How is the planting calendar worked out?

Extension guides give timing relative to frost, such as "start indoors 6-8 weeks before the last frost" or "plant 2 weeks after the last frost". The calendar applies that timing to the typical frost dates of the NOAA station that best matches your ZIP code.

Why does the calendar put tomatoes later than my last frost date?

Tomatoes, peppers, melons and other warm-season crops need warm soil as well as no frost. Where springs stay cool after the last frost, the calendar waits until the normal daily average temperature is warm enough, so tomatoes and peppers may come weeks after the frost date.

What is fall planting?

A second planting in mid to late summer of crops that mature in cool weather, such as broccoli, kale, carrots and lettuce. The calendar counts back from your typical first fall frost so they mature before hard freezes.

Does the calendar work in Florida or southern California?

Where frost happens in fewer than half of years, frost-based dates do not apply, and the calendar says so. Each plant guide then gives the planting months recommended by Extension services in Florida, Texas, Arizona, California and Hawaii.

Planting Calendar by ZIP Code: What to Plant Each Month

How the calendar is built

Extension services time most vegetables, herbs and annual flowers from two dates: the average last spring frost and the average first fall frost. Clemson’s planning guide works this way, using days to maturity and the average freeze dates to choose planting times. The calendar does the arithmetic for you. For each plant it stores the recommended timing, such as “start indoors 6-8 weeks before the last frost” or “sow 10 weeks before the first fall frost”, and applies it to the frost dates of the NOAA weather station that best matches your ZIP code, from the 1991-2020 climate normals.

Each month in your calendar lists three kinds of work:

Start indoors: seeds to sow in trays under lights, so transplants are ready at planting time. The seed-starting page covers how.
Plant or sow outdoors: transplants to set out and seeds to sow directly in the garden.
Plant for fall: a second round of crops that mature in cool autumn weather.

Cool-season and warm-season crops

Clemson divides vegetables into two groups by temperature. Cool-season crops such as peas, spinach, lettuce, cabbage, broccoli and onions grow best in cool weather and decline in summer heat. Many tolerate light frost, so they appear in the calendar weeks before your last frost date, and again in late summer for fall. Warm-season crops such as tomatoes, peppers, beans, squash, cucumbers and melons come from warmer climates, need warm soil to germinate and grow, and are injured or killed by frost. They go out after the last frost.

That is why a spring calendar starts long before the frost date. In a typical year the first outdoor sowings are hardy crops such as peas and spinach, the half-hardy crops such as lettuce and carrots follow, and the tender crops come last.

Soil temperature: the second check

The frost date is about air temperature, and soil warms more slowly. Wisconsin Extension points out that seed sown into cold soil may rot, germinate slowly or produce stunted plants. The differences are large: in a University of California trial published by Oregon State, tomato seed took about 43 days to come up at 50°F soil and about 6 days at 77°F, while bean seed barely germinated at all at 50°F.

Frost dates alone would put warm-season crops too early where spring is frost-free but cool, as along the Pacific Northwest coast. NOAA’s normals do not include soil temperature, so for crops that need warm soil the calendar also waits until your station’s normal daily average air temperature reaches the crop’s minimum soil temperature, and moves indoor seed-starting back to match.

To check your own soil, Wisconsin recommends a soil thermometer pushed 2 inches deep for seeds and 4-6 inches for transplants, read around late morning on three days in a row.

Fall planting

Late summer is the second planting season. Crops such as broccoli, kale, carrots, lettuce and turnips are sown or set out so they mature in the cool weeks around your first fall frost, and Clemson notes cool-season crops keep growing past the first light freeze but should be started early enough to mature before hard freezes. The calendar counts these dates back from your typical first fall frost. Fall crops of warm-season vegetables such as tomatoes and potatoes appear only where the season is long enough for them.

When to trust the weather over the calendar

The dates describe a typical year at your weather station. Any single spring can run weeks early or late, and a low spot in your yard can frost when the station does not. Check the forecast before setting out tender plants and cover them if frost threatens. Where frost is rare, as in much of Florida, southern Arizona and coastal California, the calendar tells you so, and the plant guides list the planting months local Extension services recommend.

Planting Calendar by ZIP Code