Start seeds indoors by planting in seed starting mix at the correct depth (usually 2x seed diameter), providing 14-16 hours of light, maintaining consistent moisture without overwatering, and transplanting after hardening off. Most vegetables should be started 6-8 weeks before your last frost date, but timing varies by crop.
Why Start Seeds Indoors?
Advantages
| Benefit | Why It Matters |
|---|---|
| Longer season | Start before outdoor soil warms |
| More varieties | Not limited to nursery selection |
| Cost savings | Seeds cost 1/10 of transplants |
| Control | Optimal conditions from day one |
| Timing | Plants ready when you need them |
What to Start Indoors
| Start Indoors | Direct Sow |
|---|---|
| Tomatoes | Beans |
| Peppers | Carrots |
| Eggplant | Radishes |
| Broccoli | Peas |
| Cabbage | Corn |
| Lettuce | Squash (can go either way) |
| Herbs (most) | Beets |
| Flowers (many) | Turnips |
Supplies Needed
Essential
| Item | Purpose |
|---|---|
| Seeds | Choose what you want to grow |
| Containers | Cell trays, pots, or recycled containers |
| Seed starting mix | Light, sterile growing medium |
| Light source | Grow light or very sunny window |
| Water source | For consistent moisture |
Helpful But Optional
| Item | Purpose |
|---|---|
| Heat mat | Speeds germination for warm-season crops |
| Humidity dome | Maintains moisture during germination |
| Thermometer | Monitor temperature |
| Labels | Track varieties |
| Fan | Strengthen seedlings |
Step-by-Step Process
Step 1: Calculate Your Start Date
Work backward from last frost:
- Find your average last frost date
- Look up "weeks to transplant" for each crop
- Count backward to find start date
| Crop | Weeks Before | Start Date |
|---|---|---|
| Tomatoes | 6-8 | March 20 - April 3 |
| Peppers | 8-10 | March 6 - March 20 |
| Broccoli | 4-6 | April 3 - April 17 |
| Lettuce | 4-6 | April 3 - April 17 |
Step 2: Prepare Containers
- Use clean containers with drainage holes
- Fill with seed starting mix (not garden soil)
- Moisten mix before planting
- Leave 1/4" from top
- Cell trays (most common)
- Peat pots
- Recycled containers (yogurt cups, egg cartons)
- Soil blocks
Step 3: Plant Seeds
Planting depth rule: 2x the seed diameter- Tiny seeds (lettuce): surface or barely covered
- Small seeds (tomato): 1/4" deep
- Medium seeds (pepper): 1/4-1/2" deep
- Large seeds (squash): 1" deep
- Plant 2-3 seeds per cell
- Thin to strongest after germination
- Better to thin than have empty cells
Step 4: Water and Cover
- Water gently after planting
- Cover with humidity dome or plastic wrap
- Place in warm location (65-75°F for most)
- Check daily for moisture and germination
Step 5: Provide Light
After germination:- Remove dome/cover
- Move to strong light immediately
- 14-16 hours of light per day
| Light Type | Distance |
|---|---|
| Fluorescent T5 | 2-4" from seedlings |
| LED panels | 4-12" (varies by intensity) |
| Shop lights | 2-3" |
| South window | As close as possible |
Step 6: Water Carefully
Key principles:- Consistent moisture, not soggy
- Water when top is dry to touch
- Bottom watering prevents damping off
- Don't let seedlings wilt
- Set tray in shallow water
- Let soil absorb from below
- Remove after 15-20 minutes
- Drain excess
Step 7: Thin Seedlings
When seedlings have first true leaves:
- Keep strongest seedling per cell
- Cut others at soil line (don't pull)
- One plant per cell ensures good growth
Step 8: Pot Up If Needed
When roots reach bottom or growth slows:
- Move to larger container
- Handle by leaves, not stems
- Use pot extensions to add depth without transplanting
Step 9: Harden Off
7-10 days before outdoor transplant:
- Gradually introduce outdoor conditions
- Start with 1-2 hours in shade
- Increase sun and time daily
- See Hardening Off Guide
Step 10: Transplant
When weather is right and seedlings are hardened:
- Plant at appropriate depth (deeper for tomatoes)
- Water thoroughly
- Protect from sun/wind for first days
Common Seed Starting Problems
Leggy Seedlings
Cause: Not enough light Fix: Lower lights, increase hours Prevention: Lights 2-4" from seedlings, 14-16 hours/dayDamping Off
Cause: Fungus in wet, stagnant conditions Symptoms: Seedlings suddenly collapse at soil line Prevention:- Sterile seed starting mix
- Good air circulation
- Don't overwater
- Bottom water instead of top
Seeds Not Germinating
Causes:- Too old (check expiration)
- Wrong temperature
- Too wet or too dry
- Planted too deep
Yellow Seedlings
Causes:- Overwatering
- Nutrient deficiency
- Not enough light
- Let soil dry slightly between waterings
- Begin dilute fertilizer after true leaves
- Improve light
Seed Starting Schedule
When to Start (Weeks Before Last Frost)
| Crop | Weeks |
|---|---|
| Onions | 10-12 |
| Peppers | 8-10 |
| Eggplant | 8-10 |
| Tomatoes | 6-8 |
| Broccoli/Cabbage | 4-6 |
| Lettuce | 4-6 |
| Basil | 4-6 |
| Cucumbers | 3-4 |
| Squash | 3-4 |
| Melons | 3-4 |
Germination Temperature
| Temperature | Crops |
|---|---|
| 60-65°F | Lettuce, spinach, onions |
| 65-75°F | Tomatoes, peppers (will germinate) |
| 75-85°F | Peppers, eggplant (optimal) |
| 80-90°F | Basil, melons, squash |
FAQ
How deep do you plant seeds indoors?
General rule: 2x the seed diameter. Tiny seeds like lettuce go on the surface, medium seeds like tomatoes go 1/4" deep, large seeds like squash go 1" deep.
How long does it take for seeds to germinate indoors?
Varies by crop: lettuce (2-7 days), tomatoes (5-10 days), peppers (7-14 days), parsley (14-28 days). Warmth speeds germination for most crops.
When should I start seeds indoors?
Count backward from your last frost date using the "weeks before transplant" for each crop. Most vegetables are 4-10 weeks before last frost.
Do seeds need light to germinate?
Most vegetable seeds germinate in darkness. Some flowers need light. After germination, all seedlings need strong light immediately.
What's the best soil for starting seeds?
Seed starting mix—a light, sterile, soilless mix. Don't use garden soil (too heavy, contains pathogens).
Can you start seeds without grow lights?
Possible with a very sunny south-facing window, but results are usually inferior. Grow lights produce stronger, stockier seedlings.
The Bottom Line
Successful seed starting requires:
- Timing — Count back from last frost
- Light — 14-16 hours, close to seedlings
- Water — Consistent moisture, not soggy
- Temperature — Match crop requirements
- Hardening — Gradual transition to outdoors
Start simple with a few varieties. Track what works and refine your process each season.
Related: When to Transplant Seedlings | How to Fix Leggy Seedlings