Seed starting indoors — complete beginner guide that works

Seed starting indoors — complete beginner guide that works

Buying a tomato transplant at a garden center costs $5-8. Starting one from a $3 seed packet gives you 30 plants, in any variety you want, with healthier roots if you do it right. If you want an indoor harvest in days rather than a transplant in weeks, the fastest payoff is microgreens grown indoors — same trays and grow light, harvest in 7 to 14 days. This guide is the start-to-finish playbook for full transplants: what to start indoors versus direct-sow outside, the equipment that actually matters, the 5-step germination method, how to harden seedlings off, and the mistakes that kill more first-year batches than any pest.

Track every variety: Add your seeds to Growli and the app sets reminders tied to your last-frost date — when to sow, when to pot up, when to harden off, when to transplant.

What to start indoors (and what NOT to)

Not every seed benefits from an indoor head start. Some plants — root crops, large-seeded annuals, and most cool-season greens — actively resent transplanting. Use this list as your default:

Start indoors (6-12 weeks before last frost):

• Tomatoes, peppers, eggplant, tomatillos — heat-lovers with long days-to-maturity. Start 6-8 weeks early.
• Basil, parsley, oregano, thyme, chives — herbs that benefit from a head start and steady warmth.
• Cabbage, broccoli, cauliflower, kale, Brussels sprouts — start 4-6 weeks early for spring planting.
• Lettuce (head varieties), leeks, onions, celery — 8-10 weeks early.
• Petunias, snapdragons, marigolds, zinnias, coleus, impatiens — flowers with slow seedling stages.

Direct-sow outside (do not start indoors):

• Carrots, radishes, beets, parsnips — taproots that fork or stunt if transplanted.
• Beans, peas, corn — germinate fast and resent root disturbance.
• Cucumbers, squash, melons, pumpkins — grow so fast they catch up if direct-sown; if you must start indoors, use peat or paper pots that go directly in the ground.
• Spinach, arugula, cilantro, dill — bolt under indoor heat; sow outdoors in cool soil.
• Sunflowers, nasturtiums, sweet peas, poppies — large seeds that prefer cool ground.

A useful rule: anything with a small, slow seedling and a long maturity benefits from indoor starting; anything with a fast-growing taproot or large seed prefers direct sowing.

When to start seeds indoors

The whole calendar pivots on your last spring frost date. Most reliable seed-starting guides — and Growli's own reminders — count backward from that date.

By US hardiness zone (typical last-frost windows):

UK gardeners: Last frost is mid- to late-May for most of England and Wales, late May to mid-June for Scotland and high-elevation areas. Start tomatoes and peppers indoors mid-March for the south, early April for the north. UK growers without a heated propagator can wait until late March even in mild regions.

For an exact date by US zip code or UK postcode, see the zone reference and the seed-starting calendar.

The classic mistake: Starting too early because the seed packet says "8-10 weeks before last frost" and you want a head start. A tomato seedling held indoors for 12 weeks gets leggy, root-bound, and transplants poorly. 6-8 weeks is the sweet spot for tomatoes, peppers, and eggplant. Bigger is not better when light is the limiting factor.

Equipment you actually need

You can spend $500 on a heated grow tent or $40 on a basic kit. Both work. Here's the minimum that produces strong seedlings in a typical home:

Essential (about $40-80 total):

1. Seed trays with cells and a tray below. 72-cell or 50-cell plastic trays are the standard. Reusable trays last 3-5 seasons. Soil block makers are an upgrade for experienced growers.
2. Humidity dome (clear plastic lid). Keeps surface moisture in until germination. Remove the dome as soon as the seedlings emerge to prevent damping off.
3. Seed-starting mix. Fine, light, sterile, peat-free if possible. Do not use garden soil or general potting compost — too coarse, often contaminated, and holds too much water.
4. A grow light. A basic LED panel ($20-40) clipped 5-10 cm above the seedlings, on a timer for 14-16 hours a day. A south-facing window alone is usually not enough — seedlings stretch toward the light and become leggy. The single biggest difference between strong and weak seedlings is light intensity.
5. A watering can with a fine rose or a spray bottle. Heavy watering displaces tiny seeds.

