Starting seeds indoors: when and how
Starting seeds indoors gives slow crops like tomatoes and peppers a head start, so they're ready to plant when the weather is. The trick is timing: count back from the day they'll go outside.
Work out the day they'll go outside
Every seed-starting date begins with a planting-out date, and that depends on how much cold the crop can take. Colorado State University Extension's planting guide gives these rules, counted from your typical last frost date:
- Broccoli and cabbage (hardy): plant out as early as 2 to 4 weeks before the typical last frost.
- Cauliflower (semi-hardy): up to 2 weeks before it.
- Tomatoes, peppers and eggplant (very tender): 2 weeks or more after it.
Then count back by the seedling's age
The same guide gives the age a transplant should be when it goes outside:
- Broccoli, cabbage and cauliflower: 5 to 7 weeks.
- Tomatoes: 5 to 7 weeks.
- Peppers: 6 to 8 weeks.
- Eggplant: 6 to 9 weeks.
Put the two together and you have your seed-starting window. Tomatoes, for example, go out 2 weeks after the typical last frost and should be 5 to 7 weeks old, so you start them 3 to 5 weeks before the typical last frost. Here's what that works out to in a few places:
| Place | Typical last frost | Start tomatoes indoors | Earliest peas and spinach | Plant tomatoes out from |
|---|---|---|---|---|
| Houston, TX | January 30 | December 26 to January 9 | January 2 to 16 | February 13 |
| Atlanta, GA | March 24 | February 17 to March 3 | February 24 to March 10 | April 7 |
| Kansas City, MO | April 4 | February 28 to March 14 | March 7 to 21 | April 18 |
| Columbus, OH | April 18 | March 14 to 28 | March 21 to April 4 | May 2 |
| Boston, MA | April 4 | February 28 to March 14 | March 7 to 21 | April 18 |
| Minneapolis, MN | April 23 | March 19 to April 2 | March 26 to April 9 | May 7 |
Your own dates are on your ZIP code's planting calendar.
Some crops are better not started indoors at all. Colorado State says cucumbers, squash and melons have roots that hate being disturbed and do best sown straight into the garden. If you do start them inside, plant them out young, no more than 2 to 3 weeks after sowing.
What you need
The University of Minnesota Extension's guide to starting seeds indoors covers the setup:
- Seed-starting mix, not garden soil. The commercial mixes, usually peat and vermiculite, are sterile, light and free of weed seeds.
- Small, separate containers, one seedling each. Seedlings sown together in one big pot tangle their roots and get hurt when you separate them.
- Lights. Seedlings grow much better under fluorescent or LED lights than on a windowsill. Ordinary shop fixtures with "cool white" tubes are enough.
- A timer, so the lights go on and off by themselves.
Warmth
Minnesota Extension advises keeping the air above 60°F. The mix in the containers can run as much as 5°F cooler than the room, and most seeds sprout sooner and grow healthier roots in warm mix. A heat mat made for seed starting helps, and also helps prevent "damping off," when tiny seedlings collapse from disease at the soil line. If your lights are on a timer, Minnesota Extension adds, don't plug the heat mat into it.
Colorado State lists the best soil temperature for sprouting most vegetables at around 80°F, and gives 60°F as the lowest for peppers and eggplant.
Light
Lack of light is the main cause of tall, skinny, floppy seedlings, says Minnesota Extension. Its rules:
- Keep the lights no more than 4 inches above the tops of the seedlings. Two inches is ideal. Hang them on chains so you can raise them as the plants grow.
- Give 12 to 16 hours of light a day, but not 24. Plants need a dark period each night.
Water and food
Keep the mix moist while seeds sprout. Mist the surface gently, or pour water into the tray and let the mix soak it up from below; then pour off what's left so the roots don't sit in water. Once seedlings are growing, feed them once a week with a general-purpose water-soluble fertilizer mixed at a quarter of the label strength, and use plain water the rest of the week.
Potting up
When seedlings outgrow their cells, move them to containers 1 to 2 inches wider. Handle them by a leaf, never by the stem, which is easily crushed.
Hardening off
Seedlings raised indoors have never felt sun, wind or a cool night. Put straight into the garden, their leaves can scorch and they may wilt and die. Minnesota Extension's routine for "hardening them off":
- Start two weeks before your planting-out date.
- Set them outside for a few hours in the shade on a warm afternoon, out of the wind, and bring them in before it cools off.
- Each day, leave them out a little longer, with a little more sun.
- By the end of two weeks, unless a freeze is forecast, they can stay out in a sunny spot until you plant them.
Plant on a cloudy day or late in the afternoon, after the sun has passed its peak. A little wilting the first day is normal.
A note on starting too early
A plant started much earlier than the transplant ages above simply spends extra weeks indoors, under lights, waiting for the weather. The ages are Colorado State's recommendation; counting back from them gets your plants to the garden at the right size on the right day.
Sources
- Vegetable Planting Guide (CMG GardenNotes #720), Colorado State University Extension. Checked October 25, 2026.
- Starting seeds indoors, University of Minnesota Extension. Checked October 25, 2026.
More guides: What your last frost date really means · Hardiness zones, and what changed on the 2023 map · How to protect plants from a late frost · What to plant, month by month