Pepper seedlings while starting seeds in hydroponics

Starting seeds without soil

Germination temperatures and germination times: all values for your sowing

Light or dark germinator, how warm, how long and how much light afterwards: my table for 39 plants from my book and more, plus my tips for reliable germination in foam cubes.

💧

Moisture

the cube stays moist, but not wet

🌡️

Heat

every plant has its comfort temperature

🌗

Light or dark

light germinators open, dark germinators under the lid

🚫

No fertiliser yet

germination happens in plain water only

What a seed needs to germinate

Every seed needs four things. First Moisture: the seed takes up water, swells and only then becomes active. Keep the cube evenly moist but not wet, because seeds need air too. Then Heat, which gets germination going. Every plant has its own comfort temperature. For me lettuce germinates best at about 15 °C, tomatoes like it around 25 °C.

The third is light or darkness, depending on whether your plant is a light germinator or a dark germinator. And the fourth: no fertiliser yet. The seed has its own nutrient reserve, in many plants in the seed leaves. Fertiliser consists of salts, and too much salt makes it hard for the seed to take up water. That is why I germinate in plain water only. Only when the seed leaves are open does the seedling get a weak nutrient solution.

Peppers germinating in a foam cube in hydroponics — Peppers are dark germinators and like it really warm to germinate.

Light and dark germinators: how I do it

Light germinator such as lettuce, basil or rocket need light to germinate. I put the seed into the slit of the foam cube and only press it in lightly. Nothing is covered. The box stands open under the grow light, 14 to 16 hours a day.

Dark germinator such as tomato, pepper or parsley germinate better in the dark. You do not need to press them in. I put the seed into the slit in the same way and simply put the lid on the propagation box. It is dark underneath, and the moisture stays in. Check dark germinators every day. As soon as they have germinated, the lid comes off and they need light immediately, otherwise they grow long and thin.

Light germinator

press lightly, open

Dark germinator

do not press in, lid on

Check

every day

Germinated

into the light immediately

The propagation station is for light germinators

If you sow directly into the net pots of the propagation station, everything stays open under the lamp. That suits light germinators. I start dark germinators in the propagation box with a lid and only move them into the station after germination.

My table: germination temperature, germination type and light

Here are the values from my book. The column Germinate at shows the temperature at which the seed germinates best. The column PPFD afterwards shows how much light the growing plant needs indoors under the lamp. At the start all seedlings need equally little; 100 to 150 PPFD is enough. The germination time is only given where I measured it myself. For all others, check the seed packet.

Leafy plants and herbs

Plant	Germination type	Germinate at	Germination time	PPFD afterwards
Anise	Dark germinator	18 to 22 °C	see packet	200 to 300
Basil	Light germinator	20 to 25 °C	about 3 days	200 to 300
Cauliflower	Dark germinator	18 to 22 °C	see packet	250 to 400
Broccoli	Dark germinator	18 to 22 °C	see packet	250 to 400
Dill	Light germinator	15 to 20 °C	about 6 days	200 to 300
Fennel	Dark germinator	18 to 22 °C	see packet	200 to 300
Kohlrabi	Dark germinator	20 to 25 °C	about 5 days	200 to 300
Coriander	Dark germinator	15 to 20 °C	see packet	200 to 300
Leek	Dark germinator	15 to 20 °C	see packet	250 to 400
Lovage	Light germinator	15 to 20 °C	see packet	200 to 300
Chard	Dark germinator	15 to 20 °C	see packet	200 to 300
Mint	Light germinator	18 to 22 °C	about 7 days (peppermint)	200 to 300
Pak choi	Dark germinator	20 to 25 °C	see packet	175 to 225
Parsley	Dark germinator	15 to 20 °C	about 7 days	200 to 300
Radish	Dark germinator	15 to 20 °C	see packet	250 to 350
Rhubarb	Dark germinator	15 to 20 °C	see packet	200 to 300
Brussels sprouts	Dark germinator	18 to 22 °C	see packet	250 to 400
Beetroot	Dark germinator	15 to 20 °C	see packet	250 to 350
Rocket	Light germinator	15 to 20 °C	see packet	200 to 300
Lettuce	Light germinator	15 to 20 °C	about 3 days	200 to 300
Sage	Light germinator	18 to 22 °C	see packet	250 to 400
Chives	Dark germinator	15 to 20 °C	see packet	200 to 300
Celery	Light germinator	18 to 22 °C	see packet	200 to 300
Spinach	Dark germinator	15 to 20 °C	see packet	200 to 300
Thyme	Light germinator	18 to 22 °C	see packet	250 to 400
Onion	Dark germinator	15 to 20 °C	see packet	250 to 400

