How GrowByDate Calculates Frost Dates, Growing Degree Days and Crop Timing

Frost timing

How frost dates are used

Frost dates are used as planning reference points, not promises about the weather in a specific year. GrowByDate pages commonly use a typical last spring frost date, a typical first fall frost date, and a derived frost-free season length to describe the outdoor growing window for a location.

A frost-date average is best treated as a planning middle point. Gardeners who want a safer planting date usually wait longer after the typical last spring frost, while gardeners using protection, transplants, raised beds, or warm microclimates may start earlier.

Seasonal heat

How growing degree days are used

Growing degree days, often shortened to GDD, estimate how much useful heat a crop receives over time. GrowByDate uses GDD because two places with the same frost-free season length can feel very different to tomatoes, peppers, melons, corn, squash, and other heat-sensitive crops.

The site most often compares locations using base-50°F growing degree days, a common garden-friendly threshold for many warm-season crops. The exact base temperature is a simplification: different crops respond differently to cool and warm conditions, but a consistent base makes locations easier to compare.

Crop assumptions

How crop heat requirements are estimated

Crop heat requirements are planning estimates used to compare a crop's expected needs with a location's typical seasonal heat. They are not exact biological limits. Variety speed, transplant size, soil temperature, daylight, stress, irrigation, pests, and harvest stage can all change the real-world result.

GrowByDate uses crop-level heat targets to help answer practical questions such as whether a crop is usually comfortable, tight, risky, or unrealistic outdoors in a given location. A fast cherry tomato, a large slicing tomato, and a paste tomato may not behave exactly the same, so crop-level targets should be read as planning signals rather than guarantees.

Planting windows

How planting windows are built

Planting windows combine frost timing, crop temperature tolerance, and the way the crop is usually started. Cool-season crops can often be planted before or after the frost-free window. Tender warm-season crops usually depend more heavily on the last spring frost date, soil warmth, and the time left before fall frost.

Local crop fit

How GrowByDate thinks about margin

Many GrowByDate crop-location pages are built around margin: the difference between what a crop usually needs and what a location usually provides from a realistic planting window. A large positive margin means the crop has more room for slow starts, cooler weeks, and normal garden mistakes. A narrow or negative margin means variety speed, transplant timing, protection, and site warmth matter more.

Derived datasets

How rankings and comparison pages are created

GrowByDate ranking pages are derived from the same underlying planning signals used across the site: frost-free days, growing degree days, crop heat requirements, crop timing margin, and short-season risk. Rankings are designed to make comparisons easier, not to declare one city or crop universally better than another.

For example, a city may have a long frost-free season but modest heat accumulation, while another city may have fewer frost-free days but stronger summer heat. That is why GrowByDate separates season length from seasonal warmth instead of treating them as the same thing.

Important limits

What the data cannot know about your garden

Location-based planning data is a starting point. It cannot fully account for the differences between two gardens in the same city, or even two beds in the same yard.

• Microclimates: slopes, cold pockets, shelter, wind exposure, pavement, buildings, and tree cover can shift local frost risk and warmth.
• Soil temperature: warm air does not always mean warm soil, especially in spring.
• Elevation and water effects: hills, valleys, lakes, oceans, and rivers can change frost and heat patterns.
• Weather in the current year: a late frost, cold June, heat wave, hailstorm, or smoky period can overpower the average.
• Variety differences: days-to-maturity labels and crop heat needs vary by cultivar and seed source.
• Garden management: transplants, row cover, mulch, irrigation, pruning, fertility, pest pressure, and disease can all change outcomes.

Practical use

How gardeners should use GrowByDate data

1. Use the local page as your baseline. Start with your city or nearest comparable location.
2. Check both frost dates and GDD. Do not assume a long season is always a warm season.
3. Choose the right crop strategy. Tight crops need faster varieties, earlier starts, warmer beds, or season extension.
4. Adjust for your actual garden. A protected raised bed may behave differently than an open, windy, low-lying garden.
5. Watch the current forecast. GrowByDate helps with planning; it does not replace short-term weather decisions.

Explore the data

Related GrowByDate references