How to Sow Seeds in a Greenhouse: The Complete Beginner's Guide!

🌱 Introduction

Growing your own seedlings is one of the most magical experiences in gardening. Watching a tiny seed transform into a vigorous plant, then into a vegetable or flower, is an incomparable satisfaction. And the polycarbonate greenhouse is the ideal tool to master this crucial stage of cultivation.

In France, millions of gardeners buy plants from garden centers every spring. However, growing your own seedlings in a greenhouse can save 60 to 80% of your plant budget, provide access to rare varieties not found in garden stores, and allow you to start your season 4 to 8 weeks earlier than gardeners who sow directly in the ground.

Whether you want to sow tomatoes, peppers, annual flowers, or aromatic herbs, this comprehensive guide will give you all the keys to successfully growing your seedlings in a greenhouse from the very first attempt. No frustration, no mysteriously dying plants: just concrete results and abundant harvests!

🌟 Why Grow Your Seedlings in a Greenhouse?

✅ Advantages of growing seedlings in a greenhouse

📅 Start the season earlier This is the number one advantage! When sowing directly in the ground, you have to wait until the risk of frost has passed (mid-May in most French regions). In a greenhouse, you can start your seedlings as early as January-February for the earliest species, thus gaining 2 to 3 months on the season.

💰 Achieve substantial savings A packet of tomato seeds costs €2 to €4 and contains 15 to 30 seeds. A single tomato plant at a garden center costs €2 to €4. By growing your own seedlings, you can reduce your plant budget by 5 to 10 times!

🌿 Access rare varieties Garden centers only offer a limited number of varieties, often standardized for mass distribution. By sowing yourself, you gain access to hundreds of old, regional, or exotic varieties: Black Krim tomato, Nice Round zucchini, Michelet bean, Chantenay carrot... A world of flavors opens up to you!

🌡️ Control germination conditions In a greenhouse, you control temperature, humidity, and light, the three key factors for germination. This allows you to optimize the germination rate and obtain more vigorous plants than by sowing directly in the ground.

🌱 More robust plants Plants grown from home-sown seeds, gradually accustomed to your garden's real conditions, are generally more robust and better adapted than garden center plants, which are often produced under intensive artificial conditions.

Discover our polycarbonate garden greenhouses to create optimal conditions for your seedlings all year round.

🛠️ Essential Equipment for Greenhouse Seedlings

Before you start, make sure you have the basic equipment. It's not necessary to invest in expensive materials to get started; here's what's essential.

🪴 Seedling containers

Seed trays and flats Seedling trays (about 40 x 25 cm) are perfect for mass sowing: salads, flowers, aromatic herbs. They allow you to sow many seeds in a small space.

Individual pots Essential for species that do not tolerate transplanting well: zucchini, cucumbers, squash, peas, beans. Use pots 6 to 10 cm in diameter depending on the size of the plant.

Cell trays (speedling) Trays with small individual cells, perfect for tomatoes, peppers, eggplants, and flowers. Each cell holds one seed and becomes a small individual pot. Very practical for producing many plants in a confined space.

Biodegradable pots Made of peat or coconut fiber, these pots can be transplanted directly into the soil without disturbing the roots. Ideal for species sensitive to transplanting.

🌱 Sowing tools

• Sprayer: for gentle watering without dislodging seeds
• Labels and marker: to identify varieties (essential!)
• Sieve: to refine the substrate and lightly cover seeds
• Ruler or stick: to draw regular furrows in the substrate

🌡️ Temperature accessories

• Min/max thermometer: to monitor nocturnal temperatures
• Heating mat: for species that require more than 20°C to germinate (tomatoes, peppers, basil)
• Mini-greenhouse or clear dome: to maintain heat and humidity for your seedlings

🌍 Choosing and Preparing Your Substrate

🌱 The ideal seedling substrate

Seedling substrate is different from regular potting soil. It must be:

• Fine and homogeneous: to allow small roots to develop easily
• Light and airy: to prevent compaction and promote root oxygenation
• Low in nutrients: seeds contain their own nutritional reserves; a substrate that is too rich burns young roots
• Draining: to prevent waterlogging (which causes damping-off)

🌿 Homemade substrate recipes

Basic recipe:

• 50% sieved universal potting soil
• 30% vermiculite or perlite
• 20% very fine mature compost

Economical recipe:

• 60% finely sieved garden soil
• 40% fine river sand (not beach sand!)

Recipe for delicate seedlings (orchids, begonias, fine seeds):

• 40% sieved potting soil
• 40% vermiculite
• 20% fine sand

💧 Preparing the substrate before sowing

Moisten your substrate before filling the containers, not after. Pre-moistened substrate prevents seeds from being dislodged during the first watering. The ideal consistency: the substrate should hold together in a ball when squeezed in your hand, but crumble when you release the pressure.

🌱 Sowing Techniques

📏 Row sowing (for trays)

Ideal for: salads, carrots, aromatic herbs, flowers.

