When does your elderberry bloom?

Have you ever noticed that trees seem to green up earlier in cities than in the countryside? If yes, it is not just a feeling. Urban areas have higher temperatures earlier during the spring. Moreover, it is already known that plants have the tendency to leaf out and bloom earlier in cities.

The phenology of plants refers to the timing of events important in the life cycle of plants such as the bud bursting, leaf development, or the opening of flowers. Phenological studies aim to better understand how the timing of such events is influenced by variations in environmental conditions. However, phenological studies in urban environments usually rely on measurements of temperature and soil moisture at regional scales.

Phenological stages of the elderberry flower, from 0% open on the left to fully blooming on the right.

That’s a big limitation, but we realized that we were having a unique opportunity at hand: we could benefit from the incredible level of precision of the data acquired by the thousands of participants in the ‘CurieuzeNeuzen in de Tuin‘ citizen science project in Flanders and let them track phenology in their garden. This way, we can link the precise temperature and soil moisture data provided by the 3000 mini weather stations spread throughout Flanders to the exact timing of flowering of plants close to them.

Will these local patterns in daytime (left) or nighttime (right) temperatures be reflected in the timing of flowering onset? We’re going to find out!

The set-up is simple: participants with a mini weather station ánd an elderberry (Sambucus nigra) or butterfly bush (Buddleja davidii) in their garden are asked to report on the unfolding of the flowers of these shrubs every three days, until they are fully in bloom. This data on flowering time will then be linked to the actual temperatures in their garden.

Flower stages of the butterfly bush

What we hope to find? For example, we want to determine at what degree the temperature differences between the city environment and the countryside become sufficient to trigger earlier blooms.

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