Fruiting plants

Plant	Germination type	Germinate at	Germination time	PPFD afterwards
Aubergine	Dark germinator	24 to 30 °C	see packet	350 to 500
Beans	Dark germinator	20 to 25 °C	see packet	300 to 450
Chilli	Dark germinator	24 to 30 °C	see packet	350 to 500
Peas	Dark germinator	10 to 20 °C	see packet	250 to 400
Strawberry	Light germinator	18 to 22 °C	see packet	300 to 350
Cucumber	Dark germinator	20 to 25 °C	see packet	300 to 500
Pumpkin	Dark germinator	20 to 25 °C	see packet	350 to 500
Sweetcorn	Dark germinator	20 to 25 °C	see packet	400 to 600
Melon	Dark germinator	24 to 30 °C	see packet	350 to 500
Peppers	Dark germinator	24 to 30 °C	see packet	350 to 500
Tomato	Dark germinator	22 to 28 °C	6 to 10 days	350 to 500
Watermelon	Dark germinator	24 to 30 °C	see packet	350 to 500
Zucchini	Dark germinator	20 to 25 °C	see packet	350 to 500

The seed packet comes first

Germination times and exact details vary by variety. If your seed packet says something different from my table, go by the packet. By the way, you do not need special seeds for hydroponics. Seed is seed; perfectly normal vegetable and herb seeds work.

Germination time: how long it takes for me

Patience is part of it. I measured these times in my own seed starting at about 21 to 23 °C:

Plant	Germinated after
Lettuce, basil	about 3 days
Kohlrabi, borage	about 5 days
Lemon balm, dill, summer savory	about 6 days
Parsley, rosemary, peppermint	about 7 days
Tomato (warm on the radiator)	6 to 10 days

If it is cooler, it takes longer. Parsley, for example, needs 1 to 4 weeks depending on temperature, often 7 to 14 days at 20 to 22 °C. Some herbs take two to three weeks, and not every seed is equally quick. So do not give up too early. Only when the usual germination time is clearly over is a new attempt worthwhile.

How germination works

Place the seed: The seed goes into the moist cube. Light germinators stay open, dark germinators go under the lid.
The seed swells: It takes up water, gets bigger, and its nutrient reserves become active.
The radicle appears: First a small root grows out, downwards into the cube. It anchors the seedling.
The seed leaves are there: The first little leaves open. Now the seedling needs light and gets its first fertiliser.

The germination test: how good is your seed?

If your seeds germinate poorly, do a germination test. Put 10 seeds on damp kitchen paper, cover them and keep them warm. After a few days, count. If 8 out of 10 germinate, your seed has a germination rate of 80 percent. If fewer than half germinate, better put 2 seeds per cube or buy new seed.

How many seeds per cube?

That depends on what you want to harvest. Plants that later grow individually get one seed per cube: lettuce, pak choi, kohlrabi, tomato, pepper, chilli, cucumber and basil. With many herbs, on the other hand, I want a small bunch. Parsley, chives, dill, coriander and rocket therefore get 3 to 5 seeds per cube.

Cucumbers straight in the cube

I start cucumbers directly in the foam cube. Their roots are sensitive and should not have to be freed from soil and washed out later.

From my shop

Everything for your soil-free start

HydroFlex Tower for indoors and outdoors, seedling box, seedling station, support rings and my own nutrient. After you buy, I’ll help you on WhatsApp.

To the shop ›

Using warmth correctly

Tomatoes, peppers and chillies only germinate well when it is really warm. Peppers and chillies need 24 to 30 °C, and hardly any room is that warm. The most precise way is with a heat mat with thermostat. I put the sensor in the water of the box, where it measures the temperature right at the cube.

The box can also go on the radiator. Two pens underneath provide 0.5 to 1 cm of distance, and a film over it keeps the moisture in. But if the heating turns down at night, the temperature fluctuates. The heat mat is better for that.

