Raising seedlings: mix, light, water and avoiding damping off

Growing your own seedlings costs a fraction of buying punnets, and it opens up varieties you'll never find at the garden centre. It takes a little attention, but the basics are simple.

Safety first

Seed-raising mix, potting mix and compost can carry Legionella longbeachae, the bacteria behind a serious lung infection. South Australia has had cases linked to potting mix. A few simple habits lower the risk:

• Wear gloves and a well-fitting dust mask when handling mix.
• Open bags with scissors, slowly, and away from your face.
• Work outdoors in fresh air, not in a closed shed.
• Wash your hands well afterwards, even if you wore gloves.

If you get flu-like symptoms, a cough or shortness of breath in the days after gardening, see a doctor and mention you've been using potting mix.

Use seed-raising mix

Seeds want a fine, free-draining, low-nutrient mix. Seed-raising mix is made for this. Garden soil is too heavy, sets hard and can carry disease. Ordinary potting mix is often too coarse for small seeds. Fill containers, tap them to settle the mix, and water well before sowing.

Choosing containers

Toilet-roll tubes can go straight into the ground, so they suit crops that dislike root disturbance. Tear the bottom open a little when planting so roots can escape.

Reusing old punnets? Scrub them in hot soapy water first.

Watering from below

Watering from the top can wash tiny seeds away or knock seedlings flat. Instead:

1. Sit trays in a shallow tray or tub of water for 10 to 20 minutes.
2. Let the mix soak up water until the surface looks damp.
3. Lift them out and let them drain.

Keep the mix evenly moist, never soggy. A clear lid or plastic bag over the tray helps hold moisture until the seeds sprout. Take it off as soon as they come up.

Light

Once seedlings appear, they need plenty of light. Leggy seedlings, tall, pale and floppy, are stretching for light they're not getting. They rarely recover well.

• A bright windowsill works, but turn trays daily so they don't lean.
• Outdoors in a sheltered, bright spot is often better once days are mild.
• Grow lights help indoors in winter.
• Avoid harsh afternoon sun on tiny seedlings in hot weather.

Warmth

Warm-season crops such as tomato, capsicum and eggplant need warmth to germinate. To get a head start on spring, gardeners often sow them in late winter indoors. A warm sunny window, a mini greenhouse or a seedling heat mat makes a real difference. Capsicum and eggplant are slow and fussy, so start them a little earlier than tomatoes.

Once they're up, seedlings need a bit less warmth but just as much light.

Thinning

If several seeds come up in one cell, keep the strongest and snip the rest at soil level with scissors. Pulling them can disturb the roots of the one you're keeping.

Pricking out

Seeds sown thickly in a punnet need separating once they have their first true leaves. The first leaves you see are seed leaves. True leaves come next and look like the adult plant.

1. Water the punnet an hour before.
2. Ease a clump out and gently tease seedlings apart.
3. Always hold seedlings by a leaf, never the stem. A torn leaf is fine; a crushed stem is not.
4. Make a hole in fresh mix with a pencil or dibber, lower the roots in, and firm gently.
5. Water in and keep out of strong sun for a day or two.

Potting on

When roots start poking out the bottom, or the seedling looks big for its pot, move it into a bigger pot with potting mix. Tomatoes can be planted a little deeper each time. This keeps them growing strongly until it's warm enough to plant out.

Feeding

Seed-raising mix has very little food in it. Once seedlings have a couple of true leaves, start feeding with a liquid fertiliser at half strength about once a week. Seaweed solution helps with root growth and general health. Don't overdo it; soft, lush seedlings are more prone to pests and disease.

Damping off

Damping off is when healthy-looking seedlings suddenly collapse at the base, as if pinched. It's caused by fungi in the mix, and it thrives in cool, wet, still conditions. You can't cure it, but you can avoid it.

• Use fresh seed-raising mix and clean containers.
• Don't overwater. Let the surface dry slightly between waterings.
• Give seedlings good airflow. Remove lids once seeds sprout.
• Don't sow too thickly.
• Water in the morning so the surface dries before night.

If it strikes, remove the affected seedlings and the mix around them, improve airflow, and water less.

When they're ready to go out

Seedlings are ready to plant out when they have several true leaves and roots that hold the mix together. Before that, they need hardening off for a week or two so they cope with sun and wind. The guide to hardening off and transplanting walks you through it.