1. Fill the tray with substrate to 2 cm from the edge
2. Draw parallel furrows with a ruler or stick, spaced 3-5 cm apart
3. Sow to a depth of 2 to 3 times the diameter of the seed (if in doubt, sow to a depth of 5 mm)
4. Place the seeds in the furrows, evenly spaced
5. Lightly cover with sieved substrate
6. Lightly tamp and water with the sprayer

🪴 Cluster sowing (for pots)

Ideal for: tomatoes, peppers, zucchini, cucumbers, squash.

1. Fill the pot with substrate
2. Make a small hole in the center (with a pencil or your finger) to the appropriate depth
3. Place 1 to 3 seeds per pot (2 to be safe, 3 for fragile seeds)
4. Cover and lightly tamp
5. Water gently

After germination: if several plants have germinated in the same pot, remove the excess plants by cutting them at soil level with scissors (do not pull them out!). Keep the most vigorous one.

🌿 Broadcast sowing (for large areas)

For salads, radishes, or lamb's lettuce in greenhouse beds.

1. Prepare a well-leveled seedbed
2. Spread the seeds evenly over the surface
3. Cover with a thin layer of substrate (1-2 mm for small seeds)
4. Lightly tamp with a board
5. Water with a fine spray

🌡️ Managing Heat and Humidity

🌡️ Germination temperatures

Each species has an optimal germination temperature. Here are the main ones:

💡 For species that require significant heat (tomatoes, peppers, eggplants, basil), place your seedlings in the warmest area of your greenhouse or use a heating mat. Germination is directly linked to temperature: too cold = slow or no germination, too hot = abnormal germination.

💧 Maintaining humidity

After sowing, the number one enemy is substrate drying out. Germinating seeds should never lack water.

Techniques:

• Cover trays and pots with plastic wrap or a transparent dome to maintain humidity
• Remove the cover as soon as the first shoots appear to prevent rot
• Always water with a fine sprayer, never with a watering can, which could dislodge the seeds

💡 The importance of light after germination

As soon as the first leaves appear, light becomes crucial. A lack of light causes etiolation: plants stretch and weaken as they search for light. Place your seedlings in the brightest area of your greenhouse, facing south if possible.

In winter, short days may be insufficient. LED grow lights (12 to 16 hours of light per day) solve this problem and allow for vigorous seedlings even in January-February.

🌿 Transplanting and Acclimatization

🌱 When to transplant?

Transplanting (moving young plants to a larger container or directly into the ground) should occur when:

• The plant has developed 2 to 4 true leaves (beyond the cotyledons, the first "embryonic" leaves)
• Roots begin to emerge from the bottom of the pot
• The plant is 8 to 15 cm tall

🔧 Transplanting technique

1. Water the plants the day before to make removal easier
2. Prepare the new containers or the location in the greenhouse with richer substrate (potting soil + compost)
3. Gently remove the plant by turning the pot upside down and pressing on the bottom
4. Dig a hole in the new substrate, large enough to accommodate the entire root ball
5. Plant slightly deeper than in the pot (especially for tomatoes which develop roots on the buried stem)
6. Water generously after transplanting
7. Place in the shade for 2-3 days to reduce stress

🌬️ Acclimatization ("hardening off")

If you plan to transplant your plants outdoors (outside the greenhouse), they must be gradually acclimatized to outdoor conditions. This operation is called "hardening off".

7 to 14-day process:

1. Open greenhouse windows and doors for a few hours a day (days 1-3)
2. Move plants outdoors for a few hours in a sheltered spot (days 4-7)
3. Leave plants outside all day, bring indoors at night (days 8-10)
4. Leave outside day and night (days 11-14), provided there is no frost

Our automatic door and window opening system greatly facilitates this gradual acclimatization phase by allowing gradual opening without constant supervision.

📅 The Greenhouse Sowing Calendar

❄️ January - February: Ultra-early sowings

In a heated greenhouse (minimum 15°C at night):

• Tomatoes: sow in pots (early varieties)
• Peppers and chilies: they need a lot of heat, this is the ideal time
• Eggplants: early sowing for plants ready in May
• Celery: long germination, sow early

In an unheated greenhouse:

• Lettuce and lamb's lettuce: cold-resistant
• Spinach: excellent winter vegetable in a greenhouse

🌸 March - April: The main sowing season

This is the most active period for sowing in a greenhouse!

