Every hydroponic crop begins with a germinated seed. Because no soil is used in a hydroponic system, you germinate the seeds in a clean, inert medium. This guide shows how to do it reliably and when the seedlings are ready to go into the system.
Propagation is where most beginner mistakes happen – yet with a few simple rules it’s easy to master.
Why not just germinate in soil?
Soil would introduce dirt, germs and fine particles into the system that clog pumps and lines and carry in diseases. That’s why you germinate in inert media that release no nutrients of their own but provide moisture and support, and can later be placed into the system together with the seedling.
Suitable propagation media
- Rockwool cubes: a classic, they hold moisture very well and are clean – but they need pre-treatment.
- Coco or peat swell pellets: easy to handle, good germination rates, ideal for beginners.
- Perlite or expanded clay: very airy, better suited to transplanting and support than to germinating.
Preparing rockwool correctly
Rockwool is naturally alkaline and must be adjusted before use. Soak the cubes for several hours in water that you’ve set to a pH of around 5.5. This prevents the high pH from disrupting germination and the fine roots. Afterwards, don’t squeeze the cubes out – they should be moist, but not dripping wet.
Germinating step by step
Place one or two seeds into the cube’s ready-made hollow. Keep the medium evenly moist but not wet, and provide warmth (around 20–25 °C). A transparent dome or a mini greenhouse keeps humidity high and speeds up germination. Depending on the plant species, seeds germinate in the dark or need light – check the seed packet for this.
From germination to the first nutrient solution
As long as the seedlings are living off their cotyledons alone, they don’t yet need fertiliser – clear, pH-adjusted water is enough. As soon as the first true leaves appear, give a heavily diluted nutrient solution (about half to quarter strength) so as not to overwhelm the delicate roots.
Transplanting seedlings into the system
Transplant once the seedlings have formed their first true leaves (after the cotyledons) and the roots poke out of the cube. Then set the whole cube along with the seedling into the net pot – without exposing or damaging the roots in the process.
Common propagation mistakes
- Too wet: waterlogging leads to damping-off, where seedlings collapse at the base of the stem.
- Too little light: the seedlings become leggy (long, thin and unstable). As soon as they germinate, they need bright light.
- Fed or transplanted too early: too much nutrient or moving them too soon overwhelms the sensitive roots.
Improving the germination rate: pre-soaking and warmth
Some seeds germinate reluctantly. It helps to pre-soak them for a few hours in lukewarm water before sowing – this softens the seed coat. Even warmth is just as important: a heat mat under the propagation tray maintains the ideal germination temperature and noticeably speeds up germination, especially for heat-loving species.
Propagation station and lighting
A small propagation kit of tray, transparent dome and an LED lamp creates consistent conditions. The dome keeps humidity high until the seeds germinate; after that it’s removed so no rot develops. From the moment of germination, the seedlings immediately need bright light directly above the little plants.
From seedling to sturdy young plant
For seedlings to grow stocky rather than long and thin, enough light is crucial. A gentle breeze (for example from a small fan) additionally strengthens the stems. Only once the roots are growing strongly out of the cube and true leaves are present does it go into the heavily diluted nutrient solution and later into the system.
Avoiding common propagation mistakes
Too many seeds per cube lead to competition and weak plants – when in doubt, keep the strongest seedling after germination. A medium that’s too cold or too wet slows germination or lets seeds rot. And transplanting too early often costs more seedlings than a little patience.
Frequently asked questions about propagation
How long does germination take?
Depending on the plant species, usually a few days to two weeks. Lettuce and many herbs germinate quickly, while other species take longer.
Do I absolutely need rockwool?
No. Coco or peat swell pellets are an easier alternative, especially for beginners, and require no pH pre-treatment.
Do the seeds need light to germinate?
That depends on the plant. Many germinate in the dark, others are light germinators. After germination, however, all seedlings immediately need bright light.
Which plants are easiest to start with?
Lettuce, basil and other fast-germinating herbs are ideal for your first attempts, because they germinate quickly and reliably.
Should I soak seeds before sowing?
For hard-coated or reluctant seeds, yes – pre-soaking for a few hours in lukewarm water improves the germination rate. Fine seeds usually don’t need it.
Why are my seedlings getting long and thin?
That’s a lack of light (legginess). Move the light source closer and provide more brightness as soon as the seeds have germinated.
How many seeds should I sow per cube?
One or two are enough. If both germinate, keep the stronger seedling after germination and remove the weaker one to avoid competition.