How to Grow Tomatoes from Seed: A Week-by-Week Guide

Set Your Timeline and Gather Supplies

Tomatoes want a head start indoors, and the calendar works backward from your last expected spring frost. Count back six to eight weeks from that date and mark it as sowing day. Start earlier and your plants outgrow their pots before it is warm enough to move them out; start later and you give up weeks of harvest.

Before you sow, gather the essentials:

• Fresh seed-starting mix, light and sterile, not garden soil or heavy potting soil
• Small cells, plug trays, or 2- to 3-inch pots with drainage holes
• A shallow tray to hold them and water from below
• A bright grow light, ideally adjustable in height
• Plant labels and a waterproof marker
• Optional but useful: a seedling heat mat and a clear humidity dome

Determinate varieties stay compact and set fruit in a concentrated window, which suits containers and canning. Indeterminate types keep growing and producing until frost. Choose based on your space and how you plan to eat them.

Week 1: Sowing

Moisten your mix until it feels like a wrung-out sponge, then fill your cells and firm them lightly. Sow two seeds per cell about a quarter inch deep, cover, and label each variety before you forget which is which. Two seeds is insurance; you will thin to the stronger seedling later.

Warmth drives germination far more than light at this stage. Aim for a mix temperature of 70 to 80 degrees Fahrenheit. A heat mat under the tray speeds things along and makes germination more even. Cover the tray with a humidity dome or loose plastic to hold moisture, and set it somewhere warm. You will not need the grow light running yet, but have it ready.

Week 2: Germination and First Light

Most tomato seeds break the surface in five to ten days. The moment you see green loops pushing up, take two actions right away: remove the dome and turn on the light. Seedlings left under a dome or in dim conditions stretch fast, ending up tall, pale, and weak.

Those first two leaves are the cotyledons, or seed leaves. They are smooth and simple, and they are not the true leaves yet. Keep the grow light close, roughly two to three inches above the tops, and run it 14 to 16 hours a day on a timer. If your seedlings lean hard toward one side or the stems look stretched, the light is too far away or too weak. Water from the bottom by adding to the tray, and let the surface dry slightly between waterings to discourage damping-off, a fungal collapse at the soil line that kills seedlings quickly.

Weeks 3-5: True Leaves, Potting Up, and Feeding

After the cotyledons, the first true leaves appear, jagged-edged and recognizably tomato. This is your cue to thin. Snip the weaker seedling in each cell at the soil line with scissors rather than pulling it, which protects the roots of the keeper.

Once seedlings have one or two sets of true leaves and roots fill their starter cells, pot them up into larger containers. Bury the stem deeper than it sat before, right up to the lowest leaves. Tomatoes are unusual this way: the tiny hairs along the buried stem grow into roots, building a stronger plant.

Seed-starting mix carries little nutrition, so begin feeding once true leaves are established. Use a balanced liquid fertilizer diluted to about half the label strength, roughly once a week. Keep the light close as the plants gain height, raising it to stay a few inches above the canopy. Brushing your hand across the tops once or twice a day, or aiming a gentle fan at them, thickens the stems and prepares them for wind outdoors.

Weeks 6-7: Hardening Off

Seedlings raised indoors have never felt direct sun, real wind, or a cold night. Moving them straight outside shocks and often scorches them. Hardening off eases the transition over seven to ten days.

Start by setting the plants outdoors in a sheltered, shaded spot for an hour or two, then bring them back in. Each day, add time and gradually more direct sun and exposure. Keep them out of strong midday sun and hard wind at first. Watch the water closely: plants outdoors dry out much faster than they did indoors. By the end of the stretch they should be spending full days out and, if nights are mild enough, staying out overnight.

Week 8: Transplanting Out

Tomatoes are cold-sensitive, so wait until the danger of frost has passed and nighttime lows stay reliably above 50 degrees Fahrenheit. Cold soil stalls them, and patience here pays off in stronger growth.

Plant deeply, just as you did when potting up. Set each seedling so that about two-thirds of the stem is buried, stripping the leaves that would end up underground, or lay the rootball in a shallow trench and bend the top gently upward. Both methods create that deep, stem-borne root system. Space determinate plants about two feet apart and indeterminate plants two to three feet, and put supports such as cages or stakes in at planting time so you are not driving them through roots later. Water in well, and mulch once the ground has warmed to hold moisture and keep soil from splashing up onto the leaves.

Quick recap

• Count back six to eight weeks from your last frost to pick sowing day
• Germinate warm at 70 to 80 degrees, then get bright light on seedlings immediately
• Thin to the strongest seedling, then pot up burying the stem deep
• Harden off over seven to ten days before moving plants outside
• Transplant only after frost has passed and nights stay above 50 degrees, planting deep with support in place