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 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 symptoms. Buffering soaks the coir in a calcium (often calcium nitrate) 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 (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 is steered (crop steering) |
| Runoff EC 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
- Prep: hydrate the brick, rinse until runoff EC is below ~0.5 mS/cm, then soak 8–24 h in a buffering solutionbeing reviewed and rinse again. Pre-buffered commercial coco can skip this.
- First feed: full mineral nutrients from day one at the seedling EC. Coco has no starter charge.
- Irrigate when the pot has lost ~30–50 % of its available water, to light runoff. Never let it dry out fully.
- Measure runoff EC and pH a few times a week. Adjust volume, not just strength.
- 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 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 | 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. 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, at any stage (see the flush debate).
- 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).
- 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.
