How seeds germinate: warmth, water, air and timing

A seed is a baby plant packed with its own lunch, waiting for the right conditions. Get those conditions right and most seeds will come up happily. Get them wrong and they sit and sulk, or rot.

What a seed needs

Every seed needs three things to wake up:

• Moisture. The seed takes up water, swells and switches on. The soil should be damp like a wrung-out sponge, not soggy.
• Warmth. Each crop has a temperature range it likes. Too cold and nothing happens. Too hot and some seeds refuse to start.
• Air. Seeds breathe. Waterlogged or compacted soil starves them of oxygen and they rot.

Some seeds also care about light. A few, such as lettuce, need some light to germinate, which is why they're sown very shallow or pressed onto the surface. Others, such as coriander, prefer darkness and a proper covering. The seed packet usually tells you which.

Soil temperature beats air temperature

This is the one most new gardeners miss. A warm spring day doesn't mean warm soil. Soil warms and cools more slowly than air, so after a cold snap it can stay chilly for days even when the afternoons feel lovely.

Warm-season crops like tomato, capsicum, eggplant, pumpkin, zucchini, cucumber and basil want warm soil. Sow them into cold ground and they sit there, or rot before they sprout. Capsicum and eggplant are the fussiest and like it warmer than tomatoes.

Cool-season crops like lettuce, spinach and coriander have the opposite problem. They germinate well in cool soil but struggle when the soil gets hot. Lettuce in particular can go dormant in hot soil and simply not come up. In an Adelaide summer, sow lettuce in a shady spot, in the cool of the evening, or in a punnet indoors.

Patchmates' sowing calendar uses soil temperature for your suburb, not just the month, so it tells you when the ground is actually ready. If you want to check for yourself, a cheap soil thermometer pushed in to about 10 cm deep in the morning gives a useful reading.

How deep to sow

A good rule of thumb is to sow a seed about two to three times as deep as it is wide.

• Tiny seeds like lettuce, carrot and many herbs go barely under the surface. A light sprinkle of fine mix or vermiculite is plenty.
• Medium seeds like beetroot, silverbeet and tomato go about 0.5 to 1 cm deep.
• Big seeds like beans, peas, pumpkin and corn go about 2 to 3 cm deep.

Too deep and small seeds run out of energy before they reach the light. Too shallow and they dry out. Firm the soil gently over the seed so it has good contact with the damp soil around it.

Soaking and nicking hard seed

Some seeds have a tough coat that keeps water out. You can help them along.

1. Soaking. Beans, peas and sweet peas can be soaked in water for a few hours or overnight before sowing. Don't leave them for days, or they can start to rot.
2. Nicking. For very hard seeds, rub one side lightly with sandpaper or nick the coat with nail clippers, away from the "eye" where the root comes out. This lets water in.
3. Hot water. Many Australian natives, especially wattles and peas like purple coral pea, have very hard coats. A common method is to pour hot (just off the boil) water over the seed, leave it to soak overnight as it cools, then sow the seeds that have swollen.

Peas and beans can rot in cold, wet soil, so in a cold spell some gardeners skip soaking and sow into soil that is just damp, then don't water again until the shoots appear.

Smoke water for some natives

In the bush, many native plants germinate after fire. Some of their seeds respond to chemicals in smoke rather than heat. Smoke water, sold at nurseries, or smoke-infused discs mimic this. Soak the seed or water the sown tray as the product directs. Not every native needs it, so check the advice for your species. Native seed suppliers usually say which treatment works best.

Seed viability and storage

Seeds are alive, and they don't last forever. Some, such as parsnip and onion, lose their vigour quickly and are best bought fresh each year. Others, like tomato and pumpkin, keep for several years if stored well.

To store seeds well:

• Keep them cool, dry and dark.
• Use paper envelopes inside a sealed jar or container.
• Label everything with the crop and the year.
• Avoid the shed or garage if it gets hot in summer.

A quick germination test

Not sure if an old packet is still good? Test it before you waste a season.

1. Dampen a sheet of paper towel.
2. Lay out ten seeds and fold the towel over them.
3. Put it in a zip-lock bag or container and keep it somewhere warm, such as the top of the fridge.
4. Check every day or two and keep it damp.
5. After the normal germination time for that crop, count how many have sprouted.

If seven or eight come up, the seed is fine. If only a few do, sow more thickly or buy a fresh packet. You can even plant the sprouted seeds carefully into a punnet.

When seeds don't come up

If nothing appears, think back through the basics. Was the soil warm enough for that crop? Did the surface dry out, even once, while the seeds were starting? Was it sown too deep, or did heavy rain or watering wash it away? Did slugs, snails or birds get there first? Most failures come down to one of these, and the next sowing usually goes better.