Starting Seeds Indoors 2026: Complete Guide from Setup to Transplant

Starting Seeds Indoors Saves Money and Extends Your Season by 6-8 Weeks

A packet of 30 tomato seeds costs $3. Thirty tomato transplants at a nursery cost $90-180. Starting seeds indoors is the highest-ROI skill in gardening. It also gives you access to hundreds of varieties you will never find as transplants. This guide covers the exact setup, timeline, and technique.

The Minimal Viable Seed-Starting Setup

You need four things. Everything else is optional.

1. Light (Most Important)

A sunny windowsill is NOT enough. Seedlings need 14-16 hours of bright light, and windowsills provide 4-6 hours of weak, directional light. The result: leggy, stretched, weak seedlings reaching for light.

The $40 setup: Two 4-foot LED shop lights (5000K "daylight" color temperature, 4000+ lumens each) hung from adjustable chains. Hang 2-3 inches above seedlings — basically touching them. Raise the lights as plants grow. The lights must be adjustable — if they are fixed at the top of a shelf, seedlings stretch to reach them.

The upgrade: Full-spectrum LED grow lights ($60-100). These provide the red and blue wavelengths plants need for compact growth. T5 fluorescent shop lights also work but use more electricity and generate heat.

2. Heat Mat

Most seeds germinate best at 70-85°F soil temperature. A heat mat ($15-25) provides consistent bottom heat. Place the thermostat probe in the soil. Set to 70-75°F. REMOVE the heat mat after germination — continued heat makes seedlings leggy. The heat mat is for germination only (days 3-14), not for growing on.

3. Seed-Starting Mix

Do NOT use garden soil (compacts, contains pathogens, poor drainage). Do NOT use potting mix (too coarse, contains fertilizer that burns seedlings). Use SEED-STARTING MIX — a sterile, fine-textured blend of peat moss or coco coir, vermiculite, and perlite. It holds moisture, drains well, and is pathogen-free. Moisten the mix BEFORE filling cells — dry mix repels water. It should feel like a wrung-out sponge.

4. Containers with Drainage

Options: (1) 72-cell plug trays ($3-5 each, reusable for years), (2) soil blocks (compressed cubes of seed-starting mix — no plastic, no transplant shock, requires a soil blocker tool $30), (3) recycled containers with drainage holes punched in the bottom (yogurt cups, egg cartons — the classic frugal option).

Do NOT use: Eggshells (too small, roots become rootbound in days), Jiffy peat pellets (the mesh does not decompose as advertised and restricts roots), or any container without drainage holes.

The Seed-Starting Timeline

| Crop | Weeks Before Last Frost to Start Indoors | Notes | |------|----------------------------------------|-------| | Onions, leeks | 10-12 weeks | Slowest. Start in January/February. | | Peppers, eggplants | 8-10 weeks | Need warmth. Bottom heat essential. | | Tomatoes | 6-8 weeks | Do not start too early — leggy tomatoes are weak. | | Broccoli, cabbage, kale | 6-8 weeks | Cool-season crops. Can go outside earlier. | | Lettuce, Swiss chard | 4-6 weeks | Fast. Start multiple successions. | | Basil | 4-6 weeks | Needs warmth. Do not rush. | | Cucumbers, squash, melons | 3-4 weeks | LARGE seeds. Start in larger pots (3-4 inch). | | Direct-sow only (do not start indoors): | — | Beans, peas, corn, carrots, radishes, beets, spinach, dill, cilantro. These resent transplanting. |

Step-by-Step Seed Starting

Moisten the seed-starting mix in a bucket. It should clump when squeezed but not drip water.
Fill containers loosely — do not pack down. Level the surface.

Plant seeds at the correct depth. The rule: plant at a depth 2-3× the seed's diameter. Tiny seeds (lettuce, basil, petunias): surface-sow — sprinkle on top, press gently into contact with soil, do NOT cover (they need light to germinate). Large seeds: poke a hole with a pencil, drop seed in, cover with mix.

Label everything. Seed variety AND date. Plastic plant tags. Pencil or paint pen (permanent marker fades under grow lights). You will not remember which tray is "Brandywine" and which is "Cherokee Purple" in 6 weeks.

Water from the bottom. Place the tray in a shallow container of water for 15-30 minutes. The soil wicks water up. Remove when the surface is damp. Bottom-watering prevents fungal diseases (damping off) and does not disturb seeds. Do not top-water with a watering can — the stream dislodges seeds.

Cover with a humidity dome (clear plastic lid or plastic wrap with air holes). This maintains humidity for germination. REMOVE the dome as soon as the first seeds sprout — continued high humidity causes damping off (a fungal disease that kills seedlings at the soil line).

Place on heat mat, under lights. Lights on for 14-16 hours/day. A simple outlet timer ($5-8) automates this. Consistent light cycles matter.

Check daily. Once seeds germinate: remove dome, remove heat mat, keep lights 2-3 inches above plants. Water from the bottom when the soil surface looks dry.

Potting Up

When seedlings have 2 sets of true leaves (the second pair of leaves after the seed leaves/cotyledons), they need more space. Transplant into larger containers — 3-4 inch pots or solo cups with drainage holes. Use POTTING MIX (not seed-starting mix) — it contains nutrients for the growing plant. Bury the stem deeper (tomatoes, peppers, eggplants — adventitious roots form on buried stems). This is the second transplant before the final transplant into the garden.

Hardening Off

One week before planting outdoors: begin hardening off. Day 1: 1 hour in dappled shade. Day 2: 2 hours. Day 3: 3 hours including morning sun. Day 4-5: 4-5 hours. Day 6-7: All day in partial sun, brought in at night. Day 8: Overnight if temperatures stay above 50°F. Then transplant to the garden. Hardening off is NOT optional — skipping it causes sunburn, windburn, and transplant shock.

Common Seed-Starting Problems

Leggy seedlings (tall, thin, weak): Insufficient light. Lights are too far away or not bright enough. Fix: lower lights to 2 inches above plants. Run lights 16 hours/day. A small fan on low (oscillating, 2-3 feet away) strengthens stems — the gentle movement simulates wind.

Damping off (seedlings collapse at soil line): Fungal disease from: too much humidity (dome left on too long), poor air circulation, or contaminated soil. Prevention: remove humidity dome immediately after germination, use sterile seed-starting mix, provide airflow (fan), bottom-water. There is no cure for damping off — remove affected seedlings immediately.

Yellow leaves: Overwatering or nutrient deficiency. If soil is wet: water less. If soil is dry and seedlings are 3+ weeks old: they need fertilizer. Apply half-strength liquid fertilizer (fish emulsion or balanced 10-10-10) every 2 weeks starting when true leaves appear.

Mold on soil surface: Overwatering + no airflow. Scrape off the mold. Water less. Increase airflow.

Key Takeaway

The seed-starting essentials are: (1) bright lights 2 inches above plants, (2) heat mat for germination only, (3) sterile seed-starting mix, (4) bottom-watering, (5) remove humidity dome immediately after germination, (6) harden off for 7-10 days before transplanting. A $60 setup (lights + heat mat + trays + mix) produces hundreds of transplants per year. It pays for itself in one season.