How to Start Seeds Indoors
Starting seeds indoors extends your growing season by weeks and gives you access to hundreds of varieties that nurseries never stock. It's also significantly cheaper than buying transplants — a single seed packet often costs less than one nursery six-pack.
Here's everything you need to know, from timing and supplies to hardening off and transplanting.
When to Start
Count backward from your last spring frost date. Most warm-season crops (tomatoes, peppers, eggplant) start 6–8 weeks before transplant date. Cool-season crops (broccoli, cabbage, kale) start 4–6 weeks before. Seed packets list specific timing — follow those recommendations over generic advice.
Supplies You Need
- Seed starting mix: Not potting soil — seed starting mix is finer, sterile, and drains well. Brands like Espoma, FoxFarm, and Pro-Mix make reliable options.
- Cell trays or peat pots: 72-cell trays work for most seeds. Peat pots let you transplant without disturbing roots.
- Light source: A south-facing window works for some crops, but most seedlings need 14–16 hours of light. A basic LED shop light hung 2–4 inches above the seedlings outperforms any window. Full-spectrum T5 or T8 LEDs are the standard.
- Heat mat (optional but helpful): Warm-season seeds (tomatoes, peppers) germinate faster at 70–80°F soil temperature. A seedling heat mat placed under trays speeds germination by days.
- Spray bottle or bottom-watering tray: Seedlings need consistent moisture without being blasted by a watering can.
Step-by-Step Process
- Fill trays with moistened seed starting mix. Press gently to eliminate air pockets but don't compact it.
- Plant seeds at the depth specified on the packet — usually 2–3 times the seed's diameter. Tiny seeds (lettuce, basil) go on the surface with a light dusting of mix.
- Cover with humidity dome or plastic wrap until seeds germinate. Remove immediately once you see green sprouts.
- Provide light for 14–16 hours daily. Keep lights 2–4 inches above the seedlings and raise them as plants grow.
- Water from the bottom when the surface feels dry. Set trays in a shallow dish of water for 10–15 minutes and let the mix wick moisture upward.
- Fertilize lightly after the first true leaves appear (not the initial seed leaves). Use a diluted liquid fertilizer at one-quarter strength.
- Thin to one plant per cell if multiple seeds germinated. Snip extras at the soil line with scissors rather than pulling (which disturbs roots of the keeper).
Hardening Off and Transplanting
Seedlings grown indoors need a gradual transition to outdoor conditions. This process — called hardening off — takes about 7–10 days.
Start by placing seedlings outdoors in a sheltered, shaded spot for 2–3 hours. Each day, increase the time and sun exposure. By day 7–10, they should be spending full days outdoors, including some direct sun. Bring them in if nighttime temperatures drop below 50°F for warm-season crops.
Transplant on a cloudy day or in the late afternoon to reduce transplant shock. Water deeply after planting and consider a light shade cover for the first 2–3 days if the weather turns hot and sunny.
Common Seed Starting Mistakes
Most seed starting failures trace back to a handful of avoidable errors. Here's what to watch for.
Planting Too Deep
Small seeds like lettuce, basil, and celery need light to germinate and should be pressed onto the surface with only the thinnest dusting of mix. Burying them half an inch deep is a death sentence. As a general rule, plant seeds at a depth of two to three times their diameter — for tiny seeds, that means essentially surface sowing.
Overwatering Seedlings
The most common killer of indoor seedlings is damping off — a fungal disease that causes young stems to rot at the soil line. It thrives in wet, stagnant conditions. Bottom-watering (setting trays in a shallow dish of water for 10–15 minutes) delivers moisture without saturating the surface. Ensure trays have drainage holes and never let seedlings sit in standing water.
Insufficient Light
Leggy, stretched-out seedlings with pale stems are light-starved. A south-facing window looks bright to your eyes but often delivers only 4–6 hours of usable light. Seedlings need 14–16 hours. A $25 LED shop light hung 2–4 inches above the foliage solves this completely. Raise the light as seedlings grow to maintain the 2–4 inch gap.
Skipping Hardening Off
Transplanting indoor seedlings directly into the garden is like walking from a climate-controlled office into a blizzard. The transition shock can stunt or kill plants that were growing perfectly indoors. The 7–10 day hardening-off process — gradually increasing outdoor exposure — allows cell walls to thicken, stomata to adjust to wind, and leaves to acclimate to UV intensity. Skipping this step wastes weeks of indoor growing effort.
Starting Too Many Varieties
It's tempting to order twenty seed varieties from a catalog and start them all at once. The problem is that each crop has different germination temperatures, light needs, and timing requirements. Managing five varieties under one light is straightforward. Managing fifteen with staggered start dates, different moisture needs, and varying light heights becomes a part-time job. Start with three to five varieties your first year and add more as your setup and confidence grow.
Frequently Asked Questions
Do I need a grow light to start seeds indoors?
For most crops, yes. A south-facing window provides some light, but seedlings typically become leggy and weak without supplemental lighting. A basic LED shop light for $20–$30 solves this problem completely.
What seeds are easiest to start indoors?
Tomatoes, peppers, basil, and marigolds germinate reliably and tolerate the indoor-to-outdoor transition well. Start with these before attempting more finicky crops like celery or artichokes.
Final Thoughts
Starting seeds indoors is a skill that pays dividends every season — earlier harvests, more variety selection, and significant cost savings over nursery transplants. The setup is simple and inexpensive, and once you've done it once, the process becomes second nature.