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Gardening glossary

Growing degree days (GDD)

Growing degree days (GDD) are how commercial growers and increasingly hobbyists predict when a crop will flower, when fruit will ripen, and when a pest will emerge — independent of the calendar. The principle: a plant accumulates physiological progress in proportion to how warm it has been, not how many days have passed. A cool spring and a hot spring will get a tomato to ripeness at very different calendar dates, but at roughly the same GDD total.

The formula in simple form:

`GDD for one day = (T_max + T_min) / 2 − T_base`

If the daily mean falls below the base temperature, the day contributes zero — temperatures below the base do nothing for the plant. Many calculators also cap T_max (typically at 30 °C), since most crops do not grow faster above that ceiling either.

Base temperatures and target GDD by crop:

- **Sweetcorn** — base 10 °C, ripens around 1,200–1,500 GDD. - **Tomato** — base 10 °C, first fruit at 1,000–1,200 GDD; full ripening 1,500–2,500 GDD depending on variety. - **Beans** — base 10 °C, harvest at 600–800 GDD. - **Lettuce** — base 4 °C, harvest at 300–500 GDD. - **Apples** — base 10 °C, "Bramley" picking window around 1,400 GDD from full bloom.

Pest emergence is also GDD-driven. Codling moth, carrot fly, and the asparagus beetle all emerge within tight GDD windows that are far more predictable than calendar dates.

How a home gardener can use GDD:

- **Decide when to expect first ripe tomato.** If your variety needs 1,200 GDD and your local average is roughly 8–12 GDD per day in midsummer, that is 100–150 days from planting out — useful for choosing varieties when your season is short. - **Plan succession sowings.** Track GDD instead of calendar weeks for more reliable harvest timing. - **Anticipate pest sprays.** Carrot root fly emerges around 300–350 GDD; codling moth around 250 GDD from January 1.

The UK Met Office, Cornell, and many extension services publish daily GDD totals by region. Apps like Growli will eventually pipe local GDD straight into your sowing calendar so you do not need to do the maths.

Where this comes up in our guides

Related terms

Hardiness zone

A hardiness zone is a geographic band defined by average annual minimum winter temperature. The USDA system uses 13 zones in 10°F steps; in the UK, RHS H1–H7 ratings serve a similar purpose for perennials and shrubs.

Growing season

The growing season is the stretch of days each year when conditions allow plants to actively grow — most commonly defined as the frost-free window between last spring frost and first fall frost. It ranges from about 90 days in cold climates to year-round in subtropical zones.

Last frost

The last frost (or last spring frost) is the average date of the final freezing temperature of spring at your location. It is the anchor date for transplanting frost-tender crops — tomatoes, peppers, basil, beans, and squash — outdoors.

First frost

The first frost (or first fall frost) is the average date of the season's first freezing temperature in autumn. It ends the growing season for warm-weather crops and is the anchor date for planning final harvests, garlic planting, and cover-crop sowing.

Chill hours

Chill hours measure the cumulative time a fruit tree spends between roughly 32°F and 45°F (0–7°C) during winter dormancy. Deciduous fruit trees like apples, peaches, and cherries need a specific minimum to break dormancy and bloom normally in spring.