How to Start Seeds Indoors

Why starting seeds indoors matters

Starting seeds indoors extends your growing season by weeks — and in northern zones, it’s the only way to harvest warm-season crops like tomatoes and peppers before fall frost.

In Zone 5, the average frost-free season is 150–160 days. Tomatoes take 70–90 days from transplant to first harvest. If you wait until the last frost date to plant nursery starts, you’re already 3–4 weeks behind the gardeners who started seeds indoors in March.

The economics also make sense. A 70-seed packet of tomatoes costs $3–5. Those seeds can produce 50–60 healthy transplants. The same number of nursery transplants costs $60–120. For gardeners who grow multiple varieties, the savings cover the equipment investment in one season.

This guide covers everything you need to produce strong, healthy transplants at home regardless of your zone or experience level.

Equipment you need

Essential

Cell trays or small pots: 6-cell packs, 72-cell trays, or 4-inch pots all work. Smaller cells dry out faster; larger cells hold more mix and need watering less often. For most vegetables, 72-cell trays are the most efficient.

Seed-starting mix: Not potting soil, not garden soil. Seed-starting mix is fine-textured, fast-draining, and sterile. It typically contains peat or coco coir, perlite, and sometimes vermiculite. Our guide to the best potting soil explains the differences between mix types.

Grow lights: The most important piece of equipment. Most US homes — even with south-facing windows — don’t provide 14–16 hours of direct light in January, February, and March. Seedlings grown in windows without supplemental light become leggy and weak. T5 fluorescent lights or LED grow lights both work. Keep lights 2–4 inches above seedlings.

Trays without holes (for bottom watering): Set trays inside a solid tray to add water below the pots. Bottom watering keeps the seed-starting mix from compacting.

Timer for lights: A simple outlet timer set to 16 hours on, 8 hours off removes one daily task and ensures consistent light duration.

Helpful but not required

Heat mat: Speeds germination for warm-season crops (tomatoes, peppers, eggplant, basil). Most effective when thermostat-controlled to maintain 75–80°F soil temperature. Without a mat, germination may take 10–14 days instead of 5–8 days.

Humidity dome: A clear plastic dome placed over trays until seeds germinate traps moisture and maintains even humidity. Remove immediately after germination to prevent damping off.

Small fan: Running a fan on seedlings for 1–2 hours per day strengthens stems (physical stress triggers thicker stem growth) and prevents the humid conditions that cause damping off.

Labels: Mark every flat at sowing. By week 2, unlabeled tomato and pepper seedlings look identical. A waterproof marker on plastic labels or popsicle sticks is sufficient.

Timing: when to start what

The seed packet says “start 8 weeks before last frost.” But what does that mean for your zone?

How to calculate your start date

1. Find your average last frost date. USDA planting zone maps and your local cooperative extension website list this by zip code.
2. Count back the number of weeks listed on the seed packet.
3. That’s your indoor start date.

Start dates by crop and zone

Zone 3–4 gardeners: Add 2–4 weeks to all timings — your last frost date falls 4–6 weeks later than Zone 5.

Zone 8–9 gardeners: Start warm-season crops in January for spring planting. Some gardeners in Zone 9 start seeds in October–November for winter gardens.

What NOT to start indoors

Some crops don’t transplant well because their taproots are disturbed during transplanting. Start these directly in the garden:

• Carrots, beets, radishes, parsnips (taproots)
• Beans, peas (germinate and grow fast enough; transplanting sets them back)
• Cucumbers, squash, melons (transplant if careful, but many gardeners direct-sow)
• Corn (requires too much root space to transplant well)
• Dill, cilantro (bolt readily after transplanting)

Step-by-step: sowing seeds indoors

Step 1: Prepare containers

Fill trays or pots to within ½ inch of the rim with damp seed-starting mix. Dry seed-starting mix is dusty and hard to wet evenly — pre-wet by adding water to the mix in a bucket and stirring before filling trays. The mix should feel like a wrung-out sponge.

Step 2: Sow seeds

Plant 2–3 seeds per cell or pot at the depth indicated on the seed packet. A general rule: plant to a depth equal to 2× the seed diameter. Tiny seeds (petunias, snapdragons) go on the surface, pressed in lightly. Large seeds (beans, squash) go ½–1 inch deep.