Lettuce likes it cool

Above 24 °C lettuce germinates worse, and with even more warmth often not at all. So do not put it on the radiator but in a cool spot around 15 °C. Remember: the grow light also gives off warmth.

Spider Farmer Heizmatte Pflanzen mit Thermostat 25x52cm Wärmematte Set | MET/UL zertifiziert, IP67 wasserdichte Heizmatte für Pflanzen, Anzucht...

Heat mat with thermostat for dark germinators such as tomato, pepper and chilli. Put the sensor in the water of the propagation box.

* Affiliate-Link: Bei einem Kauf erhalte ich eine Provision, dein Preis bleibt gleich. Preis inkl. MwSt., zzgl. Versand, Stand 8.11.2026. Bilder: Amazon Creators API

Keeping moisture in and airing correctly

The surface of the cubes dries out quickly. Mist the seeds from above every day, or put cling film or a zip bag over the box. Then the moisture stays in, nothing dries out, and you do not have to mist. Light comes through the film, which suits light germinators. For dark germinators use the light-tight lid.

As soon as the seeds have germinated, film and lid come off. For me lettuce germinates after about 3 days. If the film stays on longer, mould quickly forms in the humid air. By the way, white fuzz on the root is not mould; those are root hairs.

The fan stays off during germination, otherwise the cubes dry out. Later it makes the stems sturdier. Starting seedlings creates a lot of moisture in the air. If it stays high for long, plants quickly get powdery mildew. That is why every morning I open the window wide for about 15 minutes. After that the humidity drops from about 65 percent to below 60 percent. A hygrometer shows you the value.

Angebot

GLORIA Handsprüher Hobby 10 Flex | 1 L Sprühflasche mit 360° Sprühfunktion | Doppelhub-Pumpe & verstellbare Messingdüse | Pumpsprüher für...

Hand sprayer to keep the cubes moist from above every day.

* Affiliate-Link: Bei einem Kauf erhalte ich eine Provision, dein Preis bleibt gleich. Preis inkl. MwSt., zzgl. Versand, Stand 8.11.2026. Bilder: Amazon Creators API

Angebot

X-Sense 3 Stück WLAN Thermometer Hygrometer Innen mit X-Sense App, Mini Digital Raumthermometer, Angepasster Komfortbereich, Smiley-Indikator...

Thermometer and hygrometer for temperature and humidity at the seed station.

* Affiliate-Link: Bei einem Kauf erhalte ich eine Provision, dein Preis bleibt gleich. Preis inkl. MwSt., zzgl. Versand, Stand 8.11.2026. Bilder: Amazon Creators API

After germination: light and first fertiliser

Germinated means: into the light immediately. If seedlings get too little light, they stretch and grow long and thin. That is also why most plants like it a little cooler after germination than during germination, because the warmer it is and the less light they get, the longer the stems become. For seedlings, 100 to 150 PPFD is enough. With the box and station the lamp hangs above the plants, about 20 to 30 cm away. With a strong lamp 14 hours a day is enough; let a weaker one shine for 16 hours, but never longer.

Once the seed leaves are open, I add 0.5 ml of Frankie’s liquid nutrient to 1 litre of water. From 2 to 3 cm plant size I use 1 ml. These amounts only apply to my fertiliser. I then put the cubes individually into net pots in my Seedling station. Those that have not germinated yet can keep germinating in the box. You can read how it continues at EC value and pH value.

Seedlings

100 to 150 PPFD

Lamp

20 to 30 cm, from above

Light

14 to 16 hrs

First fertiliser

0.5 ml per litre

PPFD and DLI briefly explained

PPFD is the amount of light that reaches the plant. You measure it at leaf height with a light meter; phone apps only show approximate values. The amount of light over the whole day is called DLI. You calculate PPFD times hours times 0.0036. An example: 250 PPFD times 16 hours times 0.0036 gives about 14. Lettuce needs about 12 to 17, tomato, cucumber and pepper 20 to 30. You will find more about lamps on my page about lighting.

MARS HYDRO VG40 LED Pflanzenlampe 60cm IP65 2er-Pack | Vollspektrum, wasserdicht IP65, für Zimmerpflanzen & Gewächshaus, für Setzlinge & Blüte...

LED grow light twin pack, good for propagation box and propagation station.

