In soil the substrate does two jobs at once: it feeds and it holds. In hydroponics the nutrient solution takes over the feeding, leaving the medium only the mechanical part. That is why growing media are not compared by “fertility” but by four measurable properties — how much water they hold, how much air they leave, whether they shift pH, and whether they fall apart.
What a good medium has to do
- Hold water. A seedling has barely any root; half a day of dryness is a total loss.
- Leave air. Roots need oxygen. A medium saturated to 100 % water suffocates a seedling in 2–3 days.
- Stay inert. It must not move the pH or EC of the solution on its own.
- Not crumble. Anything that disintegrates ends up in the pump, the filter and the drippers.
The ideal ratio for propagation is roughly 70–80 % water and 20–30 % air in the saturated medium. Rockwool and foam cubes hit that almost perfectly; clay pebbles sit at the opposite end of the scale, where nearly the whole volume is air.
The comparison table
| Medium | Water retention | Reusable | pH out of the bag | Price | Best for |
|---|---|---|---|---|---|
| Rockwool (Grodan) | Very high (~80 % by volume) | No, effectively single use | 7.5–8.5 → must be soaked | €0.10–0.20 / cube | Seeds and transplants, professional standard |
| Coco plugs | High | No, but compostable | 5.5–6.8 | €0.15–0.30 / plug | Sowing without pre-soaking, greener option |
| Foam cubes (phenolic foam) | High and very consistent | No | 6.0–6.5, ready to use | €0.20–0.35 / cube | Beginners, fast and predictable germination |
| Clay pebbles 8–16 mm | Very low | Yes, 3–5+ years | 6.5–7.5 after rinsing | €1.50–2.50 / L | Anchoring in the net cup — not for seed |
| Perlite | Medium | Partly | 7.0–7.5 | €0.60–1.20 / L | Mixes for aeration; floats and produces dust |
| Vermiculite | Very high (up to 4× its weight) | No | 6.0–7.5 | €0.80–1.50 / L | Thin cover over sown seed |
| Jiffy peat pellets | High | No | 5.3–6.0 | €0.08–0.15 / pellet | Soil growing; peat disintegrates in a loop |
Prices are typical retail figures for small packs in the EU; in 1000-piece cases rockwool drops below €0.07 per cube.
Rockwool: cheapest, but with two rules
Rockwool is basalt melted at around 1600 °C and spun into fibres. Production leaves a lime residue behind, so a dry cube sits at pH 7.5 to 8.5. Use it straight from the bag and it will drag the pH of your whole reservoir up and lock out iron and manganese within the first week.
- Soak: 24 hours in water adjusted to pH 5.5. Throw that water away afterwards — it is full of leached lime.
- Never squeeze the cube. Squeezing permanently collapses the fibre structure and the cube stays waterlogged forever.
- Never leave it standing in water. The bottom third stays saturated and the seedling rots off (damping-off).
- Exception: Grodan Next Generation cubes, per the manufacturer, only need one thorough saturation with pH 5.5 solution instead of the 24-hour bath. With unbranded rockwool, stick to 24 hours.
- The dust bites. Dry fibres irritate skin and airways — wet the cubes before cutting them.
What never belongs in a recirculating system
This is the difference between a garden bed and hydroponics that most guides skip: in a tower the water circulates. Anything that crumbles does not disappear, it accumulates. Peat from Jiffy pellets clouds the tank and gums up the pump rotor within three weeks. Fine perlite floats and ends up in the overflow. Vermiculite breaks down into a slimy layer that algae feed on.
- ❌ Peat, soil, compost — cloud the water and kill the pump.
- ❌ Unrinsed fine perlite — floats up, the fines go straight into the filter.
- ⚠️ Vermiculite only as a thin cover over seed, never as the main medium in a tower.
- ✅ Rockwool, coco, foam, clay pebbles — dimensionally stable and stay where you put them.
An honest beginner recommendation
If you want the least friction in your first season, germinate in coco or foam plugs. No soaking, the pH is right out of the bag, and overwatering is almost impossible. Once the seedling has 2–3 true leaves and 2–3 cm of root outside the plug (usually after 10–18 days), drop the whole plug into a net cup and fill around it with clay pebbles, which hold the plant upright and never break down.
Move to rockwool when unit price starts to matter and measuring pH is no longer a hurdle. It is the cheapest and most predictable medium there is — but only if you respect the two rules above.
- 32 sites × 5–6 cycles per year ≈ 160–200 plugs
- Rockwool: roughly €20–35 per year
- Coco or foam: roughly €35–65 per year
- Clay pebbles: a one-off €8–14 for 5 L (about 2.5 L fills 32 net cups of 5 cm), lasting for years
A more expensive medium does not grow a better plant. It gives you fewer steps in which you can get it wrong.