Seed Starting Guide for Beginners: When, How & What to Start Indoors
Starting seeds indoors is the single biggest money-saver in gardening. A packet of 50 tomato seeds costs $3. A single tomato transplant at the garden center costs $5. Do the math.
But seed starting has a reputation for being fussy. It doesn’t have to be. Here’s everything you need to know to start seeds indoors successfully — even if you’ve never done it before.
Why Start Seeds Indoors?
- Save money — seeds cost pennies per plant vs. $3-8 per transplant
- More variety — garden centers carry 10 tomato varieties. Seed catalogs carry 200.
- Better timing — you control exactly when plants are ready for your garden
- Head start — warm-season crops like tomatoes need 6-8 weeks indoors before they can go outside
What to Start Indoors vs. Direct Sow
Not everything should be started indoors. Some plants hate being transplanted.
Start Indoors (they need the head start)
- Tomatoes (6-8 weeks before last frost)
- Peppers (8-10 weeks before last frost)
- Eggplant (8-10 weeks before last frost)
- Broccoli, cauliflower, cabbage (6-8 weeks)
- Parsley (10 weeks — very slow to germinate)
- Basil (6 weeks)
- Flowers: petunias, snapdragons, impatiens
Direct Sow Only (they resent transplanting)
- Beans — root disturbance stunts them
- Carrots — taproots can’t handle it
- Radishes — too fast to bother (22 days to harvest)
- Corn — direct sow or it sulks
- Peas — direct sow early, they love cold soil
- Dill — bolts if transplanted
Either Way Works
- Lettuce, spinach, kale — easy either way
- Cucumbers — can transplant carefully, but direct sow is fine
- Squash, zucchini — handle transplanting if you’re gentle with roots
Timing: When to Start Seeds
Count backward from your last frost date. That’s it. No complicated math needed.
| Crop | Weeks Before Last Frost |
|---|---|
| Peppers, eggplant | 8-10 weeks |
| Tomatoes | 6-8 weeks |
| Broccoli, cabbage, cauliflower | 6-8 weeks |
| Basil | 6 weeks |
| Lettuce, kale | 4-6 weeks |
| Cucumbers, squash | 3-4 weeks |
Find your last frost date: Planting Calendar by Zone
Example Timeline for Zone 6 (Last Frost: April 1)
| Date | What to Start |
|---|---|
| Jan 20 | Peppers, eggplant, parsley |
| Feb 1 | Tomatoes |
| Feb 15 | Broccoli, cauliflower, cabbage |
| Feb 20 | Basil, herbs |
| Mar 1 | Lettuce, kale starts |
| Mar 15 | Cucumbers, squash (if transplanting) |
Supplies Checklist
You don’t need a lot. Here’s the minimum:
Must-Have
- Seed starting mix ($5-8/bag) — NOT garden soil or potting mix. Seed starting mix is sterile and fine-textured. Seeds rot in heavy soil.
- Containers — cell trays, peat pots, or even yogurt cups with drainage holes punched in the bottom
- Light — a south-facing window works for some crops, but a $30 shop light hung 2-3 inches above seedlings is dramatically better
- Water — bottom watering is best (set trays in a dish of water)
Nice-to-Have
- Heat mat ($20) — speeds germination for peppers and tomatoes by 3-5 days
- Humidity dome — clear plastic lid over the tray until seeds sprout
- Labels — you WILL forget what you planted where. Always label.
Don’t Need
- Expensive grow lights (a $30 shop light with daylight bulbs works)
- Seed starting greenhouses (overpriced, poorly ventilated)
- Growth hormones or special fertilizers (half-strength liquid fertilizer after first true leaves is enough)
Step-by-Step: Starting Seeds
1. Fill containers
Moisten seed starting mix until it feels like a wrung-out sponge. Fill cells to the top, press gently (don’t compact).
2. Plant seeds
General rule: plant 2-3x deeper than the seed’s diameter. Tiny seeds (lettuce, basil) go on the surface with a light dusting of mix. Large seeds (squash, beans) go 1 inch deep.
Plant 2-3 seeds per cell. You’ll thin to the strongest one later.
3. Cover and water
Cover with humidity dome or plastic wrap until seeds sprout. Bottom water by setting the tray in a dish of water for 15 minutes — this prevents washing seeds away.
4. Provide light
As soon as seeds sprout, they need light. 14-16 hours per day. Keep lights 2-3 inches above the seedlings — raise the light as they grow.
No light = leggy seedlings. If your seedlings are tall, pale, and leaning toward the window, they need more light. This is the #1 reason indoor starts fail.
5. Thin seedlings
When seedlings have their first true leaves (the second set — the first set are “seed leaves”), thin to one per cell by snipping the weaker ones at soil level. Don’t pull them — you’ll disturb the roots of the keeper.
6. Fertilize
Start half-strength liquid fertilizer (any balanced formula) after the first true leaves appear. Once a week is enough.
7. Pot up (if needed)
If seedlings outgrow their cells before it’s time to go outside, move them to 3-4 inch pots. This happens most often with peppers and tomatoes started early.
8. Harden off
This step is critical. Indoor seedlings will die from shock if planted directly outside. Over 7-10 days:
- Day 1-2: 1 hour outside in shade, sheltered from wind
- Day 3-4: 2-3 hours, partial sun
- Day 5-6: 4-5 hours, more sun
- Day 7-10: Full day outside, bring in at night if frost threatens
- After 10 days: plant in the garden
The 5 Most Common Seed Starting Mistakes
1. Starting too early
More seedlings die from being started too early than too late. A 12-inch leggy tomato in a tiny cell is worse than a 6-inch stocky one. Count your weeks carefully.
2. Not enough light
A sunny window is not enough for most seedlings. They need the light source close (2-3 inches) and on for 14-16 hours. A $30 shop light solves this.
3. Overwatering
Wet soil = damping off (seedlings keel over at the soil line and die). Let the surface dry slightly between waterings. Bottom water when possible.
4. Skipping hardening off
The #1 transplant killer. Those pampered indoor babies can’t handle sun, wind, and temperature swings without gradual exposure. Never skip this.
5. Using garden soil
Garden soil is too heavy, may contain pathogens, and compacts in small containers. Always use sterile seed starting mix.
Seed Starting Schedule by Zone
Don’t calculate — just look up your zone:
- Zone 3 Planting Calendar — start peppers by March 1
- Zone 4 Planting Calendar — start tomatoes by March 1
- Zone 5 Planting Calendar — start peppers by Feb 1
- Zone 6 Planting Calendar — start tomatoes by Feb 1
- Zone 7 Planting Calendar — start tomatoes by Jan 15
- Zone 8 Planting Calendar — start tomatoes by Jan 1
Track Your Seedlings
Starting seeds is the beginning of a journey. Tracking germination rates, days to sprout, and which varieties performed best is what turns a hobby into a skill.
After one season of tracking, you’ll know:
- Which seed brands have the best germination rates
- Your actual indoor start-to-transplant timeline (it’s never exactly what the packet says)
- Which varieties thrive in your specific conditions
Track your seedlings from sow to transplant with Garden.gg — free seedling nursery tracker with germination logging, hardening-off schedules, and planting calendars.