Seed Starting and Season Extension

Most seed-starting advice boils down to one date: "plant after your last frost." That date is not a fact about your garden. It is a probability pulled from about 30 years of weather records, and the version most people use is a coin flip. The other things that decide whether your seeds and transplants do well (soil temperature, light, how seedlings are moved outdoors) are measurable, and extension services have been measuring them for decades.

This hub pulls those measurements together. Every number below links to the source it came from, so you can check it.

Frost dates are probabilities, not deadlines

The frost dates you see on seed packets and planting calendars come from NOAA's National Centers for Environmental Information (NCEI). The current U.S. Climate Normals cover 1991–2020 and include "growing season length for various temperature thresholds" for thousands of stations.

NCEI does not publish one last frost date per station. It publishes a range of dates at different probability levels. NC State Extension's guide to interpreting NCEI freeze/frost probabilities explains that the tables show nine probability levels, from 10% to 90%, at several temperature thresholds: 36°F (frost), 32°F (freeze) and 28°F (hard freeze), plus lower ones.

Their worked example, for a station in Raleigh, shows how wide the spread is for spring frost (36°F):

That is almost a month between the risky date and the safe one. NOAA's Climate.gov map of the average last spring freeze is built on the 50% date: the point at which "the chances of freezing temperatures drop below 50 percent." If you plant tomatoes on that date every year, you should expect to get frosted in about half of them.

NC State frames the choice as risk tolerance. The 90% date is the aggressive choice, the 50% date is moderate, and the 10% date is conservative. For cheap, hardy crops like peas and spinach, an aggressive date is fine. For tomato transplants you spent eight weeks raising, a conservative one makes more sense.

Two more caveats. First, official air temperatures are measured about five feet above the ground. The National Weather Service notes that on clear, calm nights the air near the ground can be "several degrees colder" than what the station records, which is why frost advisories go out when forecasts are in the 33–36°F range. Second, normals describe the past 30 years, and the climate keeps shifting under them.

The full breakdown, including how to find your local table, is in Your Last Frost Date Is a Probability.

Soil temperature decides germination

Air temperature decides whether a transplant survives the night. Soil temperature decides whether a seed sprouts at all, and how fast.

The standard reference is data from J. F. Harrington at UC Davis, republished by Oregon State University Extension. It lists minimum, optimum and maximum soil temperatures for common vegetables, and the days to emergence at each temperature. The differences are large. Lettuce has a minimum of 35°F. Beans, cucumbers, peppers and squash all have a minimum of 60°F.

Speed matters as much as the threshold. In the same data, tomato seed takes 43 days to emerge at 50°F, 14 days at 59°F and 6 days at 77°F. Corn takes 22 days at 50°F and 4 days at 77°F. A seed that sits in cold, wet soil for three weeks is exposed to rot and pests for all of that time.

A cheap soil thermometer settles the question. Measure at seeding depth for direct-sown crops and deeper for transplants. The full table and measuring method are in Soil Temperature for Germinating Vegetable Seeds.

Starting seeds indoors: light is the usual problem

Seedlings that come out tall, pale and floppy are usually short of light. University of Minnesota Extension puts it directly: "Lack of light is the major cause of elongated, skinny stems." It discourages windowsills and recommends 12 to 16 hours of light a day, with fluorescent tubes no more than 4 inches above the seedlings.

The reason a window falls short shows up in the numbers. UNH Extension's seedlings-under-lights fact sheet uses a target of about 12 moles of light per square meter per day. By its estimate, a tomato seedling in full sun reaches that in about 5 hours, but under T8 fluorescent shop lights it would need about 22 hours. An LED bar needed 8 hours at 8 inches above the plants and 16 hours at 20 inches.

That last comparison is the key point about LEDs: there is no single correct hanging height, because fixtures differ so much in output. Advice that works for one light can be wrong for the next. Grow Lights for Seedlings covers how to think about intensity, hours and distance without buying a light meter.

