Indoor Seed Starting: Complete Beginner Guide

Why Start Seeds Indoors

Starting seeds indoors gives you a 6–8 week head start on the growing season. You get stronger plants, more variety than nursery transplants, and significant cost savings — a $3 packet of seeds produces 30+ plants versus $4 per transplant at the garden center.

Timing: When to Start

Count backward from your last frost date. Most vegetables need 6–8 weeks indoors before transplanting. If your last frost is May 15, start tomatoes on March 20, peppers on March 6, and broccoli on March 6. Herbs like basil need only 4–5 weeks.

Containers That Work

Seed trays with cells: The standard option. Cheap, reusable, and the right size for most seeds.

Soil blocks: Compressed soil cubes that eliminate transplant shock. No plastic waste. Require a soil blocker tool ($20–$40).

Recycled containers: Yogurt cups with drainage holes punched in the bottom work fine. Just ensure they are at least 2 inches deep.

Soil Mix

Use a sterile seed-starting mix, not garden soil or potting mix. Garden soil carries disease organisms and is too heavy. A good seed mix is 50% peat or coco coir, 25% vermiculite, and 25% perlite. Moisten the mix before filling containers.

Sowing and Germination

Plant seeds at a depth of 2–3 times their width. Tiny seeds (lettuce, petunia) need light — press them onto the surface without covering. Larger seeds (beans, squash) go 1 inch deep. Cover with a humidity dome or plastic wrap until germination. Remove the cover immediately when seedlings emerge to prevent damping-off disease.

Light Requirements

Seedlings need 14–16 hours of bright light daily. A south-facing window is rarely enough. Use LED grow lights positioned 2–4 inches above seedlings, raising the lights as plants grow. Fluorescent shop lights work as a budget alternative.

Hardening Off

Never move indoor seedlings directly outside. Over 7–10 days, gradually expose them to outdoor conditions: Day 1–2 in shade for 2 hours, Day 3–4 in morning sun for 4 hours, Day 5–6 in full sun all day, Day 7–10 leave out overnight. Transplant on Day 11.