Seed does not go straight into an NFT channel. A germinating seed needs constant moisture and no flowing film to wash it away; it is raised in a plug and transplanted once it has roots that can reach the solution. This lesson covers that whole path.
Choosing a plug medium
| Medium | Strengths | Watch out for |
|---|---|---|
| Rockwool cubes | The commercial standard; excellent air-to-water ratio; consistent | Alkaline as supplied — must be conditioned before use. Wear a dust mask when handling dry. |
| Coco plugs | Renewable; forgiving; easy to handle | Holds more water than rockwool; easier to overwater at the seedling stage |
| Oasis / phenolic foam | pH neutral out of the bag; no conditioning | Not reusable; less widely stocked |
Conditioning rockwool: new rockwool has a pH around 7.5 – 8.5, well above anything a seedling wants. Soak the cubes for several hours in water adjusted to pH 5.5, then let them drain. Do not squeeze them — compressed rockwool loses its air spaces permanently and the plug will stay waterlogged.
Germination
- Pre-soak the plugs in pH-adjusted plain water — no nutrient yet. A seed carries its own food reserve for the first several days and nutrient at this stage does more harm than good.
- Sow one to two seeds per plug, about 5 mm deep. Sowing two and removing the weaker seedling insures against poor germination.
- Keep them warm and humid. Most salad and herb seed germinates best at 18 – 22 °C. A propagator lid or loose plastic cover holds humidity; ventilate it daily to prevent damping off.
- Light on emergence. The moment shoots appear, they need light — bright indirect daylight or a propagation light. Seedlings kept dark for even a day or two stretch into thin, weak stems that never fully recover.
- Keep plugs damp, never saturated. A plug sitting in standing water will rot the stem at its base.
Feeding seedlings
Once the first true leaves appear — the second pair, distinct from the rounded seed leaves — begin feeding at quarter to half strength: 0.8 – 1.2 mS/cm. Full-strength nutrient will burn young roots.
Knowing when to transplant
Timing matters more here than anywhere else in the cycle. Transplant too early and the roots never reach the film; too late and the plug is root-bound and stressed.
A seedling is ready when all four are true:
- It has two to three sets of true leaves
- Roots are visibly emerging from the base or sides of the plug — this is the critical one
- It stands up on its own without flopping
- It is a compact plant with short internodes, not a stretched, leggy one
For most leafy crops this is two to three weeks from sowing. Trust the roots, not the calendar.
Transplanting into the channel
- Check the solution first. pH 5.8 – 6.2, EC at the low end of the crop's range, temperature under 24 °C, film running in every channel. Fix any of these before the plants go in.
- Handle seedlings by the leaves, never the stem. A damaged leaf is replaced; a crushed stem kills the plant.
- Seat the plug in the net pot so its base sits in contact with the channel floor — this is the whole point. The emerging roots must reach the film immediately. A plug held high and dry above the flow is the number one transplant failure.
- Tuck the emerging roots downstream, pointing along the direction of flow rather than back against it.
- Do not fill the net pot with loose media. Clay pebbles around the plug are optional and only for stability on top-heavy plants; NFT roots want air, not packing.
- Transplant in the evening or on an overcast day. Midday transplanting in full sun stresses plants that have no established root contact yet.
- Space to the plan from Part 2. The temptation to squeeze in extra plants is strong and always regretted.
The first week after transplant
This is the highest-risk week in the cycle. Roots are short, contact with the film is marginal, and a pump interruption now is fatal rather than merely damaging.
- Check daily that every plug is still in contact with the film. Plugs shift.
- Keep EC at the low end for the first week, then bring it up to the crop's normal range.
- Provide shade in strong sun for the first two or three days.
- Expect a brief check in growth. Plants pause for a few days while roots establish, then accelerate. Some wilting on day one is normal; wilting that persists past day three is not — check film contact first.
- Do not transplant immediately before a long power outage. Wait until roots are into the channel.
Succession planting
Do not plant the whole system at once unless you genuinely want 24 lettuces in the same week. Sow a small batch every one to two weeks and transplant as sites free up. You get a continuous harvest, and a single mistake costs one batch rather than the entire system.
Your crop is in. Part 6 covers the weekly routine that keeps it there, and how to diagnose things when they go wrong.
Basic Hydroponics Training — the full series
- Part 1 — NFT Hydroponics Explained: How the Nutrient Film Technique Works
- Part 2 — Planning Your NFT System: Space, Water, Power and Crop Choice
- Part 3 — Assembling a Vertical NFT System: Channels, Slope and Plumbing
- Part 4 — Commissioning Your NFT System: First Fill, Nutrients, pH and EC
- Part 5 — Planting Out: From Seed to Channel (you are here)
- Part 6 — Weekly Maintenance and Troubleshooting for NFT Systems
Own our wall-mounted system? Setting Up the Hydro Works Wall-Mounted NFT System gives you the build and commissioning steps for that specific unit.