After sowing, press the mix gently around seeds for good soil contact. Don’t tamp hard — you want loose, air-filled mix.

Step 3: Water in and cover

Water lightly by misting the surface, or set trays in 1 inch of water and let them absorb for 20–30 minutes. Cover with a humidity dome or plastic wrap to retain moisture.

Place trays on a heat mat if germinating warm-season crops. Keep at 70–80°F.

Step 4: Watch for germination

Check trays daily. Most vegetable seeds germinate in 5–14 days when soil is at the right temperature:

Remove the humidity dome immediately when the first seedling appears in each tray. Leaving it on after germination creates humid conditions that cause damping off.

Step 5: Move under lights

Move seedlings under grow lights as soon as they emerge. Position lights 2–4 inches above the top of the seedlings and adjust upward as they grow. Lights too far away produce leggy, pale growth.

Run lights 14–16 hours daily on a timer.

Step 6: Thin to one per cell

When seedlings have their first true leaves (the leaves after the rounded seed leaves), thin to one seedling per cell by snipping the weakest seedlings at soil level with scissors. Don’t pull — pulling can disturb the roots of the seedling you’re keeping.

Step 7: Feed at 3–4 weeks

Seed-starting mix contains little to no fertilizer. Start feeding with half-strength liquid fertilizer (fish emulsion, liquid kelp, or balanced 20-20-20) every 7–10 days starting 3–4 weeks after germination. Full-strength fertilizer on young seedlings causes salt burn.

Step 8: Bottom water consistently

After germination, water from below by adding 1 inch of water to the tray under pots and letting it absorb. This encourages roots to grow downward toward moisture.

Let the top inch of mix dry between waterings. Constantly wet mix invites damping off and root rot.

Preventing the most common problems

Damping off

Damping off is a fungal disease that kills seedlings at the soil line — stems pinch and topple overnight. Caused by Pythium, Rhizoctonia, or Fusarium fungi that thrive in cool, wet, stagnant air.

Prevention:

• Use sterile seed-starting mix (never garden soil)
• Sanitize reused trays with diluted bleach
• Water less — let the top inch dry between waterings
• Run a fan for 1–2 hours per day
• Remove humidity domes immediately after germination

Leggy seedlings

Pale, thin, stretching seedlings mean not enough light. The fix is simple: lower the lights or add more.

Prevention: Keep grow lights 2–4 inches above seedling tops and run 14–16 hours daily. A small fan also helps strengthen stems.

Seeds that don’t germinate

Low soil temperature: Most common cause. Soil below 65°F severely reduces germination rates in warm-season crops. Use a heat mat.

Old seed: Test old seed by wrapping 10 seeds in a damp paper towel, placing in a plastic bag, and checking after 7–10 days. If fewer than 7 germinate, the seed is old and you should sow more densely or get fresh seed.

Too deep: Check the packet depth recommendation. Large seeds planted too shallow may also fail.

Hardening off

Hardening off is non-negotiable. Every seedling grown indoors must be gradually exposed to outdoor conditions before transplanting. Skipping this step causes sunscald (white, bleached patches on leaves), windburn, and transplant shock.

The 7-10 day hardening schedule

Bring plants back inside if frost is forecast at any point during hardening off.

After hardening off is complete, your seedlings are ready to transplant to their final location in the garden. For timing and spacing, check our guide on growing kale for cool-season crops or how to grow tomatoes for warm-season crops.

What to do if you fall behind schedule

If you miss your optimal start date:

• 1–2 weeks late: Plant anyway. You’ll get a slightly later harvest, but it’s worth starting.
• 3–4 weeks late (warm-season crops): Buy nursery transplants instead of starting from seed. A 3-week-old nursery transplant of a tomato will be further along than a 3-week-old seedling you start now.
• More than 4 weeks late for tomatoes/peppers in Zone 4–5: Buy transplants. There’s not enough season left to get value from late-started seeds.

For cool-season crops (broccoli, kale, lettuce), you can direct-sow when temperatures allow. They germinate quickly and don’t need the head start that tomatoes do.

Sources: USDA Home Gardening Survey; University of Minnesota Extension Seed Starting Guide; Penn State Extension Starting Plants from Seed.