Timing matters too. Starting too early produces overgrown transplants that sit waiting for warm weather. UMN's planting table, written for Minnesota, suggests about 5–6 weeks indoors for tomatoes, 9 weeks for peppers and eggplant, and 14–15 weeks for onions and leeks. Count backward from the frost date you have chosen, not from the calendar.

Hardening off: one to two weeks, not one afternoon

Seedlings raised indoors have never felt wind, full sun or a cold night. Moving them straight into the garden is how gardeners end up with bleached, scorched or snapped plants. Extension guidance on hardening off is consistent on the basics:

• Length: one to two weeks, per University of Maryland Extension.
• Start small: a shaded, protected spot on warm days, bringing plants in at night and increasing sun each day.
• Cold limit: keep tender seedlings in when temperatures are below 45°F or it is windy, according to Maryland.
• Water: water less often to slow growth, but do not let plants wilt.

Hardening off changes the plant physically. Nebraska Extension lists thicker leaf wax, more lignin in cell walls, more carbohydrates and faster root growth. It does not make tomatoes frost-proof. Nebraska notes warm-season plants still need warm nights, at least 60°F, and cannot survive freezing even after hardening.

The step-by-step schedule, and the crops that are hurt by over-hardening, are in Hardening Off Transplants.

Season extension: a few degrees, used well

Row covers, low tunnels and cold frames let you plant earlier and harvest later. They are also oversold. The honest figure is a few degrees of frost protection, and sources don't agree on exactly how many.

The spread reflects real differences in fabric weight, whether the cover touches the leaves, and whether the frost is a calm radiation frost or a windy arctic outbreak. Plan on the low end.

Warming the soil can matter more than the air. The University of Alaska Fairbanks reports that soil under clear plastic was 8 to 10 degrees warmer than bare soil. That is often enough to move a warm-season crop from "slow and risky" to "germinates in a week" on the Harrington table.

The main way these tools fail is heat, not cold. A closed cover or cold frame on a sunny spring day can cook plants. UNH reports temperatures under row cover as much as 15°F above outside air and recommends removing covers when it gets above 90°F. Details, including cold frame venting, are in Cold Frames, Row Covers and Low Tunnels. Row covers also keep insects out, which is covered in Row Covers for Pest Control.

A practical sequence for spring

1. Pick a frost date at the risk level you can live with. Use the 10% date for tender transplants you care about, the 50% or 90% date for hardy crops and cheap seed.
2. Count back to indoor sowing dates. Use extension timing tables for your region.
3. Give seedlings real light. Close, intense and on a timer for 12–16 hours a day.
4. Harden off over 1–2 weeks. Start in shade, add sun and time daily, and keep plants in below 45°F.
5. Check the soil, not just the calendar. Take soil temperature readings on three consecutive days before direct-sowing warm-season crops.
6. Keep a cover handy. A row cover buys a few degrees on a borderline night. It will not save tomatoes from a hard freeze.

Healthy soil helps all of this. Seedlings establish faster in beds with good structure and drainage. See Building Garden Soil for what the research supports there.

Where the sources disagree

Most of the guidance above is consistent from one extension service to the next. A few points are not, and it is better to know where the uncertainty is:

• Optimal germination ranges. Extension tables built from different data sets disagree on some crops' optimal ranges, though they generally agree on minimums. The details are in the soil temperature article.
• Light distance. Advice ranges from a few inches for fluorescent tubes to 20 inches for one LED bar in UNH's comparison. The right distance depends on the fixture.
• Row cover protection. Reported values run from about 2°F to 10°F, depending on fabric weight and conditions.

Where to go from here

• Your Last Frost Date Is a Probability — how NCEI freeze tables work and how to choose a date at the risk level you want.
• Soil Temperature for Germinating Vegetable Seeds — minimum and optimal soil temperatures by crop, plus how to measure.
• Hardening Off Transplants — a day-by-day schedule and the temperature limits extension services use.
• Grow Lights for Seedlings — why windowsills fall short and how to set up fluorescent or LED lights.
• Cold Frames, Row Covers and Low Tunnels — how many degrees each one really buys, and how to avoid cooking your plants.

Once plants are in the ground, the next questions are usually about soil and pests. Start Here covers how the whole site is organized.