Useful upgrades (about $20-50):

• Heat mat — maintains 21-24°C (70-75°F) under the tray, which speeds germination 2-3x for peppers, eggplant, and tomatoes. Worth it for warm-season crops.
• Small fan — gentle air movement strengthens stems and prevents damping off. Set on the lowest setting, pointed across (not at) the tray, for a few hours a day.
• Soil thermometer — confirms the heat mat is in the right range. Cheap and useful.

Not needed for most home gardeners:

• Grow tent, full T5 light rig, automated watering — overkill for under 200 seedlings.
• Specialty heated propagators with built-in lights — convenient but expensive; a basic tray plus a heat mat plus a separate light costs half as much.

The 5-step seed-starting method

This is the method that produces strong transplant-ready seedlings in 4-8 weeks for most warm-season crops.

Step 1 — Sow

1. Fill cell trays with damp seed-starting mix. The mix should be damp like a wrung-out sponge — squeeze a handful and a few drops should release. Bone-dry mix repels water; soaked mix kills seedlings.
2. Use a finger or pencil to make a small dimple in each cell at the depth the seed packet specifies (usually 2-3x the seed's diameter).
3. Drop 2-3 seeds per cell. You'll thin to the strongest later. Larger seeds (squash, sunflowers) — one per cell.
4. Cover lightly with more mix and press gently. Some seeds (lettuce, snapdragon, petunia) need light to germinate — read the packet and leave those on the surface.
5. Mist the surface, set the humidity dome on top, label every cell, and place on the heat mat.

Step 2 — Germinate

Most seeds germinate in 5-14 days at the right soil temperature. Keep the dome on, keep the surface damp (mist, don't water), and resist the urge to peek every hour.

The moment you see green emerging through the soil, two things happen:

• Remove the dome. Permanent humidity now causes damping off.
• Turn the grow light on. New seedlings need light immediately; even a 24-hour delay produces leggy stems.

Step 3 — Light, light, light

The hours of light in the first three weeks define whether your seedlings are sturdy or spindly.

• Grow light intensity: 5-10 cm above the leaves. Closer than you think. Raise it as the plants grow.
• Hours per day: 14-16, on a cheap timer. Skip the "sun + grow light" combo — be consistent.
• Window-only gardeners: if you have no grow light, choose the brightest south-facing window you have and rotate the tray 90 degrees every day. Expect leggier seedlings.

Leggy means the stem stretches and falls over. Once it happens, plant a leggy tomato deep at transplant time and it recovers. A leggy pepper or basil does not recover — keep the light close.

Step 4 — Water and thin

• Water from the bottom. Pour water into the tray, let cells wick it up for 15-20 minutes, then drain any excess. Bottom watering keeps surfaces dry (no damping off, no fungus gnats) and grows deeper roots.
• Frequency: every 2-4 days, depending on heat and airflow. Lift the tray — light = needs water, heavy = wait.
• Thin to the strongest seedling once each cell has 2 true leaves. Snip the extras at the soil line with small scissors. Do not pull — pulling disturbs the root of the survivor.

Step 5 — Pot up (optional, for slow-growing crops)

Peppers, eggplant, and tomatoes started 8 weeks before last frost will outgrow a 72-cell tray. Pot them up into 9 cm (4-inch) pots about 4 weeks after germination, when they have 3-4 true leaves. Use a slightly richer mix (seed-starting mix with 25% potting compost). Bury tomatoes deeper at this stage — they root from the buried stem.

Hardening off (transition to outdoors)

This is the step that ruins more home gardeners than any other. Indoor seedlings have soft tissue, untrained stems, and zero UV tolerance. Planted straight into the garden, they wilt, sunburn, and either die or stall for 2-3 weeks.

