Starting Seeds Indoors: A Beginner's Guide
The Short Answer
Starting seeds indoors gives heat-loving plants like tomatoes and peppers a head start before outdoor conditions are warm enough. You'll need seed starting mix (not garden soil), containers with drainage, light (a sunny south-facing window or grow lights), and a schedule based on your last frost date. Start most warm-season seeds 6-8 weeks before your last frost.
Why Start Indoors?
Some plants need a longer growing season than your outdoor weather allows. Tomatoes, for example, need 60-85 days from transplant to harvest, plus 6-8 weeks to grow from seed to transplant size. In zones with short growing seasons, that math only works if you start seeds indoors while it's still winter outside.
Indoor starting also gives you more variety selection. Nurseries carry a handful of tomato varieties as transplants. Seed catalogs offer hundreds. If you want to grow a specific heirloom or specialty variety, starting from seed is often the only option.
What You Need
Seed starting mix is lightweight and sterile — designed for germination. Regular garden soil or potting mix is too heavy and can harbor disease organisms that kill tiny seedlings (a condition called "damping off"). Seed starting mix is worth the small investment.
Containers can be anything with drainage holes: cell trays, peat pots, recycled yogurt cups with holes poked in the bottom, even egg cartons. Drainage is non-negotiable — seeds sitting in standing water will rot.
Light is where most beginners struggle. A south-facing window provides some light, but seedlings often get leggy (tall, thin, and weak) because window light isn't intense enough. Inexpensive shop lights with daylight-spectrum LED bulbs positioned 2-3 inches above seedlings produce much stockier, healthier plants. Run lights 14-16 hours per day.
Warmth matters for germination. Most vegetable seeds germinate best at 65-75°F soil temperature. A heat mat under your seed trays can speed germination significantly, especially for heat-lovers like peppers and eggplant.
The Process
Fill containers with moistened seed starting mix — it should feel like a wrung-out sponge. Plant seeds at the depth indicated on the packet (usually 2-3 times the seed's diameter). Cover loosely with plastic wrap or a humidity dome to retain moisture until seeds germinate.
Once sprouts appear, remove the cover and get light on them immediately. Keep the soil consistently moist but not waterlogged. Water from the bottom by setting containers in a tray of water and letting the mix wick it up — this prevents disturbing tiny seedlings and reduces disease risk.
When seedlings develop their second set of "true leaves" (the ones that look like the actual plant, not the first rounded seed leaves), they're ready to be transplanted into slightly larger containers if needed.
Hardening Off
This is the step people skip — and it's the one that kills their plants. Seedlings grown indoors have never experienced wind, direct sun, temperature fluctuations, or the real world. If you move them directly from your kitchen to the garden, they'll go into shock.
Hardening off means gradually introducing seedlings to outdoor conditions over 7-10 days. Start with an hour of outdoor time in a sheltered, shaded spot. Increase by an hour each day, gradually introducing more sun and wind. By the end of the period, seedlings should be spending full days outside and tolerating direct sun.
Then transplant on a calm, overcast day if possible. Water deeply after transplanting.
When to Start
The timing is based on your frost date. Use our Planting Date Finder to get exact indoor seed starting dates for any specific plant in your zip code. As a general guide, most warm-season vegetables are started 6-8 weeks before the last frost date. Starting too early is actually worse than starting too late — overgrown, root-bound seedlings perform poorly after transplanting.