Starting seeds indoors is one of the most satisfying parts of gardening. It gives you a head start on the season, lets you grow varieties you can’t find in nurseries, and ensures stronger, earlier harvests. After more than 30 years of year-round gardening in Sonoma Valley—and starting thousands of seedlings—I’ve honed a simple, reliable method that consistently produces strong, healthy transplants.

Here’s my step-by-step guide with the practical tips that have worked for me season after season.

Table of Contents

1. Gather Your Seed-Starting Supplies

You don’t need anything fancy—just a few essentials.

What I use in my own garden:

Seed trays or cell packs
Clean pots (2–4 inch) for potting up
Humidity dome or clear cover
Seed-starting mix (light, sterile, fine-textured)
Spray bottle for gentle watering
Heat mat (optional, but excellent for warm crops)
Labels and a pencil
Grow lights or a bright south-facing window

Experience tip:
Avoid garden soil for indoor sowing—it compacts easily and carries disease. A fresh, sterile mix prevents damping off.

2. Moisten the Seed-Starting Mix First

Before filling your trays, pre-moisten the mix so it feels like a wrung-out sponge.

Experience tip:
Dry mix repels water and leaves air pockets that prevent even germination. Moistening beforehand creates the perfect seed bed.

3. Fill Containers and Level the Surface

Fill your trays or cells gently. Do not pack tightly.

Then level the surface with your hand.

Experience tip:
A firm but not compacted surface helps seeds make contact with the mix and sprout evenly.

4. Sow Seeds at the Correct Depth

Most seed packets list the proper sowing depth. As a general rule:

Tiny seeds: Press on the surface, do not cover.
Small seeds: Cover with ⅛ inch of mix.
Larger seeds: Cover with ¼–½ inch.

Experience tip:
“Plant twice as deep as the seed is wide” works well—except for light-dependent seeds (lettuce, many herbs), which should stay uncovered.

5. Apply Gentle Moisture

Use a spray bottle or fine mist to moisten the surface after sowing.

Experience tip:
Overhead watering can wash tiny seeds into corners; bottom watering is best once seedlings emerge.

6. Cover Trays and Add Warmth

Use a humidity dome or plastic cover to trap moisture until germination.

Set trays on a heat mat for warm-season crops such as:

Tomatoes
Peppers
Eggplant
Basil
Cucumbers
Squash
Melons

Experience tip:
Heat mats speed up germination dramatically. Tomatoes that take 7–10 days at room temperature often sprout in 3–4 days with bottom heat.

7. Provide Strong Light Immediately After Germination

Once seedlings appear, remove the dome and move trays under bright lights.

Light guidelines:

Keep lights 2–3 inches above seedlings
Run lights 14–16 hours per day
Raise lights as seedlings grow

Experience tip:
Most home seedlings become leggy because the light source is too far away. Keep lights close for compact, sturdy growth.

8. Water from the Bottom

Place trays in a shallow pan and let the soil wick up water.

Experience tip:
Bottom watering prevents fungal diseases and keeps leaves dry—especially important for peppers, tomatoes, and herbs.

9. Start Airflow

Use a small fan set to low to move air across the seedlings.

Experience tip:
Gentle airflow strengthens stems and prevents damping off. I run my fan for a few hours daily.

10. Feed Seedlings Lightly

When seedlings develop their first true leaves, start feeding with a diluted liquid fertilizer (¼ strength).

Experience tip:
Seedlings in sterile mix need nourishment early. Weak, pale growth usually means lack of nutrients.

11. Pot Up When Needed

When roots fill the small cells, transplant seedlings into 3–4 inch pots.

This especially applies to:

Tomatoes
Peppers
Eggplant
Broccoli
Cabbage

Experience tip:
I bury tomato stems deeply when potting up—new roots form along the stem, creating stronger plants.

12. Harden Off Before Transplanting Outdoors

About 7–10 days before planting out:

Start seedlings in shade for 2 hours
Increase outdoor time daily
Leave plants out overnight at the end of the week

Experience tip:
Skipping hardening off is one of the quickest ways to shock or stunt seedlings. Go slow for the best results.

13. Transplant on a Cloudy Day

Plant seedlings outdoors once:

Frost danger has passed (for warm-season crops)
Soil has warmed
Weather is mild

Experience tip:
Cloudy days with light drizzle are ideal. Bright sun the first day can stress young plants.

Final Thoughts

Starting seeds indoors is one of the most personal parts of gardening—your hands, your light, your soil, your timing. After decades of doing this every winter through spring, I still count the moment seedlings emerge as one of the best parts of the season.