Everything gardeners need to know: how GDD is calculated, what the 10 phenological seasons mean and when you can safely plant which crops.
The Growing Degree Days value, or GDD, is one of the most important metrics in phenology — the science of how climate influences the development of plants and animals. It indicates how much warmth has accumulated since January 1st of the current year.
Unlike the calendar, GDD does not follow fixed dates — it follows the actual weather. A mild January pushes the GDD value up early; a cold spring keeps it flat. For gardeners, this is far more meaningful than simply looking at the date.
💡 In short: GDD is a heat account that starts at zero on January 1st and increases daily by the mean temperature — but only when that temperature is above 0°C.
The calculation is straightforward: all daily mean temperatures above 0°C are summed from January 1st onwards. Days with negative temperatures do not contribute — but they do not reset the GDD either.
In Germany, the GDD value at year's end typically ranges between 2,500 and 3,800, depending on the region. The Rhine Valley, Kaiserstuhl and Cologne Bay reach significantly higher values than Bavaria or northern Germany.
⚠️ Why do neighbouring locations differ so much? Because small temperature differences accumulate every single day. A location that is 1°C warmer each day will have a 200 GDD advantage after 200 days. Urban heat islands, valley floors and south-facing slopes make this effect particularly noticeable.
The German Meteorological Service (DWD) divides the year into 10 phenological seasons. Unlike the four astronomical seasons, they reflect the true state of nature's development. Each season is defined by specific plant events — flowering, budding or ripening.
The GDD ranges shown below are reference values for the German average. In warmer regions (Rhine, Moselle, Lake Constance) the phases occur at lower values; in cooler areas slightly later.
| # | Season | GDD (approx.) | Indicator plants | Typical period (DE) |
|---|---|---|---|---|
| 1 | Early Spring | 0 – 50 | Hazel blooms, snowdrops | February |
| 2 | First Spring | 50 – 150 | Forsythia, goat willow blooms | March – mid-April |
| 3 | Full Spring | 150 – 300 | Apple blossom, lilac | April – mid-May |
| 4 | Early Summer | 300 – 500 | Black elderflower blooms | May – mid-June |
| 5 | High Summer | 500 – 900 | Linden blooms, wheat ripens | June – August |
| 6 | Late Summer | 900 – 1,200 | Elderberries ripen, heather | August – September |
| 7 | Early Autumn | 1,200 – 1,500 | First leaf colouration | September |
| 8 | Full Autumn | 1,500 – 1,900 | Main leaf fall | October |
| 9 | Late Autumn | 1,900 – 2,200 | Winter linden leafless | October – November |
| 10 | Winter | 2,200+ | No growth, dormancy | November – January |
This is the most practical application of GDD for gardeners. Instead of giving a fixed date — which is unreliable due to weather variability — GDD reflects the true ripeness of the season. Always additionally check the latest frost forecast for your region.
🌡️ Important: The GDD values in the planting calendar are reference values for the north German lowland average. In warmer regions such as the Rhine Valley or wine-growing areas, values may be 20–30% lower. Always additionally check the local 10-day frost forecast.
The calendar gives fixed dates — nature does not. An early spring in 2024 meant that tomatoes could safely be planted outdoors as early as mid-April in some regions, while a late spring in 2021 pushed planting back to the end of May.
GDD solves this problem: a GDD value of 300 always means the same thing — regardless of whether it is reached on April 25th or May 10th. This makes GDD the most reliable tool for garden planning.
| Region | Annual GDD (approx.) | Climate character |
|---|---|---|
| Upper Rhine / Kaiserstuhl | 3,400 – 3,800 | Warmest region in Germany |
| Rhine-Main / Cologne | 3,000 – 3,400 | Mild, few frost days |
| Berlin / Brandenburg | 2,700 – 3,100 | Continental, drier |
| Bavaria (lowlands) | 2,800 – 3,200 | Warmer summers, cold winters |
| Northern Germany / coast | 2,500 – 2,900 | Cool, more consistent |
| Pre-Alps / uplands | 1,800 – 2,500 | Cool, high precipitation |
| Vienna / eastern Austria | 3,200 – 3,600 | Continental, warm |
| Zurich / Swiss Plateau | 2,600 – 3,000 | Temperate |
GDD is not only relevant for vegetable gardeners. Anyone growing plants outdoors that require a long, warm summer — whether Mediterranean herbs, chilli varieties or other heat-loving crops — benefits greatly from GDD as a planning tool.
The basic rule for all heat-loving outdoor crops: do not transplant before GDD 280–300, and always keep an eye on the 14-day weather forecast. Late frosts can still occur into mid-May — the so-called "Ice Saints" (11–15 May) are a well-known example.
GrowPilot.guide calculates the current GDD for your location directly in the app — based on your postcode. At a glance you can see which phenological season is currently active in your area.
GrowPilot.guide shows you the current Growing Degree Days for your location — including phenological season, flowering calendar and outdoor growing recommendations.
Show GDD for my location →GDD always starts at January 1st at zero and is summed daily. There is no reset during frost — negative temperatures are simply ignored (no negative contribution).
A GDD value of 200 corresponds to Full Spring in most regions. More robust plants such as lettuce, spinach, carrots and kohlrabi can already be planted. Heat-loving crops such as tomatoes or cucumbers should wait a little longer.
The German Meteorological Service (DWD) publishes official phenological observation data. For daily GDD calculation, platforms like GrowPilot.guide use weather data from the Open-Meteo API, which is based on ECMWF data and local weather stations.
Because the count starts on January 1st and the first weeks of January are often below 0°C. Even if there are occasional warm days, very little accumulates in the first two weeks of the year. This is physically normal and a deliberate part of the concept.