Outdoor

How do you start seeds indoors?

By Roof & Router Team. Updated October 11, 2026.

When should you start seeds indoors?

Colorado State University Extension says most seeds should be started four to eight weeks before the last killing frost. The seed packet gives the best timing for each variety.

Count backward from your average last frost date. Faster crops need fewer weeks. As one example, University of Minnesota Extension’s schedule keeps tomatoes indoors about 5 to 6 weeks.

Dates are simple subtraction (56 and 28 days) in a non-leap year; in a leap year, February dates shift one day later. Check the packet before you sow.

What containers and soil should you use?

• Drainage holes are required. UMN Extension says all seed-starting containers must have drainage holes at the bottom.
• One seedling per container is easiest to manage and transplant.
• Clean reused pots. UMN suggests soaking them for 30 minutes in a bleach or other disinfectant solution.
• Use a seed-starting mix, not garden soil. UMN recommends a commercial peat and vermiculite mix. It is sterile, lightweight and free of weed seeds.

How much light do seedlings need?

If you lack a bright window, CSU suggests a fluorescent fixture hung close to the plants. UMN gives similar guidance for grow lights.

UMN also says seedlings need a dark period each night. A plug-in timer makes this automatic. Raise the lights as the plants grow so the gap stays small.

What temperature do seeds need to germinate?

CSU Extension says most seeds germinate best at 70-75°F. Tomato-family seeds (tomatoes, peppers, eggplant) do better when the soil is close to 80°F.

UMN Extension says to keep the air above 60°F. It notes the potting mix can run as much as 5°F cooler than the room.

A seed-starting heat mat under the tray supplies bottom heat. UMN says this helps germination and root growth. UMN also warns not to plug the heat mat into the light timer.

Electrical safety: Keep heat mats, lights and power strips away from standing water. Plug them into a GFCI-protected outlet where possible, and follow the maker’s instructions.

How should you water seedlings?

• Keep the mix evenly moist while seeds germinate. Do not let it dry out.
• Drain excess water from the trays so roots do not sit in water.
• After seedlings have several sets of true leaves, UMN suggests a water-soluble fertilizer at 1/4 strength once a week. Use plain water the rest of the week.

What is damping off and how do you prevent it?

UMN defines damping off as the death of tiny seedlings from pathogens at the surface of the potting mix.

CSU lists excess moisture, high temperatures and poor light as conditions that make seedlings susceptible. To lower the risk:

• Start with a sterile mix and clean containers.
• Avoid overwatering and drain trays.
• Give strong light, kept close to the plants.
• Keep seedlings out of cold drafts.
• Use bottom heat; UMN says it helps prevent damping off.

How do you harden off seedlings before transplanting?

Indoor seedlings need about two weeks to adjust to sun, wind and cooler nights. UMN’s method:

1. Start with a few hours outdoors in afternoon shade, sheltered from wind.
2. Bring plants in before night temperatures drop.
3. Increase outdoor time and sun exposure each day.
4. After about two weeks, plants can stay out in a sunny spot unless frost is forecast.

A cold frame also works if you vent the lid a little more each day. For warm-season crops, see when to plant tomatoes before moving them to the garden.

Quick answers

How many weeks before the last frost should I start seeds? Most seeds go in 4 to 8 weeks before the last frost, per CSU Extension. Follow the seed packet for each variety.

Can I use garden soil to start seeds? Extension guidance favors a sterile seed-starting mix instead. It is lightweight and free of weed seeds.

How far should grow lights be from seedlings? About 2 to 4 inches above the tops of the plants. Raise them as seedlings grow.

How many hours of light do seedlings need? About 12 to 16 hours a day, with a dark period at night.

What kills young seedlings indoors? Damping off, a disease at the surface of the mix, is a common culprit. Extension sources link it to excess moisture, heat, poor light and cool soil.

How long does hardening off take? About two weeks of gradually longer time outdoors.