* Affiliate-Link: Bei einem Kauf erhalte ich eine Provision, dein Preis bleibt gleich. Preis inkl. MwSt., zzgl. Versand, Stand 8.11.2026. Bilder: Amazon Creators API

Spider Farmer 4 Stock Growzelt Klein Gewächshaus mit Anzucht Regal Indoor | 120W Pflanzenlampe Vollspektrum, Grow Tent mit Transparenter Vorhang und...

Four-tier grow shelf, handy if you start lots of young plants.

* Affiliate-Link: Bei einem Kauf erhalte ich eine Provision, dein Preis bleibt gleich. Preis inkl. MwSt., zzgl. Versand, Stand 8.11.2026. Bilder: Amazon Creators API

Why I do not germinate directly in the tower

My first attempt was exactly that: seed into the cube, cube into the tower. Many seeds did not germinate, and what came up got long, thin stems. In the tower the cube sits several centimetres deep in the net pot, in other words in a dark hole. Light germinators get too little light there, dark germinators still get some from above. On top of that, the tower needs a pump, light and electricity just for starting seedlings.

That is why today I germinate in the propagation box with lid and 25 cubes: the 25 cubes are joined as a mat and keep each other moist, a small lamp is enough, and the lid makes it suitable for dark germinators too. For many plants I use the Foam mat for 100 seeds. Rinse the mat in hot water before first use and then rinse it cold, so no manufacturing residues remain and the seeds do not go into a hot mat. Then add enough water for the mat to stand about one third deep in water.

Do not sow too early

Under the lamp everything grows fast. Once I sowed tomatoes as early as the end of February and sowed again three or four weeks later. By the start of the season they were all the same size. That is why today I sow tomatoes from about 20 March. Sowing too early only uses up space and artificial light.

More plants from my overview

These plants are not in my book. The values are guidelines from the specialist literature on germination type and temperature. For germination time and everything else, check the seed packet.

Herbs

Plant	Germination type	Germinate at	Good to know
Chamomile	Light germinator	18 to 21 °C	do not cover
Oregano	Light germinator	18 to 21 °C	do not cover
Rosemary	Light germinator	18 to 21 °C	often only germinates after a cold period, for me after about 7 days
Lavender	Light germinator	21 to 29 °C	often only germinates after a cold period

Vegetable

Plant	Germination type	Germinate at	Good to know
Lamb’s lettuce	see packet	15 to 18 °C	germinates poorly from 20 °C
Endive	see packet	20 to 25 °C	–
Carrots	Dark germinator	18 to 25 °C	–
Radish	Dark germinator	18 to 25 °C	–
Kale	Dark germinator	18 to 21 °C	–

Flowers

Plant	Germination type	Germinate at	Good to know
Snapdragon	Light germinator	around 21 °C	do not cover
Sunflower	Dark germinator	around 21 °C	–

Sweet chilli peppers germinate like chilli, celeriac like celery; see the table above. French tarragon hardly sets seed in our climate. You grow it from cuttings or by dividing the root.

Frequently asked questions about germination temperature and germination time

What does a seed need to germinate?

Four things: moisture, warmth, light or darkness depending on the plant, and no fertiliser yet. The seed swells in the moist cube and becomes active with warmth. It brings its own nutrients.

Why do I not give fertiliser during germination?

The seed has its own nutrient reserve. Fertiliser consists of salts, and too much salt makes it harder for the seed to take up water. That is why Frank germinates in plain water only and only feeds once the seed leaves are open.

What is a light germinator?

A seed that needs light to germinate. It is placed in the slit of the cube, only pressed in lightly and not covered. The box stands open under the lamp. Examples are lettuce, basil and rocket.

How do I handle a dark germinator in a foam cube?

Put the seed into the slit; you do not need to press it in. Then put the lid on the propagation box, and it is dark underneath. As soon as it has germinated, the lid comes off and the seedling goes into the light immediately.

Is parsley a light or a dark germinator?

Parsley is a dark germinator and germinates at 15 to 20 °C. Frank puts 3 to 5 seeds into a cube so a bunch grows.

Is basil a light or a dark germinator?

Basil is a light germinator. It germinates best at 20 to 25 °C and for Frank had germinated after about 3 days.

At what temperature does lettuce germinate best?

