How to Grow Tomatoes From Seeds

How long does it take to grow tomatoes from seed?

Start tomato seeds 6–8 weeks before your last frost date. Total time from seed to first harvest is 75–130 days — roughly 6 weeks to grow a transplant-ready seedling, then 60–100 days to first fruit depending on variety. Most US gardeners who start seeds in late February or March see first harvest in late July.

The date of your last spring frost is the anchor for the entire seed-starting calendar. You can find your specific date by zip code using the USDA Plant Hardiness Zone Map and your state’s cooperative extension service. In Zone 5 (central Iowa, Indiana, Illinois), last frost typically falls May 10–20. In Zone 7b (Nashville, Washington DC, central Virginia), it falls March 25 to April 15. In Zone 9b (Central Valley California), frost is rarely a limiting factor.

Seed packet “days to maturity” counts from transplant, not from seeding. Add 6–8 weeks to that number for total time from seed. A packet labeled “70 days to maturity” means 70 days after the transplant goes in the ground — making total seed-to-harvest time about 112–126 days.

For full care from transplant through harvest, see our how to grow tomatoes guide.

What do you need to start tomatoes from seed?

You need five things: sterile seed-starting mix, cell trays, a heat mat, a grow light, and liquid fertilizer. Name-brand seed-starting kits and specialized pots are optional extras — the five essentials are what actually determine results.

Everything you need:

• Sterile seed-starting mix — not potting soil or garden soil. Both compact in small cells and may carry fungal pathogens that kill seedlings before the true-leaf stage. Seed-starting mixes are lighter, drain better, and are sterile.
• 72-cell or 128-cell plug trays — small cells force a tight root ball. Large pots let roots sprawl through excess soil that stays wet too long, increasing damping off risk.
• Heat mat — the single highest-impact tool for reliable germination. Tomato seeds germinate best at 70–80°F soil temperature. Most homes run 62–68°F in late winter, which slows germination significantly.
• Fluorescent or LED grow light — a 4-foot T5 shop light or equivalent LED panel works well. You do not need specialized horticultural lighting. Position it 2–4 inches above seedling tops and run 16 hours daily on a timer.
• Liquid fertilizer — half-strength balanced formula (10-10-10 or 20-20-20) applied weekly once true leaves appear.

Does a heat mat really matter?

Yes — measurably. In germination trials conducted by Cornell Cooperative Extension, tomato seeds at 75°F soil temperature germinated at 85% within 7 days. The same seed lot at 65°F took 14–18 days with 60% germination. A heat mat costs $20–$30 and eliminates the most common seed-starting failure.

Which tomato varieties grow best from seed?

Choose based on your USDA zone and intended use. Indeterminate varieties produce continuously until frost and need staking. Determinate varieties stop growing at a fixed height and ripen most fruit in a concentrated window — better for short seasons and canning.

Seed for all of these is available from US mail-order seed companies year-round and from local garden centers January through March.

For detailed growing guides on specific varieties, see Better Boy tomato, Cherokee Purple tomato, and Sun Gold tomato.

How do you plant tomato seeds step by step?

Fill a 72-cell tray with moist seed-starting mix, press two seeds ¼ inch deep into each cell, cover with a thin layer of dry mix, firm gently, and place on a heat mat at 75°F. Germination takes 5–7 days at this temperature.

Step-by-step:

Step 1: Pre-moisten the mix. Add warm water to the bag of seed-starting mix and knead until it holds together when squeezed but does not drip. Filling trays with dry mix and then watering produces uneven moisture and patchy germination.

Step 2: Fill trays and press lightly. Fill each cell to the rim and press gently to eliminate large air pockets. Do not compact the mix — roots need aerated soil.

Step 3: Plant two seeds per cell, ¼ inch deep. Two seeds per cell ensures that at least one germinates. Thin to the strongest seedling per cell at the 2-leaf stage by snipping the weaker one at soil level — pulling it out damages the remaining roots.

Step 4: Label every row immediately. Tomato seedlings are indistinguishable by variety for the first three weeks. Losing track of variety identity mid-season is a common and annoying mistake.

Step 5: Cover with a thin layer of dry mix and firm gently. This improves seed-to-mix contact and prevents the surface from crusting.

Step 6: Place on a heat mat. Cover loosely with a clear dome or plastic film to retain humidity during germination. The cover does not need to be sealed — just draped over the tray.

