Starting seeds indoors gives you a head start on the growing season, a much wider choice of varieties than garden centers offer and the satisfaction of growing plants from the very beginning. It can also save money if you grow many plants.
Seed starting is not complicated, but a few details make the difference between strong, stocky seedlings and weak, leggy ones. This guide walks through the process step by step.
Which Seeds to Start Indoors
Some crops benefit from an indoor start, while others do better sown directly in the garden.
Good Candidates for Starting Indoors
- Tomatoes
- Peppers
- Eggplant
- Broccoli, cabbage, cauliflower and kale
- Many herbs, such as basil and parsley
- Onions and leeks
- Many annual flowers
Usually Better Sown Directly
- Root vegetables such as carrots, radishes and beets, which often dislike transplanting
- Beans and peas, which grow quickly
- Corn
- Squash, cucumbers and melons can be started indoors but grow fast and resent root disturbance, so many gardeners direct sow them or start them only a few weeks early in biodegradable pots
Step 1: Figure Out Timing
Seed packets usually say how many weeks before the last frost to start seeds indoors. Count back from your area’s average last frost date. Your local cooperative extension service publishes regional planting calendars.
Common General Timing
- Onions and leeks: Often started early, around 8 to 12 weeks before the last frost
- Peppers and eggplant: Often around 8 to 10 weeks before
- Tomatoes: Often around 6 to 8 weeks before
- Brassicas: Often around 4 to 6 weeks before transplanting
- Squash and cucumbers: Often only 2 to 4 weeks before
Always follow the specific packet directions.
Starting Too Early
Seedlings started too early can outgrow their containers and become root-bound or leggy before it is warm enough to plant them outside.
Step 2: Gather Supplies
Containers
- Seed trays with cells
- Small pots
- Soil blocks
- Recycled containers such as yogurt cups, cleaned and with drainage holes poked in the bottom
All containers need drainage. Clean reused containers with soapy water before use.
Seed-Starting Mix
Use a fine, sterile seed-starting mix rather than garden soil. Garden soil can compact in containers and may contain disease organisms. Seed-starting mixes are light and drain well.
Light
A sunny windowsill is often not enough, especially in late winter. Seedlings that do not get enough light become tall, pale and floppy. Many gardeners use simple LED or fluorescent shop lights or grow lights hung a few inches above the seedlings.
Other Helpful Supplies
- Labels and a waterproof marker
- A spray bottle or watering can with a gentle rose
- Humidity domes or plastic wrap
- A seedling heat mat for warmth-loving crops
- A small fan for air movement
- A timer for lights
Step 3: Sow the Seeds
- Moisten the mix before filling containers. It should be damp, not dripping.
- Fill containers and gently firm the surface.
- Sow seeds at the depth listed on the packet. A common rule of thumb is about two to three times the seed’s width. Some seeds need light to germinate and should be pressed onto the surface; the packet will say.
- Sow two or three seeds per cell if germination may be uneven, then thin later.
- Label everything with the variety and date.
- Cover with a humidity dome or plastic wrap to keep moisture in until seeds sprout.
Step 4: Provide Warmth for Germination
Many seeds germinate best in warm soil. Tomatoes and peppers especially benefit from a heat mat. Lettuce and some other cool-season crops germinate at cooler temperatures. Check the packet.
Remove humidity domes as soon as seedlings emerge to reduce the risk of disease.
Step 5: Give Seedlings Plenty of Light
Once seedlings appear, they need strong light right away.
- Keep lights a few inches above the seedlings and raise them as plants grow, following the light manufacturer’s guidance.
- Many gardeners run lights for about 14 to 16 hours a day using a timer.
- If using a window, rotate trays daily so seedlings do not lean.
Step 6: Water Carefully
- Keep the mix evenly moist, not soggy.
- Watering from below, by placing trays in a shallow pan of water for a short time, helps avoid disturbing seeds and keeps leaves dry.
- Empty excess water so containers do not sit in it.
Preventing Damping Off
Damping off is a fungal problem that causes seedlings to suddenly collapse at the soil line. To reduce the risk:
- Use clean containers and fresh seed-starting mix
- Avoid overwatering
- Remove humidity domes after germination
- Provide air movement with a small fan on a low setting
Step 7: Thin the Seedlings
When seedlings develop their first true leaves (the leaves that appear after the initial seed leaves), thin to one plant per cell. Snip extra seedlings at the soil line with scissors rather than pulling, which can damage roots of the keeper.
Step 8: Feed and Pot Up
Seed-starting mixes contain few nutrients. Once seedlings have true leaves, many gardeners begin feeding with a diluted fertilizer, following label directions.
If seedlings outgrow their cells before it is time to plant outdoors, move them to larger pots. Tomatoes can be planted deeper when potting up, burying part of the stem.
Step 9: Harden Off
Seedlings grown indoors are not used to sun, wind and temperature swings. Hardening off gradually prepares them:
- Start with an hour or two outdoors in a sheltered, shady spot.
- Gradually increase time outdoors and sun exposure over about a week to ten days.
- Bring plants in if frost or very cold weather is expected.
- Water as needed, since outdoor conditions dry containers quickly.
Step 10: Transplant
Transplant on a cloudy day or in the late afternoon to reduce stress. Water well before and after planting. For warm-season crops, wait until after the last frost and until soil has warmed.
See our raised bed plans for layout ideas and tomato growing mistakes to give your tomatoes a strong start.
Troubleshooting
| Problem | Likely cause | Fix |
|---|---|---|
| Tall, leggy seedlings | Not enough light | Lower lights or add stronger light |
| Seeds did not sprout | Too cold, too dry, too deep or old seeds | Check temperature, moisture and packet depth |
| Seedlings collapse at soil line | Damping off | Reduce watering, improve airflow, use clean supplies |
| Yellowing leaves | Lack of nutrients or overwatering | Feed lightly, adjust watering |
| White fuzz on soil | Too wet, poor airflow | Let surface dry slightly, add a fan |
Frequently Asked Questions
Can I start seeds on a windowsill?
You can, but seedlings may get leggy without enough light. A south-facing window in a sunny climate works better than most. Supplemental lights help a lot.
Do I need a heat mat?
Not always. Heat mats help warmth-loving crops germinate faster. Cool-season crops usually do not need them.
How long are seeds viable?
It varies by crop and storage conditions. Store seeds in a cool, dry place. Do a germination test with a few seeds on a damp paper towel if you are unsure.
Can I reuse seed-starting mix?
It is best to use fresh mix to reduce disease risk. Old mix can be added to the compost pile.
When should I move seedlings outdoors?
After hardening off, and after your last frost date for warm-season crops. Cool-season crops can often go out a few weeks earlier.
The Bottom Line
Starting seeds indoors is easy with the right timing, a clean seed-starting mix, strong light, careful watering and gradual hardening off. Start with a few easy crops like tomatoes, basil and lettuce, take notes on what works and expand next year.
Editorial note: Backyard Bloomer is independent and reader-supported. This guide is based on research into manufacturer documentation, published specifications and widely accepted best practices; we have not been paid to mention any product. If we add affiliate links in the future, they will be clearly disclosed per our Affiliate Disclosure.