Growing Season Calculator

Frost-free days and succession rounds from two frost dates.

What it does

The growing season is the frost-free gap: days between the last spring frost and the first fall frost. A garden at Apr 15 to Oct 20 holds 188 days, enough for tomatoes plus three rounds of beans. One date subtraction plans the whole year.

The tool above counts that gap in UTC days so time zones never blur it. Enter both median frost dates and the crop days to maturity, and it prints season days, weeks, and succession rounds. Defaults show 188 days with 3 rounds of a 60-day crop.

NOAA frames the inputs. Its freeze pages define first and last freeze at 32 F from 1991-2020 Climate Normals, with median maps meaning the date held in 5 of 10 years. South Dakota State Extension defines the frost-free season exactly as the days between those two medians.

Succession turns days into harvests. Minnesota Extension times fall plantings backward from the average first frost using days to maturity: bush beans at 45 to 65 days die at frost while kale at 40 to 65 days stands to about 20 F. Count rounds, then sow the last one its maturity days before the fall date.

How to use

1. Enter the last spring frost date (median, 32 F).
2. Enter the first fall frost date.
3. Enter days to maturity for the crop.
4. Read season days first, then weeks.
5. Read succession rounds for repeat sowings.
6. Time fall crops backward from the first frost.

Formula

last frost

Median last 32 F date in spring.

first frost

Median first 32 F date in fall.

crop days

Days from sowing to harvest for the crop.

NOAA normals use the 1991-2020 period with median dates. SDSU defines the frost-free season exactly as this difference.

Worked example

Reference table

Limits

• Station medians only. Yards vary by days to weeks.
• Single 32 F line; killing frost at 28 F differs.
• Ignores heat, rain, and day length limits on crops.
• Normals shift as climate periods update.

Common mistakes

• Using average dates as guarantees instead of medians.
• Planning tender crops to the 28 F killing frost.
• Ignoring days to maturity on fall sowings.
• Trusting one station for a slope or valley garden.
• Skipping hardening on late transplants.

Try it yourself

Quick checks. Answers sit tucked under each question.

Questions

Sources

• NOAA NCEI: When to Expect the First Fall Freeze
• NOAA NCEI: When to Expect Your Last Spring Freeze
• SDSU Extension: Last Spring Frost Dates (median method)

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