How to Grow Seeds in a Greenhouse

How to start seeds in a greenhouse for beginners

Before sowing your first seed, optimise your greenhouse environment. Position seed trays on the south-facing side where they'll receive maximum natural light. Install shelving to maximise vertical space—a standard 8x6 greenhouse can house 12+ seed trays when properly configured.

Insulate against temperature fluctuations by using bubble wrap on greenhouse walls during February–April sowings. This simple step maintains 3–5°C warmer night-time temperatures, crucial for heat-loving seedlings like tomatoes and peppers.

Create dedicated zones within your greenhouse: germination areas (warmest spots), growing-on sections (moderate temperatures), and hardening-off spaces (cooler areas near vents). This temperature gradient accommodates different growth stages simultaneously.

Install automatic vent openers (£25–40) to prevent overheating—temperatures above 29°C kill seedlings faster than cold. Position a thermometer at seedling level, not eye level, as floor temperatures often run 5°C cooler than the greenhouse ceiling.

Setting Up Your Greenhouse for Seed Starting Success

Introduction to Greenhouse Seed Starting

Growing seeds in a greenhouse transforms ordinary gardeners into plant production powerhouses. The controlled environment of a greenhouse—with its regulated temperature, humidity, and light—creates ideal conditions for seed germination and seedling development that outdoor gardens simply cannot match.

This guide delivers precise techniques for starting seeds in your greenhouse, whether it's a grand Victorian structure or a compact mini greenhouse tucked against a wall. You'll learn exactly when to sow, how to prevent common killers like damping off, and how to transition seedlings successfully to garden beds.

Unlike generic advice that leaves you guessing, this guide provides exact measurements, specific timing, and science-backed techniques that professional growers use. Master these methods and your greenhouse becomes a seedling factory that cuts costs while delivering healthier, more productive plants than any garden centre could provide.

Why Grow Seeds in a Greenhouse? [Benefits + ROI]

Starting seeds in your greenhouse costs pennies compared to buying seedlings. A £2 packet contains 25–100 seeds, while nursery seedlings cost £3–5 each. For tomatoes alone, you'll save £75+ per season.

Beyond cost, your greenhouse gives you access to 400+ tomato varieties versus the standard 5–10 at garden centres. Rare heirlooms, speciality peppers, and unusual flowers become instantly possible.

Disease prevention provides another compelling advantage. Commercial seedlings harbour pathogens. Your greenhouse-started seeds begin life in controlled, clean conditions, slashing blight, mosaic virus, and root rot risks.

Start seeds in your greenhouse during February–March and you'll harvest weeks before neighbours who plant in May. For cool-season crops like kale, this means two full harvests yearly instead of one.

A greenhouse environment extends your growing season by 8–12 weeks annually, turning a short British summer into a nearly year-round growing operation.

Essential Supplies: The No-Nonsense List

Success requires specific tools. Here's what you actually need:

Must-Haves:

• Sterile seed starting mix (£5–8)
• Containers with drainage (£3–15)
• Clear plastic cover or cling film (£2–5)
• Labels and waterproof marker (£3)
• Spray bottle or bottom-watering tray (£2–4)

Worth the Investment:

• LED grow light (£25–85)
• Heat mat with thermostat (£20–40)
• Fan for air circulation (£10–15)
• Seedling heat mat thermostat (£15–30)

A dedicated workspace transforms results – a potting shed is perfect for organising supplies and protecting delicate seedlings from household traffic.

Skip the compressed peat pellets and "complete kits" with flimsy trays. They fail more often than they succeed.

Seed Starting Mix: What Actually Works

Standard potting soil kills seedlings. It's too dense and harbours fungi that cause damping off. You need a sterile, lightweight mix.

Commercial seed starting mixes contain:

• Fine peat moss or coco coir (holds moisture)
• Perlite or vermiculite (provides drainage)
• Small amount of compost (supplies nutrients)
• Limestone (balances pH)

Make your own superior mix at half the cost:

• 3 parts fine coco coir (sustainable alternative to peat)
• 1 part perlite
• 1 part vermiculite
• ¼ part worm castings

Pre-moisten this mix until it holds shape when squeezed but releases no water droplets. This achieves the 70–80% moisture level seeds require for germination.

