Frost dates and growing degree days are two different planning clocks for a garden. The frost date calculator answers “when”: when does frost risk typically end in spring, and does a crop have time to mature before the first fall frost? The plant growth calculator answers “how much”: how much heat has accumulated for a crop whose base temperature you enter. One works with the calendar; the other with sums of temperature.
What each calculator does
The gardening frost date calculator takes a USDA hardiness zone (displayed for context only), a last spring frost date, a first fall frost date, and crop switches. The frost-free season is the calendar-day difference between first fall frost and last spring frost. For each crop it computes outdoor planting as last frost plus 7 × weeks-before-frost, indoor start as last frost minus 7 × indoor-start weeks, and harvest start as planting plus days to maturity, with the harvest window ending 30 days later or at first frost, whichever comes first. Peas and spinach are scheduled six weeks before the last frost, tomatoes and peppers on the last frost date, while squash and corn currently schedule two weeks before the last frost — the page flags that negative offset as likely backwards for heat-loving crops and advises caution. It also shows cold-frame coverage from 28 days before last frost through 28 days after first frost, and row cover from 14 days before through 14 days after.
The plant growth calculator takes a day’s minimum and maximum temperatures in °C, a base temperature from a named crop or project reference, and the previous cumulative growing degree days. It computes the daily mean as (minimum + maximum) ÷ 2, daily GDD as max(0, mean − base), and the new cumulative total as the previous total plus daily GDD, displayed to one decimal place. In its example, a day with 10°C and 24°C against a 10°C base produces a mean of 17°C and 7.0 daily GDD. The method applies no upper-temperature cutoff, hourly integration, or date projection.
Side-by-side
| Frost date calculator | Plant growth calculator | |
|---|---|---|
| Inputs | Hardiness zone (context), last frost date, first frost date, crop switches | Daily minimum and maximum °C, base temperature, previous cumulative GDD |
| Primary output | Frost-free days and per-crop planting, indoor start, and harvest dates | Daily and cumulative growing degree days |
| Formula basis | Calendar-day differences; planting = last frost + 7 × weeks offset; harvest = planting + days to maturity | mean = (min + max) ÷ 2; daily GDD = max(0, mean − base) |
| Time basis | Calendar dates — seasonal timing | Accumulated heat units over a temperature series |
| Crop context | Six built-in crops with fixed offsets and maturity values | User-entered base from a named crop or project reference |
| Scope limits | No southern-hemisphere years crossing New Year, no soil temperature, no cultivar-specific maturity | No upper-temperature cutoff, no hourly integration, no date projection |
When to use which
Start with the frost date calculator to build the calendar framework: whether a crop fits the frost-free season, when to start seeds indoors, when to plant outdoors, and when to consider row cover or a shorter-season variety. Frost dates tell you the safe window.
Once plants are in the ground, switch to the plant growth calculator to log heat accumulation. Add each day’s minimum and maximum against the crop’s base temperature, carry the running total forward, and compare it with a crop-specific threshold only when that threshold and this exact temperature method are supported by the same applicable reference. Frost dates tell you when the window opens and closes; growing degree days tell you how much of the window’s warmth has actually arrived. Both depend on inputs you supply or source locally — neither is a forecast, and neither should replace current frost and freeze warnings.
Limits and disclaimer
Both pages are planning tools, not guarantees. Frost dates are averages: individual years can bring late cold snaps, early freezes, or protected microclimates, and the page itself notes that the squash and corn offsets currently schedule heat-loving crops before the last frost, which should be treated cautiously. Growing degree days are method-dependent — a base from one crop, cultivar, location, or method should not be silently reused for another, and small measurement error near the base changes whether a day contributes zero or a positive amount. Neither tool diagnoses plant condition, watering, nutrient, pest, or harvest guidance.