/knowledge/glossary/salt-build-up.md), at any stage (see [the flush debate](https://genetixbay.com/knowledge/harvest-cure/flush-debate.md)).
- **Root-zone flushing coco with plain water.** It strips Ca and Mg from the exchange sites. Flush with a light solution (EC ~0.4–0.8 mS/cm, pH ~5.8–6.0) and check runoff EC.
- Feeding coco like soil (plain water days, low EC).
- Skipping buffering, then adding more cal-mag blindly instead of checking runoff.
- Blaming the LEDs for Ca/Mg spotting. That's a *Myth*: in coco the usual drivers are unbuffered coir, RO water, low pH and the cultivar's own demand ([Cal-Mag](https://genetixbay.com/knowledge/nutrients/cal-mag.md)).
- Letting coco dry fully between waterings "to make roots search".
- Reading the label pH 6.5 from soil guides.

## Evidence

Coir's high K/Na and exchange behaviour is *Established* [1][4]. Buffering practice is *Supported* by industry and substrate experts, though some argue well-processed coir needs no calcium buffering [2]. Cannabis responses to organic feed rates in coir-based substrates are *Supported* by [3]. The pH band (soilless 5.5–6.5, coco sweet spot 5.8–6.2) is accepted practice from extension and grower consensus; EC bands and irrigation frequency are *Grower practice*.

## Sources

1. Abad M, Noguera P, Puchades R, Maquieira A, Noguera V, 2002. Physico-chemical and chemical properties of some coconut coir dusts for use as a peat substitute for containerised ornamental plants. Bioresource Technology 82(3):241–245. https://doi.org/10.1016/S0960-8524(01)00189-4
2. Greenhouse Grower, 2023. Washed, low-EC, or buffered? How to specify coco substrate for your crop. https://www.greenhousegrower.com/?p=180860
3. Caplan D, Dixon M, Zheng Y, 2017. Optimal rate of organic fertilizer during the vegetative-stage for cannabis grown in two coir-based substrates. HortScience 52(9):1307–1312. https://doi.org/10.21273/HORTSCI12401-17
4. Raviv M, Lieth JH, Bar-Tal A (eds), 2019. Soilless Culture: Theory and Practice, 2nd ed. Elsevier. https://doi.org/10.1016/C2015-0-01807-5
5. GenetixBay Jardin engineering notes, 2026 (system profiles and failure margins).
6. Poorter H, Bühler J, van Dusschoten D, Climent J, Postma JA. 2012. Pot size matters: a meta-analysis of the effects of rooting volume on plant growth. Functional Plant Biology 39(11):839-850. https://doi.org/10.1071/FP12049