Containers That Guarantee Success

Not all seed starting containers perform equally. Here's the truth:

Best Performers:

1. Cell trays with 3.8–5cm cells (ideal for most vegetables)
2. 7.5cm square pots (perfect for tomatoes, peppers, cucumbers)
3. Root trainers (essential for peas, beans, sunflowers)
4. Soil blocks (eliminates transplant shock completely)

Adequate Options:

• Clean yoghurt pots with drainage holes
• Egg cartons (for short-term seedlings only)
• Toilet paper tubes (for direct transplanting)

Limited on space? A mini greenhouse provides perfect conditions for seed trays while taking minimal room on patios or balconies.

Avoid shallow seed trays without dividers—they force seedling roots to tangle.

Every container needs drainage. No exceptions. Drill holes in repurposed containers using a 6mm bit.

Seed Starting Timeline: Precision Planning

Time your seeding with mathematical precision:

1. Find your region's last frost date (between April–May for most UK areas)
2. Count backward according to these guidelines:
   • 10–12 weeks: onions, leeks, celery
   • 8–10 weeks: peppers, eggplant, petunias
   • 6–8 weeks: tomatoes, broccoli, cabbage, marigolds
   • 4–6 weeks: cucumbers, melons, zinnias
   • 2–4 weeks: squash, sunflowers, beans

Some seeds (tomatoes, peppers) germinate slowly (7–14 days) while others (lettuce, radishes) sprout in 3–5 days. Factor this into your timing.

For continuous harvests, start seeds in 2-week intervals. Three sowings of carrots, spaced 14 days apart, provide 30+ days of fresh harvests instead of a single overwhelming crop.

Step-by-Step Planting Process

1. Fill containers with pre-moistened mix to 0.6cm below rim
2. Compress slightly (not firmly) to eliminate air pockets
3. Place seeds at these exact depths:
   • Dust-like seeds (petunias): surface sow, no covering
   • Small seeds (lettuce, basil): 0.3cm deep
   • Medium seeds (tomatoes, brassicas): 0.6cm deep
   • Large seeds (beans, cucumbers): 1.3cm deep
4. Cover with mix to proper depth using a fine-mesh sieve
5. Mist gently with water or bottom water by placing in 1.3cm of water
6. Cover with plastic to retain humidity (remove once sprouted)
7. Label immediately with variety and date

For tiny seeds, mix with fine sand (1:4 ratio) for even distribution.

After sowing, place warm-germinating seeds (tomatoes, peppers) on a heat mat at 21–24°C. Cool-germinators (lettuce, spinach) need 15–18°C.

Light Requirements: The Science of Seedling Growth

Seedlings need more light than most gardeners provide. Insufficient light causes leggy, weak plants that never recover.

Light requirements by stage:

• Germination: Most seeds don't need light (exceptions: lettuce, petunias)
• Post-emergence: 14–16 hours of direct light daily
• Hardening off: Gradual increase to full sunlight

Window light rarely suffices—even south-facing windows provide ¼ the intensity of proper grow lights. Position LED grow lights 10–15cm above seedlings for 14–16 hours daily. Raise lights as plants grow to prevent leaf burn.

For optimal results, use fixtures delivering 65–85 ?mol/m²/s PAR (Photosynthetically Active Radiation) at seedling level.

Budget option: Standard shop lights with full-spectrum LED bulbs work surprisingly well when positioned 5–7cm from seedling tops.

Temperature Control: The Critical Variable

Temperature affects germination more than any other factor. Each species has specific requirements:

Warm-germinators (21–26°C):

• Tomatoes, peppers, eggplant
• Cucumbers, melons, squash
• Basil, marigolds

Medium-germinators (18–21°C):

• Broccoli, cabbage, kale
• Lettuce, chard, spinach
• Snapdragons, cosmos

Cool-germinators (13–16°C):

• Peas, carrots, radishes
• Spinach, leafy greens
• Sweet peas, calendula

After germination, reduce temperatures by 3–5°C to prevent leggy growth. Night temperatures should drop 5–8°C below day temperatures to simulate natural conditions.

Digital thermometers with probes placed in the soil provide the only accurate measurement of germination conditions. Room temperature gauges mislead—soil temperatures typically run 3–5°C cooler than ambient air.

