Starting Seeds Indoors: Germination Guide

Successful seed starting requires three things in balance: correct temperature for the species (typically 20–28°C), consistent moisture in the germination medium without waterlogging, and high humidity during germination that gradually drops after emergence. Most seeds germinate in 3–10 days under optimal conditions.

What Germination Media Should You Use?

The germination medium is the material in which seeds first germinate and establish their first roots. It must hold moisture consistently, allow adequate oxygen to roots, and be free of pathogens that can cause damping off. Three materials dominate indoor seed starting:

1. Rockwool (stone wool) cubes Made from spun volcanic rock, rockwool holds water while retaining ~30% air space. It is the standard in commercial hydroponic propagation.

Pre-soak rockwool in pH 5.5–6.0 water for 30–60 minutes before use
Squeeze gently once to remove excess water (should drip slowly, not pour)
Do not squeeze hard — this removes the air pockets that prevent root rot
Best for: hydroponic transplants (kratky, NFT, DWC)
Limitation: not biodegradable; less forgiving of over-watering than organic media

2. Peat-based seed starting mix Fine-textured peat with perlite is the classic home gardener's germination medium.

Lightweight, widely available, low cost
pH tends to be acidic (5.5–6.0) — suits most vegetables
Water-holding characteristics are excellent for surface-sown seeds
Best for: soil transplants, general home use
Limitation: peat is a non-renewable resource; coco coir is a sustainable alternative

3. Coco coir (compressed bricks or loose) Coco coir has replaced peat in many commercial operations. Near-neutral pH (5.8–6.8), excellent moisture retention, naturally antifungal properties.

Rehydrate compressed bricks with warm water; break up clumps
Mix with 20–30% perlite for improved aeration
Best for: hydroponic or soil systems, environmentally-conscious growers
Limitation: slightly lower nutrient holding capacity than peat

Medium	pH	Water Retention	Air Space	Reusable	Best For
Rockwool 1.5" cubes	7.0 (pre-soak to 5.5)	High	Medium	No	Hydroponic propagation
Peat seed mix	5.5–6.0	High	Low–Medium	No	General seed starting
Coco coir + perlite	5.8–6.8	Medium–High	Medium–High	Limited	Hydroponic or soil
Vermiculite (fine)	7.0–7.5	Very High	Medium	No	Covering seeds
Sphagnum moss	3.5–4.5	Very High	Low	No	Orchids, carnivorous plants

What Temperature Do Different Seeds Need to Germinate?

Temperature is the most common reason for poor germination indoors. Planting lettuce seeds in a 28°C medium will reduce germination significantly; lettuce has built-in thermodormancy above 25°C.

Crop	Optimal Germination Temp (°C)	Min Viable Temp	Max Viable Temp	Days to Germination
Lettuce	15–20	5	25	2–5
Spinach	15–20	5	24	5–10
Tomato	22–28	15	32	5–10
Pepper	25–30	18	35	7–14
Cucumber	22–28	15	32	3–7
Basil	22–26	18	30	5–10
Broccoli	18–25	7	30	4–8
Peas	15–22	5	28	5–10
Swiss chard	18–24	5	28	5–10
Carrots	18–24	7	30	7–21

A seedling heat mat set to the target temperature range is one of the highest-value investments for indoor seed starting. Without supplemental heat, a basement or garage environment may be 5–10°C too cool for warm-season crops.

How Do Humidity Domes Help Germination?

Humidity domes maintain high relative humidity (80–95%) around seeds and young seedlings, dramatically reducing moisture loss from the germination medium surface. This prevents the number one cause of failed germination: the surface of the medium drying out between the point of sowing and the point of root establishment.

Humidity dome protocol:

Sow seeds and water medium to field capacity
Place dome over tray immediately — before the medium surface begins to dry
Check daily; condensation on the inside of the dome is normal and indicates adequate humidity
Ventilate briefly (30 seconds, once or twice daily) to prevent excessive CO2 buildup and fungal spore accumulation
Remove dome as soon as 50–70% of seeds have germinated — keeping the dome on longer causes weak, etiolated seedlings and increases damping off risk

Humidity domes are available as hard plastic propagation lids (standard 10×20 inch flat size) or improvised from cling wrap. Invest in a rigid lid with adjustable vents — the vent control is more useful than it seems for dialling in humidity without full removal.

What Light Timing Do Seedlings Need and When Do You Transplant?

Light requirements for seedlings:

Young seedlings do not need high-intensity light. Excessive light intensity before seedlings have established a root system causes photoinhibition — the plant cannot process the light fast enough and downgrades its photosynthetic apparatus.

Recommended light levels:

Germination: No light required (covered with dome or in dark)
Days 1–5 after emergence: 100–150 µmol/m²/s, 16 hours
Days 6–14 (early seedling): 150–250 µmol/m²/s, 16–18 hours
Transplant preparation: 200–350 µmol/m²/s, 16–18 hours

Begin "hardening off" seedlings going to outdoor settings by gradually increasing light intensity and reducing humidity over 7–10 days before transplant.

When to transplant:

Indicator	Signal
First true leaves visible	Seedling is past cotyledon stage — safe to handle
Roots visible at drainage holes	Pot-bound; transplant immediately
Stem diameter	At least 2–3 mm for most vegetables
Root ball cohesion	Roots hold growing medium in a solid ball when inverted

Transplant on an overcast day or in the evening to reduce transplant shock. Water in with a dilute root-stimulating solution (seaweed extract or mycorrhizal inoculant) to support root establishment.

Frequently Asked Questions

Why are my seedlings leggy (tall and thin)?

Legginess is etiolation — the plant stretching toward an insufficient light source. The fix is almost always to move the light source closer or increase its output. Move T5 lights to within 5–10 cm of the seedling tops; LEDs to the manufacturer's recommended seedling distance. A secondary cause is excessive heat without adequate light. If leggy seedlings cannot be salvaged, pinch back to the second node and the plant will branch more compactly.

How long can I store seeds before they stop germinating?

Germination viability varies significantly by species. Onion and parsnip seeds are viable for 1–2 years; tomatoes, peppers, and cucumbers remain viable for 4–6 years when stored correctly. Correct storage means cool (below 10°C), dry (below 8% moisture), and dark. A sealed container in the refrigerator is ideal. Do not store seeds in a warm kitchen. For best results, perform a germination test (10 seeds on a damp paper towel for 7 days) on any seeds older than 3 years before committing trays to them.

Is it worth using a seedling heat mat, or can I germinate at room temperature?

For cool-season crops (lettuce, spinach, brassicas), room temperature of 18–22°C is adequate and a heat mat is optional. For warm-season crops (tomatoes, peppers, basil, cucumbers), a heat mat is close to essential in most home environments. Without consistent 24–28°C soil temperature, pepper germination can drop from 95% to below 50%, and time to germination can stretch from 7 days to 21+ days. Heat mats cost $15–30 and pay for themselves quickly in faster, more uniform germination.

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Frequently bought with:

GRODAN A-OK Rockwool Starter Cubes (1.5-inch, Sheet of 98 Plugs)

Grodan Rockwool Starter Cubes (1-inch, 50-pack)

Barrina T5 LED Grow Light Strips (4-pack, 4ft)