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Spring index

The spring index shows the difference (in days) in spring vegetation growth onset compared to the long-term average for the period 1991 to 2020. The phenological spring phases are summarized in the annually determined index. Since temperature is one of the central factors influencing vegetation growth onset, the spring index is an effective tool for measuring the effects of climate change on vegetation.


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The spring index is determined by means of the first ten phenological spring phases of the year and updated at the end of each May. It draws on the observations documented in the corresponding year at the nearly 80 stations in the phenological measurement network, using datasets of adequate length.

The spring of 2023

In 2023, spring vegetation developed 3 days earlier than the long-term average from 1991-2020 and can thus be classified as "normal". From the beginning of January, the hazel bushes flowered earlier than at any time since the beginning of their observation in 1953. Overall, the hazel flowering had a lead of 23 days over the long-term mean. The coltsfoot flowered from mid-February and the wood anemone from mid-March with an advance of 6 - 9 days. The fruit trees flowered in April within the normal time frame, as the cool April weather slowed down the development of the vegetation. Also within the normal time frame, the leaves of the deciduous trees sprouted from the beginning of April, with a slight delay of 0 - 4 days. The unfolding of the beech leaves was observed more frequently from 20 April.

Calculating the spring index

The following ten phenological phases are used to characterise the phenological spring as a whole; they occur between January and May:

  • Hazel bush flowering
  • Coltsfoot flowering
  • Wood anemone flowering
  • Cherry tree flowering
  • Leaf unfolding in horse chestnut
  • Leaf unfolding in hazel bush
  • Needle appearance in larch
  • Dandelion flowering
  • Lady’s smock flowering
  • Leaf unfolding in beech

The deviation from the average occurrence date is determined by means of a main component analysis. This method is practical for structuring, simplifying and visualising complex datasets. In addition, it can be used to filter spatial and chronological dependencies. The result of the first main component is then converted back into number of days’ deviation from the average.

Further information

Monthly and annual reports (German, French, Italian)