The index is always updated at the end of May. Ten different phenological phases are taken into account for the purpose of determining the spring index:

• 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

These observations come from the MeteoSwiss phenological monitoring network.
Around 80 stations with sufficiently long measurement series are taken into account in the calculation of the spring index. The onset of spring vegetation growth is heavily dependent on temperature.