Seed Starting Calendar and Frost Date Guide: When to Sow Every Crop

A seed starting calendar tells you exactly when to sow each crop indoors so seedlings are the right size on planting day. Start too early and plants get leggy and root bound. Start too late and you lose weeks of growing season. The method is simple: find your average last frost date, then count backward.

This guide gives you a seed starting calendar you can use anywhere, a table of weeks before last frost for common crops, a worked example for a May 15 frost date, and tips on light, temperature and hardening off. It works for gardens in any climate, from cold homesteads to mild regions.

Quick answer: how to use a seed starting calendar

1. Find your average last spring frost date from your local extension office or weather service.
2. Look up the weeks before last frost for each crop.
3. Count back from the frost date to find the sowing date.
4. Sow indoors, give strong light, and harden off before transplanting.
5. Direct sow cold-hardy crops outdoors as soon as the soil can be worked.

Seed starting calendar: weeks before last frost

Crop	Start indoors (weeks before last frost)	Transplant outdoors
Onions and leeks	10 to 12	4 to 6 weeks before last frost
Peppers and eggplant	8 to 10	1 to 2 weeks after last frost
Celery and celeriac	8 to 10	At last frost
Tomatoes	6 to 8	1 to 2 weeks after last frost
Broccoli, cabbage, cauliflower	5 to 7	2 to 4 weeks before last frost
Basil	4 to 6	1 to 2 weeks after last frost
Lettuce and kale	4 to 6	2 to 4 weeks before last frost
Melons	3 to 4	2 weeks after last frost
Cucumbers and squash	2 to 4	1 to 2 weeks after last frost

Crops like carrots, beets, peas, radishes and beans are usually direct sown and not on the indoor seed starting calendar. Sow peas and spinach as soon as the soil can be worked, and warm-season beans after the last frost.

Worked example: last frost on May 15

Crop	Weeks before	Sowing date
Onions	10	March 6
Peppers	8	March 20
Tomatoes	7	March 27
Broccoli	6	April 3
Lettuce	5	April 10
Cucumbers	3	April 24

Count back from May 15: 7 weeks is 49 days, which lands on March 27 for tomatoes. Write the dates on a wall calendar and update them each year if your local frost date shifts. Many gardeners also add a second, cautious date one week later in case of a late frost.

Find your last frost date

• Search for your location in a local extension office frost date table.
• Use the 50 percent probability date as the standard average date, and the 10 percent date for a safe date.
• Observe your own garden. Cold pockets and low spots frost later than sunny slopes.
• Adjust for elevation: frost dates are later at higher altitude.

Light, temperature and soil for seedlings

• Light: seedlings need 14 to 16 hours of strong light. A sunny window is often too weak, so use a full-spectrum LED grow light 2 to 4 inches above the plants.
• Temperature: most seeds germinate best at 70 to 80 degrees Fahrenheit. After sprouting, grow seedlings cooler, around 60 to 70 degrees.
• Soil: use a sterile seed starting mix, not garden soil.
• Water: keep the mix evenly moist and water from below to prevent damping off.
• Airflow: a small fan reduces disease and strengthens stems.

If you power grow lights from a small solar system, check the load with our battery runtime calculator. A 40 W LED light running 14 hours uses 0.56 kWh per day.

Hardening off seedlings

1. One week before planting, set trays outside in shade for 1 to 2 hours.
2. Increase time and sun exposure each day.
3. Protect from wind and cold nights.
4. By day 7 to 10, plants can stay out all day and night if temperatures allow.
5. Transplant on a cloudy day or in the evening.

Extend your season with cold frames, row cover and good winter planning. Our guide on cover crops for winter helps you prepare beds for spring planting.

Common seed starting mistakes

• Starting too early, so plants become tall and weak.
• Weak light that produces leggy seedlings.
• Overwatering and cold soil that cause damping off.
• Skipping hardening off.
• Planting warm-season crops before nights stay above 50 degrees Fahrenheit.

Related guides

• Crop Rotation for a Small Vegetable Garden
• Best Cover Crops for Winter
• Passive Solar Greenhouse Design
• How to Grow Garlic Organically

Frequently Asked Questions

When should I start tomato seeds indoors?

Start tomatoes 6 to 8 weeks before your last frost date, then transplant 1 to 2 weeks after it, when nights are reliably mild.

How do I make a seed starting calendar?

Find your last frost date, list your crops, and subtract the recommended weeks from the frost date for each crop. Write the sowing dates on a calendar.

Can I start seeds without a grow light?

A south-facing window can work for some crops, but seedlings are often leggy. A simple LED grow light gives much sturdier plants.

What if my frost date changes each year?

Use the average date, plan a second sowing a week later, and watch the local forecast. Row cover protects transplants from a late frost.

Sources and further reading

• Old Farmer’s Almanac: Frost dates
• NOAA: Climate and weather data
• University of Minnesota Extension: Starting seeds indoors

A seed starting calendar turns a vague planting season into a clear plan. Know your last frost date, count back by crop, give seedlings strong light, harden them off carefully, and your garden will start the season strong.