How to Start Seeds Indoors: A Beginner's Timeline

Starting seeds indoors gives slow-growing vegetables like tomatoes and peppers enough of a head start to ripen before your season ends. It also opens up far more varieties than any garden center carries, and once you’re growing more than a handful of plants, it costs a fraction of buying nursery starts. Most failures come down to a few predictable problems: too little light, too much water and skipping the transition outdoors. All of them are easy to avoid once you know them.

Quick answer: Find your average last frost date and count backward using the weeks listed on each seed packet. That’s typically 6-8 weeks for tomatoes and peppers and 4-6 weeks for many flowers. Sow in a fine seed-starting mix, keep it warm and evenly moist until sprouts appear, then give seedlings strong light for most of the day, ideally from a grow light a few inches above them. Pot up when true leaves appear, feed lightly, and harden seedlings off outdoors for about a week before planting.

What You’ll Need

• Containers with drainage: cell trays, small pots or recycled containers with holes poked in the bottom
• Seed-starting mix: a light, fine, sterile mix, not garden soil or dense potting soil
• A light source: a basic LED shop light or grow light on a timer works far better than most windowsills
• A clear dome or plastic wrap to hold humidity during germination
• A heat mat (optional): speeds up germination for warmth-loving crops like peppers and eggplant
• A small fan (optional): gentle airflow strengthens stems and discourages fungus
• Labels and a waterproof marker: seedlings of different varieties look nearly identical early on

What’s Worth Starting Indoors (and What Isn’t)

Not every crop benefits from an indoor start, and some actually do worse after transplanting.

Start indoors	Sow directly outdoors	Either works
Tomatoes, peppers, eggplant	Carrots, radishes, beets, parsnips	Lettuce and other greens
Broccoli, cabbage, cauliflower, kale	Beans, peas	Cucumbers, squash, melons (only 2-4 weeks early, in large cells)
Onions and leeks (from seed)	Corn	Basil and many herbs
Many slow flowers (petunias, snapdragons)	Sunflowers, nasturtiums	Zinnias, marigolds

Root crops dislike transplanting because disturbing the developing taproot causes forked or stunted roots. Beans, peas and corn germinate quickly outdoors in warm soil, so starting them indoors gains little.

Build Your Timeline From the Last Frost Date

Look up your area’s average last frost date, a quick search for your location plus “last frost date” will find it, then count backward using each packet’s instructions. A simple calendar keeps staggered start dates from slipping.

Weeks before last frost	Typical tasks
10-12 weeks	Onions and leeks from seed; slow perennials and some flowers
8-10 weeks	Peppers and eggplant (they’re slow and like warmth)
6-8 weeks	Tomatoes; broccoli, cabbage and kale for spring planting
4-6 weeks	Many annual flowers, basil, lettuce for transplanting
2-4 weeks	Cucumbers, squash and melons in large cells, if you start them indoors at all
1-2 weeks	Begin hardening off cool-season transplants
At or after last frost	Harden off and plant warm-season crops once nights stay warm

Starting too early is a common mistake. Seedlings held indoors too long become root-bound and leggy, and often transplant worse than younger, sturdier plants.

Step 1: Sow the Seeds

1. Pre-moisten the mix until it feels like a wrung-out sponge, then fill the cells without packing them down hard.
2. Sow at the depth on the packet. A common rule of thumb is about twice the seed’s diameter. Tiny seeds like petunias and lettuce often need light to germinate and should only be pressed onto the surface.
3. Sow two or three seeds per cell if seed is plentiful, then thin later by snipping the extras at soil level rather than pulling them.
4. Label immediately. You won’t remember which tray was which.
5. Cover and keep warm. Most common vegetable seeds germinate well in a warm room. Warmth-loving crops like peppers germinate much faster on a heat mat.

Step 2: Give Strong Light the Moment Sprouts Appear

This is where most indoor seedlings go wrong. A windowsill that seems bright to you usually isn’t strong enough, and seedlings stretch toward it into thin, floppy stems.

• Remove the dome as soon as most seeds have sprouted. High humidity after germination invites fungal disease.
• Turn on the lights. Keep a grow light or LED shop light just a few inches above the seedlings and run it for most of the day on a timer, roughly 14 to 16 hours. Raise the light as the plants grow.
• Add gentle airflow. A small fan on low for part of the day mimics wind, strengthens stems and dries the soil surface.

Step 3: Water, Feed and Pot Up

Water from below when possible. Set trays in a shallow tray of water for a few minutes until the surface darkens, then drain. This keeps stems and the soil surface drier, which helps prevent damping off.

Feed lightly once true leaves appear. The first leaves (cotyledons) are part of the seed’s stored food. When the second set, the “true leaves”, appear, start a diluted liquid fertilizer at around quarter to half strength every week or two.

Pot up when true leaves are established or roots fill the cell. Move each seedling into a larger pot and handle it by a leaf, never the stem. A torn leaf regrows; a crushed stem usually kills the seedling. Tomatoes can be potted up deeper than they were growing, since their buried stems form extra roots.

Step 4: Harden Off Before Planting Outside

Seedlings raised indoors have never felt direct sun, wind or cool nights. Moving them straight outside can scorch or kill them within days. Harden them off over about a week:

Day	Exposure
1-2	A couple of hours outdoors in shade, sheltered from wind
3-4	Half a day, with some gentle morning sun
5-6	Most of the day, including some afternoon sun
7	All day and overnight if nights are mild; then transplant

Bring them in if frost or strong wind is forecast, and watch for wilting or white, scorched patches on leaves. Pull back exposure if you see them. A cold frame makes this step much easier because you can vent it by day and close it at night.

Troubleshooting Seedling Problems

Symptom	Likely cause	Fix
Tall, thin, pale, leaning seedlings	Not enough light	Lower the light to a few inches above the plants; run it longer
Seedlings collapse at the soil line	Damping off (fungal)	Improve airflow, water from below, let surface dry slightly; use clean trays next time
Uneven or no germination	Too cold, too dry, too wet or old seed	Check warmth and moisture; test old seed on a damp paper towel
White fuzz on soil surface	Too wet, too little airflow	Let the surface dry, run a fan, water from below
Yellowing lower leaves	Hungry seedlings or overwatering	Start dilute feeding; let the mix dry slightly between waterings
Purple undersides on tomato leaves	Often cool temperatures	Keep seedlings warmer; usually resolves

FAQ

Can I use a sunny window instead of a grow light?

You can, but results are usually weaker. Even a south-facing window rarely provides enough light, especially in late winter. Rotate trays daily if you use a window, and consider an inexpensive LED shop light as a supplement.

How do I know if old seeds are still good?

Fold ten seeds into a damp paper towel, seal it in a bag and keep it warm. Check after the typical germination time. The number that sprout tells you roughly what share of the packet is still viable.

Do I need a heat mat?

Not for most crops in a warm house. It makes a noticeable difference for peppers, eggplant and other warmth-loving seeds, especially in cool rooms. Remove seedlings from the mat once they sprout.

When should I transplant tomatoes outside?

After your last frost date, once nights stay reliably mild and the seedlings are hardened off. Tomatoes are easily set back by cold nights. For growing them in containers, see our guide to growing tomatoes in pots, and for planning beds, our companion planting guide.