• Tomatoes, peppers, eggplants (if not sown in Jan-Feb)
• Cucumbers and zucchini (from mid-March)
• Squash and pumpkins
• Basil (from mid-March)
• Annual flowers: zinnias, cosmos, marigolds
• Salads continuously

☀️ May - June: Summer sowings

• Beans: direct sowing in a greenhouse or in pots
• Sweet corn: sow in pots for planting in June
• Sunflowers
• Cilantro: re-sow every 3-4 weeks
• Carrots and radishes: direct sowing

🍂 July - September: Autumn-winter sowings

• Lamb's lettuce and arugula: for autumn salads
• Spinach: winter production
• Cabbages: for planting in a greenhouse in autumn
• Parsley: for winter production
• Winter salads: batavia, Reine de Mai lettuce

To discover in detail which species to plant in a greenhouse according to the season, consult our article "What to plant in your greenhouse in autumn for successful crops"

⚠️ Mistakes to Avoid

❌ Mistake 1: Sowing too early without a heat source

Many beginner gardeners sow their tomatoes in January in an unheated greenhouse. Result: the seeds don't germinate (too cold) or the plants become leggy due to lack of sufficient heat and light. Without heating, wait until March for heat-loving species.

❌ Mistake 2: Overwatering

Damping off (young plants collapsing) is almost always caused by overwatering. The substrate should be moist, not soggy. Always check before watering.

❌ Mistake 3: Sowing too deep

General rule: sow to a depth equal to 2 to 3 times the diameter of the seed. A tomato seed (1mm) is sown 2-3mm deep. A squash seed (15mm) is sown 3-4 cm deep. Too deep = difficult or impossible germination.

❌ Mistake 4: Not labeling your seedlings

You think you'll remember what you sowed where... but with 20 different varieties, forgetting is inevitable! Always label your seedlings with the variety name and sowing date.

❌ Mistake 5: Transplanting too early outdoors

Plants from a greenhouse transplanted too early outdoors can die from a cold snap or suffer from thermal shock. Respect the acclimatization period and wait until all risk of frost has passed.

❌ Mistake 6: Forgetting pollination

For species that flower in a greenhouse (tomatoes, peppers, eggplants), don't forget manual pollination if your greenhouse is closed. Gently shake the plants every morning or brush the flowers with a paintbrush.

To avoid these and many other mistakes, consult our article "Garden greenhouse: 7 mistakes that ruin your harvests."

❓ FAQ - Your Questions about Greenhouse Sowing

🌡️ What temperature should the greenhouse be maintained at for seedlings? It depends on the species. For the vast majority of vegetables (lettuce, cabbage, spinach), 10-15°C at night is sufficient. For Mediterranean species (tomatoes, peppers, eggplants, basil), a minimum of 18-20°C at night must be maintained. In practice, a min/max thermometer in the greenhouse is essential to monitor actual temperatures.

💡 Can you sow in a greenhouse without electricity? Yes! For hardy species (lettuce, spinach, carrots, parsley), an unheated polycarbonate greenhouse is sufficient from March. For heat-loving species, you can use passive techniques: place seedlings against the south-facing wall of the greenhouse, use black water bottles filled with water that accumulate daytime heat and release it at night, or cover seedlings with a forcing fleece at night.

🌱 How to prevent damping off? Damping off is caused by a fungus (Pythium) that thrives in humid and warm conditions. To prevent it: use sterile substrate (commercial potting soil), do not overwater, maintain good even light ventilation, and sprinkle a little fine sand on the surface of the substrate after sowing. If damping off appears despite everything, treat with a natural fungicide based on Trichoderma.

📦 Which containers should be used first for sowing? For beginners, cell trays (speedlings) are the best compromise: they allow you to produce many plants in a small space, with easy transplanting. For species that don't like to be transplanted (zucchini, beans, squash), use individual 8-10 cm pots. Our greenhouse shelves allow you to organize and optimize your sowing space by creating multiple cultivation levels.

🌿 Can you keep your seeds from one year to the next? Yes, if they are well preserved! Store your seeds in paper envelopes (never plastic) in a cool, dry, dark place. A refrigerator is ideal. Storage duration varies by species: 3-4 years for tomatoes and squash, 2-3 years for carrots and onions, 1-2 years for leeks and onions. Test viability by germinating 10 seeds on damp paper: if more than 7 germinate, the seeds are still good.

💧 Can tap water be used for seedlings? Tap water is very suitable for seedlings in most regions. However, if your water is very hard (hardness > 30°F), it can make the substrate alkaline and hinder nutrient absorption. In this case, prefer collected rainwater (ideal!) or let tap water sit for 24 hours in a watering can before use so that the chlorine evaporates. Our 10-liter water tank is practical for storing water at room temperature inside the greenhouse.

🎯 Conclusion

Sowing in a greenhouse is one of the most valuable skills a gardener can acquire. This ancestral practice, made more accessible and efficient thanks to modern polycarbonate greenhouses, allows you to fully control your plant production, save on your gardening budget, and discover an infinite variety of tasty varieties.

Start simply with a few easy species (lettuce, radishes, aromatic herbs), gradually equip yourself (shelves, watering system, automatic opening) and you will quickly realize how much the greenhouse transforms your way of gardening.

Ready to embark on the sowing adventure? Discover our complete range of polycarbonate garden greenhouses and our accessories to optimize your sowing space. Happy germination! 🌱

Sources: INRAE, Fédération Nationale des Jardins Familiaux et Collectifs, Société Nationale d'Horticulture de France (SNHF)