Seed Starting Guide
Start your growing season early by sowing seeds indoors. This complete guide covers timing, supplies, technique, and hardening off.
Why Start Seeds Indoors?
Starting seeds indoors gives you a 6-10 week head start on the growing season. Plants like tomatoes, peppers, and eggplant need warm soil and a long growing season — by the time outdoor conditions are right, they are already well-established transplants ready to produce.
Save Money
A $3 seed packet grows 20-50 plants vs $4-6 per transplant at the nursery
More Varieties
Nurseries carry 5-10 tomato types. Seed catalogs offer hundreds.
Earlier Harvests
Transplants produce 2-4 weeks earlier than direct-sown seeds
Better Survival
Transplants are larger and more resilient against pests and weather
When to Start Seeds Indoors
Count backwards from your last frost date. Here are the typical lead times:
| Weeks Before Last Frost | Plants to Start |
|---|---|
| 10-12 weeks | Peppers, eggplant, celery, leeks, onions (from seed) |
| 8-10 weeks | Tomatoes, broccoli, cauliflower, cabbage |
| 6-8 weeks | Kale, lettuce, Swiss chard, basil, parsley |
| 4-6 weeks | Cucumbers, squash, melons, pumpkins (optional — can direct sow) |
| 2-4 weeks | Sunflowers, zinnias, marigolds (optional — can direct sow) |
Seed Starting Supplies
Seed starting trays or cell packs
Essential72-cell trays are standard. Choose trays with drainage holes. Reusable plastic or biodegradable peat pots both work.
Seed starting mix
EssentialUse a lightweight, sterile, soilless mix (peat moss, vermiculite, perlite). Do NOT use garden soil — it compacts, drains poorly, and can carry disease.
Grow lights
EssentialSeedlings need 14-16 hours of light daily. A simple T5 or LED shop light positioned 2-4 inches above seedlings works well. Windowsills rarely provide enough light.
Heat mat
HelpfulWarms soil to 70-80F for faster germination. Especially important for tomatoes, peppers, and eggplant. Remove after seeds sprout.
Spray bottle or bottom-watering tray
EssentialGentle watering prevents displacing tiny seeds. Bottom watering (filling the tray below) is ideal and prevents damping off disease.
Plant labels
EssentialMark every tray with plant name and sowing date. Seedlings look very similar early on — you will not remember what is what without labels.
Humidity dome or plastic wrap
HelpfulCovers trays to maintain moisture during germination. Remove as soon as seeds sprout to prevent mold and legginess.
Small fan
HelpfulGentle air circulation strengthens stems and prevents damping off fungal disease. Run on low for a few hours daily once seedlings emerge.
Step-by-Step Seed Starting Process
Calculate your start date
Count backwards from your last frost date. Most seeds need 6-8 weeks indoors before transplanting. Check the seed packet for specific timing. Starting too early leads to leggy, stressed seedlings.
Tip: Use our frost dates tool to find your local last frost date.
Prepare containers and mix
Fill cells or pots with pre-moistened seed starting mix. The mix should be damp like a wrung-out sponge — not soaking wet. Lightly press down to eliminate air pockets but do not compact it.
Tip: Pre-moistening the mix in a bucket is much easier than watering dry mix in cells.
Sow seeds at the right depth
General rule: plant seeds at a depth of 2-3 times their diameter. Tiny seeds (lettuce, basil) go on the surface with a light dusting of mix. Larger seeds (tomato, pepper) go 1/4 inch deep. Press gently to ensure seed-to-soil contact.
Tip: Plant 2-3 seeds per cell and thin to the strongest seedling after germination.
Provide warmth for germination
Most seeds germinate best at 70-80F soil temperature. Place trays on a heat mat or on top of a warm appliance (refrigerator). Cover with a humidity dome or plastic wrap to retain moisture. Check daily and mist if the surface dries out.
Tip: Peppers and eggplant need 80-85F for best germination. Without a heat mat, they can take 2-3 weeks.
Add light immediately after sprouting
As soon as you see green, remove the humidity dome and place seedlings under grow lights for 14-16 hours per day. Keep lights 2-4 inches above the tops of seedlings. Raise lights as plants grow. Insufficient light causes tall, weak, leggy stems.
Tip: A simple timer ($5) automates your light schedule and ensures consistency.
Water carefully and feed lightly
Bottom water by filling the tray beneath the cells. This prevents damping off and encourages roots to grow downward. Once seedlings have 2-3 true leaves, feed with a half-strength liquid fertilizer (like fish emulsion or liquid kelp) every 1-2 weeks.
Tip: If seedlings develop a white fuzzy growth at the base (damping off), improve air circulation and reduce moisture.
Thin and up-pot as needed
Once seedlings have their first set of true leaves (not the initial seed leaves), thin to one seedling per cell by snipping extras at the soil line. If roots outgrow their cell before transplant time, move to a larger pot (3-4 inch).
Tip: Snip unwanted seedlings instead of pulling — pulling can disturb the roots of the one you want to keep.
Hardening Off Seedlings
This is the most commonly skipped step — and the #1 reason transplants fail. Hardening off gradually acclimates seedlings to outdoor sun, wind, and temperature swings over 10-14 days.
Warning: Skipping hardening off causes transplant shock, sunburned leaves, and can kill seedlings you spent weeks growing. Be patient — this step is not optional.
| Timeline | What to Do |
|---|---|
| Days 1-2 | Place seedlings outdoors in a sheltered, shady spot for 1-2 hours. Bring inside. |
| Days 3-4 | Move to a spot with morning sun for 3-4 hours. Protect from wind. Bring inside at night. |
| Days 5-6 | Increase to 5-6 hours of direct sun. Allow some gentle wind exposure. Bring inside at night. |
| Days 7-8 | Full day outdoors in sun. Bring inside only if nighttime temperatures drop below 50F. |
| Days 9-10 | Leave outdoors overnight if temperatures stay above 50F. Water as needed. |
| Day 11+ | Transplant into the garden. Water deeply after planting. Provide shade cloth for the first 2-3 days if temperatures exceed 85F. |
Direct Sow vs. Transplant
Some plants prefer to be started indoors and transplanted, while others do best when sown directly in the garden. Root crops and fast growers generally prefer direct sowing.
Best Started Indoors (Transplant)
These plants need a long growing season or warm soil to germinate. Start indoors and transplant after last frost.
Best Direct Sown in Garden
These plants have taproots that dislike transplanting, or they grow so fast that indoor starting provides little advantage.
Common Seed Starting Mistakes
Starting too early
Seedlings that outgrow their pots before transplant time become root-bound and stressed. Follow the timing chart above.
Insufficient light
Windowsills rarely provide enough light. Leggy, stretched seedlings are the #1 sign of inadequate light. Use grow lights.
Overwatering
Soggy soil causes damping off — a fungal disease that kills seedlings overnight. Bottom water and allow the surface to dry slightly between waterings.
Planting too deep
Tiny seeds like lettuce and basil need light to germinate. Press them onto the surface with a light dusting of mix — do not bury them.
Skipping hardening off
Indoor seedlings have never experienced direct sun or wind. Planting them straight into the garden causes severe shock and often death.
Using garden soil
Garden soil compacts in containers, drains poorly, and contains weed seeds and pathogens. Always use sterile seed starting mix.
Continue Your Growing Journey
From seed to harvest — explore our complete guide collection.