Watering Techniques That Prevent Damping Off

More seedlings die from improper watering than any other cause. Master these techniques:

Bottom watering reigns supreme:

1. Place containers in a tray with 1.3cm water
2. Allow 10–30 minutes for absorption
3. Remove containers once the surface darkens
4. Let soil approach dryness before watering again

This method forces roots to grow downward seeking moisture and prevents the constantly wet surface that triggers fungal diseases.

If you must top-water, use a fine mister or spray bottle. Water early in the day so foliage dries before night.

The soil surface should dry slightly between waterings, but deeper layers should remain moist. Check by weight—dry containers feel noticeably lighter.

Feeding Your Seedlings

Seedlings need minimal nutrition until they develop true leaves (the second set that appears). Excessive fertiliser kills young plants.

1. Mix organic liquid fertiliser at ¼ recommended strength
2. Apply once weekly during regular watering
3. Increase to ½ strength after 2–3 weeks

Fish emulsion and seaweed extract provide balanced nutrition without burning tender roots. Chemical fertilisers work but require extreme precision in dilution.

Stop fertilising 5–7 days before transplanting to prepare seedlings for the shock of moving outdoors.

Hardening Off: Guaranteed Transplant Success

Hardening off transitions seedlings from protected indoor conditions to harsh outdoor reality. Follow this 10-day schedule exactly:

• Day 1–2: Place seedlings in a sheltered outdoor spot for 1–2 hours in indirect light
• Day 3–4: Increase to 3–4 hours, introducing morning direct sunlight
• Day 5–6: Extend to 4–6 hours with more direct light exposure
• Day 7–8: Leave seedlings out for the full day, bring in at night
• Day 9–10: Leave seedlings out day and night unless frost threatens

During this process, reduce watering slightly but never allow plants to wilt. Provide protection from wind, which dehydrates seedlings rapidly.

Skip this process and you'll lose plants. No exceptions.

Troubleshooting Common Problems

Leggy Seedlings

• Cause: Insufficient light
• Solution: Move lights closer (5–10cm above plants), increase duration to 16 hours

Damping Off (Seedlings Fall Over)

• Cause: Fungal infection from overwatering/poor air circulation
• Solution: Improve ventilation, reduce watering, apply cinnamon (natural fungicide)

Yellow Leaves

• Cause: Overwatering or nutrient deficiency
• Solution: Let soil dry more between waterings; apply dilute fertiliser if true leaves have developed

White Fuzzy Mold on Soil

• Cause: Excessive moisture, poor air circulation
• Solution: Remove visible mould, apply a thin layer of dry vermiculite, add a fan for air movement

No Germination

• Cause: Old seeds, improper temperature, too deep planting
• Solution: Try fresh seeds, check soil temperature requirements, plant at proper depth

Curled Leaves

• Cause: Heat stress, light too close
• Solution: Move lights farther away, reduce temperature, ensure adequate ventilation

Seed Starting Calendar: Your Year-Round Blueprint

Month	Essential Tasks	What to Start	Greenhouse Management
January	Order seeds	Onions, leeks	Clean and sterilise containers
February	Inventory supplies	Peppers, eggplants, cool-season flowers	Test heat mats and lights
March	Prepare potting area	Tomatoes, brassicas, first herbs	Monitor night temperatures
April	Begin outdoor prep	Cucumbers, melons, squash	Start hardening off early seedlings
May	Final frost watch	Heat-loving flowers, beans	Transplant hardened seedlings outdoors
June–July	Summer maintenance	Succession lettuce, fall brassicas	Shade new transplants as needed
August–Sept	Collect open-pollinated seeds	Fall/winter crops, cold frames	Clean empty containers
Oct–Dec	Evaluate results	Winter greens (with protection)	Store equipment, plan next season

Conclusion

Growing seeds in your greenhouse transforms your gardening experience from spectator to creator. The £30–50 investment in basic supplies pays back tenfold in the first season alone.

Begin with easy-to-germinate vegetables like tomatoes, lettuce, and squash in your greenhouse. Add challenging varieties only after mastering the fundamentals.

Record your greenhouse growing results methodically—variety name, sowing date, germination rate, and transplant success. This data becomes invaluable in subsequent years.

The greatest gardens don't come from buying plants. They emerge from greenhouse-started seeds grown with precision, patience, and proper technique.