At 15 to 20 °C, for Frank best around 15 °C. Above 24 °C lettuce germinates worse, and with even more warmth often not at all. That is why lettuce does not belong on the radiator.

How warm do peppers and chillies need to germinate?

24 to 30 °C. Both are dark germinators. Hardly any room is that warm, so a heat mat with thermostat or a spot on the radiator helps.

At what temperature do tomatoes germinate and how long does it take?

Tomatoes germinate at 22 to 28 °C, ideally around 25 °C. Warm on the radiator it takes 6 to 10 days for Frank. Tomatoes are dark germinators.

How long does lettuce take to germinate?

For Frank about 3 days, measured at 21 to 23 °C. Then the film has to come off, otherwise mould forms in the humid air.

How long does it take for parsley?

For Frank about 7 days. Depending on temperature, however, parsley can need 1 to 4 weeks, often 7 to 14 days at 20 to 22 °C. If it is cooler, it takes longer.

Where do I find the germination time for my variety?

On the seed packet. Germination times and exact details vary by variety. If the packet differs from Frank’s table, go by the packet.

When is a new attempt worthwhile if nothing germinates?

Only when the usual germination time is clearly over. Some herbs need two to three weeks, and at cooler temperatures everything takes longer.

How does a germination test work?

Put 10 seeds on damp kitchen paper, cover them and keep them warm. After a few days, count: if 8 out of 10 germinate, your seed has an 80 percent germination rate. If fewer than half germinate, put 2 seeds per cube or buy new seed.

How many seeds go into one cube?

Plants that grow individually get one seed, for example lettuce, tomato, pepper, cucumber and basil. Herbs that should grow as a bunch, such as parsley, chives, dill, coriander and rocket, get 3 to 5 seeds.

Is white fuzz on the root mould?

No. White fuzz on the radicle is root hairs. Mould mainly forms when film or lid stays on the box too long after germination.

Can the fan run during germination?

No, it stays off, otherwise the cubes dry out. It only comes in later, when it makes the stems sturdier.

How much light do seedlings need?

All seedlings need the same amount; 100 to 150 PPFD is enough. With box and station the lamp hangs above, about 20 to 30 cm away, and shines for 14 to 16 hours, never longer.

Why are my seedlings getting long and thin?

They get too little light and then stretch. Give them light immediately after germination. Too much warmth after germination also encourages long, thin stems.

When and how much do I feed after germination?

As soon as the seed leaves are open. Frank then adds 0.5 ml of his liquid fertiliser to 1 litre of water, from 2 to 3 cm plant size 1 ml. These amounts only apply to his fertiliser.

What does PPFD in the table mean?

PPFD is the amount of light that reaches the plant. In the table it shows how much light the growing plant needs indoors under the lamp, for example 200 to 300 for lettuce. It is measured at leaf height with a light meter.

How do I calculate DLI?

PPFD times hours times 0.0036. Lettuce needs about 12 to 17, tomato, cucumber and pepper 20 to 30.

Why do I not germinate directly in the tower?

In the tower the cube sits deep in the net pot, in other words in the dark. Light germinators get too little light, and the seedlings grow long and thin. In the propagation box with a small lamp it works more reliably and saves electricity.

At what temperature does lamb’s lettuce germinate?

Cool, at about 15 to 18 °C. From 20 °C lamb’s lettuce germinates poorly, so it does not belong on the radiator.

Why do rosemary and lavender often germinate so slowly?

Both are light germinators and often only germinate after a cold period. For Frank rosemary germinates after about 7 days; at cooler temperatures it takes longer.

Do I need special seeds for hydroponics?

No. Seed is seed; perfectly normal vegetable and herb seeds work. You will find the important details on the seed packet.

You will find more about growing individual plants under vegetable growing in the hydro tower and on my page about growing media.

Continue with your own growing

My book

Step by step into hydroponics

168 pages, simply explained for beginners and with videos on the topics (in German). Exactly how I grow myself: from sowing to harvest.

More about the book ›

Free

Which tower suits you?

A few questions about space, location and what you want to harvest, and you get a recommendation.

To the tower advisor ›

Questions about your setup? Message me on WhatsApp.

Tapping opens WhatsApp (provider: WhatsApp Ireland Ltd.). More in the privacy policy.