Seed starting dates

Frost dates

Your last frost date comes from about 30 years of weather records at a station near you. NOAA works out how often the last spring freeze came on or after each day of the year. The date most charts print is the 50% date: in half of those years the last freeze had already come, and in the other half it came later.

So the average last frost date is a coin toss, not a safe day. NC State Extension shows how wide the spread runs. At the Raleigh station, there is a 90% chance of a 36 °F frost on or after March 28, a 50% chance on or after April 12, and still a 10% chance on or after April 26. The safer date is two weeks past the average one.

Raleigh, North Carolina, 36 °F frost, from NC State Extension.

The temperature matters as much as the odds. NOAA lists dates for several thresholds. NC State calls 32 °F a freeze and 28 °F a hard freeze, and uses 36 °F for frost, since frost depends on humidity as well as temperature and the risk climbs once the air drops below 36. Here is the station at Montana State University in Bozeman, all at the 50% level:

Bozeman, Montana (ZIP 59715), last spring date in half of years, NOAA 1991 to 2020 normals.

Five weeks lie between the hard freeze and the last light frost in Bozeman. That is why a mountain gardener with a "May 26 frost date" still covers the tomatoes in June.

Counting back

A seed packet says something like "start indoors 6 to 8 weeks before transplanting." The count runs back from the day the seedling goes outside, and that day is not the frost date for most crops. Cabbage can go out a month before the last frost. Peppers go out two weeks after it. UNH Extension gives both numbers for each crop:

Before the frost date Just after Two weeks or more after

Weeks from seed to transplant, and weeks before or after the last frost to set out. UNH Extension.

So there are two steps. Find the set-out date from the frost date, then count back the seedling weeks from there. For a tomato in Bozeman:

Indoors under lights Hardening off Set out

Try it

Set your frost date and pick a crop

Average last frost

May 26

Crop Onion Cabbage Lettuce Broccoli Tomato Pepper Basil

Sow indoors

Apr 14

Set out

Jun 2

Under lights

7 weeks

Goes out 1 week after the frost date. Frost-tender, so watch the forecast for two weeks after that.

Soil temperature

Seeds sown straight into the garden go by soil temperature, not the calendar. Cold soil slows germination and leaves seeds sitting where they can rot. Colorado State Extension gives the lowest soil temperature each crop will sprout in, and the temperature it sprouts best in. Measure at 8 a.m., 4 inches deep (6 inches for beans), with a soil thermometer.

Minimum and optimum soil temperatures for germination, Colorado State Extension.

Colorado State also sorts crops by frost tolerance, which sets how early they can go in. Hardy crops like lettuce, peas and spinach can go in 2 to 4 weeks before the last frost. Semi-hardy crops like beets, carrots, potatoes and chard go in 0 to 2 weeks before. Tender crops like beans, corn, cucumbers and summer squash are sown around the frost date. Very tender crops like tomatoes, peppers, melons and winter squash go in 2 or more weeks after it.

Seedlings

University of Minnesota Extension gives the basics for seedlings indoors. Keep the air above 60 °F, and remember the mix in the tray can run 5 °F cooler than the room. Give them 12 to 16 hours of light a day, with grow lights no more than 4 inches above the tops and 2 inches if you can. Lack of light is the main reason seedlings come out tall and thin. Seedlings on a windowsill tend to stretch.

Give each seedling its own cell or pot. Roots that grow into each other tear when the plants are pulled apart. Squash and cucumbers do not like their roots disturbed, so start them in paper or fiber pots that go into the ground with the plant.

Short seasons

Days to maturity on a packet count from the day the crop goes in the ground. For tomatoes and peppers that is the transplant day, so Colorado State's 65 days for tomatoes comes on top of the 7 weeks indoors. For crops sown in place it counts from sowing.

The packet figure also assumes an ordinary summer. Plants grow with warmth more than with the calendar, so in a cool place the same variety takes longer. The calculator adjusts for local heat with a Bizon estimate. For Bozeman it puts a 65-day tomato at about 83 days from transplant, ripening around late August.

Some crops do not make it at all. A 115-day pumpkin like Howden, sown in Bozeman on June 9, does not get enough heat before the September 21 frost. There are two fixes. Choose a variety with fewer days, or start the pumpkin indoors 3 to 4 weeks ahead, as UNH suggests for squash and pumpkins, to put those weeks back.

Fall crops

A fall crop is timed back from the first fall frost. University of Nebraska-Lincoln Extension adds up five things:

Leaf lettuce in Bozeman works out like this: 60 days to harvest, plus 21 days of picking, plus the 14-day fall factor, is 95 days. Lettuce tolerates light frost, so no frost-tender factor. Counting 95 days back from September 21 lands on June 18. Hardy crops like kale, cabbage and carrots keep growing through light frosts, so they suit a fall planting best.

Sources

University extension services and NOAA, the same ones the Seed Starting Calculator uses.

• NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020 ↗. Bozeman Montana State Univ station, spring dates at 28, 32 and 36 °F.
• Nelson, N., El-Khoury, L. and Boyette, M., NC State Extension AG-873 (2020), "Interpreting Freeze/Frost Probabilities from the National Centers for Environmental Information" ↗. The 10, 50 and 90% dates at Raleigh, and the 36, 32 and 28 °F thresholds.
• Downing, K. and Saunders, O., UNH Cooperative Extension (2016), "Growing Vegetables: When to Plant your Vegetable Garden" ↗. Weeks from seed to transplant and when to set out.
• Colorado State University Extension, "Vegetable Planting Guide" ↗. Frost tolerance classes, germination soil temperatures, and days to harvest.
• University of Minnesota Extension, "Starting seeds indoors" ↗. Air and mix temperature, light hours and height, separate containers, and hardening off over two weeks.
• Lott, D. and Lindgren, D., University of Nebraska-Lincoln Extension G1343 (2012), "Fall Vegetable Gardening" ↗. Counting back from the first fall frost, and frost tolerance of fall crops.
• University of Kentucky, IPM-12, pumpkin varieties ↗. Howden, 115 days.