Step 7: Check daily. Remove the dome the moment seedlings emerge and move trays immediately under grow lights. Seedlings left under a dome stretch toward ambient light within 24 hours.

How do you care for tomato seedlings indoors?

Tomato seedlings need 14–16 hours of bright light daily, consistent moisture at the root zone, and fertilizer starting at the 4-leaf stage. Drop heat mat temperature after germination — seedlings grow well at 65–70°F air temperature.

Light

Most windows are insufficient in late winter. A south-facing window in February provides 6–8 hours of direct sun and lower light intensity than the June sun seedlings will face after transplant. Seedlings grown in window light become leggy — tall, thin, and weak — even when supplemented with the brightest window available.

Position grow lights 2–4 inches above seedling tops and raise them as plants grow to maintain that gap. Run 16 hours daily on a timer. This produces stocky, compact seedlings with strong stems.

Watering

Bottom watering is more reliable than top watering for seedlings. Fill a tray with 1/2 inch of water and let cells absorb from below for 30 minutes, then drain. This keeps the soil surface drier, which prevents damping off — a fungal disease that collapses stems at soil level and kills seedlings overnight.

Water when the top of the mix feels dry to the touch, typically every 1–2 days in heated indoor conditions.

Fertilizing

Seed-starting mix contains no significant nutrients. Begin fertilizing with half-strength balanced liquid fertilizer once the first true leaves appear (the second set of leaves, which look like small tomato leaves, not the smooth oval seed leaves). Apply weekly.

When purple color appears on the undersides of leaves, it indicates phosphorus stress — apply fertilizer immediately and move seedlings to a warmer spot. Our tomato fertilizer guide covers the full fertilizing schedule from seedling through harvest.

When should you pot up tomato seedlings?

Pot up from cell trays into 4-inch pots when roots begin emerging from drainage holes, or when seedlings reach 3–4 inches tall — typically 3–4 weeks after germination. Waiting until plants are rootbound slows growth that is difficult to recover.

Use a standard potting mix (not seed-starting mix) for the 4-inch stage. Seedlings outgrow the nutrient content of seed-starting mix at this point.

Plant deeper than the seedling was growing in the cell — bury the stem up to just below the lowest leaves. Tomatoes produce roots along the buried stem, building a more extensive root system that improves drought tolerance and nutrient uptake.

If outdoor transplanting is still weeks away and seedlings have grown very tall, pot up a second time into 1-gallon containers, again planting deeply. Tall seedlings planted to their first leaf set develop the deepest root systems.

How do you harden off tomato seedlings?

Harden off over 7–10 days by placing seedlings outdoors in increasing sun and wind exposure, starting with 1–2 hours of indirect light and building to full days outdoors. Skipping hardening off or rushing it causes sunscald and wind damage that sets plants back 2–3 weeks.

Indoor seedlings have soft cell walls adapted to low light and still air. Direct outdoor exposure ruptures cell walls (sunscald appears as white or bleached patches on leaves) and desiccates foliage faster than roots can replace moisture.

The 10-day hardening schedule:

Skip any day when temperatures drop below 50°F or wind gusts exceed 15 mph. Both conditions set back unhardened seedlings significantly.

How do you transplant tomatoes outside?

Transplant on a cloudy day or in late afternoon, never in midday sun. Plant into a prepared bed with 2–3 inches of compost worked in, space 24–36 inches apart depending on variety, and bury two-thirds of the stem. Water in with half-strength fertilizer solution.

For specific spacing by variety and training method, see how far apart to plant tomatoes. For bed preparation and overall vegetable garden setup, see how to start a vegetable garden.

After transplanting:

• Water every 1–2 days for two weeks until roots establish
• Stake, cage, or install trellising at planting — see tomato trellis options for a comparison of support systems
• Begin deep, infrequent watering once plants show new growth — a sign that roots are establishing

What are common problems when growing tomatoes from seed?

Leggy seedlings from insufficient light, damping off from overwatering, and germination failure from cold soil account for most seed-starting setbacks. All three are preventable.

For problems that appear after transplant, see yellow leaves on tomato plants for causes and solutions to the most common post-transplant issues.

For container growing, see how to grow tomatoes in pots — seed-starting in containers follows the same schedule but uses a self-contained potting mix from the 4-inch stage forward.

For pruning once plants are established, see how to prune tomatoes.