Growing Tomatoes From Seed in Australia

Timing, seed-raising mix, warmth, light, potting on, hardening off and how to beat damping off

Raising your own tomatoes from seed is the single best upgrade most home gardeners can make. A punnet of seedlings gives you four plants of whatever the nursery happened to grow. A single seed packet gives you dozens of plants of exactly the variety you want, for a fraction of the price.

This guide walks through the whole journey, from choosing a sowing date with the interactive calculator below, through germination warmth and light, to potting on, hardening off and planting out. Get the timing and the warmth right and tomato seed is one of the easiest things to grow.

Why grow tomatoes from seed

• Variety choice. Nurseries stock a handful of common types. From seed you can grow heirlooms like Black Russian and Brandywine, the sweetest cherries like Sungold and Tommy Toe, and paste tomatoes like San Marzano for sauce. See our how to grow tomatoes guide and its variety picker for ideas.
• Cost. One packet of 20 to 50 seeds costs about the same as a single punnet of four seedlings, and tomato seed stays viable for years if stored cool and dry.
• Stronger plants. You control the light, warmth and watering from day one, so you raise sturdy seedlings rather than leggy, root-bound punnet plants, and you avoid bringing home pests or disease.
• Seed saving. Grow an open-pollinated variety and you can save seed from your best fruit for next year. See our seed saving guide.

Step 1: Seed-raising mix

Sow into a proper seed-raising mix, not garden soil or general potting mix. Seed-raising mix is fine, light and low in nutrients, which is exactly what tiny roots want, and it drains freely so seedlings do not sit wet. Garden soil compacts, holds disease, and is far too rich and heavy for germinating seed.

Fill clean punnets, cells or a seed tray, firm the mix lightly and water it before sowing. Sow seed about 5 mm deep, cover lightly, and label every variety. Tomato seeds are small but easy to handle, so sow two per cell and thin to the stronger one later.

Step 2: Warmth for germination

Warmth is the biggest single factor in good germination. Tomato seed comes up best at a soil temperature of about 20 to 27 degrees Celsius. In that range it germinates evenly in 6 to 10 days. In cold mix it is slow, patchy, and far more likely to rot before it sprouts.

Indoors in late winter and early spring, a sunny windowsill is often too cold at night. A heat mat under the tray gives the steady bottom warmth tomato seed loves and is the most reliable way to get an even strike. The top of the fridge or a warm laundry also works. Keep the mix moist but never soggy until the seedlings emerge.

Step 3: Light (and beating leggy seedlings)

The moment seedlings emerge they need bright light, fast. Tomato seedlings that get too little light stretch toward the window with long, weak, pale stems. These leggy seedlings flop over and are slow to establish.

A bright windowsill is rarely enough in late winter, especially indoors. If your seedlings are stretching, give them more light:

• Move them to the brightest possible spot and turn the tray daily so they grow evenly.
• Better still, use a grow light positioned 5 to 10 cm above the seedlings for 14 to 16 hours a day, raising it as they grow.
• Drop the temperature a little once they are up. Warm, dim conditions are what cause stretching.

Step 4: Potting on

Once seedlings have their first one or two sets of true leaves (the second pair, which look like real tomato leaves rather than the rounded seed leaves), pot them up individually into 7 to 10 cm pots. This gives the roots room to develop into strong, stocky plants.

Handle seedlings by a leaf, never the fragile stem, and lift them out with a dibber or the handle of a spoon. Plant each one deeper than it was, burying the stem up to the lowest leaves, and water gently. Move them into individual pots of standard potting mix now that they need more nutrients than seed-raising mix provides.

Step 5: Preventing damping off

Damping off is the heartbreak of seed raising. Healthy seedlings suddenly keel over, pinched and rotted at soil level, often overnight. It is caused by soil-borne fungi that thrive in cool, wet, still, overcrowded conditions. You cannot cure it, only prevent it:

• Use fresh, sterile seed-raising mix and clean trays. Reused mix and dirty pots carry the fungus.
• Sow thinly so seedlings are not crowded, and thin early.
• Water from the base by standing the tray in shallow water, and let the surface dry slightly between waterings.
• Give good airflow. A gentle breeze or a small fan keeps the surface drier and stems stronger.
• Remove humidity covers as soon as seeds germinate, and never let the mix stay waterlogged.

Step 6: Hardening off

Seedlings raised indoors are soft and have never felt wind, full sun or cold. Throwing them straight into the garden shocks them badly. Harden them off over 7 to 10 days by gradually exposing them to outdoor conditions.

Start by putting them outside in a sheltered, partly shaded spot for an hour or two, then bring them back in. Each day leave them out a little longer and in slightly more sun and wind, until after a week or so they are spending all day and night outside. By the end they are toughened up and ready to plant.

Step 7: Transplanting deep

Plant out once all frost risk has passed, the soil is consistently above 16 degrees Celsius, and seedlings are 15 to 20 cm tall. Cold soil stalls tomatoes for weeks, so wait for warmth rather than rushing.

The golden rule is to plant tomatoes deep. Bury the stem up to the first set of leaves, or lay a leggy plant in a shallow trench with just the top poking out. Every part of the buried stem grows new roots, giving you a much stronger, more drought-resilient plant. Water in well and mulch, leaving a small gap around the stem.

When to sow in your region

Use the calculator above for your exact dates, and pick your region for a live tomato calendar in the app or on Find My Region.

Frequently Asked Questions

See also: How to Grow Tomatoes, Seed Raising Guide and Tomato in the Plant Library