The fix is a 7-10 day harden-off routine. Start about a week before your planned transplant date, and only on days warmer than 10°C (50°F):

• Day 1: Set the tray outside in dappled shade for 1 hour. Bring it in.
• Day 2: 2 hours in dappled shade.
• Day 3: 3 hours, with the last hour in direct morning sun.
• Day 4-5: 4-5 hours, mostly in direct sun.
• Day 6-7: Full day outside in a sheltered spot.
• Day 8-10: Leave the tray out overnight if no frost is forecast.
• Day 10+: Transplant.

If a frost is forecast, bring trays inside or cover with a frost cloth. The harden-off process kills more first-year seedlings than any other indoor failure mode.

Common mistakes (and the fix)

1. Starting too early. Seed packets are written for the latest-zone gardener; back the date off and aim for 6-8 weeks before last frost for most warm-season crops. An over-grown leggy seedling transplants worse than a younger compact one.
2. Not enough light. "Sunny window" usually means 2-3 hours of direct light, far below what seedlings need. Add a grow light — it's the single biggest upgrade.
3. Damping off. Seedlings flop over with a pinched, brown stem at the soil line. Caused by overwatering plus poor airflow. Prevent with bottom watering, a fan on low, and dome removal at germination.
4. Forgetting to thin. Leaving 3 seedlings in a cell means 3 weak plants instead of 1 strong one. Snip extras at the soil line.
5. Skipping the harden-off. Tender indoor seedlings sunburn and shock when planted out cold. Run the 7-10 day transition every time.
6. Using garden soil or generic potting mix. Too coarse, often contaminated, holds too much water. Use a sterile seed-starting mix.
7. Soggy soil. Seedlings drown before they germinate. The mix should be damp-not-wet at sowing, then bottom-watered every few days.
8. Skipping the label. You will not remember which cell is the cherry tomato and which is the beefsteak by week 4. Label every row at sowing.

What good seedlings look like at transplant

Strong, transplant-ready seedlings share five visible traits:

• Short and stocky — the stem is thicker than a pencil lead, with internodes (gap between leaves) tight, not stretched.
• Dark green leaves — pale or yellow leaves mean too little light or too few nutrients (start a half-strength balanced fertilizer at the 2-true-leaf stage).
• 4-6 true leaves — at least two pairs above the seed leaves before transplant.
• Roots filling the cell but not circling — slide one out; you should see a healthy root mat, not a tight spiral root-bound plug.
• Hardened off — 7-10 days of progressive outdoor exposure, no longer flopping in the breeze.

If your seedlings hit those five marks, your transplant survival rate will be over 95%.

Action plan — the next 8 weeks

• Week 0 (today): Look up your last frost date for your zip code or postcode. Count back the right number of weeks for each crop. Order seeds.
• Week 1: Buy seed-starting mix, trays, a grow light, and a heat mat. Set up the station near a window or in a basement room.
• Week 2-3: Sow tomato, pepper, and eggplant seeds first (the longest lead time). Add lettuce, brassicas, and herbs over the following 2 weeks.
• Week 4-5: Thin to the strongest seedling per cell. Begin half-strength fertilizer at the 2-true-leaf stage.
• Week 6-7: Pot up the largest seedlings into 9 cm pots if they're outgrowing the trays.
• Week 7-8: Begin the 7-10 day harden-off. Check the weather forecast daily.
• Last frost date: Transplant after the last expected frost. Water in. Watch for the first 3-5 days; any wilting calls for shade cloth.

Related articles

• How to grow tomatoes — complete guide — what to do after transplant
• When to plant tomatoes by zone — exact dates for your area
• How to grow basil from seed to harvest — the easiest indoor-start herb
• How to start a vegetable garden — broader first-year playbook
• How to grow peppers — complete guide — peppers need extra warmth
• Easiest vegetables to grow from seed — start here in year one
• Hardiness zone reference — find your zone for the right timing
• Seed-starting and planting calendar — month-by-month tasks for your area

Reviewed and updated by the Growli editorial team. For questions about anything here, open Growli and ask — or email hello@getgrowli.app.