Sowing Seeds Indoors: How to Germinate, Prick Out and Harden Off
Sowing seeds indoors gives chillies, tomatoes, herbs and salads a head start in a warm, bright spot you control. This guide covers germination and the seedling stage - pricking out, potting on, feeding and hardening off - with the propagators, plugs, pots and feeds we sell. For cuttings, see our propagators and cuttings guide.
How to germinate seeds
A seed germinates, putting out its first root and shoot, when it has moisture, warmth and air. Most need no light until they are up. Follow the packet for depth and temperature: fine seed goes shallow or on the surface.
Gardeners commonly cover seed with a thin layer of Vermiculite, because it stays evenly moist (see our perlite and vermiculite guide). Water from below so seed is not washed out: stand trays in a garden tray of water until the surface darkens, then lift them out to drain.
Or sow into plugs. Stand Jiffy 7 Peat Pellets hole side up in tepid tap water for about 20 minutes, then sow in a hole about 5 mm deep, says the Jiffy 7 Peat Pellets (X1000) page. They are pH 5.3 and fully fertilised, it adds, so need no pH adjusting or feed at first.
Seedlings that collapse at soil level usually have damping off, a fungal problem of wet, crowded trays: sow thinly, do not overwater and get air moving.
Which seed propagator?
- On a windowsill: the Jiffy Botanian Mini Greenhouse Kit comes with Jiffy pellets under a clear lid that, its page says, keeps them warm and humid. In strong sun, prop the lid open so it does not overheat.
- In a warm room or heated greenhouse: Garland Unheated Propagators, which their page calls ideal for just that.
- In a cold room, or for chillies: the Garland XL High Dome Heated Propagator warms the compost on a thermostat and has twin vents. Keep it out of direct sun and cold draughts, its page says.
More propagators, with sizes, are in the propagators and cuttings guide. A ThermoPro TP358 by the tray records the lowest temperature over 24 hours; if the room is cold, see our heaters guide.
Once seedlings are up, weak light makes them tall and floppy. Give them 18 hours of light a day on a timer, the usual 18/6 schedule for growth in our growers' glossary. Among our propagation lights, the Gorilla Propagation LED 12W is safe for close positioning, its page says (the power lead is sold separately), and the Maxibright LED T5 Propagation Lights run cool.
How to prick out seedlings
Pricking out moves seedlings from a crowded tray into pots of their own, once the first true leaves show above the seed leaves. Left longer, the roots tangle. Seedlings in pellets and plugs skip this step: if two come up in one, snip off the weaker.
- Water the tray, and fill small pots with moist light-mix soil: the Plagron Lightmix Soil page calls it ideal as a potting soil for young plants.
- Ease a clump out from below and tease one seedling free, holding it by a seed leaf, never the stem.
- Lower it into a hole made with a dibber or pencil, up to the seed leaves. Firm gently and water from below.
- Keep it out of strong sun until it perks up.
When to transplant seedlings
Pot on when roots show at the drainage holes or through the plug. Go up a size or two at a time: a small plant in a big pot stays wet longest, as our root rot guide says.
| Stage | What to use | Growing medium |
|---|---|---|
| Sowing | Jiffy 7 pellets, Root Riot cubes or a seed tray | As supplied, or light-mix soil |
| Pricked out | Jiffy Botanian Peat Free Pots or Jiffy Coir Pots | Light-mix soil or coco |
| Out of the plug, or potted on | 1 litre Square to Round Pots or 1l Air Pots, then 2-3 litre Square to Round | Light-mix soil or coco |
| Final pot | A bigger plastic pot or fabric pot | Biobizz All Mix or another richer soil, or coco |
Jiffy Botanian and Coir Pots are planted whole, so the roots are barely disturbed. The 1l Air Pots page warns they tend to dry out faster than standard pots. The Biobizz All Mix page says to establish plants in a light mix in 1 or 2 litre pots first. When you pot on, the Great White page says to dust the transplant hole.
Feeding seedlings
Start feeding plants in Jiffy 7 pellets once roots show and they are potted on, as our propagators and cuttings guide says. The Biobizz Light Mix page says you can start feeding once plants are established; the Plagron Lightmix page says you will need a soil nutrient. In unfertilised plugs, coco or hydroponics, use a propagation feed:
| Feed | Dose on its page | What its page says |
|---|---|---|
| Canna Start | 2-4 ml per litre, or EC 1.0-1.2 | Combined with Canna Rhizotonic or another root stimulant; pH 5.3-6.3 by medium |
| Formulex | See the label | One-part feed for seedlings and transplants; pH 6 in tap water |
| Intense Kickstart | 3-4 ml per litre | Until plants move to the main base feed |
| Vitalink Plantstart | See the label | Boron and calcium for very young plants |
Once plants are growing away, move to a base feed: see our guide to base feeds.
How to harden off seedlings
Hardening off is getting young plants used to tougher conditions gradually. It starts in the propagator: open the vents a little at a time, then leave the lid off for longer each day.
For a greenhouse or outdoors, stand plants in a sheltered spot by day and bring them in at night, then leave them out longer and finally overnight. Tender crops such as tomatoes, chillies and basil go out only after the last frost. Keep small pots watered: wind dries them fast.
Chillies, tomatoes, herbs and microgreens
- Chillies: slow to germinate and to grow, so sow them before your tomatoes, in a heated propagator.
- Tomatoes: quick to stretch, so get them under the light the day they show. They root along a buried stem, so plant them deeper at each move.
- Herbs: basil likes warmth, like chillies. Coriander dislikes being moved, so sow it in a Jiffy Coir Pot. Formulex, its page says, works as a full-cycle feed for herbs and leafy greens.
- Microgreens: sow thickly on a shallow tray and cut with scissors at the seed-leaf stage. The Autopot Tray2Grow, its page says, works with a microherb tray and waters from a raised reservoir you connect, with no pumps, timers or electricity.
Quick answers
How do I germinate seeds indoors?
Sow into moist plugs or light-mix soil at the depth and temperature on the packet, under a propagator lid, and water from below. Give light as soon as the first seedling shows.
When should I prick out seedlings?
When the first true leaves appear above the seed leaves. Hold each seedling by a seed leaf, never the stem.
When should I transplant tomato seedlings?
Pot them on, deeper each time, once roots show through the plug or pot. Gardeners commonly plant them in their final pot or bed when the first flowers show, once hardened off and past the last frost.
More Growers' Guides
- Propagators and cuttings
- Perlite vs vermiculite
- Root stimulants and rooting gels
- Which nutrients? Soil, coco and hydro
- Fungus gnats (sciarid flies)
Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.